RAPID CRACK-PROPAGATION IN SMALL-DIAMETER THERMOPLASTIC PIPE

The S4 (small scale steady state) test, measuring resistance to fast-running fracture, is to become a specifying standard for PE gas pipe. To extend understanding of the method, and to assess its broader usefulness in polymer testing. S4 tests were carried out on commercial 50 mm diameter pipe of 3-6 mm nominal thickness to compare MDPE with ten other polymers used in process pipework. Results were normalised, on a hoop stress basis, to a mean radius to thickness ratio of 5 (SDR 11), and must also be factored up to estimate any service critical pressure. On this basis, MDPE, with a 0 degrees C S4 critical pressure of 2.8 bar, was matched by PVDF and slightly bettered by PB. ABS performed very well, exceeding 20 bar. PP homopolymer, PA6 and PTFE also out-performed MDPE, but PC, modified PVC and PP random copolymer failed to survive 2 bar. The critical pressure of POM was too low to measure.