Damage mechanisms of adhesively bonded joints of thin tow-based discontinuous composites

• The adhesive bonding of thin Tow-Based Discontinuous Composites (TBDCs) was studied experimentally • High-toughness adhesives promoted damage within the TBDC micro-architecture, reaching fracture toughness values of 800 J/m² • The fracture toughness of TBDC joints was significantly higher than that of UD and cross-ply laminates • Thin TBDCs can be used as toughening layers in composite bonded structures due to their high fracture toughness • Acoustic Emission and fractography revealed complex co-existing mechanisms, dominated by matrix cracking and fibre/matrix debonding Tow-Based Discontinuous Composites (TBDCs) are a new class of composite materials that combine high strength and stiffness with in-plane isotropy making them of interest in high-end structural applications. Despite their potential, efficient connection methods are currently lacking and the adhesive bonding behaviour of TBDC structures remains unexplored. This work, therefore, seeks to address this gap by analysing the quasi-static performance of TBDC adhesive joints under mode I loading condition. Double Cantilever Beam (DCB) tests were performed using two adhesives with contrasting toughness levels: a moderate (∼600 J/m 2 ) and a high toughness adhesive (> 2400 J/m 2 ). When a moderate-toughness adhesive was used, a combination of cohesive failure and composite damage was observed, with only a small scatter in the experimental results. In contrast, the use of the high-toughness adhesive led to a shift in damage mechanisms towards the composite micro-architecture, resulting in fracture toughness values in the region of 800 J/m 2 , with a larger experimental scatter. Acoustic Emission analysis identified matrix cracking and fibre/matrix debonding as the dominant damage mechanisms. These findings were validated by the post-mortem fractography analysis via Scanning Electron Microscopy. This work therefore provides the first detailed analysis of the damage mechanism in adhesively bonded TBDCs, which have potential in aerospace and automotive applications.