ABSTRACT
Internal (residual) stresses build up in a thermosetting composite as the matrix shrinks during cure, and again as the composite is cooled to ambient from its elevated processing temperature. These stresses can be significant and can distort the dimensions and shape of a cured part as well as initiate damage in off-axis plies, either during fabrication or under the application of relatively low mechanical loads. The magnitude of these stresses depends on a number of factors including constituent anisotropy, volume fraction and thermal expansion, ply orientation, process cycle, and matrix cure chemistry. In this study embedded strain gages were employed to follow the buildup of residual strains in carbon fiber-reinforced laminates during cure. This in situ monitoring allows identification of the initiation of stress buildup, separation of shrinkage and thermal strains, and evaluation of process cycle changes on residual stresses. This technique was used to investigate residual stress buildup in carbon fiber composites of two epoxies, brittle and rubber-toughened, and a toughened bismaleimide, and the results are reported here.
