Determination of pure mode-I fracture toughness of multidirectional composite DCB specimens

•How DCB specimen parameters effect the width-wise distribution of GI.•A new method to determine the fracture toughness of multidirectional DCB specimen.•The non-uniformity ratio varies linearly with unidirectional specimen's constant Dc.•The evolution of non-uniformity ratio with the specimen width appears exponential. Strain energy release rate (SERR) along a linear crack front of a multidirectional composite DCB specimen shows an obviously non-uniform distribution, which causes the fracture toughness GIavg measured by experiments smaller than the real GIC, and so leading to a lower estimation of fracture toughness. In this study, based on the numerical model a novel semi-empirical closed-form formula was proposed for determination of pure mode I fracture toughness of a multidirectional composite DCB specimen via a width-wise non-uniform distribution parameter. These expressions translate the relationships between the width-wise non-uniform distribution ratio and the specimen width as well as the material parameters. They provide a rapid method to measure correctly the pure mode I fracture toughness of an uncoupled multidirectional composite DCB specimen by integrating the effect of non-uniform distribution of the strain energy release rate at the crack front.