The nature of craze development and breakdown in PVC under constant amplitude, variable amplitude and combined cyclic and static loading conditions was examined. In addition, the effect of thermal history and test frequency on the FCP resistance of neat and rubber-modified PVC was considered.
Thermal aging studies were conducted over a variety of quenched and sub-Tg annealing conditions. It was found that the influence of thermal history on the FCP resistance of PVC is small. This result was in good agreement with the theory that mechanically-induced chain motion increases the excess free volume in the damage zone at the crack tip and thus offsets the decrease in excess free volume due to aging below T(,g).
The nature of discontinuous growth (DG) band development and breakdown in PVC was examined by interrupting DG bands at various fractions of their total cyclic lifetime. The relative kinetics of discontinuous growth band formation under constant amplitude loading conditions were found to be independent of the molecular weight, plasticizer content and stress intensity levels examined. In addition, the majority of the cyclic lifetime of a DG band was found to be spent in breakdown and not development of the craze at the crack tip. Variable amplitude loading was found to result in a transient increase or decrease in FCP resistance, depending on the load block sequence. The effect of combined cyclic and static loading on the cyclic lifetime of the DG band was found to be a function of the time of application of the static hold period.
Fatigue crack propagation tests were conducted over a frequency range of 0.1 to 100 Hz in neat PVC and 1 to 100 Hz in rubber-modified PVC. An overall enhancement in FCP resistance was found with increasing test frequency in neat PVC at all ΔK levels and in rubber-modified PVC at low Δ levels. The beneficial effect of FCP resistance in neat PVC was attributed largely to the observation of the development of a more resistant craze structure with increasing strain rate at the crack tip. The beneficial effect on FCP resistance at low ΔK levels in rubber-modified PVC was attributed to localized crack tip heating. At increased ΔK levels, the beneficial effect of increasing test frequency on FCP resistance diminished. This result was attributed to a change from localized to generalized heating that decreased the overall elastic modulus and thus decreased the FCP resistance.