# 3R Composites: Knockdown Effect Assessment and Repair Efficiency via Mechanical and NDE Testing
## 3 Results and Discussion
### Lap Strap
#### 3.4.4 Non-Destructive Evaluation  
The repaired 3R specimens presented a rich AE activity from the beginning of the lap strap test (Figure 30). The amplitude of the hits ranged from 40 dB to 100 dB, denoting that multiple fracture mechanisms took place simultaneously, i.e., matrix cracks, fiber pull-outs, and fiber breakage. This behavior, which is distinctly different from the behavior of the initial specimen, can be attributed to the presence of the loading history due to the first mechanical test and the conditions of the repair process. At this point, it should be mentioned that the repair took place by applying pressure and heat only at the bonding area, as illustrated in Figure 31. The remaining area of the strap was not repaired; thus, the initial cracks were not eliminated. For this reason, fiber breakages occurred at low loadings at stress concentrations due to the formed cracks at the early stages of the test, as shown in Figure 30 by the accumulation of several hits with 80-100 dB amplitude. This rich acoustic activity at high dB levels, that appeared at the early stages of testing for the repaired specimens, but was not present in the case of the initial specimen, was indicative of pre-existing damage which led to early acoustic emission phenomena due to secondary damage already present, an analogue to the fidelity effect [17]. It may be postulated that this activity was due to accumulated damage at the non-repaired area of the specimen, where cracks were formed during the first loading cycle, and led to the subsequent fiber failure during the second loading cycle. This assumption is further supported via the comparison of the acoustic profile of the repaired specimen at the early stages of testing with the respective acoustic profile of the initial 3R specimen (Figure 25 (bottom left)) at the final failure stage. As can be seen, the two profiles are quite similar. The rich activity was continuously observed throughout the test of the repaired specimen and masked the final failure, where normally high amplitude AE signals are exhibited. Furthermore, by comparing the plot in Figure 30 (right) with the plot in Figure 25 (bottom right), it can be seen that both their mechanical and AE behavior presented distinct differences. The failure of the initial specimen occurred just above 12 kN, while the repaired specimen failed at almost 16 kN. However, prior to the final delamination between the lap and the strap, the repaired specimen also presented several small drops in the recorded load at lower values, i.e., at \(\sim\)10 kN, \(\sim\)12 kN, \(\sim\)14 kN, and just above 14 kN before the final  
Figure 29: Representative stress–strain plots (**left**) and repair efficiency values (**right**) of lap strap specimens. failure. Similarly, the acoustic profile of the initial specimen presented cumulative AE hits that increased until the debonding of the lap from the strap, where a small decrease in its AE activity was recorded. On the other hand, the acoustic profile of the repaired 3R specimen presented many more total hits until its final failure, as well as three distinct intervals, before the final failure, where its AE cumulative hits curve reached a plateau. These plateaus were observed at the time where the mechanical plot of the specimen showed the small drops of the load before the final load drop, denoting the failure of the specimen. It is postulated that at the first loading process (initial specimen), critical cracks appeared in the material only during the final failure just above 12 kN, while at the early stages of loading, the combined information from the mechanical and the AE profiles did not show any evidence of critical cracks. However, small noncritical cracks were formed during the early stages of loading, as indicated by the amplitude of the hits (Figure 25 (bottom left)).