Co-authors​ :

 Sina AHMADVASHAGHBASH (BELGIUM), Mahoor MEHDIKHANI (BELGIUM), Yentl SWOLFS (BELGIUM) 

A strong fibre-matrix interface produces a fibre-reinforced composite with high strength and stiffness, albeit prone to brittle failure and low energy absorption. Deliberately weakening the interface can thus increase the toughness by creating fibre-matrix debonding and the subsequent pull-out. Nonetheless, this approach may compromise the desired strength and stiffness. Atkins [1] laid the theoretical groundwork for understanding the failure mechanism of composites reinforced with long fibres, featuring alternating sections of high and low adhesion, leading to “intermittent interfaces”. This approach demonstrated the potential to enhance composite toughness without compromising stiffness and strength. The intricacies in producing alternating interfaces kept the field stagnant for an extended period. In 2018, Greenfeld et al. [2] proposed an intermittent approach on single fibres, for the first time, by creating beads of cured epoxy evenly distributed on a single glass fibre. Pull-out and fragmentation tests showed an improvement in toughness while maintaining tensile strength. This method relies on mechanical interlocking caused by bead failure, leading to energy dissipation through matrix deformation during fibre pull-out rather than the formation of a tortuous crack path (“brick and mortar” concept).

A modelling approach (as developed in [3,4] for a continuous interface) is imperative to comprehensively understand how intermittent surfaces impact the energy dissipation during the debonding between fibres and polymer matrices. Using such a debonding model, the experimental methods, such as plasma activation and deposition processes, can be guided and optimised. This research endeavours to elucidate the intricate mechanics of debonding of intermittent carbon fibre-epoxy interfaces and the effect on the fracture behaviour of model fibre-reinforced composites. The proposed numerical model serves as a platform to test virtually a set of experimentally available parameters to identify the most promising treated carbon-matrix duos. Fig. 1a-b illustrates the conceptual foundation of this numerical investigation, depicting the assigned interfacial properties for both weak and strong interfaces using cohesive surfaces in Abaqus. As seen from the longitudinal stress recovery profile of the broken fibre (Fig. 1c), the proposed intermittent approach results in a larger debond length (2.5 mm compared to 1.7 mm) and, consequently, higher energy dissipation values. Furthermore, through a comprehensive parametric study, the transition regions are positioned at diverse locations relative to the fibre break plane to construct an inclusive library of stress redistribution cases. Ultimately, within the feasible range of interfacial properties as reported in [5], this library can be subsequently utilised in the KU Leuven strength model to predict the effect on longitudinal tensile strength.