Static behavior and the effects of thermal cycling in hybrid laminates

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Abstract
The influence of hybridization with stacking sequence variation on static stiffness, strength, ultimate strain and residual properties after thermal cycling of graphite/Kevlar 49/ epoxy and graphite/S‐glass/epoxy angle‐ply laminates was investigated. Tensile stress‐strain curves to failure and uniaxial tensile properties were determined for all laminates. Theoretical predictions of modulus, Poisson's ratio and ultimate strain were made, based on linear lamination theory, constituent ply properties and measured strength. Reasonably good agreement was found. Stacking sequence variation showed no significant systematic influence on the measured results. Specimens containing only two 0‐degree Kevlar or S‐glass plies behaved linearly to failure. Specimens containing four 0‐degree Kevlar or S‐glass plies displayed characteristic non‐linearity. One group of laminates was subjected to a tensile load and to 100 thermal cycles between room temperature and 280°F, and another group to a tensile load and 100 thermal cycles between room temperature and −100°F. All surviving specimens were tested statically to failure to determine residual properties which were compared with the properties of uncycled specimens. Specimens containing two Kevlar or two S‐glass plies behaved linearly to failure. Residual moduli in these specimens were lower than for uncycled specimens but residual strengths and ultimate strains were higher. Specimens with four Kevlar or four S‐glass plies showed some nonlinear behavior. No significant differences were found between residual and uncycled values for modulus and strength.

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