Effects of ply thickness and architecture on the strength of composite sub-structures

This work presents an experimental study on the effect of: ply thickness; ply-level hybridization; and type of ply architecture on the damage mechanisms that dominate failure and strength of multidirectional laminates. Nine effectively equivalent 0°dominated multidirectional laminates made from unidirectional carbon fibre tapes (UD), non-crimp fabrics (NCF) and spread-tow fabrics (STF) with thickness ranging from 67 μm to 268 μm are defined and tested under plain strength tension and compression, open-hole tension and compression, filled-hole compression and tension-bearing. An overall improvement of the strengths with the decrease of ply thickness is observed for the UD and NCF laminates, but nearly no effect of ply thickness is detected for STF laminates. Ply-level hybridization, where thin off-axis plies are combined with thicker grade 0°plies contributes to the improvement of the laminate response, especially when thicker 0°plies are used. However, hybrid-ply laminates are more sensitive to the loading direction and are, therefore, only suitable in structures where highly oriented loadings are expected. Fabric-based laminates do not show especial susceptibility to early failure compared to UD laminates. Their use can, therefore, be an economical solution that does not compromise the structural response, ensuring the weight benefits of composite materials at lower processing costs.