Fatigue crack growth rate behavior of polyvinylchloride hidden by scaling: Reinterpreted by micro/macrocrack model

Fatigue crack growth characteristics in polymers differ from those in metals. Since crack grows at a much slower rate in the polymer, region III is not so obvious. However, regions I and II are easily identifiable when the data is more carefully analyzed for the PVC. That is the transition of microcracking to macrocracking which can also be shown to prevail for PVC by a plot of the two parameter relation of da/dN versus the range of the stress intensity factor ΔK. As further evidence of the transitory crack scale size behavior, the same data are again plotted using the dual scale micro/macrocrack model where ΔK   is replaced by ΔKmicromacro, the transition region disappears giving a straight line relation The subscript “micro” and superscript “macro” are manifestation of scale effects. Closing and opening of the crack surfaces due to tensile and compressive loading are controlled by the tightness ratio. While the macrocrack is characterized by the free–free surface condition, the microcrack is modeled by the free-fixed condition such that a small segment of the crack tip always remains open. This situation has been shown to prevail for microcracks in metals. It is also assumed to occur at the very end of the crazing/cracking zone for the polymer where crazing is initiated by microvoid formation. An equivalent crack length is invoked for the crazing/cracking region during fatigue crack growth whilst a small tip opening corresponds to the nucleation of microvoids. Micro/macrocrack transitory behavior is examined for change in stress amplitude, frequency and temperature.