Energy Emissions from Critical Phenomena and Applications to Structural Health Monitoring
The dissertation is focused on the research of critical phenomena in different size-scaled structures, from earth-scale, macro-scale to micro-scale. By analyzing the energy emission from their critical processes, some useful information or suggestion is extracted and to be provided in structural health monitoring (SHM). Three different forms of energies, acoustic emission (AE), electromagnetic emission (EM) and neutron emission (NE), are selected to be studied in our research. AE is the result of a sudden spontaneous strain energy release within a solid body due to the formation of cracks, which has been studied by many researchers in the past decades and is more developed compared with EM and NE studies. Further and advanced researches are fulfilled based on the qualitative analysis and quantitative analysis in AE study. Two contributions are especially mentioned here. On one hand, an improve approach, based on Akaike Information Criterion (AIC), is proposed to provide more reliable onset determination of AE signals. The introduced parameters, quantification of certainty degree and the apparent velocity in the improved-AIC method can help to eliminate the false or doubtful picked onset results automatically. On the other hand, to determine the active crack region plays a key role in AE monitoring analysis because it identifies the position of fractures in the damaged materials. The classical crack localization method works well in the steel and concrete materials due to their homogeneous or semi-homogeneous properties, but not in the masonry structures. A modified localization system based on the classical one is initiated in this context to offer a better crack localization results in masonry structures. AE monitoring technique is similar to the one employed in earthquake control. Damage in structural materials and earthquakes in geophysics, although they take place on very different scales, are very similar in many aspects. In this context, three particular aspects, b-value of the magnitude-frequency distribution, Yule statistics analysis and unified scaling law for recurrence-time are analyzed on both AE and earthquake. Among the researches, the robust method to estimate the b-value of magnitude-frequency distribution in earthquakes, the application of Yule statistics in earthquake prediction are both the first time to be mentioned in their research areas. EM can be observed in both natural and laboratorial conditions. In this context, models for occurrence mechanism of EM is summarized and analyzed theoretically. Experimental tests are operated to study the EM in laboratory and it is very interesting to note that the EM is generally observed only in correspondence to the sharp stress drops in the load versus time diagrams. As for NE, it is the first time for the research group of Prof. Carpinteri to talk about piezonuclear neutron emissions from very brittle failure of rock specimens in compression test. Comprehensive and detailed information about NE are provided to depict the important discovery in this context