Measuring fracture energy in a brittle polymeric material

The dynamic fracture behavior of a brittle polymer, polymethyl methacrylate (PMMA), was studied using single-edge-cracked tensile specimens and the method of caustics in combination with high-speed photography. The dynamic response of the specimen and the state of local stress near the crack tip, i.e., the stress intensity factor K, were measured. To analyze the dynamic response, the external work, U-ex, applied to the specimen was partitioned into three components: the elastic energy, E-e; non-elastic energy, E-n, due to viscoelastic and plastic deformation; and fracture energy, E-f, for creating a new fracture surface, A(s). The results showed that E-e, E-n, and E-f increased with U-ex, and the ratio E-f/U-ex was about 46% over a wide range of U-ex. Energy release rates were estimated using G(t) = U-ex/A(s) and G(f) = E-f/A(s). The mean energy release rate, Gm, during dynamic crack propagation was also determined using the value of K. A good correlation between Gf and Gm was found.