*3.3. Ultimate Failure Load Prognosis*

In an effort to perform ultimate failure load prognosis, the ultimate failure crane-load for each spar was correlated with AE activity. Extracted features from the recorded AE signals were used to compare the progressive failure of the composite spars, and to infer similarities and differences among them. The results for all spars were tabulated in Table 3 and ranked according to the ultimate load value. The target crane-load for the first damage signal, the estimated load at first AE damage signal, the AE energy behavior (classified as either progressive or significant behavior based on the cumulative AE energy profile), and the critical region defined by AE sensor locations with high-amplitude AE hit concentrations are given for each spar. All load values are defined as a percentage of the design-limit-load (DLL). Significant AE energy behavior was defined as a jump in the cumulative absolute AE energy distribution of at least 103 aJ.

Although variability is observed in all listed parameters, some patterns emerged in the information presented in Table 3. As mentioned previously and supported by the analysis of multiple AE parameters, the majority (10 out of 16) of the first damage signals occurred during the second target load of 100% DLL. It appeared reasonable to observe the damage initiation and progression once the spar limit load was reached. Notably, three of the spars did not show the first damage signal until the final loading sequence reached the target load of 150% DLL. All three of these spars displayed progressive damage behavior based on their cumulative AE energy profiles. Despite the apparently controlled damage progression, this did not result in the three highest ultimate loads. The highest recorded ultimate load resulted from delayed damage initiation and progressive damage behavior, but the next two highest showed earlier damage initiation and significant damage behavior. This uncertainty suggests that there are other factors that may additionally influence the ultimate load prediction.

The estimated crane load applied at the time of the first AE damage signal and the target crane load value were given for comparison, showing that initial damage was often observed prior to reaching the maximum load for that cycle. Moreover, 11 out of the 16 spars tested exhibited significant AE energy behavior, rather than a progressive behavior, making the appearance of the first damage signal more critical than is the case for the progressive energy accumulation. Critical region examination by AE zonal location revealed that 12 out of 16 spars experienced the majority of high-amplitude damage signals near Sensor 2, which was located approximately 6–13 inches from the spar root—as expected given the cantilever load profile. Only a few spars (four out of 16) showed a critical region other than Sensor 2, and one case (Test 2) recorded multiple critical regions, indicating widespread damage, which led to the lowest recorded ultimate load.
