This paper presents the results of several series of tests performed primarily to determine the effect of biaxial loads on the mechanical properties of specimens made from four graphite/epoxy laminates. Static strength, fatigue life, and residual strength tests were performed on a cruciform type of specimen containing a center hole. The parameters varied in these tests were the applied load biaxiality, hole diameter, and laminate layup.

The test results showed that the hole diameter had a limited influence on the static strength, when the strength was based on the net section area. The applied load biaxiality appeared to have a significant influence on both the fracture path and the magnitude of the failure load. For the quasi-isotropic [0,±45,90]_ns laminates, the static strength increased by a factor of 1.44 as the biaxiality was increased from zero to unity and the fracture path rotated from 90° to the principal loading axis to approximately 45° to it. Although fewer tests were performed on the anisotropic [0₂,±45]_ns laminates, the biaxial influence appeared to be insignificant. For some fatigue tests, increasing the biaxiality decreased the fatigue life, while for others the fatigue life was increased. This behavior appears to be governed by material anisotropy, in which certain directions (that is, perpendicular to each layer) exhibit increased sensitivity to fatigue damage. The decreasing lifetimes appeared to be associated with the rotation of the primary failure plane to coincide with one of the relatively weaker directions.