Assessing Residual Stresses in PE-RT Pipes and Annealing Temperature Sensitivity Analysis

Abstract

Abstract Pipe made from polyethylene with raised temperature (PERT) service capability offers several advantages in terms of material performance, and has broad application in the industry. However, residual stresses are typically present in PE-RT pipe after fabrication, which may affect pipe strength under service conditions. Research on this phenomenon in PE-RT pipe is limited, and hence, the present study investigated the residual stresses and means for mitigating them through annealing at elevated temperatures. Residual tensile and compressive stresses are formed on the inner and outer portions of extruded pipes, respectively, due to the rapid cooling that is typically applied on the outside of the pipes after the extrusion process. The objective of the present work was to assess experimentally residual stresses in PE-RT pipes. In addition, using the experimental methods, a sensitivity study on the annealing temperature for mitigating residual stresses was performed. The experimental procedures were adopted from the ASTM E1928-13 standard. In compliance with this standard, the PE-RT pipes under investigation can be considered as thin-walled tubes for analysis purposes. Moreover, the creep behavior of plastics was incorporated in the final calculation of residual stress values. Two types of slitting methods were applied to assess the residual stresses: (i) To determine the residual hoop stress, a 45-degree section was cut from ring samples, and then the change in ring diameter was observed; (ii) for the longitudinal residual stress, a tongue-like section was cut lengthwise into pipe samples, and the deformation of these sections was observed. For all cutting purposes, coolant was used to avoid any heating effects. The sample lengths varied from 0.2 to 3.0 times the outer pipe diameter. In this manner, effects of specimen length on the test results were ascertained. Experiments conducted for non-annealed PE-RT pipe, as well as pipe annealed at 90 degree Celsius, revealed substantial residual stresses. Using the developed procedures, the study seeks to identify the annealing temperature that yields the lowest possible residual stresses in the PE-RT pipes. Present work includes a discussion on whether the ratio of ideal annealing temperature to melting point can be extrapolated as a key parameter from PE-RT to other thermoplastic polymer materials.