O.S. David-West

A study of the effect of strain rate and temperature on the characteristics of quasi-unidirectional natural fibre-reinforced composites:

The responses of quasi-unidirectional vegetable fibres (sisal and flax) – styrene polyester matrix composites under 6-J nominal strike energy at different sample temperatures and at higher impact energies of 9 and 12 J for samples at room temperature have been studied and indentation properties realized from static loading at different cross-head displacement speeds. The findings are explained in terms of the impact characteristics. At temperatures above ambient, there were changes in the impact characteristics. The drop impact tests were carried out on samples (70 × 70 × 5 mm3) at an ambient temperature of 18 °C and elevated temperatures of 40 °C, 60 °C, 80 °C, and 100 °C using an instrumented drop tester with a 12.1-mm diameter hemispheric tup and the load history and energy profile were obtained. The actual damage response depends on many intrinsic and extrinsic factors, including the thickness of the laminate, the exact stacking sequence, the shape and kinetic energy of the impactor, and the degree to which the laminate is supported against bending. The examination of the impact and post-impact characteristics at elevated temperatures revealed a plastic mode of failure and the performance was assessed in terms of contact time, post-impact displacement, and total energy. Composites are generally brittle in nature and respond elastically with little or no plastic deformation, but this is not the case for natural fibre – styrene polyester matrix composites, especially at high temperatures, as there exists some degree of plasticity seen in the after-impact state of the samples. Low energy impact (6 J) results in cracking of the matrix leading to reduction in the strength of the composite. However, higher energy impact strikes produce cracking of the matrix and splintering of the fibres. From the load–indentation curves, average power coefficients of 3.6 and 15.4 were obtained for sisal and flax composites, respectively, as against 1.5 for the special Hertz contact of two elastic bodies.

Low-velocity heavy mass impact response of singly curved composites

This article presents an extensive experimental investigation of singly curved fibre-reinforced composite structures subjected to low energy heavy mass impact. The objective of the study is to understand the contribution the surface ply orientation makes to the resistance of the impact strike and the impact behaviour of composite cylinders of different stacking configuration. Tests were conducted on laminate configurations suitable for various structural applications, using an instrumented drop-weight (30 kg) device. Impact studies conducted were on half single curved symmetrical composite panels of (β2/452/−452/02/902)s stacking configuration, where β = 0, 30, 45, 60, 90 and also on fully single curve composite cylinders of stacking sequence (02/452/902/−452)s, (452/02/902/−452)s and (902/452/02/−452)s. The test samples were manufactured by the hand lay-up technique using unidirectional carbon fibre–epoxy matrix prepregs. The impact properties such as the absorbed energy, bending stiffness, saturation energy, damage degree and the contact period were obtained from the response data and interpreted to establish the impact effect on the composite structure. From this unique study, it was realised that surface ply orientation of half single curved composites significantly contributes to the energy absorption characteristics and that the damage effect was most severe for the (902/452/02/−452)s configuration of the composite cylinder.