A failure mechanics study of aligned brick-and-mortar discontinuous long-fibre composites

Aligned discontinuous long fibre (DLF) thermoplastic composites present a promising lightweight alternative to continuous fibre reinforced composites in high-volume applications. They offer a balance between mechanical properties and processability and provide an avenue for repurposing manufacturing waste. This study examines the mechanics of aligned DLF composites based on chopped discontinuous glass-fibre-reinforced polyamide-6 (PA6-GF60) unidirectional (UD) tapes as a function of tape length through a combined experimental and numerical simulation approach. Tensile tests, acoustic emission and 2D digital image correlation (DIC) were employed to investigate the stress-transfer and failure mechanics in aligned tape brick-and-mortar model composites, revealing critical stress-transfer lengths and initiation and propagation of various failure modes. A finite element analysis (FEA) damage model was developed to simulate the mechanical properties and stress-transfer mechanism in these composites, accounting for variability in tape properties and microstructures. The results revealed a transition from tape-tape interface failure to tape fracture as the dominant failure mode with increasing tape length, although even at higher tape lengths, failure was still initiated by tape pull-out in the skins. A critical tape length of approximately 15 mm was identified, beyond which mechanical properties no longer significantly improved. Results from both experimental and numerical methods showed good agreement, suggesting the model can be used to provide insights for property optimisation of these types of composites.