INFLUENCE OF LONG-TERM ANNEALING ON RESIDUAL STRESS DISTRIBUTION AND QUASI-BRITTLE FAILURE PROPERTIES OF TALCUM REINFORCED PIPE GRADE POLYPROYPLENE
In this study a polypropylene material with talcum reinforcement used for sewer pipes has been subjected to an annealing procedure at 80°C, roughly 60°C above the actual application temperature, in air for a time period of 18 months. As expected, examination of the material showed no significant decrease in mechanical or fracture mechanical properties due to the temperature exposure. However, samples stored at higher temperature showed better resistance against quasi-brittle failure in fatigue tests compared to unconditioned samples. This could mainly be attributed to the decrease of residual stress in the pipe wall. Even though pipes have been annealed for very long times above Tg, residual stress could not be totally relaxed within 18 months.
The determination of residual stresses in plastic pipe and their role in fracture
A method for the determination of residual stresses in plastic pipe by layer removal and subsequent slitting is described. Results are analyzed from deflections measured on a high density polyethylene pipe. The use of these data in the calculation of KI around a semi-elliptical flaw at the pipe wall is then discussed.
Micromechanisms of fatigue-crack advance in PVC
The fatigue-crack propagation characteristics in poly(vinyl chloride) (PVC) are examined in terms of fracture mechanics concepts where the crack growth rate is related to the applied stress intensity factor range. The microscopic details of fatigue crack extension are examined with the aid of light optical, scanning and transmission electron microscopes. The mechanism of crack advance is found to be that of void coalescence through craze material generated in advance of the crack tip. While the craze is shown to grow continuously with cyclic loading, the crack is found to grow discontinuously in several hundred cycle increments.