Crack closure in the fatigue delamination of composite multidirectional DCB laminates with large-scale fiber bridging

The stress ratio is commonly known to have an effect on the fatigue delamination growth (FDG) behavior of composite laminates. The crack closure phenomenon during delamination growth may change the value of stress ratio. Whether the stress ratio effect on the FDG behavior is resulted by the crack closure or not is investigated in this study and experimental evidences are presented in order to obtain a reliable conclusion. Using the quasi-static unloading approach, the crack closure loads during fatigue tests are firstly determined. The effect of crack closure on the mode I FDG in multidirectional composite laminates, which are widely used in engineering field, was experimentally studied. Test results show that the crack closure has an obvious effect on the minimum fatigue load and a linear relationship exists between the crack closure load and the delamination length. However, the influence of crack closure on the FDG rate is negligible. Further discussions illustrate that the crack closure does not fully explain the stress ratio effect in composite multidirectional laminates with large-scale bridging fibers.