Co-authors​ :

 Christopher SUTCU (UNITED KINGDOM), Ali ARAVAND (UNITED KINGDOM), Zafer KAZANCI (UNITED KINGDOM) 

Fibre bridging, where the fibres of the composite materials cross the crack path between plies and resist the propagation of crack growth causes a rising R-curve behaviour in composite materials. This can increase the fracture toughness by as much as 9 times [1]. Fibre bridging is dependent on several factors including ply orientation, laminate thickness, and mixed mode ratio. Crack Opening Displacement (COD) is a critical parameter in determining fibre bridging laws in composite materials. The bridging law is the relationship between the COD and the fibre bridging stress. COD is notoriously difficult to calculate experimentally and non-invasively without impacting fibre bridging [1]. This work presents a novel computer vision Python tool that utilises optical flow to determine the COD. It works by tracking the movement of a selected edge pixel on the screen in subsequent images. The difference in the y-direction between the pixel updates is then multiplied by the pixel density of the screen, to calculate COD. This is compared against an ImageJ technique which uses a different pixel scale from a known distance from experimental testing. Both techniques arrived very close (<4%) to each other in terms of COD and the shape of bridging law. R-curves are generated for four different laminates. Two controls using unidirectional 150 g/sqm SE84LV nano pre-preg at two different crack lengths (30 and 50 mm), which showcase normal well-documented fibre bridging and two hybrid interfaces. Complex fibre bridging was induced from 90g/sqm Plain Weave (PW) hybridised with unidirectional 150g/sqm SE84LVnano prepreg, this complex fibre bridging is shown in Fig 1. This phenomenologically seemed to stop large-scale fibre bridging from being generated with a considerably higher fracture toughness than the control. COD and the bridging laws were calculated for all laminate configurations, even in the presence of this complex fibre bridging, showcasing the ability of the custom Python tool to calculate COD even in situations with dissimilar interfaces, which has never been done before to the best of the author's knowledge. Lastly, optical microscopy was completed at 4 different stages within the Fibre Processing Zone (FPZ) to quantify the influence of each interface layer on the observed fibre bridging. Hybrid Uni-directional Woven Composite Laminates (HUWCL) are gaining popularity with respect to increasing the damage tolerance of composite laminates [2], [3]. Therefore, being able to accurately quantify the bridging law between dissimilar interfaces makes it possible to implement complex delamination cohesive zone models in the realm of HUWCL laminates. Fig 2 shows the mean of the hybrid and control 30mm crack length specimen, with the standard deviation scatter shown in the shaded region. This shows increased fracture toughness of the hybrid interface even without influence from bridging fibres away from the crack tip.