# Validation of Acoustic Emission Waveform Entropy as a Damage Identification Feature
## 3 Materials and Methods
### Experimental Evaluation Procedure
#### 3.2.3 Experimental Validation Procedure  
In order to validate the proposed idea, AE entropy was calculated and explored from the waveforms recorded during tensile and fatigue loading of a standard 316L stainless steel sample. Table 4 shows the mechanical properties of the material used in this experiment. Flat tensile and fatigue specimens were designed according to ASTM standard E8/E8M-09 and E466-15, respectively [40,41]. Figure 8 shows the dimension of the specimens that were used in the experiment. \begin{table}
\begin{tabular}{c c} \hline \hline
**Properties** & **Values** \\ \hline Young’s Modulus & 193 GPa \\ Yield Strength & 347 MPa \\ (Longitudinal) **R**p 1\% & 347 MPa \\ Tensile Strength (Longitudinal) & 613 MPa \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mechanical Properties. Figure 8: Dimension of the specimen: (**a**) tensile and (**b**) fatigue. Figure 7: Transducer arrangement for evaluation of AE entropy against the hit definition time (HDT). The tests were carried out on these specimens in an Instron servo hydraulic machine at room temperature (20 \({}^{\circ}\)C). The tensile sample was subjected to a displacement-controlled (1 mm/s) loading. The fatigue sample was subjected to sinusoidal loading of 5 Hz and maximum and minimum stress of 480 MPa and 48 MPa. For both the tests, the loading axis was parallel to the longitudinal direction of the material. Elastic waves generated in the material were captured as analogue signals with two AE sensors (VS-160, manufactured by VallenTM [42]). This sensor was chosen because it has a higher peak sensitivity than the VS-900 at 160 kHz, which is close to the frequency of waveforms generated during yielding and crack initiation in austenitic stainless steel [14; 22]. An illustration of this sensor and its frequency sensitivity spectrum is shown in Figure 9. The signals were amplified (using AEP4H amplifiers [43]). The amplified signals were fed to the AE data acquisition system (Vallen-AMSY-6). The analogue signals were sampled with a sampling frequency of 5 Mhz (larger than the 'Nyquist frequency' of waveform generated during damages in austenitic stainless steel [22]) in the AE data acquisition system. The window length of the recorded waveform was chosen to be 820 us, which is comparable to the rise-time of typical sloshing impact on the primary wall [4]. The threshold chosen for these tests was 45 dB (i.e., above the background noise of the laboratory). The rest of the settings used in the AE data acquisition system were similar to the ones mentioned in Table 2. Due to the nature of fatigue loading, unwanted noise originating from the grips can mask the useful information in the dataset [44]. To avoid this, a 1D localization filtration technique was used, which considered signals generated within the gauge section of the specimen to be useful and avoided unwanted signals from the grips. All the recorded waveform features were calculated by the AE data acquisition system, and AE entropy was calculated by a MATLAB(tm) coded algorithm (Appendix B) from the discrete values of the waveform. To correlate the material behaviour with AE activity, the specimens were monitored simultaneously with a ARAMIS-5m digital image correlation (DIC) camera [45]. The material behaviour correlated for the tensile specimen was engineering stress and strain. The use of an extensometer is a traditional method to extract the strain in the material during tensile deformation. In this study, the deformation was expected to be quite large (since the material is very ductile), and there was a possibility of the extensometer scratching against the surface of the specimen during deformation. Scratching of the extensometer on the specimen surface was expected to generate unwanted elastic waves that can potentially corrupt the dataset. The DIC provided a noncontact strain measurement method and as a result the possibility of this noise in the specimen was avoided.