45 mechanical properties. To test this all of the holes in some specimens were filled with PTFE plugs, preventing ingress of resin into the holes. Further details of the design and manufacturing can be found in [21]. Static tensile tests were performed with a fixed displacement rate of 0.2 mm/min. The load and displacement readings were obtained from the machine load cell and clip gauge extensometers, respectively. A high speed camera (16,000 frames per second) was used to image the initiation and propagation of the joint failure. 5.4 Results and Discussion of Bio-Inspired CFRP to Perforated Steel Joints 5.4.1 Load-Displacement Response Figure 5.5 shows the load versus displacement results from experimental tests on a SH joint without PTFE inserts and on a reference joint configuration. The perforated joint has only a small drop in stiffness before it fails catastrophically. By contrast, the non-perforated reference joint failed prematurely at the butt end of the joint steel part, with a crack propagating away from there to give failure of one of the two laps. This gives the sharp change in the load–displacement curve (Fig. 5.5). After this failure, the remaining intact lap was able to carry a further increased load until final failure occurred. Included in Fig. 5.5 is a simulation for the standard reference configuration, and an additional calculation where one of the laps of the reference joint was assumed to fail at the load observed in the experimental test. 5.4.2 Maximum Load Figure 5.6 summarises the average maximum loads obtained from the experiments. Compared to non-perforated joints, the perforated SH with PTFE configuration showed a 175 % increase of joint strength. The difference in the average maximum load for perforated joints with and without PTFE in the SH specimens was negligible. Thus, the hypothesis that resin ingress into the holes gives an increase in joint strength associated with mechanical interlocking is not observed for this perforation case. Instead it is presumed that the dominant factor which contributed to the significantly higher strength of the perforated joints, comparing to the non-perforated ones, is the transitional zone of stiffness. 5.4.3 Failure Observation Using High Speed Camera For all the perforated joints, high-speed camera images showed that the failure initiated at the CFRP end of the joint with a crack propagating rapidly towards the butt end. The crack propagated between the steel and composite interface. Figure 5.7 shows selected frames capturing the failure initiation and the catastrophic failure of a SH joint. On the other hand, the Fig. 5.4 ( a ) Joint geometry (dimensions in mm) and ( b ) loading and boundary conditions 5 Review of Natural Joints and Bio-Inspired CFRP to Steel joints
RkJQdWJsaXNoZXIy MTMzNzEzMQ==