Then, we applied Tsai-Hill criterion for the definition of risky areas in failure on the specimen. The stresses at each element are influenced by the interaction between elements. Especially, as mentioned later, σ2 and τ12 were the dominant stress components, so in this study the reduced Tsai-Hill criterion was used as shown in the following: where, S1 is the transverse strength of the lamina, S2 is the tensile strength along the fiber axis, and S12 is the shear strength. The strength values are given as S1 = 6.44 MPa, S2 = 232 MPa, which were obtained by normalizing the SLM experimental values to the fiber volume fraction of 0.70. S12 was estimated as a double value of S1 referring to the data of conventional fibrous composites [22].

The values of Tsai-Hill criterion vary between 0 and 1. When the value tends to approach 0, in general, this location is interpreted as a nonrisky area. In contrast, when it tends to approach 1, this location is recognized as a risky area.

#### 2.6.2. Maximum stress criterion

8-node hexahedron element was used. This mesh consists of two laminae, and it is assumed that no delamination occurs during deformation. Thus, each contact point at the interface between the laminae was bonded through one-nodal point. The number of element was 1500, and the number of nodal points was 7344. Each lamina consists of 750 elements, of which the meshing was constructed in the same way as the segments at angle measurement described in Section 2.2.

Elastic constants used here are as follows: when we set the fiber volume faction, Vf, as 0.70, E1 and E3 = 3210 MPa, E2 = 39,500 MPa, ν21 and ν23 = 0.401, ν12 and ν32 = ν21E1/E2. These original constants were obtained experimentally, as mentioned in Section 2.3, and estimated to the constant values at Vf = 0.7 through the rule of mixture and Reuss rule. G12 and G23 = 1610 MPa. These shear moduli was assumed as E1/2 through Ref. [16]. G31 was also

The boundary condition was a forced displacement, which was applied at one end of the finite

To evaluate risky area in the composites with fiber waviness, maximum stress and Tsai-Hill

Tsai-Hill criterion is extended from the distortional energy criterion of von-Mises to anisotropic

where F, G, H, L, M, and N are the material strength parameters. For the plane stress in 1–2 plane of a unidirectional lamina with fiber in the two direction, σ3 = τ13 = τ23 = 0. Eq. (8) was

þ

Then, we applied Tsai-Hill criterion for the definition of risky areas in failure on the specimen. The stresses at each element are influenced by the interaction between elements. Especially, as mentioned later, σ2 and τ12 were the dominant stress components, so in this study the reduced Tsai-Hill criterion was used as shown in the following: where, S1 is the transverse strength of the lamina, S2 is the tensile strength along the fiber axis, and S12 is the shear strength. The strength values are given as S1 = 6.44 MPa, S2 = 232 MPa, which were obtained by normalizing the SLM experimental values to the fiber volume fraction of 0.70. S12 was estimated as a double

τ12 S12 2

σ2 S2 2

3 � 2Hσ1σ2 � 2Gσ1σ3 � 2Fσ2σ3 þ 2Lσ2

4 þ 2Mσ2

¼ 1 (9)

5 þ 2Nσ2

6 ¼ 1 (8)

The angle measured in each segment was substituted for θ in Eq. (7).

64 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

element mesh along y-axis. Another end was fixed along y-axis.

failure criterions are applied for the stresses on the specimen.

2 þ ð Þ F þ G σ2

σ1 S1 2

materials. According to this criterion, the general equation is written as:

� σ1σ2 S2 2 þ

value of S1 referring to the data of conventional fibrous composites [22].

assumed as being G12/2 from Ref. [21].

2.6. Failure criterion

2.6.1. Tsai-Hill criterion

reduced and written as:

1 þ ð Þ F þ H σ2

ð Þ G þ H σ2

Maximum stress criterion is given on the assumption that, when the ratio of stress to strength achieves to 1, the failure occurs. For this criterion, we used the longitudinal, transverse strength, and shear strength to predict the risky areas on specimens. Maximum stress criterion was hereinafter denoted as MS. When the MS trends to 1, this is the risky point on the specimen. In contract, MS trends to 0 when the value on the specimen is not the risky point. The MS is described as the below equation.

$$\frac{\sigma\_1}{S\_1} = 1, \frac{\sigma\_2}{S\_2} = 1, \text{ and } \frac{\tau\_{12}}{S\_{12}} = 1 \tag{10}$$

where, S1, S2, and S12 are transverse strength, longitudinal strength, and shear strength, respectively.

#### 3. Results and discussion

#### 3.1. Tensile test results

Fiber orientation angles and tensile properties of SLM and DM are shown in Table 2. The results show that tensile strengths of DM specimens accompanied with the fiber waviness are lower than those of SLM. Young's moduli of DM are also lower. It should be noted that the fiber volume fraction of DM specimens are lower in whole than that of SLM. This is attributed to the fact that the fibers cannot be packed compactly, as compared to SLM specimens, because of irregular fiber waviness. However, the lower volume fraction does not necessarily cause lower tensile strength of DM specimens. Here, we estimate the tensile strength of SLM specimen at 61%, the average fiber volume fraction, using σc 0 ¼ (Vf 0 /Vf)σc ¼ (0.61/0.72) � 238, which is the lower bound of the law of mixture. Where, σc is the composite's tensile strength, Vf is the fiber volume fraction, and a symbol "dash" means a predicted value, such that σc 0 is a value at the fiber volume fraction Vf 0 predicted from σc. The calculated tensile strength was 202 MPa. This value is clearly higher than 175 MPa, the average strength of DM specimens, despite of the value derived from the lower bound. It is considered that, therefore, the waviness is a major factor to reduce the tensile properties in strength of the sliver-based natural fiber composite, whereas several low Vf specimens show relatively large strengths, as mentioned above, and therefore the effect of irregular fiber waviness is needed to be discussed with unified Vf.

#### 3.2. Angle distribution and analytical results of special autocorrelation

In order to make contour maps of the fiber orientation angle distribution, each measured angle was assigned to the center point of each segment. Figure 5(a) and (b) show examples of the contour color maps. The blue (upper right portion in 5(a)) and red (center portion slightly to

\*The number of SLM specimens was seven, and their average values are shown.

characterized by giving an even evaluation, irrespective of the difference between positive and negative angles. We consider, the intensity and scale of such disorders in fiber orientation is

(a) (b)

(a) (b)

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Figure 6. Contour maps of LM-I distribution. (a) Specimen 2A and (b) specimen 5A.

Figure 7. Contour maps of LG-c distribution. (a) Specimen 2A and (b) specimen 5A.

Figure 7(a) and (b) shows typical examples of analytical results of LG-c. In these figures, the range from 1 to 1 is intentionally employed to compare them with LM-I distribution. As shown in the both figures, LG-c areas higher than 0.5 are locally dispersed with about 1–3 mm scale, quite smaller than LM-I, whereas major parts of the specimens are occupied by low LG-c values. Thus, LG-c is a measure accompanied with locally

In this section, we tried to quantify the results of the above spatial analyses by using the concept of "area ratio" characterized by threshold levels. If the area ratios are well correlated with tensile strength data, the strength could be estimated from the fiber orientation angle distribution measured in advance. In this quantification, we used two imaging programs; first is "Graph R221," which was used for plotting a contour map, and the next is "Azo R235,"

For instance, when we choose temporarily threshold levels higher than 0.3 or less than 0.1, the corresponding LM-I areas can be obtained, as shown in black areas of Figure 8(a), (b). In these cases, the area ratios of specimens were calculated as 12.6% (positive side = 11.1% and negative side = 1.5%) and 9.1% (positive side = 6.8% and negative side = 2.3%) for the specimens 2A and 5A, respectively. In Figure 9(a), (b), the black areas of LG-c are higher than 0.2 were selected. The area ratios of specimens were calculated as 16.5 and 13.5% for specimens 2A

3.3. Threshold levels of LM-I and LG-c values: definition of area ratio

related with the tensile strength to some degree.

which was used for calculation of area ratio.

smaller scale.

and 5A, respectively.

Figure 5. Contour map of fiber orientation angle distribution. (a) Specimen 2A (avg. angle: 4.19 , standard deviation: 4.03 ) and (b) specimen 5A (avg. angle: 4.55 , standard deviation: 3.90 ).

the right in 5(b)) color areas show the high angle on the negative and positive sides respectively on specimens 2A and 5A, of which the fiber orientation angles vary with approximately 4.0 degree standard deviation. Such contour mapping results and statistical aspect mean that the fiber waviness occurs locally and randomly.

Figure 6(a) and (b) shows typical examples of analytical results of LM-I. As shown in both the figures, positive LM-I areas higher than 0.5 are locally distributed with agglomeration of about 5–10 mm scale, whereas several negative LM-I areas are dispersed with a smaller scale. In comparison with Figure 5, it is recognized that Specimen 5A of Figure 6(b) shows quite a similar distribution. Specimen 2A of Figure 6(a) shows that, on the other hand, both positive and negative orientation angles are evaluated with similar color levels. In other words, LM-I is Effect of Fiber Waviness on Tensile Properties of Sliver-Based Natural Fiber Composites http://dx.doi.org/10.5772/intechopen.70905 67

Figure 6. Contour maps of LM-I distribution. (a) Specimen 2A and (b) specimen 5A.

Figure 7. Contour maps of LG-c distribution. (a) Specimen 2A and (b) specimen 5A.

characterized by giving an even evaluation, irrespective of the difference between positive and negative angles. We consider, the intensity and scale of such disorders in fiber orientation is related with the tensile strength to some degree.

Figure 7(a) and (b) shows typical examples of analytical results of LG-c. In these figures, the range from 1 to 1 is intentionally employed to compare them with LM-I distribution. As shown in the both figures, LG-c areas higher than 0.5 are locally dispersed with about 1–3 mm scale, quite smaller than LM-I, whereas major parts of the specimens are occupied by low LG-c values. Thus, LG-c is a measure accompanied with locally smaller scale.

#### 3.3. Threshold levels of LM-I and LG-c values: definition of area ratio

the right in 5(b)) color areas show the high angle on the negative and positive sides respectively on specimens 2A and 5A, of which the fiber orientation angles vary with approximately 4.0 degree standard deviation. Such contour mapping results and statistical aspect mean that

).

, standard deviation: 3.90

(a) (b)

Figure 5. Contour map of fiber orientation angle distribution. (a) Specimen 2A (avg. angle: 4.19

Table 2. Fiber orientation angles and mechanical properties of flax sliver-reinforced composite laminates [17].

Specimens Tensile

S.D. of angles ()

<2> 0.55 2.62 5.22 4.19 4.03 224 26.7 <3> 0.55 5.14 4.06 5.55 3.18 158 29.6 <4> 0.55 3.05 3.22 8.76 4.18 211 27.6 <5> 0.65 1.74 3.73 2.85 2.66 158 23.3 <6> 0.65 2.10 2.62 3.07 3.48 203 26.4 <7> 0.65 1.61 2.33 2.20 2.32 153 27.5 <8> 0.65 2.08 3.47 3.76 2.95 170 28.0 <9> 0.65 1.45 3.72 5.26 4.77 173 26.8 <10> 0.65 0.76 3.50 4.55 3.90 168 28.8 Avg. 0.61 2.35 3.50 4.35 3.60 175 26.8

SLM\* — 0.72 0 — 0 — 238 40.5 DM <1> 0.55 1.99 3.54 3.30 4.28 132 22.7

Side A (lower angle) Side B (higher angle)

Avg. angle () S.D. of angles () strength (MPa)

Young's modulus (GPa)

, standard deviation:

Figure 6(a) and (b) shows typical examples of analytical results of LM-I. As shown in both the figures, positive LM-I areas higher than 0.5 are locally distributed with agglomeration of about 5–10 mm scale, whereas several negative LM-I areas are dispersed with a smaller scale. In comparison with Figure 5, it is recognized that Specimen 5A of Figure 6(b) shows quite a similar distribution. Specimen 2A of Figure 6(a) shows that, on the other hand, both positive and negative orientation angles are evaluated with similar color levels. In other words, LM-I is

the fiber waviness occurs locally and randomly.

) and (b) specimen 5A (avg. angle: 4.55

Production method

4.03

Sample no.

Fiber volume fraction

66 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

\*The number of SLM specimens was seven, and their average values are shown.

Avg. angle ()

> In this section, we tried to quantify the results of the above spatial analyses by using the concept of "area ratio" characterized by threshold levels. If the area ratios are well correlated with tensile strength data, the strength could be estimated from the fiber orientation angle distribution measured in advance. In this quantification, we used two imaging programs; first is "Graph R221," which was used for plotting a contour map, and the next is "Azo R235," which was used for calculation of area ratio.

> For instance, when we choose temporarily threshold levels higher than 0.3 or less than 0.1, the corresponding LM-I areas can be obtained, as shown in black areas of Figure 8(a), (b). In these cases, the area ratios of specimens were calculated as 12.6% (positive side = 11.1% and negative side = 1.5%) and 9.1% (positive side = 6.8% and negative side = 2.3%) for the specimens 2A and 5A, respectively. In Figure 9(a), (b), the black areas of LG-c are higher than 0.2 were selected. The area ratios of specimens were calculated as 16.5 and 13.5% for specimens 2A and 5A, respectively.

Figure 8. Binary images of Local Moran's I distribution: (a) Specimen 2A and (b) specimen 5A (threshold levels: positive LM-I = 0.30 and negative LM-I = 0.10).

Figure 9. Binary images of local Geary's c distribution: (a) Specimen 2A and (b) specimen 5A (threshold level: LG-c = 0.20).

Here, we consider the change in area ratio with change in threshold level. Regarding LM-I, we take an account of the threshold levels of negative value with a relatively wide range, similarly to that of positive values. This is because the segments with negative LM-I values have a possibility of causing premature damage in shear, although these are not so major. We consider, if appropriate threshold levels are given, there is an optimal area ratio at each specimen, which is well correlated with tensile strength. In other words, the ratio of segments suffering damage during tensile loading should be related closely with the area ratio. In the next segment, thus, the relation between area ratio and tensile strength is investigated.

> In contrast, the LG-c area ratio plots are not so correlated with tensile strength data, as shown in Figure 11(a) and (b). The correlation coefficients between the area ratio and tensile strength were obtained as 0.18 and 0.29, when setting threshold levels at 0.2 and 0.5, respectively. The both values of 0.18 and 0.29 obviously signify weak correlation. This result implies that the values of 0.2 and 0.5 are inappropriate as a threshold level, or that LG-c is a measure not to

> Figure 11. LG-c area ratio dependence on normal tensile strength. (a) Threshold levels: LG-c > 0.2 and (b) threshold levels:

Effect of Fiber Waviness on Tensile Properties of Sliver-Based Natural Fiber Composites

100

150

200

**Normallized Tensile**

**(a) (b)**

**Strength (MPa)**

250

**Normalized Tensile**

**Strength (MPa)**

**(a) (b)**

Figure 10. LM-I area ratio dependence on normal tensile strength. (a) Threshold levels: LM-I > 0.6 or LM-I < 0.1 and (b)

0 0.5 1 1.5 2 2.5 **Area ratio**

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0 0.2 0.4 0.6 0.8

**Area ratio**

Figure 12(a) shows the change in correlation coefficient with change in positive threshold level of LM-I. In this case, the negative threshold level was fixed at 0.1. It is found that the correlation coefficient decreases gradually approximately from 0.25, and shows the lowest value around 0.70, which is approximately 0.80, the highest negative coefficient meaning a strong correlation. The correlation coefficient is also sensitive for the change in negative threshold level, as shown in Figure 12(b). Three positive threshold levels, 0.69, 0.75, and 0.77, were chosen for simplicity. It is confirmed that the optimal threshold levels of LM-I are 0.69 at the positive level and 0.10 at the negative level. These optimal levels brought the correlation coefficient of 0.832, from which it is concluded that LM-I area ratio yields a strong correlation

match correlation with tensile strength.

100

150

200

**Normallized Tensile**

LG-c > 0.5.

**Strength (MPa)**

250

threshold levels: LM-I > 0.8 or LM-I < 0.1.

**Normalized Tensile**

**Strength (MPa)**

01234 **Area ratio**

0 10 20 30

**Area ratio**

#### 3.4. Relation between area ratio and tensile strength

To investigate the correlation between the area ratio and tensile strength, normalized tensile strength data were plotted as a function of area ratio, as shown in Figures 10 and 11. In these plots, each tensile strength value was normalized by dividing the measured strength by the fiber volume fraction Vf and then multiplying it by 0.72, which is Vf of SLM specimen. In Figure 10, it is confirmed that the normalized strength is correlated with the area ratio to some extent. The correlation coefficients of LM-I were respectively calculated as 0.44 and 0.65 when setting threshold levels at LM-I > 0.6 or LM-I < 0.1 in Figure 10(a) and LM-I > 0.8 or LM-I < 0.1 in Figure 10(b). The value of 0.65 is not so strong but presents an intermediate strong correlation. This means, if many segments in a specimen are distributed with LM-I values higher or lower than the above threshold levels, its tensile strength tends to be lowered. It also means that a rough value of tensile strength can be estimated through the least-squares regression line between area ratio and tensile strength.

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Figure 10. LM-I area ratio dependence on normal tensile strength. (a) Threshold levels: LM-I > 0.6 or LM-I < 0.1 and (b) threshold levels: LM-I > 0.8 or LM-I < 0.1.

Here, we consider the change in area ratio with change in threshold level. Regarding LM-I, we take an account of the threshold levels of negative value with a relatively wide range, similarly to that of positive values. This is because the segments with negative LM-I values have a possibility of causing premature damage in shear, although these are not so major. We consider, if appropriate threshold levels are given, there is an optimal area ratio at each specimen, which is well correlated with tensile strength. In other words, the ratio of segments suffering damage during tensile loading should be related closely with the area ratio. In the next

(a) (b)

Figure 9. Binary images of local Geary's c distribution: (a) Specimen 2A and (b) specimen 5A (threshold level:

Figure 8. Binary images of Local Moran's I distribution: (a) Specimen 2A and (b) specimen 5A (threshold levels: positive

To investigate the correlation between the area ratio and tensile strength, normalized tensile strength data were plotted as a function of area ratio, as shown in Figures 10 and 11. In these plots, each tensile strength value was normalized by dividing the measured strength by the fiber volume fraction Vf and then multiplying it by 0.72, which is Vf of SLM specimen. In Figure 10, it is confirmed that the normalized strength is correlated with the area ratio to some extent. The correlation coefficients of LM-I were respectively calculated as 0.44 and 0.65 when setting threshold levels at LM-I > 0.6 or LM-I < 0.1 in Figure 10(a) and LM-I > 0.8 or LM-I < 0.1 in Figure 10(b). The value of 0.65 is not so strong but presents an intermediate strong correlation. This means, if many segments in a specimen are distributed with LM-I values higher or lower than the above threshold levels, its tensile strength tends to be lowered. It also means that a rough value of tensile strength can be estimated through the least-squares regression line between area ratio and

segment, thus, the relation between area ratio and tensile strength is investigated.

3.4. Relation between area ratio and tensile strength

LM-I = 0.30 and negative LM-I = 0.10).

68 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

LG-c = 0.20).

tensile strength.

Figure 11. LG-c area ratio dependence on normal tensile strength. (a) Threshold levels: LG-c > 0.2 and (b) threshold levels: LG-c > 0.5.

In contrast, the LG-c area ratio plots are not so correlated with tensile strength data, as shown in Figure 11(a) and (b). The correlation coefficients between the area ratio and tensile strength were obtained as 0.18 and 0.29, when setting threshold levels at 0.2 and 0.5, respectively. The both values of 0.18 and 0.29 obviously signify weak correlation. This result implies that the values of 0.2 and 0.5 are inappropriate as a threshold level, or that LG-c is a measure not to match correlation with tensile strength.

Figure 12(a) shows the change in correlation coefficient with change in positive threshold level of LM-I. In this case, the negative threshold level was fixed at 0.1. It is found that the correlation coefficient decreases gradually approximately from 0.25, and shows the lowest value around 0.70, which is approximately 0.80, the highest negative coefficient meaning a strong correlation. The correlation coefficient is also sensitive for the change in negative threshold level, as shown in Figure 12(b). Three positive threshold levels, 0.69, 0.75, and 0.77, were chosen for simplicity. It is confirmed that the optimal threshold levels of LM-I are 0.69 at the positive level and 0.10 at the negative level. These optimal levels brought the correlation coefficient of 0.832, from which it is concluded that LM-I area ratio yields a strong correlation

3.5. Normalized stress distributions and maximum stress criterion

> 0 0.05 0.1 0.15 0.2 0.25

Correla�on between normalized

distribution.

tensile strength and area ra�o

Figure 14 shows typical computed stress distributions. Figure 14(a) and (b) shows σx and σy contour maps, respectively, which were divided by the maximum value of all normal stress components. σz contour map is omitted in Figure 14 because it is quite the same as σx contour map. Figure 14(c–e) are τxy, τyz, and τzx contour maps, respectively, which are divided by the

(a) (b)

(c) (d)

(e)

Figure 14. Contour map of normalized stress distributions (Specimen 2A). (a) Contour map of σx distribution; (b) contour map of σy distribution; (c) contour map of τxy distribution; (d) contour map of τyz distribution and (e) contour map of τzx

Figure 13. Correlation coefficients between LG-c area ratio and normalized tensile strength vs. threshold level.

0.1 0.2 0.3 0.4 0.5 0.6

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Figure 12. Correlation coefficients between LM-I area ratio and normalized tensile strength. (a) Correlation coefficient vs. positive threshold level. (b) Correlation coefficient vs. negative threshold level.

with tensile strength. In other words, tensile strength can be estimated to some degree through the least-squares regression line when setting the optimal threshold levels of LM-I. It is also expected that the present procedure is applied as an effective screening method extracting low quality pre-pregs at quality inspection.

Figure 13 shows the relation between correlation coefficient and threshold level of LG-c. Although the coefficient varies with change in the threshold level of LG-c, the highest coefficient is only 0.14 at threshold level of 0.36. This coefficient value is quite a weak correlation, and therefore it is concluded that LG-c area ratio is not a parameter to match correlation with tensile strength.

Effect of Fiber Waviness on Tensile Properties of Sliver-Based Natural Fiber Composites http://dx.doi.org/10.5772/intechopen.70905 71

Figure 13. Correlation coefficients between LG-c area ratio and normalized tensile strength vs. threshold level.

#### 3.5. Normalized stress distributions and maximum stress criterion

with tensile strength. In other words, tensile strength can be estimated to some degree through the least-squares regression line when setting the optimal threshold levels of LM-I. It is also expected that the present procedure is applied as an effective screening method extracting low

Figure 12. Correlation coefficients between LM-I area ratio and normalized tensile strength. (a) Correlation coefficient vs.

Figure 13 shows the relation between correlation coefficient and threshold level of LG-c. Although the coefficient varies with change in the threshold level of LG-c, the highest coefficient is only 0.14 at threshold level of 0.36. This coefficient value is quite a weak correlation, and therefore it is concluded that LG-c area ratio is not a parameter to match correlation with

quality pre-pregs at quality inspection.

positive threshold level. (b) Correlation coefficient vs. negative threshold level.

70 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

tensile strength.

Figure 14 shows typical computed stress distributions. Figure 14(a) and (b) shows σx and σy contour maps, respectively, which were divided by the maximum value of all normal stress components. σz contour map is omitted in Figure 14 because it is quite the same as σx contour map. Figure 14(c–e) are τxy, τyz, and τzx contour maps, respectively, which are divided by the

Figure 14. Contour map of normalized stress distributions (Specimen 2A). (a) Contour map of σx distribution; (b) contour map of σy distribution; (c) contour map of τxy distribution; (d) contour map of τyz distribution and (e) contour map of τzx distribution.

maximum value of all shear stress components. The shear stress components were replaced by absolute values. It is proved that σy distribution is much larger than σx and σz, because the specimen is reinforced along the loading axis (y-axis). Regarding the case of shear stress, τxy distribution working in the plane is also much higher than the others. Shear stress τyz slightly occurs in the specimen, while τzx components are negligibly small.

To evaluate the degree of contribution of on-axis stress components to damage occurrence, the maximum stress criterion was used, in which the on-axis stresses, σ1, σ2, σ3, τ12, τ23, and τ31, were divided by the failure stresses, respectively.

$$\tilde{\sigma}\_1 = \frac{\sigma\_1}{S\_1}, \tilde{\sigma}\_2 = \frac{\sigma\_2}{S\_2}, \tilde{\sigma}\_3 = \frac{\sigma\_3}{S\_1}, \tilde{\tau}\_{12} = \frac{\tau\_{12}}{S\_{12}}, \tilde{\tau}\_{23} = \frac{\tau\_{23}}{S\_{12}}, \text{and } \tilde{\tau}\_{31} = \frac{\tau\_{31}}{S\_{12}} \tag{11}$$

also confirmed in the other specimens. Thus, tensile strength of the specimens may be affected by local shear damage mode, according to the maximum stress criterion. But, we should also focus on the possibility of tensile fracture, because the size of the risky area is larger than the

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3.6. Comparison between spatial autocorrelation analyses, maximum stress criterion and

Figure 16(a) and (b) shows the fractured specimen (Sample No.2) and the contour map of Tsai-Hill criterion, respectively. As is seen, the latter resembles Figure 15(c). This means, the obtained result from Tsai-Hill criterion is occupied by in-plane shear stress component. In comparison between Figure 16(a) and (b), the both shear fracture portions of the upper and lower sides (see, arrows) in Figure 16(a) may be slightly different from those of Figure 16(b), but the locations are quite close. It is considered that, on the other hand, it may be severe to cause the whole fracture from such small damage areas. It also looks in Figure 16(a) that tensile fracture occurs between the two shear damage areas. Figure 16(c) shows σy stress distribution of specimen 2B. It is confirmed that the tensile fracture portion is loaded more largely, whereas σy stress distribution of specimen 2A in Figure 14(b) is reduced (see both regions surrounded by an ellipse). It is estimated that such an unbalanced σy stress distribution between laminae 2A and 2B causes the final tensile fracture. In other specimens, the similar unbalanced σy stress distributions were confirmed (it is omitted because of limited space). It is predicted that, thus, although the flax sliver-based composite with irregular fiber waviness may receive some small damages firstly, finally this fractures due to the unbalanced normal

(a)

(b) (c)

Figure 16. Comparison between fractured specimen, Tsai-Hill criterion and normal stress distribution. (a) Fractured specimen; (b) contour map of Tsai-Hill criterion (specimen 2A) and (c) contour map of σy stress distribution (specimen

shear damage area, as seen from comparison between Figure 15(b) and (c).

Tsai-Hill criterion with the fracture specimen

stress distribution between laminae.

2B) divided by the same maximum value as (specimen 2A).

The normalized stresses in Eq. (11) were furthermore unified by dividing them by α (α: the maximum value of σ~1, σ~2, σ~3, τ~12, τ~23, and τ~31) in order to deal with the maximum as 1.0. Contour map of σ~3 is again omitted because of the less stress values. In comparison between Figure 15(a–e), the maximum normalized stress was found in Figure 15(c). It means that the damage or failure may occur in τ~12, although the area scale is small. The second risky normalized stress was found in σ~2 of Figure 15(b). σ~1 in Figure 15(a) are still affecting on the specimens but they do not have influence so much. Regarding τ~23 and τ~31, they are also quite small, and therefore we can neglect them from the fracture criterion. The same tendency was

Figure 15. Contour maps of unified normalized stress-distribution based on the maximum stress criterion (Specimen 2A). (a) Contour map of unified σ~1 distribution; (b) contour map of unified σ~2 distribution; (c) contour map of unified τ~12 distribution; (d) contour map of unified τ~23 distribution and (e) contour map of unified τ~31 distribution.

also confirmed in the other specimens. Thus, tensile strength of the specimens may be affected by local shear damage mode, according to the maximum stress criterion. But, we should also focus on the possibility of tensile fracture, because the size of the risky area is larger than the shear damage area, as seen from comparison between Figure 15(b) and (c).

maximum value of all shear stress components. The shear stress components were replaced by absolute values. It is proved that σy distribution is much larger than σx and σz, because the specimen is reinforced along the loading axis (y-axis). Regarding the case of shear stress, τxy distribution working in the plane is also much higher than the others. Shear stress τyz slightly

To evaluate the degree of contribution of on-axis stress components to damage occurrence, the maximum stress criterion was used, in which the on-axis stresses, σ1, σ2, σ3, τ12, τ23, and τ31,

> , τ~12 ¼ τ12 S12

The normalized stresses in Eq. (11) were furthermore unified by dividing them by α (α: the maximum value of σ~1, σ~2, σ~3, τ~12, τ~23, and τ~31) in order to deal with the maximum as 1.0. Contour map of σ~3 is again omitted because of the less stress values. In comparison between Figure 15(a–e), the maximum normalized stress was found in Figure 15(c). It means that the damage or failure may occur in τ~12, although the area scale is small. The second risky normalized stress was found in σ~2 of Figure 15(b). σ~1 in Figure 15(a) are still affecting on the specimens but they do not have influence so much. Regarding τ~23 and τ~31, they are also quite small, and therefore we can neglect them from the fracture criterion. The same tendency was

(a) (b)

(c) (d)

(e)

Figure 15. Contour maps of unified normalized stress-distribution based on the maximum stress criterion (Specimen 2A). (a) Contour map of unified σ~1 distribution; (b) contour map of unified σ~2 distribution; (c) contour map of unified τ~12

distribution; (d) contour map of unified τ~23 distribution and (e) contour map of unified τ~31 distribution.

, ~τ23 ¼ τ23 S12 , and τ~31 ¼ τ31

S12

(11)

occurs in the specimen, while τzx components are negligibly small.

, σ~3 ¼ σ3 S1

were divided by the failure stresses, respectively.

72 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

, σ~2 ¼ σ2 S2

σ~1 ¼ σ1 S1

#### 3.6. Comparison between spatial autocorrelation analyses, maximum stress criterion and Tsai-Hill criterion with the fracture specimen

Figure 16(a) and (b) shows the fractured specimen (Sample No.2) and the contour map of Tsai-Hill criterion, respectively. As is seen, the latter resembles Figure 15(c). This means, the obtained result from Tsai-Hill criterion is occupied by in-plane shear stress component. In comparison between Figure 16(a) and (b), the both shear fracture portions of the upper and lower sides (see, arrows) in Figure 16(a) may be slightly different from those of Figure 16(b), but the locations are quite close. It is considered that, on the other hand, it may be severe to cause the whole fracture from such small damage areas. It also looks in Figure 16(a) that tensile fracture occurs between the two shear damage areas. Figure 16(c) shows σy stress distribution of specimen 2B. It is confirmed that the tensile fracture portion is loaded more largely, whereas σy stress distribution of specimen 2A in Figure 14(b) is reduced (see both regions surrounded by an ellipse). It is estimated that such an unbalanced σy stress distribution between laminae 2A and 2B causes the final tensile fracture. In other specimens, the similar unbalanced σy stress distributions were confirmed (it is omitted because of limited space). It is predicted that, thus, although the flax sliver-based composite with irregular fiber waviness may receive some small damages firstly, finally this fractures due to the unbalanced normal stress distribution between laminae.

Figure 16. Comparison between fractured specimen, Tsai-Hill criterion and normal stress distribution. (a) Fractured specimen; (b) contour map of Tsai-Hill criterion (specimen 2A) and (c) contour map of σy stress distribution (specimen 2B) divided by the same maximum value as (specimen 2A).

#### 4. Conclusions

The effect of irregular fiber waviness on tensile strength of a flax sliver-based biodegradable thermoplastic resin composite laminate was clarified. The fiber orientation angles were measured at fine segments on the both surfaces of the composite specimen. As a first approach, two representative spatial analyses, Local Moran's I and Local Geary's c, were carried out for the quantification of the fiber waviness. Then, we calculated correlation coefficients between tensile strength and various area ratios obtained by changing the ranges of Local Moran's I and Local Geary's c. The results showed that Local Moran's I was correlated well with tensile strength of the composite specimens when appropriate ranges were selected. On the other hand, Local Geary's c was not well correlated with tensile strength. Thus, it is concluded that the analysis method used in this study is an effective tool of predicting roughly the tensile strength of natural fiber-sliver-based composites.

References

2005;65:1211-1225

Part A. 1996;27:931-941

2006;37:2178-2185

[1] Compositesworld.com, The Making of Carbon Fiber: Composites World. 2015. [Online].  The recycling of comminuted glass-fiber-reinforced resin from electronic waste. Journal of the Air & Waste Management Association. 2010;60:532-539 [5] Liu L, Yu J, Cheng L, Yang X. Biodegradability of poly(butylene succinate) (PBS) composite reinforced with jute fibre. Polymer Degradation and Stability. 2009;94:90-94

[6] Alix S, Marais S, Morvan C, Lebrun L. Biocomposite materials from flax plants: Prepara-

[7] Lodha P, Netravali AN. Characterization of stearic acid modified soy protein isolate resin and ramie fiber reinforced 'green' composites. Composites Science and Technology.

[8] Serizawa S, Inoue K, Iji M. Kenaf-fiber-reinforced poly(lactic acid) used for electronic

[9] Gomes A, Matsuo T, Goda K, Ohgi J. Development and effect of alkali treatment on tensile properties of curaua fiber green composites. Composites. Part A. 2007;38:1811-1820 [10] Goda K, Cao Y. Research and development of fully green composites reinforced with natural fibers. Journal of Solid Mechanics and Materials Engineering. 2007;1:1073-1084

[11] Hsiao HM, Daniel IM. Elastic properties of composites with fiber waviness. Composites:

[12] Karami G, Garnich M. Effective moduli and failure considerations for composites with

[13] Anumandla V, Gibson RF. A comprehensive closed form micromechanics model for estimating the elastic modulus of nanotube-reinforced composites. Composites: Part A.

[14] Allison BD, Evans JL. Effect of fiber waviness on the bending behavior of S-glass/epoxy

[15] Bogetti TA, Gillespie JW JR, Lamontia MA. The influence of ply waviness with nonlinear shear on the stiffness and strength reduction of composite laminates. Journal of Thermo-

[2] Peijs T. Composites for recyclability. Materials Today. 2003;6:30-35

tion and properties. Composites: Part A. 2008;39:1793-1801

products. Journal of Applied Polymer Science. 2006;100:618-624

periodic fiber waviness. Composite Structures. 2005;67:461-475

composites. Materials and Design. 2012;36:316-322

plastic Composite Materials. 1994;7:76-90

ceramic materials. Journal of Environmental Protection. 2012;3:740-747

As a next approach, we evaluated stress distributions in the composite specimen using a three-dimensional finite element analysis (3-D FEA) based on the orthotropic theory, in which measured fiber orientation angles were substituted for the finite elements. Results showed that σy distribution was much larger than σx and σz, which means that the specimen is reinforced along the loading axis (y-axis) despite of irregular fiber orientation. Regarding the other stresses, in-plane shear stress, τxy, was much higher than the others. Shear stress τyz slightly occurred in the specimen, whereas τzx components were negligibly small. For these results, the maximum stress criterion was firstly applied, in which the on-axis stresses, σ1, σ2, σ3, τ12, τ23, and τ31, (where 1 and 3 are the transverse direction of the fiber axis, and 2 is the fiber axis) were divided by the failure stresses, respectively. As a result, the normalized maximum stress was found on τ12 distribution map. It means that a small damage may occur in τ12. The fiber axis stress, σ2, occupied a relatively large stress area, although the stress itself is smaller than the normalized maximum stress of τ12. Tsai-Hill criterion, was also applied to predict more dangerous damage areas in the composite specimen. These results were compared with fracture paths of the actual specimens. It was estimated that, finally, fracture occurred by enhanced tensile stress occurring in the counterpart of the composite laminate, which was caused by partially off-axial fiber area in a lamina.

#### Author details

Taweesak Piyatuchsananon1 , Baosheng Ren2 and Koichi Goda3 \*

\*Address all correspondence to<EMAIL_ADDRESS>

#### References

4. Conclusions

Author details

Taweesak Piyatuchsananon1

strength of natural fiber-sliver-based composites.

74 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

The effect of irregular fiber waviness on tensile strength of a flax sliver-based biodegradable thermoplastic resin composite laminate was clarified. The fiber orientation angles were measured at fine segments on the both surfaces of the composite specimen. As a first approach, two representative spatial analyses, Local Moran's I and Local Geary's c, were carried out for the quantification of the fiber waviness. Then, we calculated correlation coefficients between tensile strength and various area ratios obtained by changing the ranges of Local Moran's I and Local Geary's c. The results showed that Local Moran's I was correlated well with tensile strength of the composite specimens when appropriate ranges were selected. On the other hand, Local Geary's c was not well correlated with tensile strength. Thus, it is concluded that the analysis method used in this study is an effective tool of predicting roughly the tensile

As a next approach, we evaluated stress distributions in the composite specimen using a three-dimensional finite element analysis (3-D FEA) based on the orthotropic theory, in which measured fiber orientation angles were substituted for the finite elements. Results showed that σy distribution was much larger than σx and σz, which means that the specimen is reinforced along the loading axis (y-axis) despite of irregular fiber orientation. Regarding the other stresses, in-plane shear stress, τxy, was much higher than the others. Shear stress τyz slightly occurred in the specimen, whereas τzx components were negligibly small. For these results, the maximum stress criterion was firstly applied, in which the on-axis stresses, σ1, σ2, σ3, τ12, τ23, and τ31, (where 1 and 3 are the transverse direction of the fiber axis, and 2 is the fiber axis) were divided by the failure stresses, respectively. As a result, the normalized maximum stress was found on τ12 distribution map. It means that a small damage may occur in τ12. The fiber axis stress, σ2, occupied a relatively large stress area, although the stress itself is smaller than the normalized maximum stress of τ12. Tsai-Hill criterion, was also applied to predict more dangerous damage areas in the composite specimen. These results were compared with fracture paths of the actual specimens. It was estimated that, finally, fracture occurred by enhanced tensile stress occurring in the counterpart of the composite

laminate, which was caused by partially off-axial fiber area in a lamina.

\*Address all correspondence to<EMAIL_ADDRESS>
1 Automotive Engineering Department, Siam University, Thailand 2 Department of Mechanical Engineering, University of Jinan, China

3 Department of Mechanical Engineering, Yamaguchi University, Japan

, Baosheng Ren2 and Koichi Goda3

\*

[16] Ren B, Noda J, Goda K. Effects of fiber orientation angles and fluctuation on the stiffness and strength of sliver-based green composites, zairyo. Japan: Journal of the Society of Materials Science; 2010;59:567-574

**Chapter 5**

**Provisional chapter**

**Mechanical and Fracture Surface Analysis of Higher**

**Mechanical and Fracture Surface Analysis of Higher**

This study experimentally characterizes the effect of multiwalled carbon nanotubes (MWNTs) reinforced to higher viscous aircraft thermoset polymer epoxy. The effects of MWNTs weight percentage (wt%) to flexural and fracture toughness properties were investigated via Mode I fracture behaviour. This experiment found that the average increment in fracture toughness of 0.1 and 0.3 wt% MWNTs reinforced to epoxy is 62.7 and 31.8%, respectively. However, shifting to a higher viscosity epoxy lead to some difficulties like to remove void formed in matrix and harder to achieve appropriate carbon nanotubes (CNTs) dispersion due to limited pot life and working time. Morphological study analysis on fracture surface using field emission scanning electron microscopic (FESEM) shows that the mechanical properties enhancement was attributed to crack pinning, crack path deflection and localized inelastic matrix deformation due to agglomerated CNTs. The study concluded that the key important to the extent the strength and fracture toughness is by finding the appropriate processing method to achieve adequate state of CNTs dispersion within the matrix.

Thermosetting polymers have been extensively used in aviation industry as a replacement for historic manufactured materials such as steel and aluminium due to the remarkable advantages in strength and stiffness properties. Today, the incorporation of composites has become a common practice in aircraft design and manufacturing [1–7]. In the next generation

**Keywords:** fracture, impact, carbon nanotube, nanocomposites

© 2016 The Author(s). Licensee InTech. This chapter is 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.

© 2018 The Author(s). Licensee IntechOpen. This chapter is 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.

DOI: 10.5772/intechopen.75992

**Viscous Epoxy/Multiwalled Carbon Nanotube**

**Viscous Epoxy/Multiwalled Carbon Nanotube**

**Nanocomposites Subjected to Flexural Loading**

**Nanocomposites Subjected to Flexural Loading**

Aidy Ali, Risby M. Sohaimi and Ahmad H. Muhammad Ismail

Aidy Ali, Risby M. Sohaimi and Ahmad H. Muhammad Ismail

http://dx.doi.org/10.5772/intechopen.75992

**Abstract**

**1. Introduction**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

#### **Mechanical and Fracture Surface Analysis of Higher Viscous Epoxy/Multiwalled Carbon Nanotube Nanocomposites Subjected to Flexural Loading Mechanical and Fracture Surface Analysis of Higher Viscous Epoxy/Multiwalled Carbon Nanotube Nanocomposites Subjected to Flexural Loading**

DOI: 10.5772/intechopen.75992

Aidy Ali, Risby M. Sohaimi and Ahmad H. Muhammad Ismail Aidy Ali, Risby M. Sohaimi and Ahmad H. Muhammad Ismail

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.75992

#### **Abstract**

[16] Ren B, Noda J, Goda K. Effects of fiber orientation angles and fluctuation on the stiffness and strength of sliver-based green composites, zairyo. Japan: Journal of the Society of

[17] Ren B, Mizue T, Goda K, Noda J. Effects of fluctuation of fibre orientation on tensile properties of flax sliver-reinforced green composites. Composite Structures. 2012;94:

[18] Fortin M-J, Dale M. Spatial Analysis. New York: Cambridge University Press; 2005. p. 111 [19] Tsai C-H, Zhang C, Jack DA, Liang R, Wang B. The effect of inclusion waviness and waviness distribution on elastic properties of fiber-reinforced composites. Composites:

[20] Jones RM. Mechanics of Composite Materials. 2nd ed. Philadelphia: CRC Press; 1999. p. 117 [21] Younes R, Hallal A, Fardoun F, Chehade FH. Comparative review study on elastic properties modeling for unidirectional composite materials. In: Hu N, editor. Composites

[22] Matthews FL, Rawlings RD. Composite Materials: Engineering and Science. Cambridge:

and Their Properties. Croatia: Intech; 2012. p. 391-408

Woodhead Publishing; 1999. p. 270

Materials Science; 2010;59:567-574

76 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

3457-3464

Part B. 2011;42:62-70

This study experimentally characterizes the effect of multiwalled carbon nanotubes (MWNTs) reinforced to higher viscous aircraft thermoset polymer epoxy. The effects of MWNTs weight percentage (wt%) to flexural and fracture toughness properties were investigated via Mode I fracture behaviour. This experiment found that the average increment in fracture toughness of 0.1 and 0.3 wt% MWNTs reinforced to epoxy is 62.7 and 31.8%, respectively. However, shifting to a higher viscosity epoxy lead to some difficulties like to remove void formed in matrix and harder to achieve appropriate carbon nanotubes (CNTs) dispersion due to limited pot life and working time. Morphological study analysis on fracture surface using field emission scanning electron microscopic (FESEM) shows that the mechanical properties enhancement was attributed to crack pinning, crack path deflection and localized inelastic matrix deformation due to agglomerated CNTs. The study concluded that the key important to the extent the strength and fracture toughness is by finding the appropriate processing method to achieve adequate state of CNTs dispersion within the matrix.

**Keywords:** fracture, impact, carbon nanotube, nanocomposites

#### **1. Introduction**

Thermosetting polymers have been extensively used in aviation industry as a replacement for historic manufactured materials such as steel and aluminium due to the remarkable advantages in strength and stiffness properties. Today, the incorporation of composites has become a common practice in aircraft design and manufacturing [1–7]. In the next generation

© 2016 The Author(s). Licensee InTech. This chapter is 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. © 2018 The Author(s). Licensee IntechOpen. This chapter is 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.

of aircraft, the use of polymer composites is expected to hit more than 50% of the overall aircraft's structures. This transformation was developed due to the desire of reducing costs and weight while not losing the strength and stiffness of the materials. However, polymer composites have deprived in fracture toughness compared to steel and aluminium, and this became an impediment to polymer composites to become the so-called dominant materials on aircraft structures. Over the years, tremendous efforts [1–13] have been conducted to alter epoxy system by adding either micron-sized soft (rubber or thermoplastic) or rigid (glass or ceramic) particles into the polymer matrix. These filler particles are then expected to provide extrinsic toughening mechanism through crack bridging, crack pinning, crack path deflection, localized inelastic matrix deformation and void nucleation [8].

**2. Materials and methods**

tively. The mix ratio is 100:58.

**Figure 1.** SEM picture of as received MWNTs.

placing the mixture inside a vacuum container for 20 min.

An industrial grade multiwalled carbon nanotubes (MWCNTs) produced by Timesnano, China with >90% purity, outer diameter of 10–30 nm and length of 10–30 μm were used in this experiment. The morphology of the product was inspected with Nova NanoSEM 240 (**Figure 1**). The Hysol EA956 A/B, an aviation grade epoxy system, obtained from Henkel Corporation Aerospace Group, USA. It is a two-component adhesive, Part A (epoxy resin) and Part B (hardener) with the viscosity of 35 Pa.S (Pascal-second @ kg/ms) and 2 Pa.S, respec-

Mechanical and Fracture Surface Analysis of Higher Viscous Epoxy/Multiwalled Carbon…

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79

For the preparation of control samples (0% CNTs), neat resin specimen was prepared by mixing Part A epoxy resin and Part B hardener thoroughly. The slurry was then placed inside a vacuum container to remove the entrapped air for 20 min before transferring into a mould for further curing processes. For the CNT-based polymer composites, MWCNTs (0.1% and 0.3%) were dispersed into Part B via ultrasonication process by Qsonica Q700 for 30 min at 10% of power amplitude. After sonication, Part A epoxy resin was added to the CNT-Part B slurry and stirred for 10 min at 550 rpm using a mechanical stirrer. Air bubbles were removed by

Three samples were prepared for each sample types in according with ASTM 5045–99. After the samples cured and de-mould, 2 mm notch tip was generated mechanically using Ray-ran Motorspeed Notching Cutter. The tests were conducted in a 50 kN Instron 5569A servo-hydraulic testing machine with crosshead speed of 1.0 mm/min. A crack opening displacement (COD) gauge was fixed to the samples to measure the initial fracture displacement during the testing. The morphology of MWNTs in epoxy was observed under field emission

In recent years, carbon nanotubes (CNTs) have attracted a great interest for nanoparticlereinforcing candidates instead of those particles on polymer matrices due to proven superior mechanical, electronic and thermal properties [9]. Number of researches has been conducted to ascertain the mechanical characteristic of these nanopolymer composites. Certain properties pertaining to the aircraft structural composite, which are flexural strength and fracture toughness, have become an important basic property to predict the life of the composite structure. In the works of evaluating the effect of CNTs incorporation to polymer epoxy's fracture toughness, Thostenson et al. [10] reported adding 0.1 and 2.0 wt% into epoxy resin shows an increase in fracture toughness from 0.66 to 0.73 and 0.84 MPa.m1/2, respectively. In the work of Yu et al. [11], the mode I fracture toughness of 1.0–3.0 wt% of multiwalled carbon nanotubes (MWNTs) reinforced epoxy-matrix composites is experimentally determined via compact test, and it is in the range of 0.53–0.73 MPa.m1/2. Other work by Zhou et al. [12] conducted flexural and fracture toughness tests and reported that the strength and fracture toughness of MWNTs reinforced epoxy are 120 MPa and 150 MPa.mm1/2, respectively, for 0.3 wt% MWNTs. These variations of results reported are due to the different types of epoxy system and nanocomposite processing techniques used.

Basically, CNTs improve high mechanical properties at low volume fractions due to the high aspect ratio to volume ratio of the nanosized particles. However, high specific ratio has a strong tendency to agglomerate and has an adverse effect that decreases the strength of the nanocomposite due to the stress concentration effect [12]. Thus, the significant enhancement in polymer-CNTs composites is generally dependent to the degree of dispersion and interfacial adhesion conditions between CNTs and matrix. Srivasta and Singh [13] and Gkikas et al. [14] reported that the strength and fracture toughness enhancement in 0.5 and 1.0 wt% MWNTs inclusion have a wide range of enhancement or detraction value, which is very sensitive to the dispersion technique applied. This study is an attempt to apply CNTs into the epoxy resin, Hysol EA956 A/B that has been practically used for modern aircrafts. The primary objective is to determine the effect of adding CNTs to strength and fracture toughness properties of epoxy, in which the viscosity is higher than any resin previously studied. The fracture behaviour and mechanisms of reinforced materials will be inspected using FESEM. FESEM is able to provide morphological evidence of CNTs in epoxy resin that helps to assess the dispersion of CNTs.

## **2. Materials and methods**

of aircraft, the use of polymer composites is expected to hit more than 50% of the overall aircraft's structures. This transformation was developed due to the desire of reducing costs and weight while not losing the strength and stiffness of the materials. However, polymer composites have deprived in fracture toughness compared to steel and aluminium, and this became an impediment to polymer composites to become the so-called dominant materials on aircraft structures. Over the years, tremendous efforts [1–13] have been conducted to alter epoxy system by adding either micron-sized soft (rubber or thermoplastic) or rigid (glass or ceramic) particles into the polymer matrix. These filler particles are then expected to provide extrinsic toughening mechanism through crack bridging, crack pinning, crack path deflec-

In recent years, carbon nanotubes (CNTs) have attracted a great interest for nanoparticlereinforcing candidates instead of those particles on polymer matrices due to proven superior mechanical, electronic and thermal properties [9]. Number of researches has been conducted to ascertain the mechanical characteristic of these nanopolymer composites. Certain properties pertaining to the aircraft structural composite, which are flexural strength and fracture toughness, have become an important basic property to predict the life of the composite structure. In the works of evaluating the effect of CNTs incorporation to polymer epoxy's fracture toughness, Thostenson et al. [10] reported adding 0.1 and 2.0 wt% into epoxy resin shows an increase in fracture toughness from 0.66 to 0.73 and 0.84 MPa.m1/2, respectively. In the work of Yu et al. [11], the mode I fracture toughness of 1.0–3.0 wt% of multiwalled carbon nanotubes (MWNTs) reinforced epoxy-matrix composites is experimentally determined via compact test, and it is in the range of 0.53–0.73 MPa.m1/2. Other work by Zhou et al. [12] conducted flexural and fracture toughness tests and reported that the strength and fracture toughness of MWNTs reinforced epoxy are 120 MPa and 150 MPa.mm1/2, respectively, for 0.3 wt% MWNTs. These variations of results reported are due to the different types of epoxy system and nano-

Basically, CNTs improve high mechanical properties at low volume fractions due to the high aspect ratio to volume ratio of the nanosized particles. However, high specific ratio has a strong tendency to agglomerate and has an adverse effect that decreases the strength of the nanocomposite due to the stress concentration effect [12]. Thus, the significant enhancement in polymer-CNTs composites is generally dependent to the degree of dispersion and interfacial adhesion conditions between CNTs and matrix. Srivasta and Singh [13] and Gkikas et al. [14] reported that the strength and fracture toughness enhancement in 0.5 and 1.0 wt% MWNTs inclusion have a wide range of enhancement or detraction value, which is very sensitive to the dispersion technique applied. This study is an attempt to apply CNTs into the epoxy resin, Hysol EA956 A/B that has been practically used for modern aircrafts. The primary objective is to determine the effect of adding CNTs to strength and fracture toughness properties of epoxy, in which the viscosity is higher than any resin previously studied. The fracture behaviour and mechanisms of reinforced materials will be inspected using FESEM. FESEM is able to provide morphological evidence of CNTs in epoxy resin that helps

tion, localized inelastic matrix deformation and void nucleation [8].

78 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

composite processing techniques used.

to assess the dispersion of CNTs.

An industrial grade multiwalled carbon nanotubes (MWCNTs) produced by Timesnano, China with >90% purity, outer diameter of 10–30 nm and length of 10–30 μm were used in this experiment. The morphology of the product was inspected with Nova NanoSEM 240 (**Figure 1**). The Hysol EA956 A/B, an aviation grade epoxy system, obtained from Henkel Corporation Aerospace Group, USA. It is a two-component adhesive, Part A (epoxy resin) and Part B (hardener) with the viscosity of 35 Pa.S (Pascal-second @ kg/ms) and 2 Pa.S, respectively. The mix ratio is 100:58.

For the preparation of control samples (0% CNTs), neat resin specimen was prepared by mixing Part A epoxy resin and Part B hardener thoroughly. The slurry was then placed inside a vacuum container to remove the entrapped air for 20 min before transferring into a mould for further curing processes. For the CNT-based polymer composites, MWCNTs (0.1% and 0.3%) were dispersed into Part B via ultrasonication process by Qsonica Q700 for 30 min at 10% of power amplitude. After sonication, Part A epoxy resin was added to the CNT-Part B slurry and stirred for 10 min at 550 rpm using a mechanical stirrer. Air bubbles were removed by placing the mixture inside a vacuum container for 20 min.

Three samples were prepared for each sample types in according with ASTM 5045–99. After the samples cured and de-mould, 2 mm notch tip was generated mechanically using Ray-ran Motorspeed Notching Cutter. The tests were conducted in a 50 kN Instron 5569A servo-hydraulic testing machine with crosshead speed of 1.0 mm/min. A crack opening displacement (COD) gauge was fixed to the samples to measure the initial fracture displacement during the testing. The morphology of MWNTs in epoxy was observed under field emission

**Figure 1.** SEM picture of as received MWNTs.

scanning electron microscopy (FESEM) using Nova NanoSEM 240. After mechanical test, all the sample fracture surfaces were coated with a thin gold layer by sputtering process to make them conductive for SEM analysis.

#### **3. Results and discussion**

**Table 1** and **Figure 2** show that by addition of MWNTs to epoxy system, the flexural strength of MWNT-based composites was found to exhibit non-linear behaviour with respect to MWNTs wt% inclusion. The optimal improvement was found at 0.1 wt% MWNTs, whereas a slight reduction was found at 0.3 wt% MWNTs. This trend was also observed for the flexural modulus but a significant reduction was observed at 0.3 wt% MWNTs content. The mode I fracture critical stress intensity factor (KIC) of the specimen is listed in **Table 2**. As shown in **Figure 3**, the fracture toughness of MWNTs reinforced epoxy was improved by 62.7% at 0.1 wt% CNTs and 31.8% at 0.3 wt%. This result showed a different trend compared to the works reported by Zhou et al. [12] using Epon 862 epoxy with viscosity 4.5 Pa.S, Li et al. [15] using Epoxy 828 with viscosity 15 Pa.S and Gojny et al. [8] using L135i epoxy with viscosity 0.25 Pa.S (**Figure 3**).

**Table 1.** Flexural strength and modulus properties of epoxy system.

**Figure 4** shows SEM images of the fracture surfaces of neat epoxy and MWNTs reinforced composites. **Figure 4a** shows a fractography feature of neat epoxy. The distance between two cleavage steps is about 2.64–7.84 μm, and the cleavage plane between them is flat and featureless. **Figures 4b** and **3c** show the fracture surface of composites with MWNTs inclusion. The fracture surfaces of the nanocomposites show a different fractographic features. The surface roughness increased with higher CNTs content. **Figure 4b** and **4c** also indicates that the size of the cleavage plane decreased with higher CNTs content. The SEM picture in **Figure 4c** shows that the size of the cleavage plane decreased to 0.77–2.74 μm after the infusion of the 0.3 wt% CNTs.

**Load, P (N) Stress intensity factor, KIC (Mpa.m1/2) Increase in KIC (%)**

Mechanical and Fracture Surface Analysis of Higher Viscous Epoxy/Multiwalled Carbon…

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81

175.915 1.10 0.00 285.392 1.79 62.7

231.268 1.45 31.8

**Table 2.** Fracture toughness of epoxy system.

**Figure 3.** Effect of CNTs content on fracture toughness.

According to Zhou et al. [12] and Srivasta and Singh [13], the decrease of cleavage plane and the increase of surface roughness were correspond to the number of isolated and dispersed

**Figure 2.** Effect of CNTs content on flexural strength and modulus.

Mechanical and Fracture Surface Analysis of Higher Viscous Epoxy/Multiwalled Carbon… http://dx.doi.org/10.5772/intechopen.75992 81

**Table 2.** Fracture toughness of epoxy system.

scanning electron microscopy (FESEM) using Nova NanoSEM 240. After mechanical test, all the sample fracture surfaces were coated with a thin gold layer by sputtering process to make

**Table 1** and **Figure 2** show that by addition of MWNTs to epoxy system, the flexural strength of MWNT-based composites was found to exhibit non-linear behaviour with respect to MWNTs wt% inclusion. The optimal improvement was found at 0.1 wt% MWNTs, whereas a slight reduction was found at 0.3 wt% MWNTs. This trend was also observed for the flexural modulus but a significant reduction was observed at 0.3 wt% MWNTs content. The mode I fracture critical stress intensity factor (KIC) of the specimen is listed in **Table 2**. As shown in **Figure 3**, the fracture toughness of MWNTs reinforced epoxy was improved by 62.7% at 0.1 wt% CNTs and 31.8% at 0.3 wt%. This result showed a different trend compared to the works reported by Zhou et al. [12] using Epon 862 epoxy with viscosity 4.5 Pa.S, Li et al. [15] using Epoxy 828 with viscosity 15 Pa.S and Gojny et al. [8] using L135i epoxy with viscosity 0.25 Pa.S (**Figure 3**).

**CNT weight percentage (%) Flexural Strength (Mpa) Modulus (Mpa)**

0.0 14.659 917.058 0.1 23.783 952.441 0.3 19.273 857.787

them conductive for SEM analysis.

80 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

**3. Results and discussion**

**Figure 2.** Effect of CNTs content on flexural strength and modulus.

**Table 1.** Flexural strength and modulus properties of epoxy system.

**Figure 3.** Effect of CNTs content on fracture toughness.

**Figure 4** shows SEM images of the fracture surfaces of neat epoxy and MWNTs reinforced composites. **Figure 4a** shows a fractography feature of neat epoxy. The distance between two cleavage steps is about 2.64–7.84 μm, and the cleavage plane between them is flat and featureless. **Figures 4b** and **3c** show the fracture surface of composites with MWNTs inclusion. The fracture surfaces of the nanocomposites show a different fractographic features. The surface roughness increased with higher CNTs content. **Figure 4b** and **4c** also indicates that the size of the cleavage plane decreased with higher CNTs content. The SEM picture in **Figure 4c** shows that the size of the cleavage plane decreased to 0.77–2.74 μm after the infusion of the 0.3 wt% CNTs.

According to Zhou et al. [12] and Srivasta and Singh [13], the decrease of cleavage plane and the increase of surface roughness were correspond to the number of isolated and dispersed

**Figure 4.** Cleavage plane at 5000X magnification of (a) neat epoxy, (b) epoxy with 0.1 wt% MWNTs, and (c) epoxy with 0.3 wt% MWNTs.

MWNTs, which force the crack to propagate, bypassing the MWNTs and distorted the crack tip. This phenomenon indicates that the good adhesion has taken place between MWNTs and epoxy resin and fractured in different planes. The cracks were spanned by the MWNTs causing enhanced resistance to the crack propagation process. Here, MWNTs have played a role for pinning crack and carry the external force. They have created a firm connection and good interaction with the matrix. Therefore, the MWNTs have prevented the expansion of microcracks resulted from the stress concentration and thus improved the toughness of the composites. Also, some deep cleavages are seen in both neat and modified epoxy surfaces but in modified epoxy, instead of deep cleavage, the crack tip is more bifurcate and also leads to a rough surface features. The bifurcation mechanism probably has caused a dissipation of more fracture energy throughout the failure process, promotes plastic deformation and leads to the improvement of toughness and ductility [16]. Here, MWNTs have played a role of crack path deflection.

inelastic matrix deformation and void nucleation. The localized deformation converted the elastic strain energy to surface energy [8]. When the epoxy resin is subjected to mode I loading, shear stress is generated between the CNTs wall and the surrounding matrix resin due to the difference in their elastic properties [17]. If they are well bonded, CNTs are deformed together with the matrix. However, if the mode I loading exceeded to some critical value that leads to debonding of CNTs from the matrix, the CNTs will stop elongating together with the matrix and a further increase of the load applied can only result in the deformation of the matrix. Thus, the polymer starts to flow over the surface of CNTs, and deformation energy will be dissipated through the slippage between the CNTs and matrix [17]. Other related explanation by Greef et al. [18] is that CNTs have a high stiffness in comparison with the epoxy, and their presence creates an abrupt stiffness variation in the matrix. Therefore, this stiffness mismatch introduces local disturbances in the stress distribution around the CNTs and establishing a weak link to the material and increases the number of potentials for the material to have such a weak link. Thus, multiple "weak" sites are identified, and they compete with each other for becoming a crack path. Some of these sites fail earlier which made the sample fail quickly as occurred in 0.3 wt% MWNTs content. But, if they are not part of a larger chain, they do not grow into larger cracks and

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83

However, in the authors' opinion, this experiment has not yet been fully answered all the possible toughening mechanism occurred in the polymer matrix. There are further mechanisms participate in the toughening process which can be seen through other method than FESEM. Based on those uncertainties enhancement and unpredicted void existence, it is believed that there are such mechanisms lead to stress relation in the materials and decrease the stress intensity at the crack front which consumes energy rather than increasing the total fracture surface area which cannot be seen through FESEM (Wetzel [19]). As a whole, former researchers proposed that energy absorption by crack bridging and fibre pull-out of CNTs

remain isolated (**Figure 6**).

**Figure 5.** Agglomeration at 0.3 wt% MWNTs-epoxy.

In this experiment, the fracture toughness of epoxy with 0.3 wt% MWNTs content was lower than 0.1 wt% MWNTs. A possible explanation can be given for this behaviour is the not-well dispersion state in 0.3 wt% MWNTs. This condition was in agreement with the work reported by Gkikas et al. [14], where at constant processing of sonication power and time, the degree of dispersion of less CNT concentration (0.1 wt% MWNTs) is higher than the higher concentration (0.3 wt% MWNTs). Also, samples with lower CNT concentration exhibited larger modulus than samples with higher concentration. This is because at higher contents of CNTs inclusion, the amount of aggregated particles is higher; thus, the processing power and time should be increased to ensure a good state of dispersion within the matrix. Aggregated particles usually do not have the load bearing ability as individual particles and they may act as defects [14]. Therefore, the state of dispersion is crucial to the extent the strengthening effect of CNT particles as the concentration becomes larger. This means that to the extent of improvement at higher CNTs content, the processing parameters play a key important role to improve the dispersion state.

The presence of a partly agglomerated dispersion of MWNTs that found in epoxy with 0.3 wt% MWNTs can be seen in **Figure 5**. The agglomeration in turn leads to the localized Mechanical and Fracture Surface Analysis of Higher Viscous Epoxy/Multiwalled Carbon… http://dx.doi.org/10.5772/intechopen.75992 83

**Figure 5.** Agglomeration at 0.3 wt% MWNTs-epoxy.

MWNTs, which force the crack to propagate, bypassing the MWNTs and distorted the crack tip. This phenomenon indicates that the good adhesion has taken place between MWNTs and epoxy resin and fractured in different planes. The cracks were spanned by the MWNTs causing enhanced resistance to the crack propagation process. Here, MWNTs have played a role for pinning crack and carry the external force. They have created a firm connection and good interaction with the matrix. Therefore, the MWNTs have prevented the expansion of microcracks resulted from the stress concentration and thus improved the toughness of the composites. Also, some deep cleavages are seen in both neat and modified epoxy surfaces but in modified epoxy, instead of deep cleavage, the crack tip is more bifurcate and also leads to a rough surface features. The bifurcation mechanism probably has caused a dissipation of more fracture energy throughout the failure process, promotes plastic deformation and leads to the improvement of toughness and ductility [16]. Here, MWNTs have played a role of crack path

**Figure 4.** Cleavage plane at 5000X magnification of (a) neat epoxy, (b) epoxy with 0.1 wt% MWNTs, and (c) epoxy with

In this experiment, the fracture toughness of epoxy with 0.3 wt% MWNTs content was lower than 0.1 wt% MWNTs. A possible explanation can be given for this behaviour is the not-well dispersion state in 0.3 wt% MWNTs. This condition was in agreement with the work reported by Gkikas et al. [14], where at constant processing of sonication power and time, the degree of dispersion of less CNT concentration (0.1 wt% MWNTs) is higher than the higher concentration (0.3 wt% MWNTs). Also, samples with lower CNT concentration exhibited larger modulus than samples with higher concentration. This is because at higher contents of CNTs inclusion, the amount of aggregated particles is higher; thus, the processing power and time should be increased to ensure a good state of dispersion within the matrix. Aggregated particles usually do not have the load bearing ability as individual particles and they may act as defects [14]. Therefore, the state of dispersion is crucial to the extent the strengthening effect of CNT particles as the concentration becomes larger. This means that to the extent of improvement at higher CNTs content, the processing parameters play a key important role to

The presence of a partly agglomerated dispersion of MWNTs that found in epoxy with 0.3 wt% MWNTs can be seen in **Figure 5**. The agglomeration in turn leads to the localized

deflection.

0.3 wt% MWNTs.

82 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

improve the dispersion state.

inelastic matrix deformation and void nucleation. The localized deformation converted the elastic strain energy to surface energy [8]. When the epoxy resin is subjected to mode I loading, shear stress is generated between the CNTs wall and the surrounding matrix resin due to the difference in their elastic properties [17]. If they are well bonded, CNTs are deformed together with the matrix. However, if the mode I loading exceeded to some critical value that leads to debonding of CNTs from the matrix, the CNTs will stop elongating together with the matrix and a further increase of the load applied can only result in the deformation of the matrix. Thus, the polymer starts to flow over the surface of CNTs, and deformation energy will be dissipated through the slippage between the CNTs and matrix [17]. Other related explanation by Greef et al. [18] is that CNTs have a high stiffness in comparison with the epoxy, and their presence creates an abrupt stiffness variation in the matrix. Therefore, this stiffness mismatch introduces local disturbances in the stress distribution around the CNTs and establishing a weak link to the material and increases the number of potentials for the material to have such a weak link. Thus, multiple "weak" sites are identified, and they compete with each other for becoming a crack path. Some of these sites fail earlier which made the sample fail quickly as occurred in 0.3 wt% MWNTs content. But, if they are not part of a larger chain, they do not grow into larger cracks and remain isolated (**Figure 6**).

However, in the authors' opinion, this experiment has not yet been fully answered all the possible toughening mechanism occurred in the polymer matrix. There are further mechanisms participate in the toughening process which can be seen through other method than FESEM. Based on those uncertainties enhancement and unpredicted void existence, it is believed that there are such mechanisms lead to stress relation in the materials and decrease the stress intensity at the crack front which consumes energy rather than increasing the total fracture surface area which cannot be seen through FESEM (Wetzel [19]). As a whole, former researchers proposed that energy absorption by crack bridging and fibre pull-out of CNTs

**Author details**

**References**

177-185

Aidy Ali\*, Risby M. Sohaimi and Ahmad H. Muhammad Ismail

Department of Mechanical Engineering, Faculty of Engineering, Universiti Pertahanan Nasional Malaysia, National Defense University of Malaysia, Kuala Lumpur, Malaysia

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85

[1] Rassiah K, Megat Ahmad MMH, Ali Aidy, Sihombing H. 3-D cartesian method in characterisating of mechanical properties of bamboo polyester composite. Advances in

[2] Rassiah K, MMH MA, Ali Aidy. Mechanical properties of laminated bamboo strips from gigantochloa scortechinii/ polyester composite. Materials and Design. 2014;**57**:551-559

[3] Rassiah K, MMH MA, Ali Aidy, Tamizi MM. The influence of laminated layer and thickness gigantochloa scortechinii bamboo strips on mechanical performance of unsaturated

[4] Rassiah K, MMH MA, Ali Aidy.. Ballistic impact performance of the layered and laminated composites: A review. Pertanika Journal of Science and Technology. 2015;**23**(2):

[5] Rassiah K, Megat Ahmad MMH, Ali Aidy, Abdullah AH, Nagapan S. Mechanical properties of layered laminated woven bamboo Gigantochloa scortechinii/epoxy composites.

[6] Ali Aidy, Rassiah K, Othman F, Pueh LH, Earn TT, Hazin MS, Ahmad MMHM. Fatigue and fracture properties of laminated bamboo strips from *Gigantochloa scortechinii* / poly-

[7] Ali Aidy, Ng WK, Arifin F, Rassiah K, Othman F, Hazin MS, Megat Ahmad MMH. Development and mechanical characterization of green bamboo composites. AIP Conference Proceedings. 2 February 2018;**1930**. Article ID: 020012. DOI: 10.1063/1.5022906

[8] Gojny FH, Winchmann MHG, Fiedler B, Schulte K. Influence of different on mechanical properties of epoxy matrix. Composites Science and Technology. 2005;**65**:2300-2313

[10] Thostenson ET, Chou T-W. Processing structure multifunctional property relationship

[11] Yu N, Zhang ZH, He SY. Fracture toughness and fatigue life of MWCNT/epoxy compos-

[9] Harris PJF. Carbon nanotube composites. International Materials Reviews. 2004:49

in carbon nanotube/epoxy composites. Carbon. 2006;**44**:3022-3029

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polyester composites. Life Science Journal. 2015;**12**(2):182-188

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ester composites. BioResources. 2016;**11**(4):9142-9153

\*Address all correspondence to<EMAIL_ADDRESS>
Environmental Biology. 2014;**8**(8):2632-2639

**Figure 6.** Isolated microcracks found at 0.3 wt% MWNTs.

was responsible for the major increase in fracture toughness. But according to this research work, we found that crack pinning, crack path deflection, localized inelastic plastic deformation and void nucleation mechanisms were the most attribute toughening mechanism in CNTs' modified epoxy.

#### **4. Conclusions**

CNTs are qualified for the toughening of the higher viscosity epoxy resin for aircraft application because they are able to enhance damage tolerance of the composite structure. The key important factor for significant enhancement is achieving the optimum state of dispersion and interfacial adhesion by manipulating and controlling the processing method. Welldispersed MWNTs can easily alleviate the stress concentration of the matrix and eliminate the adverse effect of voids. In this study, the flexural strength and the fracture toughness at the break were clearly improved with CNTs additives. The increase of performance is because CNTs resist the matrix deformation through several toughening mechanisms. No crack bridging mechanism by MWNTs is observed in present study, and no obvious CNTs are detected on the fracture surface during microscopic analysis. However, a more thorough study on the evaluation of surfaces is necessary, and the toughening mechanisms should be further investigated in future works.

#### **Acknowledgements**

The authors would like to gratefully acknowledge the support of Fundamental Research Grant Scheme (FRGS) of the Ministry of Higher Education.

#### **Author details**

Aidy Ali\*, Risby M. Sohaimi and Ahmad H. Muhammad Ismail

\*Address all correspondence to<EMAIL_ADDRESS>
Department of Mechanical Engineering, Faculty of Engineering, Universiti Pertahanan Nasional Malaysia, National Defense University of Malaysia, Kuala Lumpur, Malaysia

#### **References**

was responsible for the major increase in fracture toughness. But according to this research work, we found that crack pinning, crack path deflection, localized inelastic plastic deformation and void nucleation mechanisms were the most attribute toughening mechanism in

CNTs are qualified for the toughening of the higher viscosity epoxy resin for aircraft application because they are able to enhance damage tolerance of the composite structure. The key important factor for significant enhancement is achieving the optimum state of dispersion and interfacial adhesion by manipulating and controlling the processing method. Welldispersed MWNTs can easily alleviate the stress concentration of the matrix and eliminate the adverse effect of voids. In this study, the flexural strength and the fracture toughness at the break were clearly improved with CNTs additives. The increase of performance is because CNTs resist the matrix deformation through several toughening mechanisms. No crack bridging mechanism by MWNTs is observed in present study, and no obvious CNTs are detected on the fracture surface during microscopic analysis. However, a more thorough study on the evaluation of surfaces is necessary, and the toughening mechanisms should be further inves-

The authors would like to gratefully acknowledge the support of Fundamental Research

Grant Scheme (FRGS) of the Ministry of Higher Education.

CNTs' modified epoxy.

**Figure 6.** Isolated microcracks found at 0.3 wt% MWNTs.

84 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

tigated in future works.

**Acknowledgements**

**4. Conclusions**

[12] Zhou Y, Pervin F, lewis l, Jelani S. Fabrication and characteristic of carbon/epoxy composites mixed with Multiwalled carbon nanotubes. Materials Science and Engineering A. 2008;**475**:157-165

**Section 2**

**Bio-based Composites and Recycled Products**

**Bio-based Composites and Recycled Products**

[12] Zhou Y, Pervin F, lewis l, Jelani S. Fabrication and characteristic of carbon/epoxy composites mixed with Multiwalled carbon nanotubes. Materials Science and Engineering A.

[13] Srivasta VK, Singh S. A micro-mechanical model for elastic modulus of multi-walled carbon nanotube/epoxy resin composite. International Journal of Composite Materials.

[14] Gkikas G, Barkoula N-M, Paipetis AS. Effect of dispersion conditions on the thermomechanical and toughness properties of multi walled carbon nanotubes- reinforced

[15] LI Y, Shimizu H. High-shear processing induced homogenous dispersion of pristine Multiwalled carbon nanotubes in a thermoplastic elastomer. Polymer. 2007;**48**:2203-2207

[16] Ayatollahi MR, Shadlon S, Shokrieh MM. Mixed-mode brittle fracture in epoxy/ multi-walled carbon nanotube nanocomposites. Engineering Fracture Mechanics.

[17] Ma P-C, Siddiqui NA, Marom G, Kim J-Y. Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review. Composites: Part A.

[18] Greef ND, Gorbatikh l, Gandra A, Mezzo l, lomov SV, Verpoest I. The effect of carbon nanotubes on the damage development in carbon fiber/epoxy composites. Carbon.

[19] Wetzel B, Rosso P, Haupert F, Friedrich K. Epoxy nanocomposite – Fracture and tough-

ening mechanisms. Engineering Fracture Mechanics. 2006;**73**:2375-2398

epoxy. Composites Part B Engineering. 2012;**43**:2679-2705

86 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

2008;**475**:157-165

2012;**2**(2):1-6

2011;**78**:2620-2632

2010;**41**:1345-1367

2011;**49**:4650-4664

**Chapter 6**

Provisional chapter

**Waste and Recycled Textiles as Reinforcements of**

DOI: 10.5772/intechopen.70620

Currently, the use of composite materials in the construction areas has had a great impact on the society; mainly, those related with sustainability and environment aspects. Daily proposals aimed at overcoming the properties of traditional materials that arise, which include emergent materials either from waste or recycled products. One of them is related to the textile materials, which include fibers such as wool, hemp, linen, and cotton. In the past decade, special attention has been focused on the used clothes, which represent a source of raw materials environmentally responsible and economically profitable. Textile materials are discarded daily around the world, representing approximately 1.5% of the generated waste. Blue jeans are the most used clothing in the world, and they are elaborated by one of the most commonly used natural textile fibers cotton. Textile materials have been reused in different applications, for example, in the production of poor-quality wires, crushed to manufacture noise and temperature insulation materials, and as fillers or reinforcements of concrete. In this chapter, different topics are described that include: (a) environmental impact of textile waste—a result of massive consumption of clothing, (b) recycling and reuse of textile waste, and (c) waste

Keywords: recycling, waste, textiles, cotton, cellulose, composites, polymer concrete,

As a consequence of the technology boom and global population growth, the environment is being seriously damaged by different types of waste. Large amounts of wastewater, polluting gases, and solid waste are being disposed of worldwide, which have degraded the ecosystem

> © The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

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

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

Waste and Recycled Textiles as Reinforcements

Patricia Peña Pichardo, Gonzalo Martínez-Barrera,

Fernando Ureña-Núñez and Liliana I. Ávila-Córdoba

and recycled textile materials used as building materials.

gamma irradiation, mechanical properties

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Patricia Peña Pichardo, Gonzalo Martínez-Barrera, Miguel Martínez-López, Fernando Ureña-Núñez and Liliana I. Ávila-Córdoba

http://dx.doi.org/10.5772/intechopen.70620

**Building Materials**

of Building Materials

Miguel Martínez-López,

Abstract

1. Introduction

Provisional chapter

#### **Waste and Recycled Textiles as Reinforcements of Building Materials** Waste and Recycled Textiles as Reinforcements of Building Materials

DOI: 10.5772/intechopen.70620

Patricia Peña Pichardo, Gonzalo Martínez-Barrera, Miguel Martínez-López, Fernando Ureña-Núñez and Liliana I. Ávila-Córdoba Patricia Peña Pichardo, Gonzalo Martínez-Barrera, Miguel Martínez-López, Fernando

Additional information is available at the end of the chapter Ureña-Núñez and Liliana I. Ávila-Córdoba

http://dx.doi.org/10.5772/intechopen.70620 Additional information is available at the end of the chapter

#### Abstract

Currently, the use of composite materials in the construction areas has had a great impact on the society; mainly, those related with sustainability and environment aspects. Daily proposals aimed at overcoming the properties of traditional materials that arise, which include emergent materials either from waste or recycled products. One of them is related to the textile materials, which include fibers such as wool, hemp, linen, and cotton. In the past decade, special attention has been focused on the used clothes, which represent a source of raw materials environmentally responsible and economically profitable. Textile materials are discarded daily around the world, representing approximately 1.5% of the generated waste. Blue jeans are the most used clothing in the world, and they are elaborated by one of the most commonly used natural textile fibers cotton. Textile materials have been reused in different applications, for example, in the production of poor-quality wires, crushed to manufacture noise and temperature insulation materials, and as fillers or reinforcements of concrete. In this chapter, different topics are described that include: (a) environmental impact of textile waste—a result of massive consumption of clothing, (b) recycling and reuse of textile waste, and (c) waste and recycled textile materials used as building materials.

Keywords: recycling, waste, textiles, cotton, cellulose, composites, polymer concrete, gamma irradiation, mechanical properties

#### 1. Introduction

As a consequence of the technology boom and global population growth, the environment is being seriously damaged by different types of waste. Large amounts of wastewater, polluting gases, and solid waste are being disposed of worldwide, which have degraded the ecosystem

© 2018 The Author(s). Licensee IntechOpen. This chapter is 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.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

to an alarming extent. Concern about the environmental deterioration is leading science to design strategies to remedy the damages caused to some extent, which seek to generate in the population an ecological awareness, aimed at the reduction of waste, recycling of materials that can be reprocessed, and the reuse of objects or materials before proceeding to final disposal.

Actually, garments are discarded more quickly; which causes the industry to streamline its production and generate more textile waste. The production of textile fibers, both natural and synthetic, has been showing sustained growth, as a result of the increase in demand and population growth. The quantities of textile fibers used worldwide, between 1980 and 2000

Waste and Recycled Textiles as Reinforcements of Building Materials

http://dx.doi.org/10.5772/intechopen.70620

91

The different environmental aspects caused by the production, use, and disposal of garments are shown in Table 1 [6]. In the textile production of garments, different impacts on health,

are shown in Figure 2.

land, water, and air are detected.

Figure 1. Solid urban waste in Mexico [4].

Figure 2. Worldwide production of textile fibers [5].

The use of waste materials or recycled materials for the creation of raw materials for the construction area is a topical issue, with a promising future and primarily aimed at environmental preservation. Concrete, wood, and steel have been the most used materials in the infrastructure of houses and buildings, but the high production costs of these are leading science to develop research aimed at developing composite materials that recycled materials, such as PET, polycarbonate, recycled tires, wood, textile fibers, etc. The use of plastics in the form of small particles to reinforce hydraulic concrete has been applied in several investigations, but the problem lies in the poor interface between the matrix and the reinforcement; which causes diminution of mechanical properties of the concrete, such as flexural and compressive strength.

The use of waste PET fibers, from bottles, as reinforcement in concrete has allowed improvement in mechanical properties. Other reinforcement materials in the processing of concrete are waste tyre particles, which contribute to diminution of the crack propagation effect and improvement of resistance to deformation of the concrete.

Some investigations are focused on the use of natural materials as reinforcements in Portland cement concrete. In special, natural fibers, such as jute, flax, coconut fiber, henequen, and cotton, as a reinforcement in building materials, which have great interest for their advantages when compared with synthetic materials, and one of their biggest benefits are the low environmental impact, low cost, and wide range of applications. Cellulose fibers mixed into the concrete improve the thermal and acoustic insulation.

In this regard, during the last decade, the construction industry is being innovated with a new material known as textile reinforced concrete (TRC): a combination of fine-grained concrete and multiaxially oriented textiles whose structural functionality, ease of production, applicability, and design are investigated [1], as a way to take advantage of the high amount of waste from the industry textile.

#### 2. Environmental impact of textile wastes

At present, there is a growing need for textiles to make yarns, fabrics, and garments of different types, shapes, and colors. Demand increases with population and trends in international fashion being the reason why the rational exploitation of the necessary inputs for this purpose has become one of the main challenges for scientists and industrialists. Wastes from the production of textiles are undesirable but are inevitable byproducts in many manufacturing processes, which are often not given the recognition and economic value that they actually have [2].

In developed countries, each person produces on average 1.5–2 kg of solid waste per day and in Latin America 1 kg per day [3]. The urban waste percentages in Mexico are shown in Figure 1, in which textile wastes represent 1.2%, and this means 1100 tons are produced daily.

Actually, garments are discarded more quickly; which causes the industry to streamline its production and generate more textile waste. The production of textile fibers, both natural and synthetic, has been showing sustained growth, as a result of the increase in demand and population growth. The quantities of textile fibers used worldwide, between 1980 and 2000 are shown in Figure 2.

The different environmental aspects caused by the production, use, and disposal of garments are shown in Table 1 [6]. In the textile production of garments, different impacts on health, land, water, and air are detected.

Figure 1. Solid urban waste in Mexico [4].

to an alarming extent. Concern about the environmental deterioration is leading science to design strategies to remedy the damages caused to some extent, which seek to generate in the population an ecological awareness, aimed at the reduction of waste, recycling of materials that can be reprocessed, and the reuse of objects or materials before proceeding to final disposal.

The use of waste materials or recycled materials for the creation of raw materials for the construction area is a topical issue, with a promising future and primarily aimed at environmental preservation. Concrete, wood, and steel have been the most used materials in the infrastructure of houses and buildings, but the high production costs of these are leading science to develop research aimed at developing composite materials that recycled materials, such as PET, polycarbonate, recycled tires, wood, textile fibers, etc. The use of plastics in the form of small particles to reinforce hydraulic concrete has been applied in several investigations, but the problem lies in the poor interface between the matrix and the reinforcement; which causes diminution of mechanical properties of the concrete, such as flexural and compressive strength. The use of waste PET fibers, from bottles, as reinforcement in concrete has allowed improvement in mechanical properties. Other reinforcement materials in the processing of concrete are waste tyre particles, which contribute to diminution of the crack propagation effect and

Some investigations are focused on the use of natural materials as reinforcements in Portland cement concrete. In special, natural fibers, such as jute, flax, coconut fiber, henequen, and cotton, as a reinforcement in building materials, which have great interest for their advantages when compared with synthetic materials, and one of their biggest benefits are the low environmental impact, low cost, and wide range of applications. Cellulose fibers mixed into the

In this regard, during the last decade, the construction industry is being innovated with a new material known as textile reinforced concrete (TRC): a combination of fine-grained concrete and multiaxially oriented textiles whose structural functionality, ease of production, applicability, and design are investigated [1], as a way to take advantage of the high amount of waste

At present, there is a growing need for textiles to make yarns, fabrics, and garments of different types, shapes, and colors. Demand increases with population and trends in international fashion being the reason why the rational exploitation of the necessary inputs for this purpose has become one of the main challenges for scientists and industrialists. Wastes from the production of textiles are undesirable but are inevitable byproducts in many manufacturing processes, which are often not given the recognition and economic value that they actually have [2].

In developed countries, each person produces on average 1.5–2 kg of solid waste per day and in Latin America 1 kg per day [3]. The urban waste percentages in Mexico are shown in Figure 1, in which textile wastes represent 1.2%, and this means 1100 tons are produced daily.

improvement of resistance to deformation of the concrete.

90 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

concrete improve the thermal and acoustic insulation.

2. Environmental impact of textile wastes

from the industry textile.

Figure 2. Worldwide production of textile fibers [5].

Table 1. Environmental impacts generated by textiles (adapted from [6]).

Water contamination carried out through the dyeing of fabrics is produced by using synthetic dyes, which has become a common practice. Such toxic nature is a cause of concern for environmentalists [7]. It has a high content of sulfur, nitrates, acetic acid, soaps, enzymes, chromium compounds, and heavy metals such as copper, arsenic, lead, cadmium, mercury, nickel, and cobalt. Also used dyes are fixing agents that are based on formaldehyde, chlorinated stain removers, hydrocarbon softeners, and nonbiodegradable chemicals, which react with many disinfectants, especially chlorine, and form carcinogenic products.

In the research work carried out [8] was evaluated physicochemical parameters in waters coming from a textile industry, as well as the soil where they are arranged. The parameters such as pH, electrical conductivity, temperature, dissolved oxygen (DO), chemical oxygen demand (COD), biological oxygen demand (BOD), and total dissolved solids (TDS), as well as N, P, and K in soils were determined. The salinity and alkalinity of the soils changed; moreover, the food crops that were in these lands reported a very low yield and negative hygienic conditions.

The fact is undeniable that almost all human activities have an environmental impact; the textile industry is no exception, but as in many other productive processes, the population is increasingly acquiring an ecological awareness for recycling, reusing, and reducing (RRR).

Types of pollution Index

Volume of wastewater 317 m3

DBO5 155 kg/ton product Suspended solids 70 kg/ton product Total dissolved solids 205 kg/ton product

Preparation of fibers and cotton thread 32 kg/ton product Fiber fabric, thread, cloth 11 kg/ton product Dyeing and finishing fabrics 7 kg/ton product Dried fibers trapped in grids 0.8 kg/ton product Natural fibers trapped in grids 2.8 kg/ton product Residual sludge (not dehydrated or treated) 2300 kg/ton product

/ton product

93

Production of particles during the process 14 kg/ton cotton Production of particles by incineration of waste 7.5 kg/ton residue Production of SO2 by incineration of waste 1.25 kg/ton residue Production of nitrogen oxides by incineration of waste 1.0 kg/ton residue Hydrocarbon production by incineration of waste 7.5 kg/ton residue

Waste and Recycled Textiles as Reinforcements of Building Materials

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At present, the consumption trend is due to the use of natural textile fibers, this as a response to the conservation of the environment, and that fashion is increasingly pronounced by com-

World cotton production in 2014 was 25.8 million tons and global production is expected to grow by 2.1% per year over the next 10 years. An important part of cotton produced worldwide is part of the textile fibers used for the production of Denim garments, which was created in 1873 by Jacob Davis, a friend of Levi Strauss, for the hard work of the mining time during the socalled gold rush in the USA. In the case of Mexico, 9000 million tons of Denim were produced in 2010. Jeans are the best-selling item in Mexico, representing 30% of the entire clothing market.

Textile waste is part of the solid waste that is generated by the population and also contains used clothing, footwear, curtains, sheets, etc. The theme of textile recycling should be taken as

fortable, light, and friendly with the skin.

Table 2. Index of contamination for the cotton industry (Adapted from [11]).

Air pollution

Water contamination

Solid waste

3. Recycling and reuse of textile waste

The cotton cultivation requires large amounts of water. The amount of cotton fibers used for textiles is only one-third of the total production of raw cotton and the remainder consists of cotton seed (which are used for the extraction of oil) and weed which is used as food for livestock. The part of the fibers that does not have the necessary quality to enter into textile production processes is used for viscose raw material and cleaning cloths [9].

Requirements for the production of garments made from virgin materials were quantified [10]. They showed that for every kilogram of virgin cotton that is replaced by second-hand clothing, electric energy is saved. The reuse and recycling of used clothing reduces the environmental burden compared with the purchase of clothing obtained from virgin fibers. Rojas et al. [11] indicated the importance of minimizing waste produced in the textile industry.

The textile industry is one of the four most representative and polluting items in Latin America, because its processing employ a great amount of chemical compounds, water, and energy [11]. The indexes of pollutants from the cotton industry include yarns and cotton cloth (Table 2).

Table 2. Index of contamination for the cotton industry (Adapted from [11]).

Water contamination carried out through the dyeing of fabrics is produced by using synthetic dyes, which has become a common practice. Such toxic nature is a cause of concern for environmentalists [7]. It has a high content of sulfur, nitrates, acetic acid, soaps, enzymes, chromium compounds, and heavy metals such as copper, arsenic, lead, cadmium, mercury, nickel, and cobalt. Also used dyes are fixing agents that are based on formaldehyde, chlorinated stain removers, hydrocarbon softeners, and nonbiodegradable chemicals, which react

Activity Environmental aspects Health Water Air Ironing Consumption of electricity and occupational diseases X X X

Cloth cut Noise generation X X

Bleaching of tissues Generation of liquid waste X Cloth dyeing Generation of liquid waste X Cloth wash (Denim) Water consumption X

In the research work carried out [8] was evaluated physicochemical parameters in waters coming from a textile industry, as well as the soil where they are arranged. The parameters such as pH, electrical conductivity, temperature, dissolved oxygen (DO), chemical oxygen demand (COD), biological oxygen demand (BOD), and total dissolved solids (TDS), as well as N, P, and K in soils were determined. The salinity and alkalinity of the soils changed; moreover, the food crops that were in these lands reported a very low yield and negative

The cotton cultivation requires large amounts of water. The amount of cotton fibers used for textiles is only one-third of the total production of raw cotton and the remainder consists of cotton seed (which are used for the extraction of oil) and weed which is used as food for livestock. The part of the fibers that does not have the necessary quality to enter into textile

Requirements for the production of garments made from virgin materials were quantified [10]. They showed that for every kilogram of virgin cotton that is replaced by second-hand clothing, electric energy is saved. The reuse and recycling of used clothing reduces the environmental burden compared with the purchase of clothing obtained from virgin fibers. Rojas et al. [11]

The textile industry is one of the four most representative and polluting items in Latin America, because its processing employ a great amount of chemical compounds, water, and energy [11]. The indexes of pollutants from the cotton industry include yarns and cotton cloth

with many disinfectants, especially chlorine, and form carcinogenic products.

Fabric and cloth production Impact

Clothing sewing Consumption of human energy, and textile and metal waste X

Table 1. Environmental impacts generated by textiles (adapted from [6]).

92 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

production processes is used for viscose raw material and cleaning cloths [9].

indicated the importance of minimizing waste produced in the textile industry.

hygienic conditions.

(Table 2).

The fact is undeniable that almost all human activities have an environmental impact; the textile industry is no exception, but as in many other productive processes, the population is increasingly acquiring an ecological awareness for recycling, reusing, and reducing (RRR).

At present, the consumption trend is due to the use of natural textile fibers, this as a response to the conservation of the environment, and that fashion is increasingly pronounced by comfortable, light, and friendly with the skin.

World cotton production in 2014 was 25.8 million tons and global production is expected to grow by 2.1% per year over the next 10 years. An important part of cotton produced worldwide is part of the textile fibers used for the production of Denim garments, which was created in 1873 by Jacob Davis, a friend of Levi Strauss, for the hard work of the mining time during the socalled gold rush in the USA. In the case of Mexico, 9000 million tons of Denim were produced in 2010. Jeans are the best-selling item in Mexico, representing 30% of the entire clothing market.

#### 3. Recycling and reuse of textile waste

Textile waste is part of the solid waste that is generated by the population and also contains used clothing, footwear, curtains, sheets, etc. The theme of textile recycling should be taken as a means to obtain economic and environmental benefits for various reasons, as it contributes to the reduction of spaces required for landfills, reduces the need to produce virgin materials and reduces problems of water pollution. The textile recycling involves the reuse of used clothing, fibrous materials, and production losses of the manufacturing process of clothing.

Some products made from synthetic fibers do not decompose naturally, which causes soil and surface water contamination problems. Although cotton fibers decompose naturally, in doing so they produce methane, which contributes to the global warming.

Textile recycling strategies are based on the knowledge of their classification: (a) post-production materials, such as yarns, textiles, fibers, which come from the loss of the production process and (b) post-consumption materials, such as clothing, carpets, shoes, and furniture, which are discarded once its useful life expires.

The traditional recycling of textiles is based on introducing them back to a productive process that contemplates: (a) the classification is made according to the conditions and type of fiber; at this stage, it is decided whether garments can be reused, crushed, or used for reprocessing, and (b) re-classification, where the granulated material is separated according to its color, to be later crushed, carded, and finally spun.

The idea of using textile waste in applications that do not involve a new industrial process results in a novel research panorama, due to the economic and environmental benefits that can be achieved, as a novel investigation about home-use products obtained from waste textile materials [12]. Table 3 shows the proposals and results of the recycling.

The results showed that eco-design is an effective tool, involving creativity and innovation, with environmental objectives. Recycling textiles are an excellent opportunity to reduce the environmental impact of commonly used products.

Improvement on the reinforcement-matrix interface is one of the most important challenges for using textile fibers as reinforcements in composite materials. Some studies are concerning to the modification of reinforcing fibers, by means of chemical or physical techniques, that look for changes in the chemical structure and in consequence to improve the interface with

1. Agglomerates The fibers of fabrics that are considered as debris are separated by rippers, and the threads

2. Art objects Textile waste considered to be waste is subjected to different procedures such as ripping or

3. Promotional objects Fabric dolls consisting of materials of waste that promote institutions or campaigns

woven by hand-form carpets for homes and offices 5.  Insulation The same procedure is followed to make felt of but in thicker layers and then subjected to

7. Cleaning cloths Used clothing, considered scrap made from cotton mainly, is cut or torn to be used like

characteristics can be used for industrial cleaning

Table 3. Proposals for replacement of virgin materials by waste textiles (adapted from [13]).

cut, to create artistic works such as paintings, sculptures, plastic art, etc.

a cut for its adaptation in walls, in cars like acoustic and thermal insulation

are joined together and under pressure and heat in the form of vapor to create textiles as

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around the type: felt bags made of textile waste that they bring logos, labels promoting

Carpets made of T-shirts, second-hand clothing or scrap, which are cut into strips and

rags or tows for cleaning. Some fabric synthetics by their oleophilic and hydrophobic

Large textile fabric is selected for be subjected to cuts with patterns and form that will be joined with the help of a sewing machine to construct canvases based on pieces.

There are several physical methods applied on fibers, as temperature plasma treatment, corona treatment, and a novel based on the use of gamma radiation. Physical methods of interface modification involve fibrillation on the fibers, causing structural and surface modification, and

In an investigation about the effects of chemical treatment (with alkali) and gamma radiation on cotton fibers, structural modification of cellulose component was evidenced by a decrease in the degree of polymerization and an increase in carbonyl content [16]. In an other investigation, irradiated cellulose fibers (by using gamma rays) showed degradation from 6 to 12% up to an applied dose of 31.6 kGy; moreover, crystallinity decrease for dosages higher than 300 kGy and finally diminution of 1% in the crystallinity at dosages up to 1 MGy are observed. In general, gamma radiation causes degradation of cellulose in shorter chains and leads to "opening of microcracks" that are easily penetrable by water molecules. The complete degradation of cellulose is given up to 6.55 MGy, where amorphous zones along the length of the microfibrils are seen, which allows penetration of chemical substances in the

the matrix.

Design alternatives matter

4. Carpets made with T-

8. Bedspreads or bedding patchwork

shirts

Proposals reuse matter

congresses, or associations

felt.

virgin

microfibrils [17].

in consequence mechanical changes are obtained.

The use of textile waste to obtain ethanol was studied. Two types of textiles were used, one containing viscose, polyester, cotton, and modal and the other containing the cellulose contents that were between 70 and 94%. The textiles were cut from 2 to 5 mm in length and subjected to an alkaline pretreatment based on sodium hydroxide, and subsequently to an enzymatic hydrolysis process. The yields of ethanol obtained at 72 h were between 15 and 36% [13].

In a study carried out by Liang and Hota concerning to fiber-reinforced polymers and their use in engineering, they show a wide range of possibilities for their use within construction industry; due their characteristics, as: lower cost for maintenance (no paint, no decay, no insect infestation), lower heating and cooling costs and high structural strength [14].

Other researches focused on the use of recycled textile fibers in composites, as reinforcing materials [15], i.e., used carpet waste as a soil reinforcement material. The reinforcing fibrous strips were 5 5 mm and lengths of 5–45 mm. They were added to the soil in concentrations from 0.4 to 1.2%. The mechanical properties of shear forces were improved, is to say, composites without fibers have a value of 300 kPa, which was improved 25% when adding 1.2% of fibers (375 MPa). Moreover, the axial strain at yield point was improved, covering from 1% for composites without fibers to 4% for those with 1.2% of fibers.

Table 3. Proposals for replacement of virgin materials by waste textiles (adapted from [13]).

a means to obtain economic and environmental benefits for various reasons, as it contributes to the reduction of spaces required for landfills, reduces the need to produce virgin materials and reduces problems of water pollution. The textile recycling involves the reuse of used clothing,

Some products made from synthetic fibers do not decompose naturally, which causes soil and surface water contamination problems. Although cotton fibers decompose naturally, in doing

Textile recycling strategies are based on the knowledge of their classification: (a) post-production materials, such as yarns, textiles, fibers, which come from the loss of the production process and (b) post-consumption materials, such as clothing, carpets, shoes, and furniture, which are discar-

The traditional recycling of textiles is based on introducing them back to a productive process that contemplates: (a) the classification is made according to the conditions and type of fiber; at this stage, it is decided whether garments can be reused, crushed, or used for reprocessing, and (b) re-classification, where the granulated material is separated according to its color, to be

The idea of using textile waste in applications that do not involve a new industrial process results in a novel research panorama, due to the economic and environmental benefits that can be achieved, as a novel investigation about home-use products obtained from waste textile

The results showed that eco-design is an effective tool, involving creativity and innovation, with environmental objectives. Recycling textiles are an excellent opportunity to reduce the

The use of textile waste to obtain ethanol was studied. Two types of textiles were used, one containing viscose, polyester, cotton, and modal and the other containing the cellulose contents that were between 70 and 94%. The textiles were cut from 2 to 5 mm in length and subjected to an alkaline pretreatment based on sodium hydroxide, and subsequently to an enzymatic hydro-

In a study carried out by Liang and Hota concerning to fiber-reinforced polymers and their use in engineering, they show a wide range of possibilities for their use within construction industry; due their characteristics, as: lower cost for maintenance (no paint, no decay, no insect

Other researches focused on the use of recycled textile fibers in composites, as reinforcing materials [15], i.e., used carpet waste as a soil reinforcement material. The reinforcing fibrous strips were 5 5 mm and lengths of 5–45 mm. They were added to the soil in concentrations from 0.4 to 1.2%. The mechanical properties of shear forces were improved, is to say, composites without fibers have a value of 300 kPa, which was improved 25% when adding 1.2% of fibers (375 MPa). Moreover, the axial strain at yield point was improved, covering from 1% for

lysis process. The yields of ethanol obtained at 72 h were between 15 and 36% [13].

infestation), lower heating and cooling costs and high structural strength [14].

composites without fibers to 4% for those with 1.2% of fibers.

fibrous materials, and production losses of the manufacturing process of clothing.

so they produce methane, which contributes to the global warming.

94 Natural and Artificial Fiber-Reinforced Composites as Renewable Sources

materials [12]. Table 3 shows the proposals and results of the recycling.

ded once its useful life expires.

later crushed, carded, and finally spun.

environmental impact of commonly used products.

Improvement on the reinforcement-matrix interface is one of the most important challenges for using textile fibers as reinforcements in composite materials. Some studies are concerning to the modification of reinforcing fibers, by means of chemical or physical techniques, that look for changes in the chemical structure and in consequence to improve the interface with the matrix.

There are several physical methods applied on fibers, as temperature plasma treatment, corona treatment, and a novel based on the use of gamma radiation. Physical methods of interface modification involve fibrillation on the fibers, causing structural and surface modification, and in consequence mechanical changes are obtained.

In an investigation about the effects of chemical treatment (with alkali) and gamma radiation on cotton fibers, structural modification of cellulose component was evidenced by a decrease in the degree of polymerization and an increase in carbonyl content [16]. In an other investigation, irradiated cellulose fibers (by using gamma rays) showed degradation from 6 to 12% up to an applied dose of 31.6 kGy; moreover, crystallinity decrease for dosages higher than 300 kGy and finally diminution of 1% in the crystallinity at dosages up to 1 MGy are observed. In general, gamma radiation causes degradation of cellulose in shorter chains and leads to "opening of microcracks" that are easily penetrable by water molecules. The complete degradation of cellulose is given up to 6.55 MGy, where amorphous zones along the length of the microfibrils are seen, which allows penetration of chemical substances in the microfibrils [17].

Industrial development and the need to protect the environment are placing escalating demands on the natural resources and improved technologies. Radiation technology has been used to produce high performance polymeric materials with unique physical and chemical properties. Now, this technology can successfully be utilized to upgrade natural polymers as well, yielding value-added products for diverse applications.

and polymer matrix influences the strength and toughness of composite materials. When an interfacial failure happens, fibers are pulled out from the polymer matrix and bridging forces are developed on the crack surface. The bridging forces shield the crack and hence reduce the stress intensity factor at the crack tip. Interfacial shear strength plays a dominant role since the bridging pressure from fiber pull-out is governed mainly by shear stress resistance between fibers and the

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Mechanical behavior of polymer concrete with textile residues was investigated. Textile wastes were recovered from lingeries, which are elaborated with cotton, polyester, silk, and rayon fibers. Two sets of resin, fine aggregate and textile fibers were elaborated, and 1 and 2 wt.% of recycled textile fibers were added. The results show diminution on the flexural strength of polymer concrete when increasing the fiber concentration. However, flexural strength values are higher than those obtained for cement concrete. Polymer concrete without fibers failed and was broken into pieces; in contrast, all fiber-reinforced specimens after reaching the maximum

Polymer concrete with olive oil from vegetable solid wastes was elaborated; for its use in the construction industry or in the manufacture of furniture. Polymer concrete specimens had from 10 to 60 wt.% of residues, and a silanized process was used to improve the interface characteristics. Silanized was made with untreated alpha-mercaptopropyltrimethoxysilane. The results show that olive oil improves the mechanical properties, and even more silanized process allows

In the search to include textile waste materials in construction materials, an investigation was carried out in which we evaluated the feasibility of using residuals and subresidues of fabrics, for thermal insulation in the construction industry [20]. This was done by making an outer double wall with an air box filled with such debris by placing two heat flow meters and four surface temperature sensors on the wall surface to determine the thermal conductivity of the waste. The results show that the application of fabrics in the external double wall increases its

In the construction industry, one of the most demanding items is the construction of floors for roads and highways, which must be able to withstand the constant impact of the traffic of people and transports. In an investigation, we used recycled carpet waste fibers to make composites of lightweight cements in a 20% fiber ratio. The flexural, tenacity, and impact properties were characterized. The results show that in the three-point flexural test, a ductile behavior and an increase in flexural strength were observed. Nevertheless, density decreases with increasing fiber concentration. Energy absorption was also measured by the weight drop impact test, but this was not very significant due to the total absorption of the impact energy of

The importance of the concentration of the load of reinforcing material in a composite is evidenced in an investigation in which it developed a polymer concrete elaborated with polyester resin, sand, and textile glass fibers (1, 2, and 3 wt.%). The properties of fibers were weight

), thickness (0.47 mm), mesh (3.5 3.5 mm), and tensile strength (1200 N/cm2

displacement curves for different reinforcement contents were obtained. The results show that

). Load-

load remained as an integral piece, with hold fibers to polymer matrix [18].

improvements on interface and in consequence higher mechanical values [19].

thermal behavior between 30 and 56%.

the specimens [21].

(160 g/m2

polymer matrix.

Cellulose is the most abundant natural polymer around the world, and its high abundance and the presence of reactive hydroxyl groups in the chain promise an array of potential applications. The modified celluloses like carboxymethyl cellulose (CMC) or hydroxyethyl cellulose (HEC) are more useful in producing specialized polymers due to their solubility in water. CMC is the most promising derivative of cellulose that can be used for the production of hydrogels, since it has an ionizable carboxyl group.

The natural polymers are presently being exploited for industrial use. However, by judicious use of radiation, these polymers can yield high value products with interesting applications. Possibility of blends of polymers with natural polymers opens up a new avenue for novel applications.

#### 4. Waste and recycled textile materials used in building materials

Fibers recovered from various waste streams are suitable for concrete reinforcement. The advantages of using such recycled fibers generally include lower cost to process than virgin fibers and the elimination of the need for waste disposal in landfills.