*Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu Province, P.R. China*

*National Key Lab of Electromagnetic Environmental Effect and Electro-optical Engineering, Nanjing,*

This work is supported by EU-FP7 SICA programme (Grant No.FP7-PEOPLE-2010-IRSES-

[2] Jones RM. Mechanics of composite materials (2nd edition). Philadelphia PA: Taylor &

[3] Miracle DB, Donaldson SL. Introduction to Composites. In: Miracle DB, Donaldson SL (ed.) ASM Handbook Volume 21: Composites. ASM International; 2001. p3-17 [4] Awad ZK, Aravinthan T, Zhuge Y, et al. A review of optimization techniques used in the design of fibre composite structures for civil engineering applications. Materials and

[5] Montalvao D, Maia NMM and Ribeiro AMR. A Review of Vibration-based Structural Health Monitoring with Special Emphasis on Composite Materials. The Shock and

[6] Qing XL, Kumar A, Zhang C. A hybrid piezoelectric/fiber optic diagnostic system for structural health monitoring. Smart Materials and Structures 2005; 14(3) S98-S103. [7] Qing XL, Chan HL, Beard SJ, et al. An Active Diagnostic System for Structural Health Monitoring of Rocket Engines. Journal of intelligent material systems and structures

[8] Ihn JB, Chang FK. Pitch-catch Active Sensing Methods in Structural Health Monitoring

[9] Lia HN, Lia DS and Song GB. Recent applications of fiber optic sensors to health monitoring in civil engineering. Engineering Structures 2004; 26(11): 1647-1657.

for Aircraft Structures. Structural Health Monitoring 2008; 7(1): 5-19.

[1] Campbell FC. Structural Composite Materials. Ohio: ASM International; 2010.

impact location in an aircraft composite wing box, are also arranged.

for the further practical SHM applications in composite materials.

Jian Cai, Lei Qiu, Shenfang Yuan, PeiPei Liu and Dong Liang *The Aeronautic Key Lab for Smart Materials and Structures,*

269202), Natural Science Foundation of China (Grant No.50830201).

**Author details**

*Jiangsu Province, P.R. China*

**Acknowledgement**

**6. References**

Francis; 1999.

Design 2012; 33(1) 534-544.

2006; 17(7): 619-628.

Vibration Digest 2006; 38(4) 295-324.

Lihua Shi

**Table 3.** The coordinates (x, y) of PZT wafers and impacts in the top panel

The impact responses are fed into the integrated SHM system. Figure 22 shows a waterfall plot of normalized responses produced by the impact at IST1. Here, the narrow-band components with central frequency of 100kHz are extracted from all the responses and their envelopes are then computed for arrival time determination, which can be performed with the complex wavelet transform [57]. After the arrival times are decided by the first peaks in the obtained envelopes, the impact can be located based on equation (3). Note that the anisotropic properties in the composite panel should be considered during impact location. The location result is given in table 4, in which the location error is defined as the spatial interval between the actual and the estimated impact sites.

**Figure 22.** Acquired impact responses [52]

**Table 4.** Impact location result in the carbon-fiber composite panel

## **5. Summary and conclusions**

SHM for composite materials is briefly described in the chapter. Firstly, an introduction involving advantages, problems and SHM requirements of composites is made. The common sensors used in SHM as well as typical SHM methods for composite materials are then introduced. Two examples, Lamb wave imaging for a glass-fiber composite plate and impact location in an aircraft composite wing box, are also arranged.

Though much development of SHM has been achieved, a great deal of work is still required for the further practical SHM applications in composite materials.

## **Author details**

Jian Cai, Lei Qiu, Shenfang Yuan, PeiPei Liu and Dong Liang *The Aeronautic Key Lab for Smart Materials and Structures, Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu Province, P.R. China*

Lihua Shi

56 Composites and Their Applications

(x, y)/(mm) (x, y)/(mm)

P1 (-210 , 225) P20 (-200 , 75) P3 (210 , -75) P22 (-200 , -225) P4 (210 , -225) IST1 (-120 , 150) P7 (-100 , 225) IST2 (0 , 150) P16 (-50 , -225) IST3 (0 , 0) P19 (-200 , 225) IST4 (0 , -150)

The impact responses are fed into the integrated SHM system. Figure 22 shows a waterfall plot of normalized responses produced by the impact at IST1. Here, the narrow-band components with central frequency of 100kHz are extracted from all the responses and their envelopes are then computed for arrival time determination, which can be performed with the complex wavelet transform [57]. After the arrival times are decided by the first peaks in the obtained envelopes, the impact can be located based on equation (3). Note that the anisotropic properties in the composite panel should be considered during impact location. The location result is given in table 4, in which the location error is defined as the spatial

**Table 3.** The coordinates (x, y) of PZT wafers and impacts in the top panel

interval between the actual and the estimated impact sites.

Impact Number Impact location result

**Table 4.** Impact location result in the carbon-fiber composite panel

Actual site Estimated site Location error

IST1 (-120mm, 150mm) (-130mm, 125mm) 27mm IST2 (0mm, 150mm) (15mm, 120mm) 16mm IST3 (0mm, 0mm) (0mm, -20mm) 20mm IST4 (0mm, -150mm) (-20mm, -120mm) 36mm

SHM for composite materials is briefly described in the chapter. Firstly, an introduction involving advantages, problems and SHM requirements of composites is made. The

**Figure 22.** Acquired impact responses [52]

**5. Summary and conclusions**

*National Key Lab of Electromagnetic Environmental Effect and Electro-optical Engineering, Nanjing, Jiangsu Province, P.R. China*

## **Acknowledgement**

This work is supported by EU-FP7 SICA programme (Grant No.FP7-PEOPLE-2010-IRSES-269202), Natural Science Foundation of China (Grant No.50830201).

## **6. References**

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[26] Quek ST, Tua PS, Jin J. Comparison of Plain Piezoceramics and Inter-digital Transducer for Crack Detection in Plates. Journal of Intelligent Material Systems and Structures

[27] Su ZQ, Ye L, Bu XZ. A damage identification technique for CF/EP composite laminates using distributed piezoelectric transducers. Composite Structures 2002; 57(1-4): 465-471. [28] Park HW, Sohn H, Law, KH, et al. Time reversal active sensing for health monitoring of

[29] Michaels JE. Detection, localization and characterization of damage in plates with an in situ array of spatially distributed ultrasonic sensors. Smart Materials and Structures

[30] Cai J, Shi LH, Yuan SF, et al. High spatial resolution imaging for structural health

[31] Wang HC, Rose JT, Chang FG. A synthetic time-reversal imaging method for structural

[32] Wang Q, Yuan SF. Baseline-free Imaging Method based on New PZT Sensor Arrangements. Journal of Intelligent Material Systems and Structures 2009; 20(14): 1663-

[33] Giurgiutiu V, Bao J. Embedded-ultrasonics Structural Radar for In Situ Structural Health Monitoring of Thin-wall Structures. Structural Health Monitoring 2004; 3(2):

[34] Wilcox PD. Omni-directional guided wave transducer arrays for the rapid inspection of large areas of plate structures. IEEE Transactions on Ultrasonics, Ferroelectrics and

[35] Yan F, Royer RL, Rose JL. Ultrasonic Guided Wave Imaging Techniques in Structural Health Monitoring. Journal of Intelligent Material Systems and Structures 2010; 21(3):

[36] Leonard KR, Malyarenko EV, Hinders MK. Ultrasonic Lamb wave tomography. Inverse

[37] Zhao XL, Gao HD, Zhang GF, et al. Active health monitoring of an aircraft wing with embedded piezoelectric sensor actuator network. Smart Materials and Structures 2007;

[38] Ayres JW, Lalande F, Chaudhry Z, et al. Qualitative impedance-based health monitoring of civil infrastructures. Smart Materials and Structures 1998; 7(5): 599-605. [39] Giurgiutiu V, Rogers C A. 1998 Recent advancements in the electro-mechanical (E/M) impedance method for structural health monitoring and NDE Proc. Conf. on Smart

[40] Gyuhae P, Sohn H, Farrar CR, et al. Overview of Piezoelectric Impedance-Based Health Monitoring and Path Forward. The Shock and Vibration Digest 2003; 35(6): 451-463. [41] Farrar CR, Doebling SW, Nix DA. Vibration-based structural damage identification. Philosophical transactions of the Royal Society. Mathematical, physical, and

[42] Messina A, Williams EJ, Contursi T. Structural damage detection by a sensitivity and statistical-based method. Journal of Sound and Vibration 1998; 216(5): 791-808.

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[11] Speckmann H, Henrich R. Structural health monitoring (SHM) - overview on technologies under development. In: Proceedings of the 16th world conference on NDT

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[13] Boller C. Next generation structural health monitoring and its integration into aircraft

[14] Raghavan A, Cesnik CES. Review of Guided-wave Structural Health Monitoring. The

[15] Sodano HA. Development of an Automated Eddy Current Structural Health Monitoring Technique with an Extended Sensing Region for Corrosion Detection.

[16] Zilberstein V, Walrath K, Grundy D, et al. MWM eddy-current arrays for crack initiation and growth monitoring. International Journal of Fatigue 2003; 25(9-11): 1147-

[17] Goldfine N, Zilberstein V, Schlicker D, et al., 2001, Surface Mounted Periodic Field Eddy Current Sensors for Structural Health Monitoring, in Proc. Of SPIE, Advanced Nondestructive Evaluation for Structural and Biological Health Monitoring, ed. T.

[18] Varadan VK, Varadan VV. Microsensors, microelectromechanical systems (MEMS), and electronics for smart structures and systems. Smart Materials and Structures 2000; 9(6):

[19] Yuan SF, Liang DK, Shi LH, et al. Recent Progress on Distributed Structural Health Monitoring Research at NUAA. Journal of Intelligent Material Systems and Structures

[20] Qing XL, Beard SJ, Kumar A, et al. Advances in the development of built-in diagnostic system for filament wound composite structures. Composites Science and Technology

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[22] Lemistre MB, Balageas DL. A Hybrid Electromagnetic Acousto-ultrasonic Method for SHM of Carbon/epoxy Structures. Structural Health Monitoring 2003; 2(2): 153-160. [23] Giurgiutiu V. Structural Health Monitoring with Piezoelectric Wafer Active Sensors.

[24] Bottai GS, Chrysochoidis NA, Giurgiutiu V, et al. Analytical and experimental evaluation of piezoelectric wafer active sensors performances for Lamb waves based

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[43] Pascual R, Golinval JC, Razeto M. A frequency domain correlation technique for model correlation and updating. International Modal Analysis Conference (IMAC), 15th, Orlando, FL; UNITED STATES; 3-6 Feb. 1997. pp. 587-592. 1997.

**Chapter 4**

© 2012 Jee and Lee, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Jee and Lee, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Acoustic Emission of Composite Vessel**

There are about 10 million vehicles run on natural gas in the world. There are about 1.7 million low-pressure LPG vehicles [1], and 17,000 high-pressure CNG vehicles running in

Generally for CNG vehicles, type II vessel, where the increase in used pressure and lighter weight are achieved through fiber-reinforced composite material, which is wrapped in hoopdirection on the steel liner is used. Since 1984, the U.S. experienced more than 80 cases of vehicle fuel tank-related accidents [3] and Korea also experienced 8 cases in which the CNG tank exploded; thus, there is a need for the development of inspection technology for highpressure fuel tanks. In the case of the U.S., the inspection technology of high-pressure fuel tanks were developed by DOD and NASA as an inspection technology for missile fuel tanks[4] but as the use of high-pressure fuel tanks for transport increased DOT executed a research on inspection technology for vehicles based on the research results of NASA and reported that among several NDT technology, Acoustic Emission(AE) has a possibility of being used as an inspection technology for vehicles[5,6]. The gas vessel, which is made of fiber-reinforced composite material, is unlike vessel made of only steel materials in that when the damage increases the acoustic generation activity increases but when the degree of damage increases even more, the acoustic generation activity rather decreases [7]. A study of defect detection and failure analysis for composite Materials using acoustic emission is progressing steadily [9-11,14].

Experiment vessel used in this research is a 64 Liter CNG fuel tank used in vehicles. The thickness of the liner in the shell is about 6 mm and was made using the DDI (Deep Drawing Ironing) method[12,13] using 34CrMo4 steel plate, and is a type-II vessel in which

Hyun-Sup Jee and Jong-O Lee

Korea and the number is increasing.[2]

http://dx.doi.org/10.5772/47877

**1. Introduction**

**2. Experiment**

**2.1. Experimental vessel**

glass fiber is hoop-wrapped on the shell of the liner.

Additional information is available at the end of the chapter

## **Chapter 4**

## **Acoustic Emission of Composite Vessel**

Hyun-Sup Jee and Jong-O Lee

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/47877

## **1. Introduction**

60 Composites and Their Applications

10(6): 713-721.

[43] Pascual R, Golinval JC, Razeto M. A frequency domain correlation technique for model correlation and updating. International Modal Analysis Conference (IMAC), 15th,

[44] Zang C, Friswell MI, Imregun M. Structural Health Monitoring and Damage Assessment Using Measured FRFs from Multiple Sensors, Part I: The Indicator of

[45] Doebling SW, Farrar CR, Prime MB. Review of Vibration-Based Damage Identification

[46] Gostautas RS, Ramirez G, Peterman RJ. Acoustic Emission Monitoring and Analysis of Glass Fiber-Reinforced Composites Bridge Decks. Journal of Bridge Engineering 2005;

[47] Nair A, Cai CS. Acoustic emission monitoring of bridges: Review and case studies.

[48] Huang M, Jiang L, Liaw PK. Using Acoustic Emission in Fatigue and Fracture Materials

[49] Groot PJ, Wijnen PA, Janssen RB. Real-time frequency determination of acoustic emission for different fracture mechanisms in carbon epoxy composites. Composites

[50] Morscher GN, Modal acoustic emission of damage accumulation in a woven SiC/SiC

[51] Surgeon M, Wevers M. Modal analysis of acoustic emission signals from CFRP

[52] Qiu L, Yuan SF, Zhang XY, et al. A time reversal focusing based impact imaging method and its evaluation on complex composite structures. Smart Materials and

[53] Silva-Munoz RA, Lopez-Anido RA. Structural health monitoring of marine composite structural joints using embedded fiber Bragg grating strain sensors. Composite

[54] Mrad N. State of Development of Advanced Sensory Systems for Structural Health Monitoring Applications. Proceedings of the NATO RTO AVT-144 Workshop on Enhanced Aircraft Platform Availability Through Advanced Maintenance Concepts and

Technologies, Vilnius, Lithuania, 3-5 October 2006 (DRDC Atlantic SL-2008-260). [55] Wishaw M, Barton DP. Comparative Vacuum Monitoring: a New Method of In-Situ Real-Time Crack Detection and Monitoring. In: Proceding of 10th Asia-Pacific

[56] Stehmeier H, Speckmann H. Comparative Vacuum Monitoring (CVM) Monitoring of fatigue cracking in aircraft structures. 2nd European Workshop on Structural Health Monitoring July 7-9, 2004 Amazeum Conference Centre at Deutsches Museum, Munich,

[57] Jeong H. Analysis of plate wave propagation in anisotropic laminates using a wavelet

Correlation Criteria. Key Engineering Materials 2003; 245-246(131): 131-140.

Orlando, FL; UNITED STATES; 3-6 Feb. 1997. pp. 587-592. 1997.

http://www.tms.org/pubs/journals/JOM/9811/Huang/Huang-9811.html

composite. Composites Science and Technology 1999; 59(5): 687-697.

Methods. Shock and Vibration Digest 1998; 30(2): 91-104.

Engineering Structures 2010; 32(6): 1704-1714.

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laminates. NDT&E International 1999; 32(6): 311-322.

Conference On Nondestructive Testing, 2001. Brisbane.

transform. NDT&E International 2001; 34(3): 185-190.

Research. JOM-e 1998; 50(11): 1-14.

Structures 2011; 20(10): 105014.

Structures 2009; 89(2): 224-234.

Germany.

There are about 10 million vehicles run on natural gas in the world. There are about 1.7 million low-pressure LPG vehicles [1], and 17,000 high-pressure CNG vehicles running in Korea and the number is increasing.[2]

Generally for CNG vehicles, type II vessel, where the increase in used pressure and lighter weight are achieved through fiber-reinforced composite material, which is wrapped in hoopdirection on the steel liner is used. Since 1984, the U.S. experienced more than 80 cases of vehicle fuel tank-related accidents [3] and Korea also experienced 8 cases in which the CNG tank exploded; thus, there is a need for the development of inspection technology for highpressure fuel tanks. In the case of the U.S., the inspection technology of high-pressure fuel tanks were developed by DOD and NASA as an inspection technology for missile fuel tanks[4] but as the use of high-pressure fuel tanks for transport increased DOT executed a research on inspection technology for vehicles based on the research results of NASA and reported that among several NDT technology, Acoustic Emission(AE) has a possibility of being used as an inspection technology for vehicles[5,6]. The gas vessel, which is made of fiber-reinforced composite material, is unlike vessel made of only steel materials in that when the damage increases the acoustic generation activity increases but when the degree of damage increases even more, the acoustic generation activity rather decreases [7]. A study of defect detection and failure analysis for composite Materials using acoustic emission is progressing steadily [9-11,14].

## **2. Experiment**

## **2.1. Experimental vessel**

Experiment vessel used in this research is a 64 Liter CNG fuel tank used in vehicles. The thickness of the liner in the shell is about 6 mm and was made using the DDI (Deep Drawing Ironing) method[12,13] using 34CrMo4 steel plate, and is a type-II vessel in which glass fiber is hoop-wrapped on the shell of the liner.

© 2012 Jee and Lee, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Jee and Lee, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Acoustic Emission of Composite Vessel 63

**Figure 2.** Diagram of Experimentation : (a) Burst and (b) fatigue test of the composite vessel

burst test was carried out. Figure 3(b) shows the conditions for pressurization.

**3. Result and research**

*3.1.1. Damage mechanism of composite vessel*

liner of that part and finally destroying the vessel.

**3.1. Burst test**

The burst pressure was estimated to be 600 bar under the pressured conditions. By raising the pressure to 30%, 50%, 60%, 70%, 80% and 90% of estimated burst pressure and keeping each pressure stage for 10 minutes as in Figure 3(a), we acquired the AE signal from each stage. As can be seen in the figure, pressure was put on the vessel with a pump to control the pressure and acoustic emission signals were detected using acoustic emission sensors attached to the vessel. And the signal were processed and analyzed after fed into AE equipment. The fatigue test repeated 20000 cycles between 0 and 207 bar, afterwards which has a used pressure of 0 and afterwards, the pressure was continuously increased and the

(b)

(a)

First, the composite materials wrapped in the metal liner were separated due to the matrix crack and then each layer was separated from each other. Then, some section of the fuel tank was weakened due to the cutting of some reinforced fiber, causing the destruction of metal

**Figure 1.** Schematic diagram of experimental vessel

**Table 1.** Chemical composition of vessel liner (unit : wt.%)

## **2.2. Method of experiment**

For the test, the acoustic emission sensor is R15I (PAC) with the resonance frequency of 150 kHz and cable of RG58A/U (10m) is put on the middle of shell using the vacuum grease.

The detected AE signal is put into the DiSP-52 Acoustic emission workstation (PAC) for processing. In addition, the water was used as medium for burst test. The threshold value of test was set at 45dB. The source of simulated sound was the destruction of the 2H Pentel pencil lead. The average sensitivity of sensor was 98dB within 1 inch from sensor.

**Figure 2.** Diagram of Experimentation : (a) Burst and (b) fatigue test of the composite vessel

The burst pressure was estimated to be 600 bar under the pressured conditions. By raising the pressure to 30%, 50%, 60%, 70%, 80% and 90% of estimated burst pressure and keeping each pressure stage for 10 minutes as in Figure 3(a), we acquired the AE signal from each stage. As can be seen in the figure, pressure was put on the vessel with a pump to control the pressure and acoustic emission signals were detected using acoustic emission sensors attached to the vessel. And the signal were processed and analyzed after fed into AE equipment. The fatigue test repeated 20000 cycles between 0 and 207 bar, afterwards which has a used pressure of 0 and afterwards, the pressure was continuously increased and the burst test was carried out. Figure 3(b) shows the conditions for pressurization.

## **3. Result and research**

## **3.1. Burst test**

62 Composites and Their Applications

**Figure 1.** Schematic diagram of experimental vessel

1.00 0.40

P+S ≤0.020

**Table 1.** Chemical composition of vessel liner (unit : wt.%)

0.40 0.10

For the test, the acoustic emission sensor is R15I (PAC) with the resonance frequency of 150 kHz and cable of RG58A/U (10m) is put on the middle of shell using the vacuum grease.

The detected AE signal is put into the DiSP-52 Acoustic emission workstation (PAC) for processing. In addition, the water was used as medium for burst test. The threshold value of test was set at 45dB. The source of simulated sound was the destruction of the 2H Pentel

pencil lead. The average sensitivity of sensor was 98dB within 1 inch from sensor.

0.38 0.25

**2.2. Method of experiment**

Max Min C Mn Si P S Cr Mo

0.015 -

0.010 -

1.20 0.80 0.40 0.15

#### *3.1.1. Damage mechanism of composite vessel*

First, the composite materials wrapped in the metal liner were separated due to the matrix crack and then each layer was separated from each other. Then, some section of the fuel tank was weakened due to the cutting of some reinforced fiber, causing the destruction of metal liner of that part and finally destroying the vessel.

Acoustic Emission of Composite Vessel 65

pressure on the vessel and keeps the pressure for some time until some sound emits from it. This is called the analysis of creep effect. The values in Figure 5 show that the pressure up to 360 bar, or 60% of the expected burst press is weak, showing that the vessel is not likely to receive significant damages. But after 70% of the expected pressure, the signals of 60dB or more are often shown, which may mean that there is a lot of creep effect and that the

(a) (b)

(c) (d)

(e) (f)

significant damages have been done to the vessel.

**Figure 3.** Loading sequence : (a) Burst and (b) fatigue test

**Figure 4.** Composite vessel after burst test

#### *3.1.2. AE signal generated during burst test*

In this test, considering the flow noise during the initial pressure, I excluded the signal during the first two minutes from the data but included the remaining 8 minutes data for evaluation. Generally, for the evaluation of the soundness of vessel, a tester imposes pressure on the vessel and keeps the pressure for some time until some sound emits from it. This is called the analysis of creep effect. The values in Figure 5 show that the pressure up to 360 bar, or 60% of the expected burst press is weak, showing that the vessel is not likely to receive significant damages. But after 70% of the expected pressure, the signals of 60dB or more are often shown, which may mean that there is a lot of creep effect and that the significant damages have been done to the vessel.

64 Composites and Their Applications

**Figure 3.** Loading sequence : (a) Burst and (b) fatigue test

Pressure

Operating Pressure (207 bar)

10 min

**Figure 4.** Composite vessel after burst test

*3.1.2. AE signal generated during burst test*

In this test, considering the flow noise during the initial pressure, I excluded the signal during the first two minutes from the data but included the remaining 8 minutes data for evaluation. Generally, for the evaluation of the soundness of vessel, a tester imposes

(a)

(b)

<sup>0</sup> 4k 8k 12k 16k 20k Cycle

Bursting

Acoustic Emission of Composite Vessel 67

**Figure 7.** Mean rise time during load holding

**Figure 8.** Mean duration during load holding

related to the growth of the cracks.

Unlike the metal vessel, the pressure vessel that is made with composite materials has various damage mechanisms including the matrix crack, crack growth, separation between layers, cutting of fiber, and destruction of metal liner. Figure 9 shows that at the load of 30 % stage, the amplitude of signal rises due to the increased sound emission from matrix crack of composite materials. It also shows that at the load of 50-60 % of the estimated burst pressure, the sound emission is weakened and thus the amplitude of the signal is also reduced. At the load of 70 % pressure, the interlayer separation and the cut of reinforced fiber start and the amplitude of the signal increases again. Then, more reinforcement fibers become severed and the metal liner begins to burst, showing a little increase in amplitude.

As shown below, the composite material pressure vessel has many damage mechanisms in it. The initial damage mechanism contains the matrix crack due to the stress as well as the crack grown. As shown in Figure 10 (a), the rise time appears at the range of 10 μs and 100 μs. This shows the creation of matrix crack and the growth of this crack. The signal from around 10 μs is related to the creation of cracks while the signals from 100 μs look to be

**Figure 5.** Amplitude vs Time plot during load holding : (a)30% (b)50% (c)60% (d)70% (e)80% (f)90% and (g)fractured

The pressure rise to the 80 % and 90 % of the expected burst pressure shows a lot of signals with high amplitudes. At the pressure stage of 540 bar or higher, where the burst is likely to occur, the signals with the amplitude of 80-100 dB continue to occur while giving the continuous leak signal from 1352 seconds to 1361 seconds. The burst occured when the pressure rapidly went down to 400 bar. Figure 6 shows the total hits from 2 minutes after the start of each load. There is a rapid increase in the number of hits when the load pressure goes over 70 % of the expected burst pressure where the significant damage is likely to happen. In the final destruction, the number of total hits is low because the time to destruction was short, considering the felicity ratio and the pressure rising up to the fracture.

**Figure 6.** Total hits during load holding

Figure 7 and 8 show that the average rise time and duration for the signals occurred during the holding time of each load. The rise time and duration become shorter up to loads up to 70 % while those from time of 80 % to the time of burst get longer, during which the damage is likely to be greater.

**Figure 7.** Mean rise time during load holding

and (g)fractured

fracture.

**Figure 6.** Total hits during load holding

is likely to be greater.

**Figure 5.** Amplitude vs Time plot during load holding : (a)30% (b)50% (c)60% (d)70% (e)80% (f)90%

(g)

The pressure rise to the 80 % and 90 % of the expected burst pressure shows a lot of signals with high amplitudes. At the pressure stage of 540 bar or higher, where the burst is likely to occur, the signals with the amplitude of 80-100 dB continue to occur while giving the continuous leak signal from 1352 seconds to 1361 seconds. The burst occured when the pressure rapidly went down to 400 bar. Figure 6 shows the total hits from 2 minutes after the start of each load. There is a rapid increase in the number of hits when the load pressure goes over 70 % of the expected burst pressure where the significant damage is likely to happen. In the final destruction, the number of total hits is low because the time to destruction was short, considering the felicity ratio and the pressure rising up to the

Figure 7 and 8 show that the average rise time and duration for the signals occurred during the holding time of each load. The rise time and duration become shorter up to loads up to 70 % while those from time of 80 % to the time of burst get longer, during which the damage

**Figure 8.** Mean duration during load holding

Unlike the metal vessel, the pressure vessel that is made with composite materials has various damage mechanisms including the matrix crack, crack growth, separation between layers, cutting of fiber, and destruction of metal liner. Figure 9 shows that at the load of 30 % stage, the amplitude of signal rises due to the increased sound emission from matrix crack of composite materials. It also shows that at the load of 50-60 % of the estimated burst pressure, the sound emission is weakened and thus the amplitude of the signal is also reduced. At the load of 70 % pressure, the interlayer separation and the cut of reinforced fiber start and the amplitude of the signal increases again. Then, more reinforcement fibers become severed and the metal liner begins to burst, showing a little increase in amplitude.

As shown below, the composite material pressure vessel has many damage mechanisms in it. The initial damage mechanism contains the matrix crack due to the stress as well as the crack grown. As shown in Figure 10 (a), the rise time appears at the range of 10 μs and 100 μs. This shows the creation of matrix crack and the growth of this crack. The signal from around 10 μs is related to the creation of cracks while the signals from 100 μs look to be related to the growth of the cracks.

Acoustic Emission of Composite Vessel 69

(f)

**Figure 10.** Rise time vs amplitude plot during load holding : (a)30% (b)50% (c)60% (d)70% (e)80%

(g)

(c) (d)

(f)90% and (g)fractured

(e)

**Figure 9.** Mean amplitude during load holding

AE signals at the load of 50% and 60% show that the matrix crack and growth occur in this range instead of the additional causes for damages. At the load of 70%, a lot of the rise time of AE signals come from 500 μs appear. It is likely to indicate that the new damage elements such as cut of reinforcement fiber appear. The rapid increase in AE signals around 10 μs shows that the growth of matrix cracks such as growth of existing cracks and interlayer separation occur at a fast pace. Then, at the load of 80~90%, it is estimated that rapid damages such as growth of the previous matrix cracks and the cutting of reinforcement fiber occur, making much sound from cutting of reinforcement fiber. The 800 μs of rise time at this stage is likely to be caused by the cutting of several lines of reinforcement fibers. Then, at 100% load, a composite of damage mechanisms occur, increasing the damage of vessel and then destroying the last metal liner.

Figure 11 shows the total count of sound emission signals which occur during the 2 minutes of holding time. These variables are better indications than mean rise time in Figure 7, mean duration of Figure 8 or mean amplitude of Figure 9. It was discovered that it was difficult to assess the damage against the vessel with variables such as mean amplitude, total hit, mean rise time, or mean duration but the total count and the total signal strength of the sound emitted when the load is pressed on, activation and vessel's damage were better indications for damages on the vessel.

**Figure 9.** Mean amplitude during load holding

for damages on the vessel.

AE signals at the load of 50% and 60% show that the matrix crack and growth occur in this range instead of the additional causes for damages. At the load of 70%, a lot of the rise time of AE signals come from 500 μs appear. It is likely to indicate that the new damage elements such as cut of reinforcement fiber appear. The rapid increase in AE signals around 10 μs shows that the growth of matrix cracks such as growth of existing cracks and interlayer separation occur at a fast pace. Then, at the load of 80~90%, it is estimated that rapid damages such as growth of the previous matrix cracks and the cutting of reinforcement fiber occur, making much sound from cutting of reinforcement fiber. The 800 μs of rise time at this stage is likely to be caused by the cutting of several lines of reinforcement fibers. Then, at 100% load, a composite of damage mechanisms occur, increasing the damage of vessel and then destroying the last metal liner.

Figure 11 shows the total count of sound emission signals which occur during the 2 minutes of holding time. These variables are better indications than mean rise time in Figure 7, mean duration of Figure 8 or mean amplitude of Figure 9. It was discovered that it was difficult to assess the damage against the vessel with variables such as mean amplitude, total hit, mean rise time, or mean duration but the total count and the total signal strength of the sound emitted when the load is pressed on, activation and vessel's damage were better indications

(a) (b)

**Figure 10.** Rise time vs amplitude plot during load holding : (a)30% (b)50% (c)60% (d)70% (e)80% (f)90% and (g)fractured

Acoustic Emission of Composite Vessel 71

 30% 50% 60% 70% 80% 90% 100%

> 30% 50% 60% 70% 80% 90% 100%

load of 30 % to 60 %, even though there was increase in the number of signals, there was not a big increase in amplitude. Accordingly, we now that up to 60 %, there is not much of a creep affect but just the low amplitude of signals. I think that this is mainly caused by the creation and growth of cracks of substrate. After pressing, there were the creep effects, which show the significant increase in AE signal and the signals with the amplitude of 85 dB. In addition, as the distribution of amplitude appears different from the previous one, I think that the damages on the vessel are significant. It seems that there are more creations and growths of cracks in the composite materials and the cutting of reinforcement fiber and peeling of substrate cause the damage mechanism. In addition, the slope of the distribution of amplitude is known to be related to the mechanism of the signals[8], In the load of 70 % or higher, the slop looks similar, meaning that the damage is caused by the similar damage

**45 50 55 60 65 70 75 80 85 90 95 100**

Amplitude (*dB*)

**0 1000 2000 3000 4000 5000**

Count

mechanism.

**Figure 13.** Amplitude distribution during load holding

**1**

**10**

**100**

Number of hits

**1000**

**10000**

**Figure 14.** Count distribution during load holding

**1**

**10**

**100**

Number of hits

**1000**

**10000**

**100000**

**Figure 11.** Total count during load holding

**Figure 12.** Total signal strength during load holding

#### *3.1.3. Distribution of AE signal during the holding time*

I did not use the signals during the initial 2 minutes among the 10 minutes to minimize the flow noise which often includes the initial stages of pressing in the pressure test. After that, I observed the number of hits and creep effect of the sound emission signals which occur during the holding time. Even though there were creation and growth of cracks on substrate under the load of 60 % of the estimated burst pressure, they did not cause much damage to the vessel. But it was found that at the load of 70 % pressure, the vessel was quickly destroyed.[4] Figure 13 shows the distribution of amplitude of signals occurring after 2 minutes of the pressure holding at each load. At the load of 60 % or less, there were not a lot of signals of 60 dB or higher but less than 7 0dB. At this time, I could estimate that at the initial stage of the vessel damage, the composite material substrate wrapping around the vessel were showing some cracks and their growth. I could observe the cracks with my own eyes at the 2000th cycle of fatigue test when the pressure was at 207 bar. The used pressure was more than 30 % of the estimated burst pressure (180 bar). During the burst test, at the load of 30 % to 60 %, even though there was increase in the number of signals, there was not a big increase in amplitude. Accordingly, we now that up to 60 %, there is not much of a creep affect but just the low amplitude of signals. I think that this is mainly caused by the creation and growth of cracks of substrate. After pressing, there were the creep effects, which show the significant increase in AE signal and the signals with the amplitude of 85 dB. In addition, as the distribution of amplitude appears different from the previous one, I think that the damages on the vessel are significant. It seems that there are more creations and growths of cracks in the composite materials and the cutting of reinforcement fiber and peeling of substrate cause the damage mechanism. In addition, the slope of the distribution of amplitude is known to be related to the mechanism of the signals[8], In the load of 70 % or higher, the slop looks similar, meaning that the damage is caused by the similar damage mechanism.

**Figure 13.** Amplitude distribution during load holding

70 Composites and Their Applications

**Figure 11.** Total count during load holding

**Figure 12.** Total signal strength during load holding

*3.1.3. Distribution of AE signal during the holding time*

I did not use the signals during the initial 2 minutes among the 10 minutes to minimize the flow noise which often includes the initial stages of pressing in the pressure test. After that, I observed the number of hits and creep effect of the sound emission signals which occur during the holding time. Even though there were creation and growth of cracks on substrate under the load of 60 % of the estimated burst pressure, they did not cause much damage to the vessel. But it was found that at the load of 70 % pressure, the vessel was quickly destroyed.[4] Figure 13 shows the distribution of amplitude of signals occurring after 2 minutes of the pressure holding at each load. At the load of 60 % or less, there were not a lot of signals of 60 dB or higher but less than 7 0dB. At this time, I could estimate that at the initial stage of the vessel damage, the composite material substrate wrapping around the vessel were showing some cracks and their growth. I could observe the cracks with my own eyes at the 2000th cycle of fatigue test when the pressure was at 207 bar. The used pressure was more than 30 % of the estimated burst pressure (180 bar). During the burst test, at the

**Figure 14.** Count distribution during load holding

Figure 14 shows the distribution of counts of AE signals coming from each load of pressure. When the signal with counts of 500 or less occurred under the load of 60 %, the damage was caused by the creation of growth of substrate crack under the load of 60 % pressure. At the pressure of 80-90 % where the significant damage is done to vessel, the number of counts was 1000 or more. At the burst stage the number of counts was 2,500 or more.

Acoustic Emission of Composite Vessel 73

 30% 50% 60% 70% 80% 90% 100%

**Figure 16.** Rise time distribution during load holding

**1**

**10**

**100**

Number of hits

**1000**

**10000**

**100000**

*3.1.4. Mean frequency of AE signal during the holding time*

hoop did not affect the burst pressure of the vessel [8].

To estimate the damage with the sound emission signal, the signal of 60 dB or higher is used. Figure 17 shows the distribution of amplitude for hits of 60 dB or higher for each load of pressure. AE time becomes lower until the pressure of 70 % but at 80 %, it becomes faster. As the pressure rises at constant pace during the test, at 70 % pressure, there is no AE signal until 60 % pressures, showing the Kaiser effect. Accordingly, up to 60 % pressure, it looks like that there was no significant damage to the vessel. As the pressure rises up to 70 % that may have damaged the vessel, it looks like that AE happened due to the Felicity effect when the pressure goes up to 80 % as the vessel was damaged during the rise up to 80 %. At the 90 and 100 % pressure, there is the Felicity effect. Particularly, in the pressure range of up to 100 %, which experienced the load of 90 % or higher, there is AE signals of 60 dB or more from the beginning, showing that the damage is significant. In the actual burst test, the damage mechanism of Type II vessel, or the matrix crack in the direction of initial hoop occurs and then the creation of cracks and its growth and the interlayer separation (which belongs to matrix crack but has different damage mode) occur [4]. At the load of 60 %, the matrix crack in the direction of hoop was shown. But, the matrix crack in the direction of

**0 100 200 300 400 500 600 700<PHONE_NUMBER> 1100**

*s*)

Rise time (

Figure 18 shows the count of hits where sound exceeds 60 dB at each stage of burst test. The rate decreases from 7 % to 1 % during the pressure of 30 % to 60 %. Then, the signals of 60 dB or higher rapidly go up to 2 0% at the load of 70 %. The rate goes up to 30 % at the load of 100 %. The signals of 60 dB or higher at an early stage may be caused by the creation of matrix crack on the composite materials. The reduction thereafter may be caused by the low amplitude (60 dB or less) rather than the creation of crack. At the load of 70 % or higher, there are not only the matrix cracks but also the damages by the new damage mechanism such as the cut of fibers, increasing the signals of 60 dB or higher. The measurement of the

count of hits which are 60 dB or higher is a good tool to assess the damage.

Figure 15 shows that the duration of signal occurs with the range of 500 μs at the load of up to 70 % and that the signal of over 10000 μs started to appear at the load of 80 % or higher. At the load of 90 %, the 20000 μs appears during the final burst stage where the signals of 25000 ~ 50000 μs appeared.

**Figure 15.** Duration distribution during load holding

Figure 16 shows the rise time of 100 μs or less and the signal of 100-200 μs at the load of up to the initial 30 %. As we understand the distribution of the amplitude from the fatigue test of 20,000 cycles, there may be no mechanism other than the creation and growth of cracks at the load of 30 % pressure. It means that the signal of 200 μs or less is caused by the creation and growth of cracks of the vessel materials. Even though there were 1 and 4 signals of 300 μs or higher at the load of 50-60 %, they are negligible considering the total number of signals. It seems that the damage was mainly caused by creation and growth of cracks on composite materials rather than by new damage mechanisms. At the load of 70 % pressure, the signal of 250-450 μs, which is longer than at previous load stage appeared, meaning that there may be new damage mechanism. It seems that it was caused by the additional damage mechanism such as the cutting of reinforcement fibers and interlayer peeling due to the increase in internal pressures. At the load of 80-90 %, the rise time of 500 μs or more is observed. It may be affected by the composite damage mechanism such as the growth of existing cracks, cutting of reinforcement fiber and inter-layer separation happening at the neighboring location. This trend is true of the distribution of counts and durations as specified above. The observation of the sound emission signals coming out of the neighboring location through the composite damage mechanism would be a good tool to assess the vessel.

**Figure 16.** Rise time distribution during load holding

25000 ~ 50000 μs appeared.

**Figure 15.** Duration distribution during load holding

**1**

**10**

**100**

Number of hits

**1000**

**10000**

**100000**

Figure 14 shows the distribution of counts of AE signals coming from each load of pressure. When the signal with counts of 500 or less occurred under the load of 60 %, the damage was caused by the creation of growth of substrate crack under the load of 60 % pressure. At the pressure of 80-90 % where the significant damage is done to vessel, the number of counts

Figure 15 shows that the duration of signal occurs with the range of 500 μs at the load of up to 70 % and that the signal of over 10000 μs started to appear at the load of 80 % or higher. At the load of 90 %, the 20000 μs appears during the final burst stage where the signals of

Figure 16 shows the rise time of 100 μs or less and the signal of 100-200 μs at the load of up to the initial 30 %. As we understand the distribution of the amplitude from the fatigue test of 20,000 cycles, there may be no mechanism other than the creation and growth of cracks at the load of 30 % pressure. It means that the signal of 200 μs or less is caused by the creation and growth of cracks of the vessel materials. Even though there were 1 and 4 signals of 300 μs or higher at the load of 50-60 %, they are negligible considering the total number of signals. It seems that the damage was mainly caused by creation and growth of cracks on composite materials rather than by new damage mechanisms. At the load of 70 % pressure, the signal of 250-450 μs, which is longer than at previous load stage appeared, meaning that there may be new damage mechanism. It seems that it was caused by the additional damage mechanism such as the cutting of reinforcement fibers and interlayer peeling due to the increase in internal pressures. At the load of 80-90 %, the rise time of 500 μs or more is observed. It may be affected by the composite damage mechanism such as the growth of existing cracks, cutting of reinforcement fiber and inter-layer separation happening at the neighboring location. This trend is true of the distribution of counts and durations as specified above. The observation of the sound emission signals coming out of the neighboring location through the

**0 10000 20000 30000 40000 50000**

*s***)**  30% 50% 60% 70% 80% 90% 100%

Duration **(**

composite damage mechanism would be a good tool to assess the vessel.

was 1000 or more. At the burst stage the number of counts was 2,500 or more.

#### *3.1.4. Mean frequency of AE signal during the holding time*

To estimate the damage with the sound emission signal, the signal of 60 dB or higher is used. Figure 17 shows the distribution of amplitude for hits of 60 dB or higher for each load of pressure. AE time becomes lower until the pressure of 70 % but at 80 %, it becomes faster. As the pressure rises at constant pace during the test, at 70 % pressure, there is no AE signal until 60 % pressures, showing the Kaiser effect. Accordingly, up to 60 % pressure, it looks like that there was no significant damage to the vessel. As the pressure rises up to 70 % that may have damaged the vessel, it looks like that AE happened due to the Felicity effect when the pressure goes up to 80 % as the vessel was damaged during the rise up to 80 %. At the 90 and 100 % pressure, there is the Felicity effect. Particularly, in the pressure range of up to 100 %, which experienced the load of 90 % or higher, there is AE signals of 60 dB or more from the beginning, showing that the damage is significant. In the actual burst test, the damage mechanism of Type II vessel, or the matrix crack in the direction of initial hoop occurs and then the creation of cracks and its growth and the interlayer separation (which belongs to matrix crack but has different damage mode) occur [4]. At the load of 60 %, the matrix crack in the direction of hoop was shown. But, the matrix crack in the direction of hoop did not affect the burst pressure of the vessel [8].

Figure 18 shows the count of hits where sound exceeds 60 dB at each stage of burst test. The rate decreases from 7 % to 1 % during the pressure of 30 % to 60 %. Then, the signals of 60 dB or higher rapidly go up to 2 0% at the load of 70 %. The rate goes up to 30 % at the load of 100 %. The signals of 60 dB or higher at an early stage may be caused by the creation of matrix crack on the composite materials. The reduction thereafter may be caused by the low amplitude (60 dB or less) rather than the creation of crack. At the load of 70 % or higher, there are not only the matrix cracks but also the damages by the new damage mechanism such as the cut of fibers, increasing the signals of 60 dB or higher. The measurement of the count of hits which are 60 dB or higher is a good tool to assess the damage.

Acoustic Emission of Composite Vessel 75

related to the source mechanism. It shows the creation and growth of matrix crack at the load of up to 60 % and the additional damage mechanism such as the cut of fiber at the load of 70 % or higher. The reverberation frequency shows a little difference between below and above 70% but is constant in most stages while the average frequency was also constant

**20 30 40 50 60 70 80 90 100 110**

**load stage (%)**

**20 30 40 50 60 70 80 90 100 110**

**load stage (%)**

**20 30 40 50 60 70 80 90 100 110**

**load stage (%)**

with a little rise at the load of initial 30 % pressure.

**200**

**Figure 19.** Mean initial frequency during each loading stage

**0**

**50**

**100**

**initial frequency(kHz)**

**150**

**Figure 20.** Mean reverberation frequency during each loading stage

**0**

**50**

**100**

**reverbration frequency(kHz)**

**150**

**200**

**Figure 21.** Mean average frequency during each loading stage

**0**

**50**

**100**

**average frquency(kHz)**

**150**

**200**

**Figure 17.** Amplitude distribution during loading at each loading stage (over 60 dB)

**Figure 18.** Ratio of hits over 60 dB / total hits during each loading stage

Figure 19-21 show the mean initial, reverberation and average frequency of AE signal at each load stage. The frequency is obtained not from the analysis of wave form but from the duration and counts. The frequency shows the difference between above and below 70 % load. As for the initial frequency, it did not show a big change at the load of up to 60 % or 100 kHz but it goes up at the load of 70 % until going down a little. This is likely to be related to the source mechanism. It shows the creation and growth of matrix crack at the load of up to 60 % and the additional damage mechanism such as the cut of fiber at the load of 70 % or higher. The reverberation frequency shows a little difference between below and above 70% but is constant in most stages while the average frequency was also constant with a little rise at the load of initial 30 % pressure.

**Figure 19.** Mean initial frequency during each loading stage

74 Composites and Their Applications

**Figure 17.** Amplitude distribution during loading at each loading stage (over 60 dB)

30

50

60

70

80

90

100

**amplitude(dB)**

0 300<PHONE_NUMBER> 1500

**time(sec)**

**Figure 18.** Ratio of hits over 60 dB / total hits during each loading stage

**0**

**10**

**20**

**hits over 60 dB/total hits (%)**

**30**

**40**

Figure 19-21 show the mean initial, reverberation and average frequency of AE signal at each load stage. The frequency is obtained not from the analysis of wave form but from the duration and counts. The frequency shows the difference between above and below 70 % load. As for the initial frequency, it did not show a big change at the load of up to 60 % or 100 kHz but it goes up at the load of 70 % until going down a little. This is likely to be

**0 20 40 60 80 100**

**load stage(%)**

**Figure 20.** Mean reverberation frequency during each loading stage

**Figure 21.** Mean average frequency during each loading stage

## **3.2. Fatigue test**

## *3.2.1. Identification and Verification of Acoustic Emission Location*

Figure 22 shows the results of measuring the elastic wave speed of the artificial acoustic emission source for the acoustic emission location on the fiber-wrapped composite material. As shown on the picture, the speed of longitudinal elastic wave was 4512 m/sec and the elastic wave speed of the wrapped direction was 5689 m/sec showing the characteristic of anisotropy. Thus, the location was identified using anisotropic vessel source location that uses the difference of time for the acoustic emission signal is to be reached.

Acoustic Emission of Composite Vessel 77

thickness of the wrapping composite material. The direction of the realized defect was

Figure 25 shows the average number of hit per sensor during the fatigue test of vessel. The vessel with the artificial defect has more number of hits compared to the sound vessel and when you see the tendency in increase and decrease, the number of hits for transverse defect vessel and sound vessel decreases as the number of cycle increases; however, the number of hits for longitudinal defect vessel decreases in 4000 cycle and then increases and then

The reason the number of hit in the early stages is big is related to the initiation and growth of the matrix rupture in the comparatively weak areas within the vessel. The number of hits decrease and the initiation and growth of ruptures are comparatively slowed until sufficient

In the case of longitudinal defects vessel, the amount of resilience needed to bring about progress in the defect (growth of rupture) is comparatively smaller than other vessel, so in the 8000th and 12000th fatigue test, the number of hits is bigger than in other vessel and because there is a need for another accumulation of resilience, the number of hits decrease afterwards. Such phenomena are clearly distinct in the composite material as mentioned in the introduction and in the early stages of damage, resilience increases like the number of hits and when there are some increases in damage, the resilience afterwards is comparatively low.

**Figure 25.** The number of hits per channel during fatigue test for artificial defect and sound vessel

Figure 26 shows the number of events extracted during the fatigue test on longitudinal defect and transverse defect vessel. An event shows the number of sources calculating the

amount of resilience is stored to bring about new initiation and growth.

longitudinal and transverse, two types of artificial defects.

**Figure 24.** Schematic diagram of artificial defect

decreases in 8000 and 12000 cycles.

**Figure 22.** The elastic wave velocity with degree between propagation and wrapping direction

Figure 23 is the location of the sensor and the result of the identification test of the location using the artificial acoustic emission source. The four sensors were attached in staggered locations in channel 5, 6, 7, 8 and channel 7 refers to the backside of 8. The acoustic emission source is located diagonally in equal intervals between channel 5 and 8 and between channel 6 and 8.

**Figure 23.** Confirm of Source location

## *3.2.2. Fatigue Test and Location of Artificial Defect*

Figure 24 shows the size of the artificial defect realized on the composite material which is wrapped and the length is 50 mm, width 3 mm, and the depth is 3 mm which is 50 % of the thickness of the wrapping composite material. The direction of the realized defect was longitudinal and transverse, two types of artificial defects.

**Figure 24.** Schematic diagram of artificial defect

76 Composites and Their Applications

*3.2.1. Identification and Verification of Acoustic Emission Location*

uses the difference of time for the acoustic emission signal is to be reached.

**Figure 22.** The elastic wave velocity with degree between propagation and wrapping direction

Figure 23 is the location of the sensor and the result of the identification test of the location using the artificial acoustic emission source. The four sensors were attached in staggered locations in channel 5, 6, 7, 8 and channel 7 refers to the backside of 8. The acoustic emission source is located diagonally in equal intervals between channel 5 and 8 and between

Figure 24 shows the size of the artificial defect realized on the composite material which is wrapped and the length is 50 mm, width 3 mm, and the depth is 3 mm which is 50 % of the

Figure 22 shows the results of measuring the elastic wave speed of the artificial acoustic emission source for the acoustic emission location on the fiber-wrapped composite material. As shown on the picture, the speed of longitudinal elastic wave was 4512 m/sec and the elastic wave speed of the wrapped direction was 5689 m/sec showing the characteristic of anisotropy. Thus, the location was identified using anisotropic vessel source location that

**3.2. Fatigue test**

channel 6 and 8.

**Figure 23.** Confirm of Source location

*3.2.2. Fatigue Test and Location of Artificial Defect*

Figure 25 shows the average number of hit per sensor during the fatigue test of vessel. The vessel with the artificial defect has more number of hits compared to the sound vessel and when you see the tendency in increase and decrease, the number of hits for transverse defect vessel and sound vessel decreases as the number of cycle increases; however, the number of hits for longitudinal defect vessel decreases in 4000 cycle and then increases and then decreases in 8000 and 12000 cycles.

The reason the number of hit in the early stages is big is related to the initiation and growth of the matrix rupture in the comparatively weak areas within the vessel. The number of hits decrease and the initiation and growth of ruptures are comparatively slowed until sufficient amount of resilience is stored to bring about new initiation and growth.

In the case of longitudinal defects vessel, the amount of resilience needed to bring about progress in the defect (growth of rupture) is comparatively smaller than other vessel, so in the 8000th and 12000th fatigue test, the number of hits is bigger than in other vessel and because there is a need for another accumulation of resilience, the number of hits decrease afterwards. Such phenomena are clearly distinct in the composite material as mentioned in the introduction and in the early stages of damage, resilience increases like the number of hits and when there are some increases in damage, the resilience afterwards is comparatively low.

**Figure 25.** The number of hits per channel during fatigue test for artificial defect and sound vessel

Figure 26 shows the number of events extracted during the fatigue test on longitudinal defect and transverse defect vessel. An event shows the number of sources calculating the

location of the acoustic emission source within vessel using the acoustic emission signal that hit the sensor and in the case transverse defect vessel which includes a transverse defect, the number of events is noticeably decreased as the number of cycle increases, but in the case of longitudinal defect vessel which has longitudinal defect, the number of events increases a lot in 8000th cycle and does not change much as the number of cycle increases.

Acoustic Emission of Composite Vessel 79

**Figure 27.** The ratio of events/ hits per sensor during fatigue test for artificial defect vessel

shown clearly.

steel liner.

*3.2.3. AE parameters during fatigue test*

In the case of c), in the 8000th fatigue cycle, 108 events were clustered around the artificial defect and as mentioned in the explanation in Figure 27 more than 50% of the occurred hits were signals related to the artificial defect. Afterwards, events that occurred in d) ~f) were those that mostly occurred near the artificial defect so we can acknowledge that the damage on the composite material is progressed around the artificial defect. Figure 29 shows the surroundings of the longitudinal artificial defect after the 20000th cycle and shows that at the end of the defect, there is a matrix rupture progressing in a hoop-direction and although not clear in the picture, at the end of the depth in the artificial defect, delaminating was observed on the overall defect. In the case of vessel with transverse defects, after the 8,000th fatigue test, less than 1 event was created and the location of the artificial defect could not be

On the other hand, after the 20000th fatigue test, in the burst test, the location of the burst is marked in c) of Figure 28 and events are also observed in a), d), e), and f). The source of the acoustic emission signal is assumed to be the fatigue rupture in the weak areas of the steel liner rather than in the composite material. The final burst in the case of the longitudinal defect accompanies matrix rupture and delaminating as mentioned above during the fatigue test and burst test so the whole vessel area, which is the length of the defect, is thinner in terms of thickness like the depth of the defect and thus is weaker than other areas and it is thought that it burst at the final burst location in which the fatigue rupture occurred in the

Figure 30 shows the relationship between the amplitude of the signal occurring during the early three cycles and the rise time, and can be clearly distinguished as around 10 μs and over 100 μs and the grey mark shows the rise time while holding the load during the three cycles and also at 90 %, which is the highest, has a rise time of about 10 μs. Generally while load holding, it can be inferred that there is likely to be growth of an existent crack rather

**Figure 26.** The number of events during fatigue test for artificial defect vessel

Figure 27 compares the number of hit per sensor and the number of events using Figure 25 and 26 and shows the event/hit ratio according to the increase in the number of cycle. This is a figure that shows the % of the number of hits that can precisely show the source among the total number of hits. In the case of longitudinal defect vessel, it was 41.8 % in the 4000th cycle, 55.7 % in the 8000th cycle. An event can be calculated using the difference in time that the resilience from the source (acoustic emission signal) sent through the walls of the vessel reaches the sensor and has to be extracted from at least 3 sensors. Hits that do not go by such standards cannot be used to calculate events and if the location was identified by simulation or if the signal is weak or is static, it cannot be recorded as an event. In the case of longitudinal defect vessel, the number of hits is small in the 4000th and 8000th cycle but the growth of the rupture is relatively easy and it sends hits to at least 3 sensors with signals with sufficient amplitude so the number of event/hit rate increases.

Figure 28 is the result of the location due to the acoustic emission test performed during the fatigue test on longitudinal defect vessel. Figure 28 (a) shows the location of 25 events during the first 3 fatigue test cycles. It does not show the location of artificial defects but shows the overall looks on the whole vessel. Such results are not shown in pictures but also in transverse defects vessel, 49 events in Figure 26 shows looks like a) on the whole vessel and it seems that signals were created on the weakest areas of the whole vessel when the first pressure was put. In the case of b), after the 4,000th cycle, in the 3rd fatigue test, 23 events were shown to be clustered around the artificial defect. As for the vessel with transverse defects, after the 4,000th fatigue test cycle, in the 3rd fatigue test, 12 events were created but they were all over the whole vessel so we could not show the location of the artificial defect.

**Figure 27.** The ratio of events/ hits per sensor during fatigue test for artificial defect vessel

In the case of c), in the 8000th fatigue cycle, 108 events were clustered around the artificial defect and as mentioned in the explanation in Figure 27 more than 50% of the occurred hits were signals related to the artificial defect. Afterwards, events that occurred in d) ~f) were those that mostly occurred near the artificial defect so we can acknowledge that the damage on the composite material is progressed around the artificial defect. Figure 29 shows the surroundings of the longitudinal artificial defect after the 20000th cycle and shows that at the end of the defect, there is a matrix rupture progressing in a hoop-direction and although not clear in the picture, at the end of the depth in the artificial defect, delaminating was observed on the overall defect. In the case of vessel with transverse defects, after the 8,000th fatigue test, less than 1 event was created and the location of the artificial defect could not be shown clearly.

On the other hand, after the 20000th fatigue test, in the burst test, the location of the burst is marked in c) of Figure 28 and events are also observed in a), d), e), and f). The source of the acoustic emission signal is assumed to be the fatigue rupture in the weak areas of the steel liner rather than in the composite material. The final burst in the case of the longitudinal defect accompanies matrix rupture and delaminating as mentioned above during the fatigue test and burst test so the whole vessel area, which is the length of the defect, is thinner in terms of thickness like the depth of the defect and thus is weaker than other areas and it is thought that it burst at the final burst location in which the fatigue rupture occurred in the steel liner.

#### *3.2.3. AE parameters during fatigue test*

78 Composites and Their Applications

location of the acoustic emission source within vessel using the acoustic emission signal that hit the sensor and in the case transverse defect vessel which includes a transverse defect, the number of events is noticeably decreased as the number of cycle increases, but in the case of longitudinal defect vessel which has longitudinal defect, the number of events increases a

Figure 27 compares the number of hit per sensor and the number of events using Figure 25 and 26 and shows the event/hit ratio according to the increase in the number of cycle. This is a figure that shows the % of the number of hits that can precisely show the source among the total number of hits. In the case of longitudinal defect vessel, it was 41.8 % in the 4000th cycle, 55.7 % in the 8000th cycle. An event can be calculated using the difference in time that the resilience from the source (acoustic emission signal) sent through the walls of the vessel reaches the sensor and has to be extracted from at least 3 sensors. Hits that do not go by such standards cannot be used to calculate events and if the location was identified by simulation or if the signal is weak or is static, it cannot be recorded as an event. In the case of longitudinal defect vessel, the number of hits is small in the 4000th and 8000th cycle but the growth of the rupture is relatively easy and it sends hits to at least 3 sensors with signals

Figure 28 is the result of the location due to the acoustic emission test performed during the fatigue test on longitudinal defect vessel. Figure 28 (a) shows the location of 25 events during the first 3 fatigue test cycles. It does not show the location of artificial defects but shows the overall looks on the whole vessel. Such results are not shown in pictures but also in transverse defects vessel, 49 events in Figure 26 shows looks like a) on the whole vessel and it seems that signals were created on the weakest areas of the whole vessel when the first pressure was put. In the case of b), after the 4,000th cycle, in the 3rd fatigue test, 23 events were shown to be clustered around the artificial defect. As for the vessel with transverse defects, after the 4,000th fatigue test cycle, in the 3rd fatigue test, 12 events were created but they were all over the whole vessel so we could not show the location of the artificial defect.

lot in 8000th cycle and does not change much as the number of cycle increases.

**Figure 26.** The number of events during fatigue test for artificial defect vessel

with sufficient amplitude so the number of event/hit rate increases.

Figure 30 shows the relationship between the amplitude of the signal occurring during the early three cycles and the rise time, and can be clearly distinguished as around 10 μs and over 100 μs and the grey mark shows the rise time while holding the load during the three cycles and also at 90 %, which is the highest, has a rise time of about 10 μs. Generally while load holding, it can be inferred that there is likely to be growth of an existent crack rather

On ris fat of av μs aft

n the other hand, e time noticeably tigue test, there a the cracks created erage rise time of and afterwards ter the 12000th cy

 in the case of a s y decreased and are not many initi d in early stages. f related hits for e decreased to 34 ycle and then decr

sound cylinder, a increased little u iations of new ma On the other han events occurred d μs after the 800 reased again.

Acoustic Em

mission of Composit

e Vessel 81

verage 4000th growth cts, the was 56 about

atigue test, the av h test. After the 4 you can see the g vessel with defec hree fatigue test w creased to 82 μs

nals of events occ and is clustered a d that the initiatio Especially, a high ect as shown in b) ke a) or d) so the g

curring around on and her rise ), c), e), growth

curring ring in

n other around vessel h other

ls, and vity of sensors

e of the signal occ of signals occurr

curs far away in e cracks occurs a trix crack in the nals overlap with

rs to detect signal cause the sensitiv d using resonant s

racks.

after the 4000th fa up to the 20000th atrix cracks and y nd, in the case of during the first th 0th cycle and inc

for sound vessel

plitude of the sign hits and rise time 00 μs. It is judged he overall vessel. r around the defe ll on the vessel lik the initiation of cr

average rise time verage rise time

vidual cracks occ rowth of multiple he growth of ma nd generated sign

.wideband sensor

als. However, bec s are also detected

ring initial 3 cycles

between the amp ssel and related h red around 100-8 n weak areas of th ce forms a cluster is scattered overal wer rise time than t

atigue cycle, the a igher than the av

growth of indiv ith defects, the gr rgy needed for th sound vessel. An ignal is extended. equency, we use of detected signa nt sensors, signals

me distribution dur

the relationship b ee cycles of the ve and is also scatter ack will happen in ed when the sourc when the source i e defect has a low

after the 4000th fa e shown to be hi

ound vessel, the case of vessel wi e the elastic ener ller than that of s the rise time of si to analysis of fre urier transform o all behind resonan

**Fig**

**gure 30.** The rise tim

gure 31 a) shows uring the first thre μs in the figure a owth of matrix cra me can be observe of Figure 31 than w cracks around the

n the other hand, defect vessel are und vessel.

the case of a so acks. But, in the c e defects because ith defect is smal gnals. Therefore, t enerally, in order alyze by the Fou deband sensors fa

Fig du 10 gro tim f) o of

On in sou

In cra the wi sig Ge an wi

**Figure 28.** The result of source location with cycle for longitudinal defect : a)0, b)4000, c)8000, d)12000, e)16000, f)20000

**Figure 29.** Longitudinal defect and matrix crack after 20000 cycle fatigue test

than initiation of a new matrix crack and thus can be said that it is a growth of crack around 10 μs. And the rise time of AE signal which occurs during the initiation of a matrix crack can be said to be more than 100μs. This accord to the result that the rise time of the AE signal occurring during the initiation of matrix crack during the burst test is around 100 μs and that the rise time of the AE signal occurring during the growth of the crack is around 10 μs. [4]

On ris fat of av μs aft n the other hand, e time noticeably tigue test, there a the cracks created erage rise time of and afterwards ter the 12000th cy in the case of a s y decreased and are not many initi d in early stages. f related hits for e decreased to 34 ycle and then decr sound cylinder, a increased little u iations of new ma On the other han events occurred d μs after the 800 reased again. after the 4000th fa up to the 20000th atrix cracks and y nd, in the case of during the first th 0th cycle and inc atigue test, the av h test. After the 4 you can see the g vessel with defec hree fatigue test w creased to 82 μs verage 4000th growth cts, the was 56 about

**Fig gure 30.** The rise tim me distribution dur ring initial 3 cycles for sound vessel

80 Composites and Their Applications

e)16000, f)20000

[4]

**Figure 28.** The result of source location with cycle for longitudinal defect : a)0, b)4000, c)8000, d)12000,

Matrix crack

Artificial defect (Longitudinal direction)

than initiation of a new matrix crack and thus can be said that it is a growth of crack around 10 μs. And the rise time of AE signal which occurs during the initiation of a matrix crack can be said to be more than 100μs. This accord to the result that the rise time of the AE signal occurring during the initiation of matrix crack during the burst test is around 100 μs and that the rise time of the AE signal occurring during the growth of the crack is around 10 μs.

**Figure 29.** Longitudinal defect and matrix crack after 20000 cycle fatigue test

Fig du 10 gro tim f) o of gure 31 a) shows uring the first thre μs in the figure a owth of matrix cra me can be observe of Figure 31 than w cracks around the the relationship b ee cycles of the ve and is also scatter ack will happen in ed when the sourc when the source i e defect has a low between the amp ssel and related h red around 100-8 n weak areas of th ce forms a cluster is scattered overal wer rise time than t plitude of the sign hits and rise time 00 μs. It is judged he overall vessel. r around the defe ll on the vessel lik the initiation of cr nals of events occ and is clustered a d that the initiatio Especially, a high ect as shown in b) ke a) or d) so the g racks. curring around on and her rise ), c), e), growth

On in sou n the other hand, defect vessel are und vessel. after the 4000th fa e shown to be hi atigue cycle, the a igher than the av average rise time verage rise time e of the signal occ of signals occurr curring ring in

In cra the wi sig the case of a so acks. But, in the c e defects because ith defect is smal gnals. Therefore, t ound vessel, the case of vessel wi e the elastic ener ller than that of s the rise time of si growth of indiv ith defects, the gr rgy needed for th sound vessel. An ignal is extended. vidual cracks occ rowth of multiple he growth of ma nd generated sign curs far away in e cracks occurs a trix crack in the nals overlap with n other around vessel h other

Ge an wi enerally, in order alyze by the Fou deband sensors fa to analysis of fre urier transform o all behind resonan equency, we use of detected signa nt sensors, signals .wideband sensor als. However, bec s are also detected rs to detect signal cause the sensitiv d using resonant s ls, and vity of sensors and the frequency is calculated by rise time, duration, and count. Frequency by calculation was used and analyzed in this research because resonant sensors were used for the source location.

Acoustic Emission of Composite Vessel 83

Figure 32 shows the distribution of initial frequency versus the amplitude with fatigue cycle but it is dispersed around 100-200 kHz, which is unrelated to the fatigue cycle. It is in accord with the result that the average frequency of signal which is generated due to initiation and growth of matrix crack is around 100 kHz in the burst test using resonant type sensors [7].

(a) (b)

(c) (d)

**Figure 32.** The initial frequency distribution of hits connected with event during fatigue cycle for

(e) (f)

longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

**Figure 31.** The rise time distribution of hits connected with event during fatigue cycle for longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

Figure 32 shows the distribution of initial frequency versus the amplitude with fatigue cycle but it is dispersed around 100-200 kHz, which is unrelated to the fatigue cycle. It is in accord with the result that the average frequency of signal which is generated due to initiation and growth of matrix crack is around 100 kHz in the burst test using resonant type sensors [7].

82 Composites and Their Applications

and the frequency is calculated by rise time, duration, and count. Frequency by calculation was used and analyzed in this research because resonant sensors were used for the source location.

(a) (b)

(c) (d)

**Figure 31.** The rise time distribution of hits connected with event during fatigue cycle for longitudinal

(e) (f)

defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

**Figure 32.** The initial frequency distribution of hits connected with event during fatigue cycle for longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

Acoustic Emission of Composite Vessel 85

on signals relating to all events of Figure 33 c) was 73 kHz and these accords to the fact that the average reverberation frequency is between 50 - 75 kHz in all stage of burst which includes the damage mechanism of composite materials in the cast of burst test.[7] On the other hand, the reverberation frequency during the burst test of the same vessel was known to occur in all stage of the burst test in the range of 150 - 350 kHz[8], and in such case, because it includes the mechanism of all damage, it is hard to differentiate damage mechanism as a frequency. There were 23 event signals that had a reverberation frequency in the 150 - 350 kHz area in Figure 33 c) and 16 of them, which are 70 %, occurred in the matrix crack area of Figure 29, which is an observation of artificial defects after the 20000th fatigue test. More than 90 % of the related 44 hits(150 - 350 kHz) were signals with rise time lower than 100 μs and average rise time of 31 μs. As mentioned in 3.2.1, it is assumed that it

Figure 34 shows the amplitude distribution of accumulated hits according to the number of fatigue test and its slope has two types of shapes. Generally, the slope in the amplitude distribution of accumulated hit is known to be related to the mechanism of the source [8] and in vessel with defects as used in the experiment, as mentioned in the previous chapter, it includes mechanisms such as the initiation of matrix crack, growth of the created cracks (including delaminating), and the initiation and growth of liner fatigue

Initiation and growth of liner fatigue crack will be mentioned in the following chapter and in the view of the estimated result of the damage mechanism according to the number of fatigue explained in the previous chapter, the case of the initiation of matrix cracks is estimated to have a slope of ①(0.04) and the growth of cracks, a slope of

In order to analyze the characteristics of acoustic emission signals that occurred in the final burst position, event signals observed around the final burst location within 200 mm hoop direction in terms of length as marked in Figure 29 were analyzed. 18 events were occurred during the 20000th fatigue cycle and the number of related hits was 59. Figure 35 shows accumulated amplitude distribution and there are only 3 that are over 60 dB and 49 of them are below 50 dB. You can see the slope as ③ but if you observe closely, it is possible to observe that the same slope exists as ① in Figure 34 and also that it include the initiation of matrix crack. Seeing it as showing the slope of ③, as a signal according to a sole damage mechanism, it is estimated to be related to liner damage and

Figure 36 shows the distribution of rise time on the amplitude of the signals occurred in the final burst location. There is almost no rise time above 100 μs and is dispersed around 10 μs. It can be thought that the rise time which occurs during the growth of steel liner fatigue

is due to the growth of matrix crack.

cracks.

②(0.12).

the size is 0.06

*3.2.4. Amplitude distribution slop during fatigue test*

crack is similar to that during the growth of matrix crack.

**Figure 33.** The reverberation frequency distribution of hits connected with event during fatigue cycle for longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

Figure 33 shows the distribution of reverberation frequency on amplitude according to the number of fatigue tests but besides the oval area in Figure 33 c) which is the result of the 8000th fatigue test, most are dispersed below 150 kHz. The average reverberation frequency on signals relating to all events of Figure 33 c) was 73 kHz and these accords to the fact that the average reverberation frequency is between 50 - 75 kHz in all stage of burst which includes the damage mechanism of composite materials in the cast of burst test.[7] On the other hand, the reverberation frequency during the burst test of the same vessel was known to occur in all stage of the burst test in the range of 150 - 350 kHz[8], and in such case, because it includes the mechanism of all damage, it is hard to differentiate damage mechanism as a frequency. There were 23 event signals that had a reverberation frequency in the 150 - 350 kHz area in Figure 33 c) and 16 of them, which are 70 %, occurred in the matrix crack area of Figure 29, which is an observation of artificial defects after the 20000th fatigue test. More than 90 % of the related 44 hits(150 - 350 kHz) were signals with rise time lower than 100 μs and average rise time of 31 μs. As mentioned in 3.2.1, it is assumed that it is due to the growth of matrix crack.

## *3.2.4. Amplitude distribution slop during fatigue test*

84 Composites and Their Applications

**Figure 33.** The reverberation frequency distribution of hits connected with event during fatigue cycle

Figure 33 shows the distribution of reverberation frequency on amplitude according to the number of fatigue tests but besides the oval area in Figure 33 c) which is the result of the 8000th fatigue test, most are dispersed below 150 kHz. The average reverberation frequency

for longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

Figure 34 shows the amplitude distribution of accumulated hits according to the number of fatigue test and its slope has two types of shapes. Generally, the slope in the amplitude distribution of accumulated hit is known to be related to the mechanism of the source [8] and in vessel with defects as used in the experiment, as mentioned in the previous chapter, it includes mechanisms such as the initiation of matrix crack, growth of the created cracks (including delaminating), and the initiation and growth of liner fatigue cracks.

Initiation and growth of liner fatigue crack will be mentioned in the following chapter and in the view of the estimated result of the damage mechanism according to the number of fatigue explained in the previous chapter, the case of the initiation of matrix cracks is estimated to have a slope of ①(0.04) and the growth of cracks, a slope of ②(0.12).

In order to analyze the characteristics of acoustic emission signals that occurred in the final burst position, event signals observed around the final burst location within 200 mm hoop direction in terms of length as marked in Figure 29 were analyzed. 18 events were occurred during the 20000th fatigue cycle and the number of related hits was 59. Figure 35 shows accumulated amplitude distribution and there are only 3 that are over 60 dB and 49 of them are below 50 dB. You can see the slope as ③ but if you observe closely, it is possible to observe that the same slope exists as ① in Figure 34 and also that it include the initiation of matrix crack. Seeing it as showing the slope of ③, as a signal according to a sole damage mechanism, it is estimated to be related to liner damage and the size is 0.06

Figure 36 shows the distribution of rise time on the amplitude of the signals occurred in the final burst location. There is almost no rise time above 100 μs and is dispersed around 10 μs. It can be thought that the rise time which occurs during the growth of steel liner fatigue crack is similar to that during the growth of matrix crack.

Acoustic Emission of Composite Vessel 87

**Figure 35.** The amplitude distribution of accumulated hits connected with event during fatigue cycle

�

�

We burst the vessel with two types of artificial defect after carrying out 20000 cycles fatigue test on a sound vessel and continuously increasing the pressure where the burst pressure is 590~615 bar with the difference in the pressure of the vessel were within 5 % and thus was

Generally, 20000 cycles of fatigue is equivalent to a vessel used for more than 50 years if you are to put pressure on the vessel once a day although, of course, in the case of a real gas vessel, gas is used as a pressure medium so it may be different from the case in which machine oil is used as a pressure medium, but if the vessel with artificial defects and sound vessel were tested in the same conditions, the burst pressure is shown to be almost the same. Thus, in the case of the size of artificial defect used in this research, the direction of defect is

**Figure 36.** Distribution of rise time on the amplitude of the signals

shown to have almost no effect on the life of the vessel.

*3.2.5. Burst test after 20000 cycles fatigue test*

irrelevant to the existence of defects.

for longitudinal defect vessel

**Figure 34.** The amplitude distribution of accumulated hits connected with event during fatigue cycle for longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

**Figure 35.** The amplitude distribution of accumulated hits connected with event during fatigue cycle for longitudinal defect vessel

**Figure 36.** Distribution of rise time on the amplitude of the signals

#### *3.2.5. Burst test after 20000 cycles fatigue test*

86 Composites and Their Applications

**Figure 34.** The amplitude distribution of accumulated hits connected with event during fatigue cycle

for longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

We burst the vessel with two types of artificial defect after carrying out 20000 cycles fatigue test on a sound vessel and continuously increasing the pressure where the burst pressure is 590~615 bar with the difference in the pressure of the vessel were within 5 % and thus was irrelevant to the existence of defects.

Generally, 20000 cycles of fatigue is equivalent to a vessel used for more than 50 years if you are to put pressure on the vessel once a day although, of course, in the case of a real gas vessel, gas is used as a pressure medium so it may be different from the case in which machine oil is used as a pressure medium, but if the vessel with artificial defects and sound vessel were tested in the same conditions, the burst pressure is shown to be almost the same. Thus, in the case of the size of artificial defect used in this research, the direction of defect is shown to have almost no effect on the life of the vessel.

Figure 37 is a picture that shows the burst location in the vessel with the artificial defect. As shown in the picture, we can see that in the case of a vessel with a transverse defect, the final burst location is in the general burst location (cylinder and head area) for a well-constructed type II vessel. However, in the case of a vessel with a longitudinal defect, in both vessels, the final burst location was within the transverse vessel in which the defect was located. We think that this is because in the case of the longitudinal defect, the wrapped fiber is cut in 3 mm depth and in 50 mm length so the effect in which the thickness of the composite material is big, but in the case of the transverse defect, the fiber is cut only in 3 mm depth and in 3 mm length so the effect is small.

Acoustic Emission of Composite Vessel 89

the vessel. If the pressure is over 420 bar, or 70 % of the estimated burst pressure, the damage to the vessel becomes greater, meaning that the creep effect becomes larger. 2. The sound emission signal variables such as mean amplitude, mean rise time, means duration, and rise time amplitude correlation can be obtained when the vessel is damaged at each stage of pressure load. Though the variables were not enough to

3. It was discovered that the total count, and total signal strength at the pressure holding stage were the sound emission variables, which represent the degree of damages on the

4. The rate of number of hits with 60dB or higher in amplitudes in the number of total hits

5. We can estimate the damage mechanism through mean rise time, mean amplitude and

After manufacturing a sound vessel and a vessel with artificial defect for the composite vessel, we executed acoustic emission test during fatigue test and came up with the

1. Vessel with two types of artificial defects (longitudinal and transverse) and a sound vessel was put in 20000 fatigue test and the pressure was continuously increased and then was burst, the burst pressure was 590~615 bar and the differences in pressure on

3. Acoustic Emission Signal, which occurs during the fatigue test, occurred more in vessel with defects rather than in sound vessel, and as the number of fatigue test accumulated, the number of hits increased more in vessel with longitudinal defects than in those with

4. In the case of vessel with longitudinal defects, events were clustered around the artificial defect and more than 50 % of the occurred hits were signals that were related to artificial defects and the source location was precisely found on the defect location but in the case of vessel with transverse defect, events rarely occurred and even if they

5. Longitudinal defect of the vessel created matrix rupture and delaminating of the composite material during the fatigue test and the burst test and the thickness of the whole vessel area became thinner like the length of the defect and thus was weaker than other areas of the vessel. And the final burst was at the location in which the

6. The position of the longitudinal defect was shown well using the identification of acoustic emission location during the fatigue test and the average rise time of acoustic emission signal related to events occurring here was about 30-90 μs and signals with a shorter rise time can be observed more in the growth rather than in the initiation of matrix cracks.

occurred, the source location relevant to a defect did not match.

fatigue rupture of the steel liner occurred.

the vessel were less than 5 % and was not relevant to the existence of defects. 2. There is a correlation between the direction of defect and the final burst location and the longitudinal defect had a greater effect on the final burst location of the vessel rather

evaluate but were effective to estimate the damage mechanism.

is likely to indicate a damage of vessel.

vessel.

**4.2. Fatigue test**

following conclusion.

than the transverse defect.

transverse defects.

frequency analysis.

In this research, we cannot precisely know how much the depth of the defect has to be in order for the final burst pressure to change; however, the direction of the defect and the final burst location do have a correlation and we can infer that the longitudinal defect has a bigger effect on the final burst location.

**Figure 37.** Position of artificial defect and burst location : a) longitudinal, b) transverse

## **4. Conclusion**

## **4.1. Burst test**

By increasing the loading of pressure up to the expected burst pressure, I obtained the sound emission signal during 10 minutes holding time after loading. Considering that there would be flow noises during the initial 2 minutes, I obtained data for the remaining 8 minutes except the initial 2 minutes to analyze the AE variables.

1. Up to 360 bar, or 60 % of the estimated burst pressure, which is equivalent to 1.8 times of the usage pressure, it seems that there is little creep effect as there is little damage to the vessel. If the pressure is over 420 bar, or 70 % of the estimated burst pressure, the damage to the vessel becomes greater, meaning that the creep effect becomes larger.

## **4.2. Fatigue test**

88 Composites and Their Applications

and in 3 mm length so the effect is small.

bigger effect on the final burst location.

**4. Conclusion**

**4.1. Burst test**

Figure 37 is a picture that shows the burst location in the vessel with the artificial defect. As shown in the picture, we can see that in the case of a vessel with a transverse defect, the final burst location is in the general burst location (cylinder and head area) for a well-constructed type II vessel. However, in the case of a vessel with a longitudinal defect, in both vessels, the final burst location was within the transverse vessel in which the defect was located. We think that this is because in the case of the longitudinal defect, the wrapped fiber is cut in 3 mm depth and in 50 mm length so the effect in which the thickness of the composite material is big, but in the case of the transverse defect, the fiber is cut only in 3 mm depth

In this research, we cannot precisely know how much the depth of the defect has to be in order for the final burst pressure to change; however, the direction of the defect and the final burst location do have a correlation and we can infer that the longitudinal defect has a

**Figure 37.** Position of artificial defect and burst location : a) longitudinal, b) transverse

Defect position

a b

minutes except the initial 2 minutes to analyze the AE variables.

By increasing the loading of pressure up to the expected burst pressure, I obtained the sound emission signal during 10 minutes holding time after loading. Considering that there would be flow noises during the initial 2 minutes, I obtained data for the remaining 8

Defect position (back side)

1. Up to 360 bar, or 60 % of the estimated burst pressure, which is equivalent to 1.8 times of the usage pressure, it seems that there is little creep effect as there is little damage to After manufacturing a sound vessel and a vessel with artificial defect for the composite vessel, we executed acoustic emission test during fatigue test and came up with the following conclusion.

7. The initial frequency is distributed around 100-200 kHz and signals with reverberation frequency higher than 150 kHz are related to the growth of matrix cracks.

**Chapter 5**

© 2012 Liu et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Liu et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In this chapter, a technique for delamination identification in laminated composites using the zero-order mode of Lamb waves, i.e., S0 mode and A0 mode, without referring to the baseline data, is described. Through measuring the propagation speed of a wave and the traveling time of a reflected wave from the delamination, the delamination position can be

**Locating Delamination in Composite**

Yaolu Liu, Alamusi, Jinhua Li, Huiming Ning, Liangke Wu, Weifeng Yuan, Bin Gu and Ning Hu

Additional information is available at the end of the chapter

**of Lamb Waves**

http://dx.doi.org/10.5772/49991

**1. Introduction**

studies.

**Laminated Beams Using the Zero-Order Mode**

To improve the safety and reliability of various engineering structure, it is essential to develop efficient techniques for non-destructive damage detection or structural health monitoring. Lamb wave can travel a long distance in plate-like and shell-like structures made of materials even with high attenuation ratio (e.g. Carbon Fibre/Epoxy Polymer composites). To take this advantage, many researchers have recently explored the possibility of using Lame waves for damage identification [1]. To date, many developed Lamb wavebased techniques are generally based on so called two-stage prediction models by which the difference in the signals between a defective structure and a benchmark (intact structure) can be evaluated. Then, the residual error is easy to be defined no matter what information extracted from the signals is used, such as the information in time domain [2, 3] or frequency domain [4, 5]. Therefore, a benchmark or baseline signal is essential for the detection, which is very reliable and suitable for monitoring the propagation of damage. Also, tremendous efforts have been put to the delamination identification, which could be treated as a problem of inverse pattern recognition using calibrated numerical methods such as artificial neural network [6]. The interaction between Lamb waves and delamination has also been investigated numerically and theoretically [7-9]. However, the complex wave scattering phenomenon in a delamination area has not been clearly understood in these

8. The slope of accumulated amplitude distribution is about 0.04 during the initiation of matrix cracks and is about 0.12 during the growth. However, signals estimated to be liner fatigue crack growth have a slope of 0.06 and a rise time similar to that of during the growth of matrix cracks.

## **Author details**

Hyun-Sup Jee and Jong-O Lee *Korea Institute of Materials Science, South Korea*

## **5. References**

## **Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves**

Yaolu Liu, Alamusi, Jinhua Li, Huiming Ning, Liangke Wu, Weifeng Yuan, Bin Gu and Ning Hu

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/49991

## **1. Introduction**

90 Composites and Their Applications

**Author details**

**5. References**

the growth of matrix cracks.

*Korea Institute of Materials Science, South Korea*

Fuel Tanks, FaAA-SF-R-97-05-04

Japan (ISIJ), 64(14),2019-2028

Composite Structures, 40(2), 149-158

[1] Statics of korean Association for Natural Gas Vehicles (2009)

quality by Acoustic emission, Journal of KSNT, 16(2), 79

emission of composite plate, Journal of KSCM, 18(5), 15-20

pressure gas cylinder, Journal of KSPE, 19(2), 177-186

Composite Science and Technology, 68, 1144-1155

of laminated composite structures, Journal of KSCM, 16(6), 16.

Hyun-Sup Jee and Jong-O Lee

7. The initial frequency is distributed around 100-200 kHz and signals with reverberation

8. The slope of accumulated amplitude distribution is about 0.04 during the initiation of matrix cracks and is about 0.12 during the growth. However, signals estimated to be liner fatigue crack growth have a slope of 0.06 and a rise time similar to that of during

[2] Mark Toughiry (2002) Examination Of The Nondestructive Evaluation Of Composite Gas Cylinders, United States Department of Transportation, NTIAC/A7621-18:CRC-CD8.1, 10 [3] General Motors Corporation (1997) Development of Inspection Technology for NGV

[4] H. S. Jee, J. O. Lee, N. H. Ju and J. K. Lee (2011) Study of acoustic emission parameters

[5] J. O. Lee, J. S. Lee, U. H. Yoon and S. H. Lee (1996) Evaluation of adhesive bonding

[6] H. S. Jee, J. O. Lee, N. H. Ju, J. K. Lee and C. H. So (2011) Development of in-service inspection for type-ll gas cylinder, Proceeding for Spring Conference of KIGAS [7] H. S. Jee, J. O. Lee, N. H. Ju, J. K. Lee and C. H. So (2011) Damage Evaluation for High Pressure Fuel Tank by Analysis of AE Parameters Journal of KSCM, 24(4), 25 [8] S. Yuyama, T. Kishi and Y. Hisamatsu (1982) Detection and Analysis of Crevice Corrosion-SCC Process by the Use of AE Technique, The Iron and Steel Institute of

[9] S. H. Paik, S. H. Park and S. J. Kim (2001) Three dimensional FE analysis of acoustic

[10] J. S. Park, K. S. Kim and H. S. Lee (2003) A study on the acoustic emission characteristics

[11] L. Dong and J. Mistry (1998) Acoustic emission monitoring of composite cylinder,

[12] J. C. Choi, J. S. Jung, C. Kim, Y. Choi and J. H. Yoon (2002) A study on the Development of computer-aided Process planning system for the deep drawing & Ironing of high

[13] Y. Choi, J. H. Yooh, Y. S. Park and J. C. Choi (2004) A study on the Die Design for manufacturing of High Pressure Gas cylinder, Journal of KSPE, 21(7), 153-162 [14] A. Bussiba, M. Kupiec, S. Ifergane, R. Piat and T. Bohlke (2008) Damage evaluation and fracture events sequence in various composites by acoustic emission technique,

during a burst test for CNG vehicle fuel tank, Journal of KSME, 35(9), 1131

frequency higher than 150 kHz are related to the growth of matrix cracks.

To improve the safety and reliability of various engineering structure, it is essential to develop efficient techniques for non-destructive damage detection or structural health monitoring. Lamb wave can travel a long distance in plate-like and shell-like structures made of materials even with high attenuation ratio (e.g. Carbon Fibre/Epoxy Polymer composites). To take this advantage, many researchers have recently explored the possibility of using Lame waves for damage identification [1]. To date, many developed Lamb wavebased techniques are generally based on so called two-stage prediction models by which the difference in the signals between a defective structure and a benchmark (intact structure) can be evaluated. Then, the residual error is easy to be defined no matter what information extracted from the signals is used, such as the information in time domain [2, 3] or frequency domain [4, 5]. Therefore, a benchmark or baseline signal is essential for the detection, which is very reliable and suitable for monitoring the propagation of damage. Also, tremendous efforts have been put to the delamination identification, which could be treated as a problem of inverse pattern recognition using calibrated numerical methods such as artificial neural network [6]. The interaction between Lamb waves and delamination has also been investigated numerically and theoretically [7-9]. However, the complex wave scattering phenomenon in a delamination area has not been clearly understood in these studies.

In this chapter, a technique for delamination identification in laminated composites using the zero-order mode of Lamb waves, i.e., S0 mode and A0 mode, without referring to the baseline data, is described. Through measuring the propagation speed of a wave and the traveling time of a reflected wave from the delamination, the delamination position can be

© 2012 Liu et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Liu et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

accurately identified. Moreover, to understand the complex interaction of Lamb waves with a long delamination damage, the numerical simulations have been carried out.

Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves 93

*tNf*

(1)

two signals can yield the pure A0 mode. In experiments, an excitation signal in the following

0.5[1 cos(2 / )sin(2 )], / ( ) 0, / *ft N ft t N f P t*

**Figure 1.** Schematic view of models for experiments using S0 mode (unit: mm)

アクチュエータ(PZT) センサ(PZT)

1005mm

**Figure 2.** Schematic view of models for experiments using A0 mode (unit: mm)

**Table 1.** Material properties of PZT and CFRP lamina

Material properties

PZT E11=62 GPa, E33=49 GPa, d33=472 pC/N, d31=-210 pC/N, *ρ*=7500 kg/m3

*ν*12=*ν*13=0.3, *ν*23=0.45, *ρ*=1600 kg/m3

4.8

擬似はく離(10/11層目)

10th and 11th plies

10

CFRP lamina E11=115 GPa, E22=E33=9 GPa, G12=G23=5.5 GPa, G13=3 GPa,

where *f* is the central frequency in Hz and *N* is the number of sinusoidal cycles within a pulse. In this experiment, the signal with *f*=100 kHz (Fig. 1 for S0 mode) or *f*=50 kHz (Fig. 2 for A0 mode) and *N*=5 was used and the electrical voltage on the actuator was 10 V. The

Without the baseline data, it is still easy to estimate the arrival of the reflections from the beam boundaries, provided that the wave propagation speed and the dimensions of the beam are known. Then, if the reflected signal from the delamination is not overlapped with the reflections from the boundaries, it can be simply collected by the sensor. Therefore, it is still a technical challenge when the delamination is located close to the beam boundaries and the reflected signal of the delamination is completely overlapped with the reflections

<sup>x</sup> <sup>z</sup> <sup>y</sup> <sup>30</sup>

Sensor (PZT) Delamination between

455 320 200

Eq. (1), was adopted to generate S0 or A0 mode.

material properties of PZT are listed in Table 1.

Actuator (PZT)

from the boundaries.

z

## **2. Detective technique without the baseline data**

## **2.1. Experiments and numerical analyses**

In general, compared with S0mode, the attenuation of A0 mode in structures is more severe. Here, S0 mode corresponds to an axial deformation mode while A0 mode corresponds to a flexural deformation mode. Therefore, it should be paid more attention to the detective capability of both modes for a delamination damage in the different interfaces along the thickness direction. Moreover, the experimental setup of S0 mode is slightly different with that of A0 mode.

## *2.1.1. Materials and experimental procedure*

For S0 mode, as shown in Fig. 1, a CFRP laminated composite beam of stack sequence of [010/9012/010] was used. An artificial delamination damage with different lengths for several cases, i.e., 30 mm, 20 mm and 10 mm, respectively, was intentionally created at the interface between the 10th and 11th plies by inserting a Teflon film with a thickness of 25 μm. A PZT actuator was attached on the top surface of the left end of the beam. The PZT actuator had a diameter of 10 mm and a thickness of 0.5 mm. In this case, the generated waves from the actuator merged into the reflected waves from the left end of beam, which leads to a simpler signal. And the same PZT unit was used as a sensor to pick up the reflected wave from the delamination damage. Because the propagation speed of S0 mode is much higher than that of A0 mode, confirmed as 4 times higher in the used composites from our testing, it is expected that in a specified time domain, no A0 mode will be collected due to its slow speed, making the analysis of S0 modes much easier.

For the A0 mode, a similar composite beam was used, as shown in Fig. 2. Two kinds of stack sequence of the laminated beam were used, i.e., [010/9012/010] and [012/04/04/012]. Note that [012/04/04/012] is a unidirectional laminate, and it can be simply expressed as [032]. To simplify the description of delamination later, this expression is used. Like the case of S0 mode, a delamination damage with different lengths, i.e., 30 mm, 20 mm and 10 mm, was intentionally created at the interface between two plies by inserting a Teflon film with a thickness of 25 μm. The distance between the center of the delamination and the left end of beam is 790 mm. To generate the A0 mode, two actuators were attached on the top and bottom surfaces of the beam with applied out-of-phase voltages because the difference of signals in the two actuators can produce the pure *A*0 mode, as shown in Fig. 2. Although this arrangement can generate a relatively pure A0 mode, there are still reflections in S0 mode due to the mode change caused by the scattering between the Lamb waves and the delamination. To pick up the pure *A0* mode, two PZT sensors were attached. We know that two components of A0 mode in the two sensor signals are of the electrical charges of opposite signs, and two components of S0 mode in the two sensor signals are of the electrical charges of same signs. Naturally, the difference of two signals can yield the pure A0 mode. In experiments, an excitation signal in the following Eq. (1), was adopted to generate S0 or A0 mode.

92 Composites and Their Applications

that of A0 mode.

accurately identified. Moreover, to understand the complex interaction of Lamb waves with

In general, compared with S0mode, the attenuation of A0 mode in structures is more severe. Here, S0 mode corresponds to an axial deformation mode while A0 mode corresponds to a flexural deformation mode. Therefore, it should be paid more attention to the detective capability of both modes for a delamination damage in the different interfaces along the thickness direction. Moreover, the experimental setup of S0 mode is slightly different with

For S0 mode, as shown in Fig. 1, a CFRP laminated composite beam of stack sequence of [010/9012/010] was used. An artificial delamination damage with different lengths for several cases, i.e., 30 mm, 20 mm and 10 mm, respectively, was intentionally created at the interface between the 10th and 11th plies by inserting a Teflon film with a thickness of 25 μm. A PZT actuator was attached on the top surface of the left end of the beam. The PZT actuator had a diameter of 10 mm and a thickness of 0.5 mm. In this case, the generated waves from the actuator merged into the reflected waves from the left end of beam, which leads to a simpler signal. And the same PZT unit was used as a sensor to pick up the reflected wave from the delamination damage. Because the propagation speed of S0 mode is much higher than that of A0 mode, confirmed as 4 times higher in the used composites from our testing, it is expected that in a specified time domain, no A0 mode will be collected due to its slow

For the A0 mode, a similar composite beam was used, as shown in Fig. 2. Two kinds of stack sequence of the laminated beam were used, i.e., [010/9012/010] and [012/04/04/012]. Note that [012/04/04/012] is a unidirectional laminate, and it can be simply expressed as [032]. To simplify the description of delamination later, this expression is used. Like the case of S0 mode, a delamination damage with different lengths, i.e., 30 mm, 20 mm and 10 mm, was intentionally created at the interface between two plies by inserting a Teflon film with a thickness of 25 μm. The distance between the center of the delamination and the left end of beam is 790 mm. To generate the A0 mode, two actuators were attached on the top and bottom surfaces of the beam with applied out-of-phase voltages because the difference of signals in the two actuators can produce the pure *A*0 mode, as shown in Fig. 2. Although this arrangement can generate a relatively pure A0 mode, there are still reflections in S0 mode due to the mode change caused by the scattering between the Lamb waves and the delamination. To pick up the pure *A0* mode, two PZT sensors were attached. We know that two components of A0 mode in the two sensor signals are of the electrical charges of opposite signs, and two components of S0 mode in the two sensor signals are of the electrical charges of same signs. Naturally, the difference of

a long delamination damage, the numerical simulations have been carried out.

**2. Detective technique without the baseline data**

**2.1. Experiments and numerical analyses**

*2.1.1. Materials and experimental procedure*

speed, making the analysis of S0 modes much easier.

$$P(t) = \begin{cases} 0.5[1 - \cos(2\pi ft / N)\sin(2\pi ft)]\nu & t \le N / f \\ 0, \quad t > N / f \end{cases} \tag{1}$$

where *f* is the central frequency in Hz and *N* is the number of sinusoidal cycles within a pulse. In this experiment, the signal with *f*=100 kHz (Fig. 1 for S0 mode) or *f*=50 kHz (Fig. 2 for A0 mode) and *N*=5 was used and the electrical voltage on the actuator was 10 V. The material properties of PZT are listed in Table 1.

Without the baseline data, it is still easy to estimate the arrival of the reflections from the beam boundaries, provided that the wave propagation speed and the dimensions of the beam are known. Then, if the reflected signal from the delamination is not overlapped with the reflections from the boundaries, it can be simply collected by the sensor. Therefore, it is still a technical challenge when the delamination is located close to the beam boundaries and the reflected signal of the delamination is completely overlapped with the reflections from the boundaries.

**Figure 1.** Schematic view of models for experiments using S0 mode (unit: mm)

**Figure 2.** Schematic view of models for experiments using A0 mode (unit: mm)

**Table 1.** Material properties of PZT and CFRP lamina

## *2.1.2. Finite element analysis procedure*

To explore the wave propagation in laminates with a long delamination case, a threedimensional 8-noded brick hybrid element proposed by the authors [10] and the explicit time integration algorithm were used in the finite element simulations without considering the dynamic contact effects like those in some previous studies [11-13]. For the delamination area, one node used on the intact interface along the through-thickness direction, and double nodes were used on the delamination interface, i.e., one belongs to the elements of upper delaminated portion, and the other belongs to the elements of lower delaminated portion. The contact effects in the delamination area were neglected. The same signal stated in Eq. (1) was also used here. Furthermore, the actuator and the sensor were discretized using 3D brick elements. If an electrical field is applied on the actuator, the tensional or compressive strains will be generated in it from a proper relation, which connects the applied voltage and the generated internal strains. The induced stresses from the generated strains can be used to calculate the elemental nodal forces, which form axial force and bending moment in the beam simultaneously. Then, both S0 and A0 modes can be automatically generated. The material properties of CFRP are shown in Table 1.

Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves 95

detect the delamination position, as shown in Fig. 4. For the 20 mm long delamination, a slightly higher error in the delamination position is observed. The reason is the overlapping between the reflected signals from the delamination and those from the right end of the beam, causing an increased error in determination of the arrival time of a reflected wave

Incident wave

**Figure 3.** Comparison between signals of delaminated and intact beams (a 30 mm delamination

Identified Identified

0 100 200 300

*f*=100kHz, *N*=5

Time (s)

30mm

Reflected wave from boundary

25mm

<sup>13</sup> mm 20mm

Reflected wave from delamination

 Delaminated beam Intact beam

from the delamination.

damage between the 10th and the 11th plies)

0.0

Voltage (V)

0.5

1.0

**Figure 4.** Delamination positions identified experimentally

## **2.2. Results of S0 mode**

## *2.2.1. Experimental analysis*

Firstly, to evaluate the wave propagation speed, an intact beam with two sensors attached, was employed. The distance between the two sensors was 420 mm. By using the wavelet transformation technique [14], the arrival times of the incident waves to the two sensors were determined and the wave propagation speed of S0 mode at *f*=100 kHz was estimated as 6210 m/s. To verify the experimental results, the theoretical wave speed of the S0 mode was estimated based on the transfer matrix method [15]. The calculated speed was 6467 m/s if the material properties of CFRP were taken as the values of Table 1. On the other hand, the corresponding propagation speed of A0 mode was estimated as 1506 m/s, much lower that the S0 mode. The higher theoretical wave speed of S0 mode may be due to the transfer matrix method used here, which is actually for an infinite plate. For a beam with a finite width, the influences of Poisson's ratio and boundary conditions are expected.

A comparison of the signals from an intact beam and a delaminated one (a 30 mm delamination damage) for the case in Fig. 1 is shown in Fig. 3. It is clear that there is a reflected wave in the signals from the delaminated beam, which is located between the incident and the reflected waves. However, no clear reflected wave could be detected when the delamination length was reduced to 10 mm. No A0 mode can be observed in Fig. 3, since its incident wave is not fast enough to reach the sensor within 300 μs. For a signal with *f*=100 kHz and *N*=5, the minimum detectable length of the delamination was 20 mm, which is about 1/3 of the wavelength of the S0 mode at 100 kHz. It is reasonable to assume that a higher excitation frequency may have the benefit of detecting smaller delamination cases. Once the arrival time of a reflected wave from the delamination is determined, the difference between the arrival times of the incident and reflected waves can be used to detect the delamination position, as shown in Fig. 4. For the 20 mm long delamination, a slightly higher error in the delamination position is observed. The reason is the overlapping between the reflected signals from the delamination and those from the right end of the beam, causing an increased error in determination of the arrival time of a reflected wave from the delamination.

94 Composites and Their Applications

**2.2. Results of S0 mode**

*2.2.1. Experimental analysis*

*2.1.2. Finite element analysis procedure*

To explore the wave propagation in laminates with a long delamination case, a threedimensional 8-noded brick hybrid element proposed by the authors [10] and the explicit time integration algorithm were used in the finite element simulations without considering the dynamic contact effects like those in some previous studies [11-13]. For the delamination area, one node used on the intact interface along the through-thickness direction, and double nodes were used on the delamination interface, i.e., one belongs to the elements of upper delaminated portion, and the other belongs to the elements of lower delaminated portion. The contact effects in the delamination area were neglected. The same signal stated in Eq. (1) was also used here. Furthermore, the actuator and the sensor were discretized using 3D brick elements. If an electrical field is applied on the actuator, the tensional or compressive strains will be generated in it from a proper relation, which connects the applied voltage and the generated internal strains. The induced stresses from the generated strains can be used to calculate the elemental nodal forces, which form axial force and bending moment in the beam simultaneously. Then, both S0 and A0 modes can be

automatically generated. The material properties of CFRP are shown in Table 1.

width, the influences of Poisson's ratio and boundary conditions are expected.

Firstly, to evaluate the wave propagation speed, an intact beam with two sensors attached, was employed. The distance between the two sensors was 420 mm. By using the wavelet transformation technique [14], the arrival times of the incident waves to the two sensors were determined and the wave propagation speed of S0 mode at *f*=100 kHz was estimated as 6210 m/s. To verify the experimental results, the theoretical wave speed of the S0 mode was estimated based on the transfer matrix method [15]. The calculated speed was 6467 m/s if the material properties of CFRP were taken as the values of Table 1. On the other hand, the corresponding propagation speed of A0 mode was estimated as 1506 m/s, much lower that the S0 mode. The higher theoretical wave speed of S0 mode may be due to the transfer matrix method used here, which is actually for an infinite plate. For a beam with a finite

A comparison of the signals from an intact beam and a delaminated one (a 30 mm delamination damage) for the case in Fig. 1 is shown in Fig. 3. It is clear that there is a reflected wave in the signals from the delaminated beam, which is located between the incident and the reflected waves. However, no clear reflected wave could be detected when the delamination length was reduced to 10 mm. No A0 mode can be observed in Fig. 3, since its incident wave is not fast enough to reach the sensor within 300 μs. For a signal with *f*=100 kHz and *N*=5, the minimum detectable length of the delamination was 20 mm, which is about 1/3 of the wavelength of the S0 mode at 100 kHz. It is reasonable to assume that a higher excitation frequency may have the benefit of detecting smaller delamination cases. Once the arrival time of a reflected wave from the delamination is determined, the difference between the arrival times of the incident and reflected waves can be used to

**Figure 3.** Comparison between signals of delaminated and intact beams (a 30 mm delamination damage between the 10th and the 11th plies)

**Figure 4.** Delamination positions identified experimentally

## *2.2.2. Numerical simulation*

A comparison between the experimental result and numerical simulation for the case of Fig. 1 is shown in Fig.5. A good agreement between two results is observed. For convenience, the amplitude of the first arrival S0 mode in the simulated waves was calibrated using the experimental data. The attenuation coefficients of the CFRP in the numerical model were determined by matching the amplitudes of the reflected waves from the right end of the beam to the experimental data. When the actual sensor thickness, i.e., 0.5 mm, was used in numerical simulations, a small reflected wave from the sensor could be observed immediately next to the incident wave. This was also confirmed by the experimental result. Corresponding to a 0.05 mm sensor thickness, however, no obvious reflection from the sensor could be observed in the numerical simulation. Therefore, the sensor thickness of 0.05 mm was eventually chosen in the simulations. Note that the selection of sensor thickness does not affect the simulation results as the calculated amplitudes will be calibrated using the experimental data. Similar to the experimental results, the numerical simulation showed that the reflected wave from the 10 mm delamination was very weak.

Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves 97

**Figure 5.** Comparison of experimental and numerical results for a 30 mm delamination damage

0 100 200 300

*f*=100kHz, *N*=5

Experimental

Time (s)

Reflected wave from delamination

 Numerical (sensor thickness=0.05 mm) Numerical (sensor thickness=0.5 mm)

**Figure 6.** Comparison of experimental and numerical results for a 30 mm delamination damage at the

0 100 200 300

*f*=80kHz, *N*=5

 Experimental Numerical

Time (s)

between the 10th and the 11th plies

mid-plane between the 16th and the 17th plies

0.0

Voltage (V)

0.5

1.0

1.5

0.0

Voltage (V)

0.4

0.8

To understand the effect of delamination position along the through-thickness direction on the propagation of a Lamb wave, a 30 mm delamination damage was created in the midplane of the laminated beam, i.e., between the 16th and the 17th plies. With the signal of *f*=80 kHz and *N*=5, the comparison between the numerical and experimental results is shown in Fig.6. In Fig.6, no obvious reflected waves from the delamination can be observed. This finding is consistent with the work of Guo et al. [7]. They showed that the delamination at the positions of zero shear stress along the through-thickness direction, such as the midplane of a beam, had no effect on Lamb wave propagation in a S0 mode.