The Effect of Temperature and Moisture Content on the Flexural Response of Kevlar/Epoxy Laminates: Part II. [ ± 45,0/90] Filament Orientation*

Abstract

This study examines the effect of temperature and moisture on the flexural properties of quasi-isotropic Kevlar 49 fabric/epoxy composite laminates. Part I of this study treated the [0/90] filament orientation. Additional variables evaluated in Part II which were not studied in Part I include the effects of voids, long term moisture exposure, freeze-thaw thermocycling, the reversibility of moisture exposure and laminate unload ing behavior. Results indicate that the [±45,0/90] laminates exhibit a de pendence on temperature and moisture similar to the [0/90] laminates, but that the quasi-isotropic orientation is somewhat less sensitive to moisture at 21 and 150°C. Strength losses in the quasi-isotropic laminates can nevertheless be substantial. At 150°C and near saturation moisture content, the strength is 55-60 percent lower and the stiffness 40 percent lower than 21°C dry values. Voids, freeze-thaw cycling of specimens with voids and moisture, and long term moisture exposure all degrade laminate strength. Specimens subjected to long term moisture exposure followed by drying displayed stiffness values equivalent to as-fabricated results, but did not totally recover in strength properties. Flexural strengths of the dried specimens were similar to those of fully moisture saturated material at 21°C (15-20 percent loss from dry values). The lack of 100 percent re covery in strength indicates that moisture effects are not totally reversible. Microscopic examination of failed specimens showed no filament buckling at 21°C. Failures are characterized by a tensile delamination mode which may be induced by edge effects produced by the off-axis filament plies.