Analysis of fatigue crack growth behavior in polymers using the unified approach

The unified approach for fatigue damage developed by the authors is applied for the analysis of fatigue crack growth in polymers. According to this approach, fatigue requires two load parameters and correspondingly there are two thresholds that must be met simultaneously for a crack to grow. The thresholds manifest in terms of stress intensity range, ΔK and maximum stress intensity, Kmax. The Kmax-threshold is normally greater than the ΔK-threshold. It is shown that fatigue crack growth behavior of a polymeric material is analogous to that of any metal or alloy. Typical L-shaped type of curves in the ΔK–Kmax plots can be seen, defining the two limiting values for each crack growth rate. The variation of these two limiting values provides a trajectory map that reflects the changing crack growth mechanisms involved. In polymers depending on molecular weight and processing conditions, creep crack growth can superimpose on fatigue crack growth. The unified approach shows clearly the degree of such superpositions and how it varies with processing conditions. Understanding of these changing mechanisms is important for development of any reliable fatigue life prediction model.