# 3R Composites: Knockdown Effect Assessment and Repair Efficiency via Mechanical and NDE Testing
## 2 Materials and Methods
### Equipment for the Mechanical Characterization
#### 2.3.4 Lap Strap Testing  
Lap strap is a geometry that can simulate the stiffening of a composite panel and can be used in order to reduce materials consumption and simplify the geometry by replacing stringer run-out configuration [38]. The method employed a simplified joint configuration usually referred to as lap strap geometry. This geometry comprises two parts, the lap and the strap, bonded together with either a 3R or conventional resin adhesive layer. During the test procedure, the lap initially debonds from the strap and then the delamination propagates further in the interply plane in the strap. It should be mentioned that failure in the lap strap geometry is manifested in mixed mode. However, at the early stages of the test, the adhesive layer between the lap and the strap fails in mode II, followed by mode I failure at higher stress levels [38]. The dimensions and the geometry are depicted in Figure 1. Mechanical testing was performed with an Instron 8001 Universal Testing Machine with a 100 kN loadcell. The selected displacement rate was 1 mm/min under tensile load, and the test was stopped when the first small drop of the load occurred, whereby the initiation of the debonding of the lap from the strap happened. The stress was calculated as the ratio of the load to the cross-sectional area (thickness \(\times\) width) and the strain as the ratio of the extension to the grip-to-grip distance. The mechanical knockdown effect and the repair efficiency (of the 3R specimens) were evaluated in terms of maximum stress (MPa). For the structural health monitoring in lap strap specimens, ERCM was utilized simultaneously with mechanical testing. Figure 2 shows the lap strap geometry and the spots where the ERCM electrodes were attached. Exploiting the electrical properties of the carbon fibers and the highly electrically insulating epoxy matrix, electrical NDE methods can be utilized for the (i) stain sensing and (ii) damage monitoring on CFRPs. At the early stages of the lap strap test, the specimen is under tensile loading. Although the carbon fibers have piezoresistive properties as a  
Figure 1: Schematic illustration and dimensions of the lap strap geometry. Figure 2: (**A**) Setup of the lap strap specimens for ERCM, (**B**) the current path marked with red arrows before damage, and (**C**) the current path after debonding of the lap from the strap. typical strain gage, i.e., the electrical resistance increases when the fibers are subjected to tensile strain, the total behavior at composite level is highly dependent on the electrical anisotropy of the composite. In a typical unidirectional composite, the resistance is expected to increase with tensile strain [39]. However, there is always the possibility that tensile strain increases the possibility of interfiber contact which potentially could reduce the resistance [40; 41]. In the presence of out-of-axis laminae such as \(\pm\)45 plies, this effect is more prominent as the fibers tend to align themselves with the loading axis during testing and, thus, the probability of the establishment of random contacts between the fibers is definitely increased [40]. As a result, these phenomena cause a decrease in the electrical resistance in the thickness direction which may surpass the strain-induced increase in the reinforcement material. On the other hand, when microcracks and delaminations are induced in the matrix of the composite, the network of fiber contact is disrupted and the length of the conductive path increases, leading to an increase in the electrical resistance. This increase is described in Figure 2B,C, whereby the conductive path between the two electrodes is marked with red arrows. Before debonding, the conductivity occurs through the shortest path, which is a straight line between the electrodes (Figure 2B). After debonding of the lap from the strap, the shortest possible conductive path is described by the curved arrow in Figure 2C. This phenomenon triggers an increase in resistance due to the appearance of damage [39; 40].