Effect of fibre architecture on the falling weight impact properties of hemp/epoxy composites

The aim of this work is studying the falling weight impact properties of three different types of hemp/epoxy composites, two of which were realised using loose hemp fibres disposed either unidirectionally (LU laminate) or in a 0/90° (LC laminate), while the third one has been obtained using hemp mat (M laminate). The maximum fibre volume which was possibly introduced using hand layup was 55 vol.% for LU, 52 vol.% for LC and only 43 vol.% for M laminate. In practice, the aim of this investigation would be evaluating whether disposing loose hemp fibres in a mat, an operation which results in the reduction of the volume of reinforcement introduced, would yield lower falling impact properties or not. Quasi-static tests results show that the best performance, but also the largest scattering in properties is obtained from the unidirectional composites (LU): this suggests the fundamental role played by fibre orientation in these materials. In contrast, the study of impact hysteresis cycles suggest that, despite the limitations owed to the presence of through thickness fibres and the lower amount of fibres introduced, hemp mat laminates (M) are superior in terms of impact properties. In particular, impact hysteresis cycles involve, in the case of M laminates, a higher stiffness in the linear-elastic phase of impact, followed by a hardly detectable load drop, and a very large rebound energy, released at quasi-constant rate after penetration. As a result, a larger amount of energy is absorbed during impact on hemp mat laminates than on the loose hemp laminates.