Crack face friction effects in the strength scaling of composites failing by kinked shear cracks under transverse compression

Abstract

Recent research showed that the failure and strength scaling of composites under transverse compression exhibits a quasibrittle behavior, characterized by a non-negligible fracture process zone (FPZ). The present work is aimed at analyzing and quantifying the contributions of crack face friction towards this quasibrittleness, by focusing on fracture plane angles and the strength scaling. The failure, frictional effects, and strength scaling are numerically predicted via the fixed crack damage model, augmented with Puck’s friction-dependent damage initiation criterion and Ragueneau’s parallel coupling model for combined damage and friction evolution. The model is calibrated and verified using available test data and then used to analyze the transverse compressive failure in unnotched specimens and strength scaling in pre-cracked specimens, with and without friction. The analyses show that the friction-augmented model can accurately predict the fracture plane angle equal to 53 degrees in unnotched specimens, and can be tuned to reproduce other angles as well. Further, in all notched specimens failure occurs via a kinked shear crack propagating at an angle to the pre-crack, and having at its tip a large FPZ. The kink angle is observed to fall in the range of 44 to 47 degrees when friction is excluded, and 54 to 58 degrees when friction is included. The specimen strengths with or without friction, scale in accordance with the energetic type II size effect law. This finding is also supported analytically using energy balance-based arguments. The results reveal that the inclusion of friction can considerably increase the strength and enlarge the FPZ size (by nearly 90%), making the fracturing behavior significantly more quasibrittle. The strength increase is size dependent for smaller sizes, and appears to become roughly constant for larger specimen sizes. The FPZ of the kinked shear cracks is seen to embody a strongly multiaxial stress state. This makes the fracture energy associated with the kinked shear crack growth much higher than the pure mode II fracture energy. The frictional dissipation can further lead to an increase in this energy, which in the present case is found to be nearly 130%. These findings serve to emphasize the importance of considering and quantifying friction effects in the transverse compressive failure of composites.