Stick- Slip Instabilities in Fracture

Abstract

Stick-slip fracture is commonly observed in fracture tests of polymers under constant applied extension rate loading conditions. It is characterized by oscillatory crack tip velocities and crack growth jumps (cycles of fast unstable crack propagation and arrest) resulting in periodic transverse crack arrest marks on the fracture surface and periodic load fluctuations (e.g., in fracture of PMMA [1], polyesters [2] and epoxy resins [3], tearing of rubber [4] as well as peeling of polymeric adhesives [5, 6]). Recently, a stick-slip fracture model was proposed based on a nonmonotonic fracture toughness-velocity curve and an “inertia dependent” modified equation of motion for the crack tip [7]. It was shown that the stick-slip fracture instabilities are associated with the negative slope region(s) in a nonmonotonic fracture toughness γ or energy release rate G vs. crack tip velocity v curve where steady (continuous) crack propagation cannot be realized and the temporal crack tip instabilities were identified as self-oscillations. However, an ad hoc “loading curve concept” was introduced in order to account for the experimentally observed decrease in the amplitude of the stick-slip fracture oscillations and the critical energy release rate at the onset of stick-slip fracture instability with increasing applied loading rates. More specifically, the loading curve provided a relationship between the energy release rate and crack tip velocity during nonsteady subcritical crack growth; thus modelling the “apparent” crack growth due to fracture processes (e.g., crack tip blunting or crazing) occurring ahead of the crack tip prior to stick-slip fracture instability.