The fatigue crack propagation (FCP) resistance and fracture surface micromorphology of poly(vinyl chloride) (PVC) was studied as a function of molecular weight (M), plasticizer content, and residual orientation. With regard to fatigue crack growth, previous studies of the effect of M on FCP were extended and earlier conclusions were verified. In the range of My between 6.0 x 10⁴ and 2.2 x 10⁵, M strongly affected the fatigue crack growth rate (da/DN) at a given stress intensity (ΔK) level and followed the relationship da/dN = Ae^1/M ΔKⁿ. Residual orientation due to processing was found to have a slight, but consistent effect on fatigue behavior, with the greatest FCP resistance occurring when crack growth was perpendicular to the direction of residual chain alignment. The effect of residual orientation on fatigue crack growth was found to be less pronounced at high M.

Regarding fatigue fracture surface micromorphology, particular attention was given as to how the above variables affected the formation of discontinuous growth bands (DCB's). DG bands, which form on a number of polymeric solids under cyclic loading, represent successive increments of crack extension through a craze zone. As expected, DGB width showed little change in size over the molecular weight range of sample orientations examined, and increased in size with increasing plasticizer content. On the other hand, the cyclic stability of these bands increased markedly with increasing molecular weight, but to a lesser extent with additions of a plasticizer. The internal structure of the bands became more ragged with increasing M, while the addition of a plasticizer re-established the crisp and well-defined micromorphology associated with DG bands in low-M polymers.