ON THE DETERMINATION OF RESIDUAL-STRESSES IN FIBER-REINFORCED THERMOSET COMPOSITES

Residual processing stresses in fiber-reinforced composites arise primarily from a mismatch in the coefficients of thermal expansion of fiber and matrix, and from the anisotropy in the thermal expansion of the individual plies. These stresses significantly influence the mechanical properties of the laminate. In this study an attempt was made to vary the processing-induced strains by curing the same graphite/epoxy (AS4/3501-6) composite system at different temperatures. Residual stress-free temperatures were determined from the deflection of unsymmetric laminate strips as a function of temperature and from residual strains in symmetric laminates measured at room temperature using a peel-ply technique. The first method gave stress-free temperatures above the maximum cure temperature, and the difference between the two is attributed to the onset of shrinkage stresses at the cure temperature. Stress-free temperatures determined by the latter technique were, for the most part, in the vicinity of the maximum cure temperature. These results were used to predict die stress at first-ply and ultimate failure of [0(2)/90(2)], laminates, and the predictions were compared with experimental results.