# Validation of Acoustic Emission Waveform Entropy as a Damage Identification Feature
## 4 Experimental Results
### Influence of HDT on AE Entropy  
The elastic wave in the metal plate (as a result of rectangular pulse excitation of an AE transducer) was captured with another AE sensor. The signal from the sensor was acquired by the AE data acquisition system three times with three different HDT settings. Figure 13 shows an acquired waveform for a HDT of 200 \(\upmu\)s. In fixed window length settings, the distribution and the total number of the discrete voltage values in a waveform are not dependent on HDT. Hence, the three waveforms recorded with the three HDT's are identical. It can be observed in Figure 13 that the entire waveform consists of three sub-waveforms. A waveform with multiple sub-waveforms was chosen for this analysis as it can appear when a material emits many elastic waves in a short interval. For instance, the cyclic plastic zone extension in austenitic stainless steel can generate a waveform containing two sub-waveforms [46]. It is also evident from Figure 13 that the peak of the first and second sub-waveform is 0.5 and 0.75 times the third waveform. This corresponds well with the amplitude of the excitation pattern used for this test. Figure 12: Influence of acquisition threshold on waveform generated from the pencil lead break (PLB) test: (**a**) rise-time, (**b**) duration, (**c**) count, (**d**) energy, (**e**) peak amplitude and (**f**) AE entropy. The AE entropy and the traditional AE features for the waveform acquired with three HDT's are shown in Figure 14. The Figure 14a-f represents the influence of HDT on rise-time, duration, count, energy, peak amplitude and AE entropy, respectively. Although the three waveforms are identical, as the HDT increases, the features extracted by the AE data acquisition also increases. The first HDT setting has the lowest values of all the recorded feature. In this setting, AE features were only calculated from the first sub-waveform from the entire acquired waveform in Figure 13. This is because the duration from when the first sub-waveform dropped below 50 dB to the onset of the second sub-waveform was greater than the first HDT (200 \(\upmu\)s). In AE, this phenomenon is termed as the expiration of HDT. Currently in AE monitoring, features are only extracted before the expiry of HDT, and the part of the waveform outside the HDT is discarded [39]. In the second HDT setting, AE features were calculated from the first and second sub-waveform. As a result, it had a higher value of duration, count and energy. It is also evident from Figure 14 that the rise-time and peak amplitude are also higher in the second HDT settings. This is because the peak voltage and its time of occurrence  
Figure 14: Influence of HDT on a realistic AE waveform. (**a**) Rise-time, (**b**) duration, (**c**) count, (**d**) energy, (**e**) peak amplitude, and (**f**) AE entropy. Figure 13: Waveform recorded by the first channel with a HDT of 200 \(\upmu\)s.  
was calculated from within the first and second sub-waveform (the second sub-waveform had a higher peak voltage compared to the first one). The second HDT (300 us) was greater than the duration from when the first sub-waveform dropped below 50 dB to the onset of the second sub-waveform. However, it was smaller than the duration from when the second sub-waveform dropped below 50 dB to the onset of the third sub-waveform. Therefore, the second HDT expired before the onset of the third sub-waveform. As a result, the feature was extracted over a larger duration compared to the first HDT setting. The third HDT (400 us) was greater than the duration between each of the pairs of sub-waveforms in Figure 13 (first and second; second and third). Therefore, it did not expire between the sub-waveforms, and the AE features were calculated from the entire duration of the waveform. The AE feature for this HDT was the largest (including the peak amplitude and rise-time, since they were extracted from the third sub-waveform, which had the largest peak voltage).