Multiscale simulation of fracture of braided composites via repetitive unit cells

Two-dimensional triaxially braided composites (2DTBCs) are attractive in crashworthiness design because their fracture can dissipate a significantly larger amount of impact energy than other light-weight materials. This paper aims at predicting the fracture energy, G_f, and the effective length of the fracture process zone, c_f, of 2DTBC composites. Since the fracture parameters are best manifested in the scaling properties and are the main parameters in the size effect law, the nominal strengths of three geometrically similar notched beams of three different sizes are simulated in a 3D finite element framework. The simulations are run for three different bias tow angles: 30°, 45° and 60° Continuum beam elements in front of the notch are replaced with repetitive unit cells (RUCs), which represent the 2DTBC's mesostructure, and are located in the region of potential cracking. Multiscale simulations, incorporating damage mechanics, are used to predict the pre- and post-peak response from three-point bending tests. Nominal stresses are calculated from the predicted peak loads and used to fit the size effect law. The dimensionless energy release rate function g(α) is determined from the J-integral. The values of G_f and c_f are then determined using g(α) and the size effect law. With some exceptions, the results in general match well with the results of size effect experiments, and particularly the strong size effect observed in the tests.