*3.1. 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

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, including those that showed some abrupt and high-amplitude activity starting early in the loading sequence (e.g., Figure 3a,b) and those that demonstrated a more gradual AE activity, which was expected, as the loading increased with time.

**Figure 3.** Sample AE activity observed during four separate spar tests. (**a**) Test 14, (**b**) Test 8, (**c**) Test 10, and (**d**) Test 4.

**Table 2.** Felicity ratio values calculated based on the four applied loading cycles.


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 (>103) 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.

**Figure 4.** AE energy evolutions for the same tests shown in Figure 1. (**a**) Test 14, (**b**) Test 8, (**c**) Test 10, and (**d**) Test 4.

Similar trends were observed in all tested spars, and the behavior was classified as progressive or significant based on the cumulative absolute energy patterns noted in Figure 4. For example, Figure 5 shows four additional spar tests to further demonstrate the identified data trends using the same 10<sup>3</sup> aJ energy increase to denote the behavior as significant.

**Figure 5.** Additional examples of AE activity during four tests with some progressive and some significant behavior observed. (**a**) Test 5, (**b**) Test 3, (**c**) Test 15, and (**d**) Test 13.

Felicity ratio values for all spars at each loading step are given in Table 2. Most test specimens resulted in FR values below 1.0 and above 0.9. Noted exceptions are Test 8 and Test 9, both of which displayed the lowest FR values and further exhibited significant behavior during LL2. Interestingly, the spars that had the lowest FR values still resulted in near-average ultimate load. It should be also noted that some high FR spars failed at lower loads than those with lower FR values, potentially suggesting that these spars may enable damage distribution that leads to higher ultimate loads. In contrast, spars with higher FR values may potentially have less damage (which could not have been confirmed during testing as tests were not interrupted for post mortem inspection), however given that some of these spars failed at lower loads, damage in them is expected to be more localized and near the actual failure zone.

It can be inferred from the parametric AE data trends presented herein that, generally, appreciable damage initiates once the design-limit-load (LL2) is reached, which further progresses until ultimate failure. In conclusion, the AE parametric analysis presented provided additional insight into damage progression and sample-to-sample variation that may otherwise be undiscernible using traditional test methodologies. For example, note the AE energy profile in Figure 4d, Test 4. This particular spar showed progressive damage, with a higher comparable FR value averaging 1.00 over all three loads. It may appear that something about this spar e.g., its design, production, or loading parameters, provided an advantage in controlling damage progression that resulted in achieving a higher ultimate load. By correlating spar-to-spar differences in damage behavior as indicated by AE data with structural testing, production or design data, additional value can be mined from such nondestructive evaluation, which may support design or process improvements.

#### *3.2. Identification of Probable Damage Regions*

As determined during pre-testing, AE wave propagation velocity fluctuated across the length and among the different spar faces due to structural and material variations; therefore, a zonal technique based on AE hit amplitude was used to identify regions along the spars related to onset of damage. The AE sensors were distributed along the length of the spar, on the top surface, in order to achieve sufficient "zonal" coverage from root to tip (recall Table 1). The authors examined high amplitude AE hits recorded by certain sensors for which the locations are known. If a sensor had a relatively high concentration of high amplitude AE hits, the sensor "zone" was considered the "critical region" as a first order approximation to damage location. For the preponderance of the test cases, high

AE amplitude behavior was observed in sensors near the spar root. As noted in Table 3, the most commonly identified was Sensor 2 (S2), which was located approximately 6–13 inches from the closest root pin. Failure near the root was anticipated, due to the loading and design of the spar. Prior testing experience indicated that common failures may be in the form of cracking or buckling of the shear web, and/or bending of the cap near the spar root. These types of damage matched the failure modes observed in similar cantilevered wind turbine blade tests [39,41].


**Table 3.** Results summary.
