Off-axis fatigue behavior of a carbon/epoxy cross-ply laminate and predictions considering inelasticity and in situ strength of embedded plies

Abstract

The off-axis fatigue behavior of a carbon/epoxy symmetric cross-ply laminate at room temperature is examined, and a ply basis fatigue life prediction method is tested with a special emphasis on consideration of the inelastic deformation and in situ strength of the inclined plies embedded in the laminate. Constant amplitude tension–tension fatigue tests are first performed on the coupon specimens of the cross-ply laminate for various fiber orientations. The fiber orientation dependence of the off-axis fatigue data on the cross-ply laminate can be removed by normalizing the maximum fatigue stress levels with the help of the associated off-axis static strengths. The distribution of the normalized off-axis fatigue data on the cross-ply laminate almost agrees with the normalized fatigue data on the unidirectional laminate made of the same prepreg tape, indicating that the relative fatigue performance of the cross-ply laminate is almost indistinguishable from that of the unidirectional laminate. A simple off-axis fatigue analysis of the cross-ply laminate is then attempted on a ply basis, in which no effects of progressive damage are assumed. A ply fatigue model that takes into account the in situ principal strengths of the plies embedded in a general laminate is developed to this end, along with an analytical procedure for identifying the in situ strength ratios of the actual principal strengths to reference values. The actual stress components in the plies embedded in the cross-ply laminate are evaluated using a laminate constitutive model based on the classical lamination theory, in which the non-linear plastic deformation of inclined plies under in-plane off-axis loading is taken into account. It is demonstrated that good guesses about in situ strength ratios can be made on the basis of elastoplastic CLT analysis, and the off-axis fatigue lives of the cross-ply laminate predicted using appropriate in situ strength ratios are shown to agree well with the experimental results.