Plastic Pipes Conference Association # 2006 Washington DC
Krishnaswamy
A comprehensive analysis of ductile and brittle failures from creep rupture testing of a wide spectrum of HDPE pipes (PE-80 and PE-100 pressure rating) was conducted; the HDPE resins studied included those polymerized using chrome-oxide, Ziegler-Natta and metallocene catalyst technology. The analysis indicates that the ductile failure of such pipes is primarily driven by the yield stress of the polymer (or pipe). Examination of ductile failure data at multiple temperatures indicates a systematic improvement in performance with increasing temperature. It is proposed that testing at higher (above-ambient) temperatures leads to progressive relaxation of the residual stresses in the pipe; this causes the pipe to perform better as residual stresses are known to help accelerate the fracture process. Finally, our investigation indicates no correlation, whatsoever, between brittle failures in pressurized pipes and the PENT (Pennsylvania EdgeNotch Tensile test; ASTM F1473) failure times; this is explained in terms of the fundamental mechanistic differences in the two tests. Therefore, one has to be extremely cautious in interpreting the true value of the PENT test when developing polymers and pipes for highperformance pressure pipe applications.
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