Effects of Stacking Sequence on the Strength of Cross-Ply and Quasi-Isotropic Carbon/Epoxy Laminates

The effect of stacking sequence on the tensile strength of carbon/epoxy laminate was studied. The differences in effective strength values between cross plies and quasi-isotropic laminates, with the same lay-up and with inverse stacking sequence, as well as the strength value differences between laminate coupons of the same stacking sequence but of different widths, were analysed by failure observations and by assessment of the edge effect. Three-dimensional state of stress near coupon free edge was deduced by the method based on force and moment equilibrium and complementary energy minimization principle. The calculated values of edge interlaminar stresses, strains and elastic strain energy stored under edge normal interlaminar stress in interlayers of tested laminates, were correlated with the appearance or absence of axial cracks in the interlayer, as well as with the measured strength values. It is shown that the edge interlaminar stress effect on crack initiation or inhibition in interlayers and consequently on measured laminate strength values, can be well correlated with the arise, under edge normal interlaminar stress, of stored elastic energy per unit volume of interlayer edge boundary region.