A method for measuring the creep behavior of pressurized polymer tubing

Polymer components have been proposed for use in domestic solar hot-water heating systems. A polymer heat exchanger is under development for such systems. For heat transfer considerations, the heat exchanger will comprise many thin-walled tubes. The heat exchanger must survive 10 years of service at high pressure (1.55 MPa) and high temperature (82°C). A novel method has been developed for evaluating the long-term performance (creep) of the polymer tubing. Traditional creep testing, performed with dog bone test specimens, cannot be applied because the thin-walled tubing has anisotropic material properties. Consequently, performance must be evaluated directly on the extruded tubing. The method entails wrapping a Constantan wire around the tube specimen to continuously record the hoop strain. For pressure loading of tubing, this method offers significant improvements over strain gage instrumentation. In this paper, the test method is described, an analysis of the strain transfer between the tubing and wire wrap is presented, and strain data for polypropylene tubing measured with a strain gage and wire wrap are compared. The data show that the wire measurement method can be successfully used for the characterization of long-term mechanical behavior of polymer tubes.