Failure Analysis of Ring Hoop Tension Test (RHTT) Specimen Under Different Loading Conditions

The thermo-mechanical history during manufacturing of plastic pipes affects their resulting material mechanical behavior. Therefore, it is necessary to demonstrate a study of the mechanical behavior of ring tensile test specimens cut from pipe to obtain proper design data during evaluations of the final product from the polymeric pipe material. Ring hoop tension test (RHTT) is one of the most important methods that can be used to measure transverse tensile properties accurately for pressure pipes. Two types of tensile ring specimens are tested; single ring hoop tensile test specimen with one configuration of Dumb-bell-shaped (DBS) specimen in the transverse direction of the pipe ring specimen and double ring hoop tensile test with two configurations of DBS cut in the collinear direction from the pipe ring specimen. RHTT specimens are cut from the pipe in circumferential (transverse) directions with similar dimensions. The material of the investigated pipe is high-density polyethylene (HDPE), which is commonly used in natural gas piping systems. The pipe has internal rated pressure P_i = 1.6 MPa, standard dimension ratio, SDR = 11 and external diameter, D_o = 90±0.5mm. All the dimensions RHTT specimens are taken according to ASTM D 2290-12 and ASTM D 638-10 standards. The ring hoop tensile tests are conducted on specimens cut out from the pipe with thickness 9.5±0.2 mm at different crosshead speeds (V_C.H = 10–1000 mm/min), and loading angle, θ equal 0° at ambient environmental temperature, T_a = 20 °C to investigate the mechanical properties of RHTT specimens. The results are compared with those obtained for DBS specimens taken along the axial pipe direction [1]. This shows the effect of DBS specimen orientation (longitudinal and circumferential) on the mechanical properties of HDPE pipe material at different crosshead speeds. The tensile testing fixture is designed specially on specified design criteria in order to test the RHTT specimen in the transverse direction. The main purpose of the design of test fixture is that it prevents the bending effect and stress concentration due to rotation of half disks during the tensile test. In order to avoid this rotation, the half disks are fixed, and their sharp ends are smoothed (rounded). The present experimental work reveals that the crosshead speeds, specimen orientation of DBS and configuration of DBS for RHTT specimens have a significant effect on the resulting mechanical behavior of HDPE pipe material.