Fracture Toughness Behaviours of Rate Sensitive Materials Under Stable Crack Extension.

By assuming that fracture toughness (crack resistance force R) is expressed by constitutive relation R=R1(a)·(a^./a^.*)m (a and a^., crack length and crack velocity, respectively m, rate sensitivity parameter), the fracture behaviour under stable crack extension in a DCB specimen is analyzed for rate-sensitive materials. Analytical results show that an initial crack begins to extend before maximum load (Pmax), and the maximum value of R appears after Pmax. Alternate stable/unstable crack extension is obtained by introducing periodic disorder in R. The analytical work explains well the serrated P-δ curve which is often observed in delamination tests of laminated carbon composites.