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 appear- ance 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 ve- locities in amorphous materials is controlled by the brittle/quasi-brittle transition studied here.
Highlights
Driven by both technological needs and the challenges of unresolved fundamental questions, dynamic fracture in brittle materials has been widely investigated over the past century
We show that the slope of the dynamic fracture energy as a function of crack speed exhibits a discontinuity at a well-defined critical velocity va well below vb
Both the experimental results and the above discussion shed light on how material defects might control the dynamic fracture of amorphous solids before the onset of micro-branching
Summary
Driven by both technological needs and the challenges of unresolved fundamental questions, dynamic fracture in brittle materials has been widely investigated over the past century. Since the pionneer work of Griffith [1], Orowan[2] and Irwin[3] a coherent theoretical framework, the Linear Elastic Fracture Mechanics (LEFM) has developed This theory is based on the fact that – in an elastic medium under tensile loading – the mechanical energy released as fracture occurs is entirely dissipated at the crack tip within a small zone so-called process zone. Defining the fracture energy Γ as the energy needed to create two crack surfaces of a unit area, the crack growth velocity is selected by the balance between the energy flux and the dissipation rate Γv. K depends only on the applied loading and specimen geometry, and characterizes entirely the stress field in the vicinity of the crack front
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