Fracture behaviours of CFRP laminates in mode I interlaminar fracture toughness testing

Fracture behaviours in mode I delamination tests of CFRP (T300/2500) were studied at cross-head velocity V=0.005 similar to 200 mm/min. In low velocity region, serrations due to repeated unstable fracture appear on load P-displacement delta curve, and they become conspicuous as V becomes smaller. The crack resistance force R evaluated by using the peak values of the serrated P-delta relation becomes larger with the decrease of V; the rate dependence of R is negative. On the other hand, at velocities higher than the critical one (V-c), there is nothing but fluctuations on P-delta relations. Although the rate dependence of R is extremely small, it is a positive value. Relaxation tests were also performed for specimens deformed at lower and higher velocities than V-c; at high velocity, load P decreases with time, while at low velocity, the load remains constant during relaxation test. The rate dependence and fracture behaviours have been discussed by using a model which takes the crack tip blunting effect into account.