A Study on Compressive Strength of Unidirectional Graphite-Epoxy Composites

Abstract

The shear moduli of the unidirectional graphite-epoxy composites and the matrix material “epoxy” under compressive load were measured by torsion test. The compressive failure tests of them were also carried out and the acoustic emission was measured. The shear modulus of epoxy did not decrease until a high compressive strain. Therefore the previous theories for the compressive strength which require the decrease of shear modulus of epoxy under the compressive stress should be denied. Young's modulus and the shear modulus of composites suddenly decreased at the load about 10% to 20% lower than the compressive failure load, and the numbers of events of acoustic emission rapidly increased. Photographs by the scanning electron-microscope have shown that the section of the failed fiber is skewed and stepwise, which is characteristics different from that of the tensile failure. The stresses caused by the difference between Poisson's ratio of the fiber and that of the matrix under a compressive load and the thermal residual stress produced in the process of curing have been analytically calculated on the assumption of the hexagonal array of fibers. The results have revealed that those two stresses cannot be the factors to determine the compressive strength of the composites. A formula to express the effect of compressive load on the torsional deformation has been obtained for the square section on the asssumption of no warping constraint.