Multi‐Relaxation Test to Characterize PE Pipe Performance

Abstract

A new concept is proposed, which uses results from a multi‐relaxation test to characterize transition of deformation mechanisms in polyethylene (PE) pipes, for plastic deformation from the amorphous phase only to the involvement of the crystalline phase. The former mechanism is believed to lead to brittle fracture, while the latter to ductile fracture. This phenomenon is believed to be related to the transition from ductile to brittle (DB) fracture that has been observed in creep tests of PE pipes by reducing the applied stress below a critical level. This paper presents results from 6 PE pipes of different density and molecular weight distribution. The results suggest that high‐density PE pipes require a higher deformation level for the DB transition than the medium‐density PE pipes. The results also suggest that the trend of change in the critical stress level for the DB transition is close to the trend of change in the hydrostatic design base, but the former takes less than two weeks to complete, while the latter more than 1 year. Therefore, the multi‐relaxation test can be used as an alternative method to characterize PE pipe performance, as a means for preliminary screening or in‐service monitoring of pipe performance.

Evaluating the Residual Stress and Its Effect on the Quasi-Static Stress in Polyethylene Pipes

“…While this phenomenon is observed, it is supposed to explain the scenario reported previously in Ref. [ 16 ], which is detailed below.…”

“…The test results were used to explain the difference in the measured critical quasi-static stresses for the deformation transitions. The latter was reported previously [ 16 ] for the same six PE pipes.…”

“…For semi-crystalline polymers, the crystalline phase is stiffer than the amorphous phase; therefore, the deformation process starts in the amorphous phase and involves the crystalline phase only when the stress level is sufficiently high. A previous study [ 16 ] proposed a multi-relaxation test (Refs. [ 16 , 24 ]) to detect the transition from the amorphous-phase-dominant deformation to the involvement of the crystalline phase in PE, on the basis of the concept that stress relaxation behavior can be used to reflect the material state of PE under tension.…”

“…In this study, the residual hoop stress in six polyethylene (PE) pipes of different material characteristics was determined using the one-slit-ring method. We note that the materials used here are the same as those used in a previous study [ 16 ], as this is a part of an ongoing study to use a short-term test to quantify the long-term behaviors of the PE pipes. First, this study briefly introduced the one-slit-ring method and then summarized the test results.…”

Determining Deformation Transition in Polyethylene under Tensile Loading

“…The main difference between the MR test and that used by Hong et al is that the former uses a single specimen for all stress relaxation stages, while the latter uses multiple specimens, one for each stress relaxation stage. The latter approach was also used in our previous work on PE pipe specimens [20], from which critical deformation for the mechanism transition was found to be consistent with that using MR test on a single specimen [21]. However, the use of a single specimen has the advantage of avoiding inconsistency in the stress decay during stress relaxation, which can be caused by, for example, dimensional inconsistency among specimens.…”