The effects of residual stress on the impact properties of the unidirectionally reinforced P 100 Gr/6061 Al metal matrix composites with different thermal histories have been investigated using an instrumented impact test method and scanning electron microscopy. The cantilever impact generally causes tensile failure at the notch and compressive loading on the opposite side of the specimen. The specimens with yield tensile matrix residual stresses have planar fracture surfaces and low impact energy due to the contribution of tensile residual stress. The specimens with small residual stresses have moderate impact energy because debonding between fibre and matrix or fibre/matrix separation also serves as an additional mechanism for energy absorption. The specimens with higher compressive matrix residual stresses have the largest maximum load of all the specimens with the same matrix treatment. The specimen with matrix compressive yield residual stress has the maximum impact energy owing to a stepwise fracture surface. It can be concluded that good impact properties of composite materials can be obtained by choosing a suitable thermal history to modify the deleterious tensile matrix residual stress.
