Damage and dissipation mechanisms in the dynamic fracture of brittle materials: Velocity driven transition from nominally brittle to quasi-brittle

Abstract

We present the results of recent dynamic fracture experiments [Scheibert et al., Phys. Rev. Lett. 104 (2010) 045501] on polymethylmethacrylate, the archetype of nominally brittle materials, over a wide range of crack velocities. By combining velocity measurements and finite element calculations of the stress intensity factor, we determine the dynamic fracture energy as a function of crack speed. We show that the slope of this curve exhibits a discontinuity at a well-defined critical velocity, below the one associated to the onset of micro-branching instability. This transition is associated with the appearance of conics patterns on the fracture surfaces. In many amorphous materials, these are the signature of damage spreading through the nucleation, growth and coalescence of micro-cracks. We end with a discussion of the relationship between the energetic and fractographic measurements. All these results suggest that dynamic fracture at low velocities in amorphous materials is controlled by the brittle/quasi-brittle transition studied here.