Fatigue crack propagation in PVC: Effects of molecular weight and specimen history

In this study, an earlier investigation of the effects of molecular weight on fatigue crack propagation, FCP, in PVC was extended and earlier tentative conclusions were verified using samples whose molecular weight distributions were relatively constant and narrow. In the range of M_w between 6 × 10⁴ and 2 × 10⁵, a strong effect of M on the FCP rate per cycle (da/dN) as a function of the range in the stress intensity factor (δ K) was observed. While the apparent fracture toughness increased moderately with M, FCP rates at constant δK decreased by three orders of magnitude over the range examined. This strong dependence, which followed the empirical relationship da/dN = A e^B/M δKⁿ, implies that cyclic disentanglement is more difficult the higher M. Preliminary observations were also made on the roles of sample orientation, mean stress and thermal history. It was found that the residual orientation that survived specimen fabrication had a slight but significant effect on FCP rates and the maximum values of δ K obtainable prior to fracture (δK _max). Thus the smaller the angle between the loading and orientation directions, the lower da/dN and the higher δK_max; the magnitude of the effect was greater, the lower the value of M. The mean stress also affected the FCP rate, sometimes positively and sometimes negatively, depending on the material. In contrast, little effect of prior thermal history of the FCP rate was noted. Just as an increase in temperature to above T _g is known to erase the effects of aging, so did cyclic loading, a t least in this case.