Analysis of Three-Dimensional Quadratic Failure Criteria for Thick Composites using the Direct Micromechanics Method

Abstract

Currently fiber composites are used in thick structures with significant out of plane stresses for which new 3D failure criteria are required. In this article the direct micromechanics method is used to determine the exact failure envelope of a unidirectional graphite/epoxy composite. A hexagonal unit cell of the composite is modeled using finite elements. Assuming that the failure criteria for the fiber and matrix materials and for the fiber-matrix interface are known, the exact failure envelope is constructed from a large number of three-dimensional stress states that correspond to failure initiation in the composite. These 3D failure stress states are then used to develop five three-dimensional phenomenological failure criteria: maximum stress; maximum strain; quadratic stress; quadratic strain; and optimized quadratic failure criteria. It is observed that the 3D quadratic stress and strain failure criteria may not always be closed, that is, they predict infinite strength in some directions. They can be made closed in combination with the maximum stress or the maximum strain failure criterion. It is found that a combination of aforementioned 3D failure criteria make failure prediction in thick composites more accurate and reliable. It is noted that the newly proposed optimized quadratic failure criteria is always closed, and is found to be more reliable than all other 3D failure criteria.