3D printed continuous fibre-reinforced composites: bio-inspired microstructures for improving the translaminar fracture toughness

Translaminar fracture toughness is a vital property governing the notch sensitivity and damage tolerance of composites. Nature has shown that incorporating material transitions can increase toughness significantly. This work presents finite element models demonstrating that such transitions can indeed increase the translaminar fracture toughness. The designed microstructures were then 3D printed using continuous glass and carbon fibres. The specimens consisted primarily of glass fibres, but with local carbon fibre strips. A new compact tension specimen with a side groove was designed to ensure proper failure. When the strips were sufficiently large, toughness improvements of 20–60% were found after the crack had grown through the strips. These results reveal a powerful strategy for locally increasing the toughness in areas where it is needed the most

The influence of the hybridisation configuration on the mechancial properties of hybrid self reinforced polyamide 12/carbon fibre composites

This paper compares and contrasts the properties of self-reinforced polyamide 12/carbon fibre hybrid composites made by three different hybridisation routes, termed intra-yarn, intra-layer and inter-layer. The starting point for each route was to manufacture layers of woven cloth (containing both components), from which the hybrid composites were manufactured using the Leeds hot compaction technique. In all cases, a carbon fibre volume fraction of around 8% was the target. On balance, the intra-layer hybrids had the best combination of properties, although all three hybridisation routes yielded interesting results. This intra-layer hybrid configuration showed a significant increase in tensile modulus and strength, bending modulus and strength and penetration impact energy compared to a pure self-reinforced polyamide sheet. The only negative aspect was a reduction in the tensile failure strain from 11 to 2%, whereas the ductility in bending was unaffected by the incorporation of the carbon fibres

Synchrotron radiation computed tomography for experimental validation of a tensile strength model for unidirectional fibre-reinforced composites

Synchrotron radiation computed tomography has been used to analyse fibre break accumulation in unidirectional composites loaded in tension. The data are compared to model predictions. The model only slightly overestimated the composite failure strain, but predictions of fibre break density were too high, which can be mainly attributed to errors in the Weibull distribution. Both the number and percentage of interacting fibre break clusters were under-predicted by the model. This was attributed to an underestimation of stress concentrations in the model. While the experimental observations revealed mainly co-planar clusters, the model predicted mainly diffuse clusters. The experiments showed that the clusters did grow any further after their formation, while the model predicted a gradual development. Both local and dynamic stress concentrations were hypothesised to be key features for further exploration. The discrepancies identified, inform suggestions for directions advancing the state-of-the-art strength models of UD composites

Morphology-induced fatigue crack arresting in carbon fibre sheet moulding compounds

Carbon Fibre Sheet Moulding Compounds (CF-SMCs) are tow-based composite materials. Interrupted fatigue tests, combined with computed tomography, were performed here to investigate the damage mechanisms in high in-mould flow CF-SMC. The tow-based microstructure created obstacles for fatigue damage propagation, increasing the CF-SMC’s resistance against cyclic loading. Failure is shown to nucleate inside the tows, but inter-tow crack propagation tends to be hindered by the presence of the other tows. Tows oriented perpendicularly to the initial fatigue crack stop the crack itself, showing an intrinsic crack arrest mechanism. Additionally, pre-existing manufacturing cracks or voids do not propagate at all. As a result, flatter slopes of the SN diagrams were observed for CF-SMC than for other carbon or glass fibre composites with short, long and even continuous fibres

Hybrid effects in thin ply carbon/glass unidirectional laminates: Accurate experimental determination and prediction

Experimental results are presented which allow the hybrid effect to be evaluated accurately for thin ply carbon/epoxy-glass/epoxy interlayer hybrid composites. It is shown that there is an enhancement in strain at failure of up to 20% for very thin plies, but no significant effect for thicker plies. Hybrid specimens with thick carbon plies can therefore be used to measure the reference carbon/epoxy failure strain. The latter is significantly higher than the strain from all-carbon specimens in which there is an effect due to stress concentrations at the load introduction. Models are presented which illustrate the mechanisms responsible for the hybrid effect due to the constraint on failure at both the fibre and ply level. These results give a good understanding of how variability in the carbon fibre strengths can translate into hybrid effects in composite laminates