Viscoelastically prestressed polymeric matrix composites – Effects of test span and fibre volume fraction on Charpy impact characteristics

A viscoelastically prestressed polymeric matrix composite (VPPMC) is produced by subjecting polymeric fibres to tensile creep, the applied load being removed before moulding the fibres into a resin matrix. After matrix curing, the viscoelastically strained fibres impart compressive stresses to the surrounding matrix, thereby improving mechanical properties. This study investigated the mechanisms considered responsible for VPPMCs improving impact toughness by performing Charpy impact tests on unidirectional nylon 6,6 fibre–polyester resin samples over a range of span settings (24–60 mm) and fibre volume fractions (3.3–16.6%). Comparing VPPMC samples with control (unstressed) counterparts, the main findings were: (i) improved impact energy absorption (up to 40%) depends principally on shear stress-induced fibre–matrix debonding (delamination) and (ii) energy absorption improves slightly with increasing fibre volume fraction, but the relationship is statistically weak. The findings are discussed in relation to improving the impact performance of practical structures.