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 stretc hing polymeric fibres under a load that is released prior to moulding the f ibres into a matrix material. On solidification of the matrix, continuing v iscoelastic recovery of the strained fibres imparts compressive stresses to the surrounding material. To evaluate the technique, batches of samples, c omprising nylon 6,6 fibres (3-4% fill) in a polyester resin matrix, were pr oduced for Charpy impact testing. Each batch consisted of five test and fiv e control samples, the latter containing the fibres in unstressed condition . Of the 15 batches tested, test samples from 11 batches absorbed, on avera ge, 25% more impact energy than their control counterparts. The remaining f our batches showed little difference between test and control samples, and this may be attributed to inadequate fibre-matrix bonding. Batches were tes ted up to 3500 h after moulding, and there was no discernible deterioration in impact performance with age. (C) 2000 Elsevier Science S.A. All rights reserved.