Measurement of Residual Strains in Boron-Epoxy and Glass-Epoxy Laminates

Embedded-strain-gage techniques were developed and used for measuring strains in composite angle-ply laminates during curing and thermal cycling. The specimens were 2.54 by 22.9 cm (1 by 9 in.) eight-ply boron-epoxy and S-glass-epoxy [0₂/±45]_slaminates. Unidirectional [0₈] specimens were also used for control purposes. Strain readings were corrected for the purely thermal output of the gages obtained from an instrumented quartz reference specimen. The strains measured during the cooling part of the curing cycle were in agreement with those recorded during subsequent thermal cycling, indicating that residual stresses induced during curing are primarily caused by differential thermal expansions of the various plies. Restraint strains, that is, the difference between unrestrained thermal strains of a given ply and the restrained strains in the laminate, were computed for the 0-deg and 45-deg plies of the angle-ply laminates tested. Residual stresses were obtained from these restraint strains using the anisotropic constitutive relations and taking into account the temperature dependence of stiffnesses and strains.