Nonlinear stress-strain behavior of graphite/epoxy laminates

The nonlinear stress-strain behavior of graphite/epox y was investigated. One hundred and eighty-one coupon specimens of the [ ±0] s family, with 0 equal to 0°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°, 75°, and 90°, were tested in uniaxial tension. All specimens were unnotched. Three types of tests were conducted: monotonic-to-failure; load, unload, reload-to-failure; and 100 load-unload cycles followed by a load-to-failure. In the latter two cases, the load cycle entered the nonlinear stress-strain region of the specimen. Two basic loadings were found to cause nonlinear stress-strain behavior in a unidirectional ply: longitudinal tension, which causes stiffening, and shear, which causes softening. These effects combined to cause overall stiffening in the [ ±8] s laminates for 0 less than 20°, and softening for 6 greater than 20°. The load, unload, reload tests showed three important results. First, there was a permanent strain left at zero load for those laminates that softened. Second, the reload path has the same modulus as the original load path and rejoins the basic stressstrain curve at the point to which the laminate was previously loaded. Third, the laminates show no memory of one previous loading after being reloaded to the original load point as further loading produces a stress-strain curve unaltered by one load, unload, reload cycle including no change in fracture stress. Implications for general laminates are discussed.