Lamination Residual Strains and Stresses in Hybrid Laminates

The influence of hybridization in angle-ply laminates on curing residual strains and stresses was investigated. The specimens were graphite/Kevlar 49/epoxy and graphite/S-glass/epoxy laminates of various stacking sequence variations of the basic [±45/0₂]_s construction. The ±45-deg plies and, in some cases, half the 0-deg plies were graphite/epoxy. Residual stains were determined experimentally following previously developed techniques by taking the difference between the unrestrained thermal strain of an individual ply and the restrained strain of that ply within the laminate. It was found that stacking sequence did not have an influence on measured residual strains. In the graphite/Kevlar/epoxy group, the transverse (ɛ₉₀) strain in the 0-deg Kevlar plies is the highest, exceeding 9 × 10⁻³, reflecting the high-transverse thermal expansion of unidirectional Kevlar. The transverse strain in the 0-deg graphite plies is much lower, reaching a peak value of 5.6 × 10⁻³. In the graphite/S-glass/epoxy group the transverse strains in the 0-deg S-glass plies are relatively low, compared to similar strains in the 0-deg graphite plies. Residual strains in the graphite plies do not seem to be influenced much by the type and number of hybridizing plies. Residual stress computations show that at room temperature the transverse to the fibers residual stresses exceed the static strength for the graphite plies and reach values up to 75 percent of the transverse strength for the glass plies. The residual stresses in the Kevlar plies are high enough to cause early transverse failure of these plies before the laminate is cooled down to room temperature.