The behaviour of boron-aluminium composites during and after impact

The behaviour of two types of aluminium alloy (1200 and 2024) reinforced unidirectionally by boron fibres has been studied and the effect of an impact on the residual properties and under fatigue conditions determined. Spherical and cylindrical indenters have been used over a range of speeds covering six orders of magnitude. It is found that the fibres fail due to bending induced by the impact, but that the matrix surrounding the breaks is not necessarily cracked. The tensile strengths of impacted specimens have been compared to strengths of similar specimens containing a partial surface notch, simulating the impact damage, and good agreement found. Impacted specimens have been tested in circular bending fatigue on a machine which has been developed so as to maintain the maximum applied load constant despite any change of specimen compliance. The matrix bridge is quickly broken under cyclic loading and the B-2024 specimens show signs of progressive fibre damage. TheS-N curve for the B-1200 specimen is much flatter due to the inability of the softer matrix to transmit high stresses to fibres neighbouring fibre breaks. The residual tensile strength of impacted boronaluminium is found to depend on the remaining intact fibres and not on the matrix. In fatigue the notch effect produced by the damage zone is reduced at long lifetimes as cracking of the matrix parallel to the fibres isolate the damaged region.