# 3R Composites: Knockdown Effect Assessment and Repair Efficiency via Mechanical and NDE Testing
## 3 Results and Discussion
### Lap Strap
#### 3.4.2 Non-Destructive Evaluation via Electrical Resistance Change Method and Acoustic Emission  
The AE activity of the 3R specimen (Figure 25 (bottom)) presented a higher population of low-amplitude hits (40-60 dB) and fewer hits at a high amplitude, which occurred at the initial failure of the lap strap joint. The low-amplitude hits were related to matrix cracking from the beginning of the lap strap test due to tensile loading. Before the debonding of the lap from the strap, high-amplitude hits (60-80 dB) were accumulated, denoting shear failure. On the contrary, the RTM6 specimens only exhibited their total AE activity before the debonding (Figure 25 (top)). Most of the recorded hits were in the range of 40-60 dB and a few in the range of 60-80 dB, indicating that the specimen failure mainly occurred by matrix cracking. Figure 24: Representative stress–strain plots (**left**) and knockdown effect values (**right**) of lap strap specimens. Figure 25: Representative plots of amplitude (**left**) and AE hits (**right**) on RTM6 (**top**) and 3R (**bottom**) lap strap specimens. Electrical Resistance Change Method  
Figure 26 (left) presents the electrical behavior during the mechanical test of the RTM6 specimen. The initial resistance of the specimen before the damage was ca. 167 kohm. This value showed that the lap was electrically isolated by the strap. As aforementioned, electrical isolation was achieved with the good impregnation of the carbon fibers with the RTM6 resin, which prevents inter-fiber contact. A slight decrease in the resistance was observed during the elastic deformation of the specimen. This is typically an indication of strain-dependent behavior on the elastic deformation, which precedes damage phenomena. A sharp drop of the resistance designated the delamination onset. As should be noted, this delamination onset was followed by the very brittle local failure of the matrix and extended bridging between the lap and the strap by bare carbon fibers, as the surrounding matrix had disappeared (Figure 27). This led to the creation of an electrical shortcut between the lap and the strap and the abrupt reduction in electrical resistance to levels comparable to that of the resistance of carbon fibers. The 3R specimen had an initial resistance of \(\sim\)2 ohm. A conclusion that can be drawn from this small value of resistance is that the impregnation of the carbon fibers with the 3R resin allowed inter-fiber contacts and thereby reducing the total resistance. This may be either attributed to unimpeded resin flow, or, conversely, to poor impregnation. As dry spots were observed in the case of the 3R composites, the second explanation is highly possible. The area where the elastic deformation took place can be clearly observed in Figure 28, with an almost proportional dependence of the resistance to the elongation of the  
Figure 27: Images from fractured surfaces of RTM6 lap strap specimen. Figure 26: Representative lap strap test of RTM6 composite and ERCM monitoring (**left**) and zoom in the resistance before failure (**right**). specimen. As expected, the delamination onset, i.e., the failure of the lap strap geometry, was detected by a discontinuity in the resistance vs. time curve. In contrast, after failure, the resistance rise was masked by the deformation sensing, and the resistance continued to have a downtrend. The analysis of the NDE results from AE and ERCM techniques led to the following conclusions. RTM6 presented a highly brittle behavior, as indicated both by the AE results as well as the electrical resistance change findings, whereby the load was accumulated in the adhesive until the point where an abrupt load release was manifested as a sudden rupture. In more detail, the AE activity of the lap strap specimens manufactured with RTM6 began as the specimen departed from the elastic deformation region and culminated to the point of its sudden failure when a part of this load was released, most probably due to the unstable crack propagation.