# Part Qualification Methodology for Composite Aircraft Components Using Acoustic Emission Monitoring
## 3 Results and Discussion  
### Progressive Damage Characterization  
Acoustic emission activity trends were used to infer progressive damage during pseudo-cyclic loading of the composite spar test specimens. Representative AE data is provided in Figure 3 for four different spar tests; the grey line provides the applied loading, while the red diamonds represent the recorded AE events correlated with the corresponding load values at which they occur. In addition, the blue markers indicate the AE activity amplitude distribution. To complement the graphical AE data trends, FR values were calculated and are reported in Table 2. All reported cases showed some AE activity prior to achieving the previous peak load, in accordance with the Felicity effect. However, Figure 3a,b shows AE events at comparatively lower loads, marked by black ellipses, compared to the test cases in Figure 3c,d. Additionally, in Figure 3b-d, it is noticeable that AE events occurred during  
Figure 2: Sample AE activity observed during Test 8 at four different load points. (**a**) Zero load, (**b**) during the first load step, (**c**) during the second load step, and (**d**) at final failure. the unloading portion of the second loading sequence. AE events during unloading can indicate the presence of damage [13; 33; 47; 54; 56], and are highlighted by green ellipses in Figure 3. Furthermore, the amplitude distributions in Figure 3a,b show relatively large spikes during the second cycle with values greater than 95 dB, while all other activity is below 70 dB for both Test 14 (Figure 3a) and Test 8 (Figure 3b). Sudden spikes of high-amplitude AE activity can correspond to material damage initiation or progression. In contrast, Tests 10 and 4 (Figure 3c,d) show a gradual increase in amplitude with each loading sequence. Based on the information presented in Figure 3 and Table 2, it can be concluded that the presented AE data visualization as a function of applied loading, in combination with the FR analysis, can provide some preliminary assessment of test item damage progression. Specifically, damage appears to initiate after the second target load (LL2) for most tests, as indicated by both the FR values, as well as the appearance of significant AE activity in the unloading portion of the applied cyclic loading. Furthermore, two distinct AE dataset classes existed among the 16 tests,  
Supplemental to the AE activity trends shown in Figure 3, cumulative absolute AE energy is plotted against crane load in Figure 4. In all loading cases observed in Figure 4, the cumulative AE energy increases during loading, while remaining relatively constant during load holds and unloading. Similar to the AE amplitude spikes shown in Figure 3a,b during the second loading sequence, Figure 4a,b show corresponding jumps in AE energy. Such AE energy accumulations are observed in subsequent loading cycles, but they are orders of magnitude lower. Note that the observed pattern in Figure 4a,b is strikingly different from the results shown in Figure 4c,d. Accordingly, the recorded datasets were classified as significant vs. progressive in terms of the corresponding spar behavior. Specifically, Figure 4a,b is characterized by a sudden, dominant, discontinuous jump in AE energy, while Figure 4c,d is characterized by a gradual AE energy accumulation that appeared to be proportional to the load increase. Hence, the gradual cumulative AE energy profile observed in Figure 4c,d was classified as \"progressive\" (damage) behavior, in contrast to the large spikes (\(>\)10\({}^{3}\)) displayed in Figure 4a,b, which were classified as \"significant\" behavior. Note that composite materials are known to exhibit progressive damage behavior and, therefore, test items that depart from this expected behavior (e.g., high intensity AE activity early in the loading cycle) may indicate design, production, or loading conditions that could lead to an early onset of catastrophic failure (i.e., lower ultimate load) when compared with similar parts.