Limit Load Determination and Material Characterization of Cracked Polyethylene Miter Pipe Bends

Abstract

The quality of Natural Gas Piping Systems, NGPS, must be ensured against manufacturing defects. The main purpose of the present paper is to investigate the effect of loading mode and load angle (30°,45°, and 60°) on the limit load of miter pipe bends, MPB, under different crack depths a/W = 0 to 0.4 at a crosshead speed 500 mm/min. The geometry of cracked and un-cracked multi miter pipe bends are: pipe bend angle, α = 90°, pipe bend factor, h = 0.844, standard dimension ratio, SDR = 11, and three junctions, m = 3. The material of the investigated pipe is a high-density polyethylene, HDPE, which is commonly used in natural gas piping systems. The welds at the miter pipe junction are produced by butt-fusion welding. For all loading modes the limit load is obtained by the tangent intersection method, TI, from the load deflection curves produced by the specially designed and constructed testing machine at the laboratory. Tensile tests are conducted on specimens longitudinally extruded from the pipe with thickness, T = 10, 30 mm, at different crosshead speeds (5–500 mm/min), and different gauge lengths (G = 20, 25, and 50 mm) to determine the mechanical properties of welded and un-welded specimens. The fracture toughness is determined on the basis of elastic plastic fracture mechanics, EPFM. Curved three-point bend specimens, CTPB, are used. All specimens are provided with artificially pre-crack at the crack tip, a/W = 0.5. The effect of specimen thickness variation (B = 10, 15, 22.5, 30, 37.5, and 45mm) for welded and un-welded specimens is studied at room temperature (Ta = 23°C) and at different crosshead speeds, VC.H, ranging from 5 to 500 mm/min. The study reveals that increasing the crack depth leads to a decrease in the stiffness and limit load of MPB for both in-plane, and out-of-plane bending moment. In case of combined load (out-of-plane and in-plane opening; mode) higher load angles lead to an increase in the limit load. The highest limit load value occurs at a loading angle, φ = 60°. In case of combined load (out-of-plane and in-plane closing; mode) the limit load decreases with increasing load angles. On the other hand, higher limit load values are proved at a load angle, φ = 30°. For combined load opening case; higher values of limit load are obtained. The crosshead speed has a significant effect on the mechanical behavior of both welded and un-welded specimens. The fracture toughness, JIC, is greater for un-welded than welded specimen.