A PRELIMINARY-STUDY INTO THE MATRIX DOMINATED NONLINEAR BEHAVIOR OF GRAPHITE-EPOXY LAMINATES

The potential of composites for weight-saving is now widely recognized and they are used extensively in both control surfaces and doors. How ever, if major weight-saving is to be realized it is essential that co mposites be used in 'primary' components, i.e. wing and fuselage skins . Before this can be achieved it must be shown that composites are an adequate substitute for thin-skinned metallic components, particularly in regions where they undergo large strains and/or post-buckling defo rmations. For stiffened composite panels one potential failure mechani sm is the separation of the skin from the stiffeners; resulting from e xcessive 'through the thickness' stresses. To address this potential i nterlaminar mechanism, the present paper discusses the non-linear, tim e-dependent, response of a T300/914C, [0/90], graphite/epoxy laminate. Initially a series of ASTM standard, D4255-83, shear rail tests were performed and the equivalent sigma(eq)-epsilon(eq) curve for the lamin ate was derived using combined experimental and computational techniqu es. In general, the shear behaviour of the laminate was very similar t o that observed for epoxy adhesives with extensive strain rate effects , stress relaxation and creep even at room temperature. To further sub stantiate these results a series of ASTM standard, D3518-76, tests wer e then performed on +/-45 degrees laminates. These tests revealed that the sigma(eg)-epsilon(eq) curve was indeed rate dependent, often with a significant inelastic behaviour. Furthermore, the failure stresses and strains were found to be strongly dependent on the loading rate.