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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 enough to 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 gauges were employed to follow, in sib, the buildup of residual strains in carbon fiber-reinforced laminates during cure. The data were compared to those from volumetric dilatometer studies to ascertain the fraction of resin shnnkage that contributed to residual stress buildup during cure. Based on earlier studies with single-fiber model composites, the process cycle in each case was then varied to determine if the cycles optimized to minimize residual stresses for isolated fibers in an infinite matrix were applicable to the reduction of residual stresses in conventional multifiber composites. The results of these studies are reported here.

This paper describes the initiation of matrix cracking for glass and glass-ceramic matrix composites reinforced with small-diameter silicon carbide and carbon fibers under uniaxial tensile loading. Acoustic emission, replication, and optical microscopy in conjunction with stress-strain curves are employed to detect the initiation of matrix cracking. The proportional limit of the stress-strain curve is found to overestimate the initiation of matrix cracking in the material systems studied. The matrix cracking initiates at axial strains from 0.07% to 0.15%. The ACK model overestimates the initiation of the matrix cracking for the material systems studied in this paper. [