On the velocity-dependent fracture toughness of epoxy resins

In a recent paper [1], Drs Selby and Miller described three methods of determining the fracture toughness (R), or equivalently, the fracture surface energy (23') for an epoxy resin. These methods are respectively (a) Berry's method, (b) Irwin-Kies equation and (c) Gurney's irreversible work area method. While the experimental data obtained from tapered double-cantilever beam (TDCB) specimens supported the equivalence of the first two methods (based on crack initiation), they apparently have difficulties in the interpretation of these data using Gurney's method because of the "crack-jumping" nature of the epoxy resin. They have found that the segmental areas give fracture surface energies (23') less than that obtained from either Berry's method or Irwin-Kies equation. It would thus appear at first sight that there is a limitation to the usefulness of the Gurney-method for fracture toughness determination for such "crack-jumping" materials. In the present note, we wish to show that such limitations really do not exist once the criteria governing stability in cracking are investigated. Conditions governing the stability of cracks under constraints of both loadand displacementcontrolled machines are given in [2 -4] . In general, the stability criteria may be written as