Effect of delamination defects on buckling and growth characteristics in composite laminates

Delaminations pose a critical threat to the safety and reliability of composite structures due to increased risk of buckling and further growth under compressive loads. Understanding the interplay between how even seemingly minor interface defects can amplify the risk of buckling and how buckling in turn can affect the growth characteristics of existing delaminations is therefore crucial during the design phase of the structure. In this study, we focus on the column buckling case to get an understanding of growth characteristics of pre-existing delamination under compression. A quasi-static “Effect of Defect” study has been conducted using geometrically nonlinear finite element (FE) simulations. The study has been performed by maintaining a constant applied load while incrementally increasing the delamination length in the models. Different through thickness delamination configurations have been investigated in a generic quasi-isotropic laminate and the configuration with delamination near the laminate surface has been found to be most critical. It is observed that as the delamination length is increased, the laminate initially experiences local buckling. This is followed by unstable growth and a mixed local/global buckling failure when the delamination length reaches a critical size. For this near-surface configuration, the influence of laminate dimensions has been investigated and a comparison has been made between the original quasi-isotropic laminate and a stiffer, zero-dominant laminate. It has been demonstrated that in the stiffer laminate, under the same applied load, local buckling occurs without leading to unstable growth, even for very large delamination sizes. Therefore, in this case, if growth occurs under a service load equalling the applied load, it is expected to remain stable without transitioning to unstable growth. By understanding these distinct growth characteristics within the context of delamination buckling, designers can more effectively develop inspection strategies that improve the monitoring of damage growth.