Biaxial Testing of Graphite/Epoxy Composites Containing Stress Concentrations. Part I.

Abstract

Abstract : An experimental program was conducted to study the behavior under uniaxial and biaxial tensile loading of quasi-isotropic graphite/epoxy plates with circular holes and through-the-thickness cracks and to determine the influence of notch size on failure. Experimental stress analysis techniques used were strain gages, photoelastic coatings and moire grids. Recorded failure strains at the notch have exceeded twice the ultimate strain of the unnotched laminate. In specimens with circular holes failure initiates at characteristic points of high strain concentration with nonlinear response located at 22.5-deg. from some or all fiber directions. Failure at the crack tip in the form of a damage zone propagates usually along one of the fiber directions. Failure occurs when this damage zone reaches some critical value. There also seems to exist a threshold notch size below which the laminate becomes notch-insensitive. At least under uniaxial loading, it was found that the notch strength for the laminate studied is independent of notch geometry. Notch size effect can be satisfactorily explained and predicted using as a failure criterion the stresses averaged over a distance of 3-5 mm from the notch boundary. The strength reduction of plates with holes under equal biaxial loading is approximately 30 percent lower than under uniaxial loading and the two cases represent upper and lower bounds.