Microstructural damage on thermoplastic hybrid composite in fatigue: Qualitative correlation with a creep strain rate fatigue criterion

This study investigates the fatigue behaviour of hybrid PA66 composites reinforced with short and continuous fibres through fatigue testing and in-situ microtomography. Damage initiates in the PA66GF50 short-fibre composite, propagates as a macro-crack towards the UD reinforcement, and leads to fibre fracture, fibre–matrix debonding, and rapid crack growth. A strong correlation exists between cyclic creep stages, at macroscopic level, and damage: primary stage with negligible damage, secondary stage, which defines the cyclic creep strain-rate criterion, is characterised by the initiation and limited progression of micro-damage, and tertiary stage with macro-crack propagation. Weaknesses lie in the short-fibre composite and fibre–matrix interface, rather than between the two polyamides.