Anisotropic strain rate effects on the fibre–matrix interface decohesion in sheet moulding compound composites

Material and structure overall response vary significantly under rapid strain as compared to quasi-static loading. The strain rate sensitivity of a discontinuous fibre-reinforced composite was investigated. The composite material consists in a sheet moulding compound (SMC-R26: weight content of 26%) with randomly oriented reinforcement. The specific aim of this study is to develop a multi-scale analyzed methodology in order to understand the physical origin of the strain rate effect. To this task, using a servo-hydraulic machine, monotonic and interrupted tensile tests were performed at different strain rates over a range from 2 × 10−4 to 2 × 102 s−1 and coupled to scanning electronic microscope observations. The obtained results at macroscopic and the microscopic scale, have shown that SMC-R26 presents a visco-damageable behaviour: damage onset and kinetic are widely sensitive to the strain rate, whereas the load rate effect, if any, is insignificant on the elastic properties. It has been established that the material macroscopic response is widely governed by the fibre–matrix interface failure strength which increases significantly when increasing the strain rate. Moreover, through the analysis of the fibre orientation influence, we have demonstrated that the strain rate effects (a delayed damage onset and a slightly reduced damage accumulation kinetic) are more important when the fibre–matrix interfaces are submitted to a pure normal stress.