Implementing structural fuses in CFRP components via microstructurally-engineered crack paths

This study aims to develop and implement actual carbon fibre-reinforced polymer (CFRP) solutions for realising structural fuses in real components. To this end, we have developed various concepts for structural fuses, applied to generic idealised components and aimed at engaging different in-plane and through-the-thickness damage propagation mechanisms. Micro-cut patterns (MCPs) / crack path combinations have been engraved on thin-ply CFRP prepregs (by using a laser cut machine) for manufacturing CFRP specimens. Afterwards, we have carried out a series of experimental studies to evaluate the fracture properties of various MCPs under three-point bending (3PB). Then, 3PB results were used to refine and down-select our concepts, for use in our generic idealised component design to test them under indentation test using a cantilever beam rig. The test results demonstrated that MCPs can provide significant control over the fracture locus and path, additionally allowing the failure initiation load and energy dissipation to be tailored

A novel profiling concept leading to a significant increase in the mechanical performance of metal to composite joints

In this work, we designed metal-CFRP joints with a profiled adherend termination to improve the mechanical performance. We have applied several profiles to the edge of titanium adherends which were adhesively bonded to CFRP substrates. We conducted finite element modelling and experimental 4PB (4-Point-Bend) testing to investigate how the geometry of the adherend edge profile effects the mechanical performance of the joint. This work shows that profiling of the metal adherend can result in increases of at least 27% in the peak load, and of at least 272% in the energy dissipated up to critical failure normalised by the mechanical energy

Bio-inspired interleaved hybrids: Novel solutions for improving the high-velocity impact response of carbon fibre-reinforced polymers (CFRP)

We propose a novel design methodology consisting of bio-inspired (BI) and interleaved layups to develop hybrid carbon fibre-reinforced polymer (CFRP) composite structures for improved high-velocity impact (HVI) performance. Firstly, we apply a BI helicoidal design method consisting of various pitch angles (considering both thick- and thin-ply CFRP) to develop BI monolithic CFRP laminates. Secondly, we apply the interleaving design method to develop BI hybrid CFRP-based laminates interleaved with blocks of BI Zylon fibre-reinforced polymers through the thickness. We evaluate their response and compare it with traditional quasi-isotropic (QI) hybrid bulk layups. In addition to hybridising with Zylon, we apply titanium (Ti) foils to both the monolithic and hybrid CFRP-based laminates to investigate and compare their response. For all our hybrids, we kept the ratio of the hybridising material(s) to be less than 50% to ensure suitable in-plane mechanical properties and aimed at a target areal weight of 0.95 g/cm2. We also manufactured QI thick- and thin-ply monolithic CFRP laminates as baselines. We tested all laminates at 170 and 210 m/s and studied their response and failure modes. Our results show that the average energy dissipation of the QI monolithic thin-ply baseline improved by up to 22% by changing the layup from QI to BI, and by about 118% by changing the baseline QI layup to BI hybrid interleaved. Post-mortem analysis reveals that there are additional failure mechanisms activated in the BI hybrid interleaved layup with respect to the baseline

Novel zone-based hybrid laminate structures for high-velocity impact (HVI) in carbon fibre-reinforced polymer (CFRP) composites

We propose novel zone-based hybrid laminate concepts for improving the high-velocity impact (HVI) response of baseline carbon fibre-reinforced polymer (CFRP) composites while maintaining similar areal weights and retaining substantial in-plane mechanical properties by requiring that about 80% (by mass) of the baseline CFRP is kept in the hybrid concepts. We identify three zones along the thickness of the laminate according to their role during HVI and implemented tailored materials in these zones to improve the HVI response. We studied a wide range of materials, including: fibre reinforcements of carbon (thin- and thick-plies), glass, Zylon and ultra-high molecular weight polyethylene (UHMWPE); a shape memory alloy/carbon fabric; and ceramic, alumina and titanium sheets. All laminate concepts have similar areal weights for a meaningful comparison. After defining the various concepts, we manufactured and measured their specific dissipated energy under HVI, and finally carried out post-mortem analysis (including C-scan and microscopy). The results show up to 95% improvement in energy dissipation over the baseline quasi-isotropic (QI) CFRP configuration

A novel profiling concept leading to a significant increase in the mechanical performance of metal to composite joints

Traditional adhesive joints with straight edged adherends suffer from a significant stress concentration in the composite coincident with the edge of the metal adherend, which can lead to accelerated translaminar failure of the substrate. In this work, we developed a novel profiling concept which improves the mechanical performance of adhesive joints between metallic adherends and composite substrates. We conducted quasi-static four-point bending (4PB) tests which showed that profiling the edge of the metallic adherend could improve the peak load by at least 27%, and that the stability of failure was simultaneously improved. We investigated varying the profile parameters and were able to conclude that further significant mechanical performance gains could be achieved by increasing any of the profile: amplitude, frequency, or number of fractal length-scales. By analysing in-situ acoustic emission (AE) monitoring data we were able to observe that profiling of the metallic adherend results in failure initiation occurring at higher loads, which suggests that the concept is successful in providing better stress distributions and lowering peak stresses. By analysing the fracture surfaces, it is apparent that the profiling concept is successful in deflecting the translaminar fracture path; and additionally that a debonding mechanism occurs at the profile tips which is thought to be an important additional mechanism for creating damage tolerant joints