Damage tolerance of filled glass/epoxy laminates
Damage assessment techniques have been applied to particulate-filled woven roving glass fibre reinforced epoxy laminates. Impact energies up to 43J were applied to approximately 8mm thick laminates, using a drop weight apparatus. An optimal testing arrangement was used to investigate the effect of particulate filler characteristics, e.g. particle strength, size, shape and surface treatment, on the compressive strength of undamaged and impact-damaged laminates. The damage to impacted laminates filled with particulate fillers was examined using optical and scanning electron microscopy and quantified by measuring the maximum damage zone size. Their damage tolerance was determined by compressive strength retention. All the particulate fillers caused a deterioration in the damage tolerance. Mineral fillers increased the size of the damage zone, and the compressive strength of unimpacted laminates was increased by incorporating hard, high modulus filler. The compressive failure modes before and after impact were distinctly different. Glass beads gave the highest damage resistance and compressive strength retention, but hollow glass microspheres and mica caused the most severe damage and a significant reduction in the retention of compressive strength after impact, while alumina trihydrate, calcium carbonate and quartz showed moderate damage tolerance. The size of the glass beads had a more significant effect on the damage resistance and post-impact compressive strength than the surface treatments. Although the laminates filled with two-dimensional glass flakes showed a lower impact damage resistance than those containing glass beads, they showed a higher compressive strength retention caused by better interlaminar shear strength. Adding rubber modifiers and combining Kevlar fabrics with glass fibre did not improve the damage tolerance of ATH-filled laminates. The use of a hot cure (piperidine) system improved the quality of the filled laminates probably by lowering the void content and/or improving fibre/matrix adhesion, and therefore improved the low energy impact resistance of laminates, but this advantage disappeared at higher impact energies. Based on a theoretical analysis of the impact damage mechanism, the fundamental matrix properties affecting the damage tolerance of laminates were identified, and verified by experimental data. There was a linear relationship between the strength of the matrix and the compressive strength retention of impact damaged laminates. The pre-impact interlaminar shear strength of laminates also correlated well with the compressive strength retention of laminates after impact. But there was no positive correlation between the modulus of the matrix and the impact damage tolerance' of laminates.