Fatigue crack propagation in unplasticized poly(vinyl chloride) (uPVC): 2. Near-threshold fatigue crack growth

A new computer-aided test method was developed to measure near-threshold fatigue crack growth rates for a 150 mm class 12 uPVC pipe. A computer program was written to drive the fatigue testing machine and an optical microscope of 0.01 mm resolution was used to determine crack growth in the single-edge-notched specimen. The efficiency and advantages of this new technique are discussed and compared to the current ASTM proposed method. For crack growth rates of the order of 10^-9 m cycle^-1 the present computerized method was approximately seven times more efficient and therefore greatly minimized the testing times required to collect near-threshold fatigue data. Using this method the effects of stress ratio and level of processing on the near-threshold fatigue crack growth were investigated. It is observed that multiple crazes were predominant at stress ratios larger than 0.6 but they apparently disappeared at lower stress ratio values. This phenomenon was explained in terms of a critical stress criterion for craze formation and the elastic stress field near the crack tip. The fatigue threshold ΔK_th at a crack growth rate of 5 × 10^-10 m cycle^-1 was found to decrease linearly with stress ratio from 0 to 0.5 and to remain constant thereafter. The two levels of processing studied for this uPVC pipe material did not produce any effect on ΔK_th.