Development of a standardized test procedure and an improved data reduction method for the mixed-mode I/II delamination in composite laminates

Abstract The original data reduction method in the ASTM standard D6671/D6671M-13 is established for unidirectional laminates free of fiber bridging. However, for multidirectional laminates, large-scale fiber bridging occurs in the wake of the crack-tip. Whether the original data reduction method can be applied for multidirectional laminates is questionable. This paper examines the effects of the complex failure mechanism on the flexural modulus of the multidirectional laminates, and how to avoid potential incorrect calculations of the fracture toughness in multidirectional laminates. To achieve this, mixed-mode bending delamination experiments are designed and conducted on specimens with two types of material combinations. An unloading-loading test procedure is proposed to determine the flexural modulus at various delamination lengths. Experimental results show that the flexural modulus increases with the delamination length rather than a constant. And a nearly linear relation between each other exists. An improved data reduction method is thus proposed to consider the effects of the fiber bridging on the flexural modulus, which can provide an accurate characterization for the fracture toughness and thus lay the foundation for the development of standard test procedures for the mixed mode I/II delamination in composite multidirectional laminates.