High-speed fracture of elastomers: Part I

The fracture behavior of elastomers has been studied in the high speed regi on immediately above a transition region where a change in the nature of th e fracture surface occurs. A video camera operating at up to 6000 frames/s was used to follow the progress of failure. The test pieces employed were o f the pure shear or tensile strip (with edge crack) varieties employed in e arlier tear studies; in order to attain the high energy release rates requi red they were usually pre-strained and then cut to initiate the crack growt h. The materials studied included various strain crystallizing and non-crys tallizing elastomers, with different glass-transition temperatures, includi ng natural rubber in normal, cis-trans isomerized or partially-epoxidized f orms, a butadiene-acrylonitrile copolymer, a styrene-butadiene copolymer an d ethylene-propylene co- and ter-polymers. The use of a fracture mechanics approach based on the strain energy release rate enables results for different test piece geometries to be brought int o agreement in the region above the transition. Fracture energies in this r egion correlate well with viscoelastic properties, but the potential for st rain crystallization to strengthen is not exhibited. This is presumably bec ause at the high failure rates involved, the loading time at the crack tip is insufficient to allow significant crystallization to occur. The correlat ion with viscoelastic behavior suggests that the material around the propag ating tip is still essentially rubbery in its behavior, although the fractu re surfaces in the high speed region have a smooth, "glassy" appearance. Fu rther evidence of this is provided by effects of crosslink type and density and filler type and loading, where again the effects seen in the high spee d failure region parallel those observed below the transition. An effect of thickness on fracture properties appears to be absent in the h igh speed region, by contrast with behavior at lower energies and at the tr ansition itself. This supports the view that the fracture surface roughness that develops in the lower energy region is due to the initiation of a pro cess akin to cavitation by through-the-thickness stresses near the crack ti p. The transition is found to vary with pressure and temperature, as well a s thickness, in a way that does not correlate with viscoelastic changes but may reflect changes in the through-the-thickness stresses. The existence of a limiting crack speed is illustrated and discussed. Fract ure under large biaxial deformations, where higher crack speeds are observe d, will be discussed in part II.