# Validation of Acoustic Emission Waveform Entropy as a Damage Identification Feature
## 5 Validation and Discussion  
### Tensile Test  
The material's response during the test (such as stress and strain) was extracted from the sequence of DIC images taken for the entire length of the test (approximately 40 minutes). Figure 15 shows the reference DIC image and the strain evaluation points in the specimen. The correlation of traditional features and AE entropy of the AE events, captured with respect to the material response, from the tensile specimen is shown in Figure 16. Figure 16a-f represents the variance of rise-time, duration, count, energy, peak amplitude and AE entropy, respectively, for the entire range of deformation. A number of researchers have concluded that during tensile deformation of austenitic stainless steel, there is remarkable AE activity in the initial stages of deformation [15; 22]. This remarkable source of AE activity is attributed to the dislocation movement. It can be observed in all the graphs of Figure 16 that from the onset of loading up to about 0.044 strain, there is plenty of AE activity. Strain of 0.044 corresponds to an apparent plasticity in the specimen (as yielding is considered at 0.02 strain). Therefore, like the previously reported research, the initial increase in AE activity observed in all the graphs of Figure 16, can be attributed to the dislocation movement as a result of yielding. It can also be observed in all the graphs of Figure 16 that after the initial increase, there was no AE activity recorded until the fracture. It is mentioned in references [20; 22] that as the austenitic stainless steel continues to deform after yielding, the glide distance of the dislocation movement decreases. Owing to this reason, the dislocation movement after significant yielding does not produce any detectable AE activity. A similar study in austenitic stainless steel concluded that failure in un-notched specimens of austenitic stainless steel occurs by extensive necking, which generates very little elastic wave [15; 16; 20]. It is evident from all the graphs in Figure 16 that at fracture, the AE activity was significantly lower as compared to the yielding. The occurrence of lower AE activity at fracture can therefore be attributed to the failure by extensive necking. It can be noticed in Figure 16c-f that count, energy and peak amplitude exhibits the same trend as AE entropy. The fracture of metals is known to produce AE events with enhanced traditional AE features (e.g., count, energy and peak Amplitude) [44]. It is evident from these results that the fracture of the specimen had a dominant AE entropy like the traditional features. However, during yielding, there were no apparent distinguishable events. During yielding, AE entropy was distributed approximately between 0 and 8, whereas the fracture was accompanied by a value beyond 10. Count and peak amplitude has been widely used to study the failure mechanism in austenitic stainless steel during tensile deformation [14; 15; 17; 20; 22]. It is clear from these results that AE entropy exhibits the same trend as AE entropy. The fracture of metals is known to produce AE events with enhanced traditional AE features (e.g., count, energy and peak Amplitude) [44]. It is evident from these results that the fracture of the specimen had a dominant AE entropy like the traditional features. However, during yielding, there were no apparent distinguishable events. During yielding, AE entropy was distributed approximately between 0 and 8, whereas the fracture was accompanied by a value beyond 10. Count and peak amplitude has been widely used to study the failure mechanism in austenitic stainless steel during tensile deformation [14; 15; 17; 20; 22]. It is clear from these results that AE entropy exhibits the same trend as AE entropy. Figure 16: Validation of AE features with material response for the tensile specimen: (**a**) rise-time, (**b**) duration, (**c**) count, (**d**) energy, (**e**) peak amplitude and (**f**) AE entropy. Figure 15: Reference digital image correlation (DIC) image of the tensile specimen.