Effects of Holes on Graphite Cloth Epoxy Laminates Tension Strength

Abstract

A series of tests was conducted on laminates made from a graphite cloth epoxy material and having circular machined holes. The tests showed room temperature behavior similar to holed tape laminates except that residual or notched-to-unnotched strength ratio of the cloth laminates were somewhat greater. Increasing temperature and degree of laminate orthotropy was demonstrated to increase the effective notch sensitivity to holes. Strain gages exhibited virtually linear response to failure except on or immediately adjacent to the hole edge, where significantly nonlinear response was evident well below the failure load. Highly localized anisotropic failures apparently are the cause of this behavior, suggesting it will be difficult to model analytically. Test results generally followed the trends predicted by the average stress failure criterion, but suggest the parameter a0 is a function of both temperature and layup. X-ray pictures of the failed specimens show that internal damage is restricted to a small distance from the failure surface.