Experimental investigation of residual strain formation mechanisms in composite laminates under cycling loading

Cycling tension tests of the specimens from symmetric angle-ply laminate based on ELUR-P unidirectional carbon fiber and XT-118 cold-hardening epoxy have been carried out. Using the obtained experimental results, the process of residual strains formation has been investigated and qualitatively analyzed. It has been shown that there is such a level of ultimate stress in cycling loading conditions for the composite under investigation in the principal material axes, below which stabilization of the strain increment parameter at each loading cycle occurs. A method has been developed for determining the residual strains at each loading cycle by introducing the secant moduli of elasticity in the loading and unloading paths and their determination from the stress-strain curve. For several loading rates, which differ significantly from each other, the experimental dependencies of secant moduli formed in the loading and unloading paths on the cycle number have been obtained. A method has been proposed, which allows to isolate a viscoelastic component from the total strain accumulated during the cycling loading. This is specified by the epoxy creeping in the shear conditions. It has been found that the total strain for the fiber reinforced plastics under consideration can be represented as a sum of the viscoelastic part, which is recoverable over time, and the residual strain, which is probably associated with unrecoverable structural changes in the composite.