Investigation into the feasibility of viscoelastically generated pre-stress in polymeric matrix composites

A method of producing compressive stresses within a composite material, to improve resistance to crack propagation, is described. This involves stretching polymeric fibres under a load that is released prior to moulding the fibres into a matrix material. On solidification of the matrix, continuing viscoelastic recovery of the strained fibres imparts compressive stresses to the surrounding material. To evaluate the technique, batches of samples, comprising nylon 6,6 fibres (3–4% fill) in a polyester resin matrix, were produced for Charpy impact testing. Each batch consisted of five test and five control samples, the latter containing the fibres in unstressed condition. Of the 15 batches tested, test samples from 11 batches absorbed, on average, 25% more impact energy than their control counterparts. The remaining four batches showed little difference between test and control samples, and this may be attributed to inadequate fibre–matrix bonding. Batches were tested up to 3500 h after moulding, and there was no discernible deterioration in impact performance with age.