Experiments on Controlled Fractures

A new type of fracture experiment is described. The specimen is a flat strip, which is split lengthwise by gradually propagating a crack down the middle. The crack is made stable in direction by holding the specimen in a state of lengthwise compression. The energy balance of the system can be established by simple beam theory. Given Young's modulus for the material, the fracture energy is found from the length of the crack and from its width at the open end. Measurements made on plastics, with specimens of varying size, show that the crack propagation is governed by a constant specific fracture energy, which is invariant with respect to scale transformations of the stresses. For a sample of polymethylmethacrylate the specific fracture energy was 4.9 × 105 dyn cm-1. For a sample of polystyrene it was 25 × 105 dyn cm-1 for slow cracks, but dropped to 3 × 105 dyn cm-1 for fast-running cracks.