On failure mechanisms of composite laminates with an open hole subjected to compressive load

In the present study, the compressive failure mechanism of quasi-isotropic composite laminates with an open hole was experimentally and numerically studied to explain the mechanical meaning of the open hole compression (OHC) strength. In the experiment, we adopted a fixture for the OHC test method proposed by the National Aerospace Laboratory (NAL III). Two types of composite systems were tested to examine the dependence of failure behavior on the material properties such as interlaminar toughness. The damage which appeared first was fiber micro-buckling in the 0° layer. Some accumulation of damage, such as further fiber micro-buckling in the 0° layers and interlaminar delaminations in several interfaces, was observed before the final unstable fracture in the laminate with high interlaminar toughness, while sudden failure occurred in the laminate with low interlaminar toughness. In the numerical study, a full three-dimensional finite element analysis was conducted. At the transverse edge of the hole, not only stress singularity at the interface but also high stress concentration at the 0° layers was obtained. Singular stress at the interfaces decreased quickly to a level much below the interlaminar strength. To consider the effect of micro-buckling on the stress redistribution and further damage accumulation, a damage analysis was conducted by reducing the stiffness components of the corresponding elements to 1/10. When one element was damaged, the maximum stress next to the damaged element became higher than the maximum stress for the intact laminate. The maximum stress decreased with the increase in number of the damaged element in the transverse direction and became lower than that for intact laminate after some extension of the damage area. This result indicates that some unstable damage progress of limited size would be expected just at the first damage occurrence and a stable increase of damage would follow it with further compressive load increase until other damage systems occurred.