Fatigue Crack Growth (FCC) and Creep Crack Growth (CCG) in Polyethylene: micromechanisms compared

摘要

1. Simple Constant Amplitude fatigue tests cannot predict creep crack growth in a consistent way. However, a fatigue load schedule is introduced which could be used to extrapolate fatigue behaviour towards creep crack growth behaviour. This worked well for two of the three materials tested. More materials need to be tested to evaluate the general validity of the extrapolation procedure. 2. For material F, creep crack growth at ambient temperature is: a) Excellent compared to material O and T, when creep crack growth behaviour at elevated temperatures is regarded to reflect the behaviour at ambient temperatures. b) Similar compared to material O and T, when fatigue behaviour at ambient temperature is considered to characterise the creep crack growth behaviour at ambient temperature. So, the creep crack growth behaviour of material F at ambient temperature (i.e. service conditions) is predicted contradictory. The general acceptance of high temperature tests to predict creep crack growth at ambient temperatures may therefore be questioned, considering the fatigue results. 3. Crack growth behaviour can be linked qualitatively to macroscopic creep properties and crack tip geometry. The occurrence of blunting at the crack tip is found to play an important role in crack growth behaviour. 4. The mechanism of fatigue in polyethylene is linked to the embrittlement of fibrils in the craze, induced by the cyclic loading.