Nonlinear Time-Dependent Mechanical Behavior of Medium-Density Polyethylene Pipe Material

Medium-density polyethylene (MDPE) pipes are extensively used for gas distribution systems in Canada and worldwide. MDPE pipe material possesses time-dependent mechanical properties that govern the performance of the pipes in service. In this research, an extensive laboratory investigation is carried out to investigate the time-dependent behavior of MDPE pipe material. Uniaxial tensile tests are conducted with samples (coupons) cut from the wall of a 60-mm diameter MDPE pipe. A tensile test with a sample of the full cross section of the pipe is also conducted to investigate the influence of sample type on the test results. The test program includes uniaxial testing at various strain rates ranging from ${10}^{-6}/\mathrm{s}$ to ${10}^{-2}/\mathrm{s}$ to capture the effects of loading rates, creep testing, and relaxation testing. The program revealed that the stress-strain responses of MDPE pipe material are highly nonlinear and strain rate-dependent. However, the strain rate effect is negligible below ${10}^{-6}/\mathrm{s}$, which is termed herein as the “reference strain rate.” A numerical technique for modeling time-dependent behavior is proposed using the features available in a commercially available finite element software, Abaqus. In this technique, strain rate-dependent stress-strain models are used to simulate loading and unloading responses, and a power-law type creep-law model is used to simulate the creep/relaxation behavior. The proposed modeling approach successfully simulated the test results.