The Role of Friction in the Strength Size Effect in Fiber Composites Failing by Compression Kink Bands

Prior studies have provided experimental evidence of the size effect on the nominal strength of fiber composites, failing by the formation of a kink band under compression. This size effect is analyzed here computationally with the goal to understand and quantify the role of crack face friction. The mechanism of kink band formation is modeled by the semi-multiscale cylindrical microplane model, recently calibrated and experimentally validated for its predictions of the failure geometry as well as strength size effect. The kink band initiation in the model is driven by damage triggered on cylindrical microplanes. The model embodies a combined damage friction formulation on the microplane level, which assumes a parallel coupling between the two mechanisms. The model is used here to predict the strength size effect with and without friction effects. In the friction-free models, the predicted strength is found to scale in accordance with linear elastic fracture mechanics, suggesting brittle fracture. On the other hand, in the models embodying friction, the fracture process zone size is found to be enlarged, and the predicted strengths are found to not only obey the Bažant size effect law, but also match the test data. The strengths are found to fall in the transitional portion, suggesting quasibrittle fracture. Thus, the inclusion of friction is found to considerably increase the degree of quasibrittleness, and the combined friction/damage based microplane model employed here, is found to be useful to quantify this increase.