Novel concepts for automated fibre placement ply-drops with higher strength

Abstract

Automated fibre placement (AFP) is key for large-scale manufacturing of composite structures, but has some drawbacks, such as the saw-tooth geometry of ply-drops. We propose and develop original concepts for AFP ply-drop configurations that demonstrate an enhanced mechanical performance compared to traditional ply-drops in tapered carbon fibre-reinforced polymer (CFRP) composites. To achieve this, we developed and manufactured two designs of tapered CFRP composites using an AFP machine. The first design (baseline) involves conventionally laying down CFRP slit-tapes, with the tape ends terminated perpendicularly (at 90°). The second design (proposed in this work) incorporates terminating the tape ends at specific angles, a unique feature integrated into our AFP machine. Following manufacturing of the tapered CFRP specimens, to investigate the performance of the developed concepts, we designed a bespoke test rig to allow off-centre point loads on cantilever-like specimens; this test configuration can represent an idealised foreign object damage on fan engine blades (caused by runway debris or bird strike events). Following mechanical characterisation, we conducted post-mortem analyses to examine the differences in failure mechanisms between the two designs. The test results show that the new concept can increase the maximum load and maximum displacement by at least 10 % compared to the conventional ply-drop design. Additionally, post-mortem analysis shows that failure changed from undesirable compressive failure at the root in conventionally designed specimens to desirable tensile failure at the ply-drop region in the novel developed specimens.