Slow crack propagation in polyethylene under fatigue at controlled stress intensity

Fatigue tests were performed on circumferentially notched bars (CNB) of high density polyethylene in order to analyse the kinetics and mechanisms of crack propagation. Tests were performed at 80 °C in order to accelerate the processes. Unlike standard fatigue procedure in which the force amplitude is constant, the original system utilised in this work was capable of imposing a constant stress intensity amplitude, K^max, during the whole propagation range. This was made possible through the real-time monitoring of crack propagation, a(N), by means of a video-controlled technique. The results of the tests, for K^max in a range from 0.2 to 0.45 MPa m^1/2, show that stress intensity is the proper variable which controls crack propagation rate since, on the overall, crack speed is constant at constant K^max and no self-acceleration is observed unlike under force control. The empirical Paris law is verified under all conditions, with a stress intensity exponent close to 4. However, it is shown that constant crack speed is obtained only for K^max<0.25 MPa m^1/2, when propagation proceeds through the continuous stretching and breaking of microfibrils in the localised craze at the tip of the crack. By contrast, at larger K^max, it is observed that crack tip successively jumps across the extended crazed zone in which very coarse fibrils were previously stretched from voids nucleated in the plane of maximum normal stress at a long distance ahead of the crack tip.