JOURNAL OF MATERIALS SCIENCE 22 (1987) 3873-3879 Effect of residual stress on crack MDPE pipes propagation in K. CHAOUI, A. CHUDNOVSKY, A. MOET* Department of Civil Engineering, and *Department of Macromo/ecular Science, Case Western Reserve University, Cleveland, Ohio 44106, USA Crack propagation behaviour in single edge notched specimens prepared from medium-density polyethylene (MDPE) pipe is examined under creep condition. The crack grown from an exterior notch (inbound) initiated faster than that grown from an interior notch (outbound). Subsequently, the outbound crack propagated monotonically to ultimate failure. The inbound crack showed anomalous behaviour involving two arrest stages prior to ultimate failure. The pipe is found to possess substantial residual stresses. The energy release rate for each case was calculated taking into account the respective residual stress distribution. The fact that the rates of crack propagation are not a unique function of the energy release rate indicates that the fracture is also influenced by morphological gradients imposed by processing conditions. 1. Introduction Residual stresses in plastic pipes are a consequence of the thermomechanical history imparted by extrusion. Different cooling rates on the inner and outer surfaces of the pipe wall create a gradient of temperature [1] which results in residual stresses and morphological differences [2-4]. Similar effects may also arise from the non-homogeneity of melt flow [5, 6]. Previous investigations by Williams et aI. [7], Brat- nagar and Broutman [8] and in our laboratory [9] show the presence of residual stresses in longitudinal and circumferential directions. Generally, com- pressive axial and circumferential components exist at the outer layers of the pipe wall. However, tensile components dominate towards the bore. It is known that the presence of these stresses is directly related to the material microstructure [6]. Consequently, the resistance of a material to crack propagation and, thus, its service lifetime will be affected by the state of stress and morphology. This paper presents results of an experiment designed to examine the influence of resi- dual stress and associated morphology variances on the crack propagation behaviour in MDPE extruded pipes. 2. Experimental procedure 2.1. Material The pipe material used in this investigation was sup- plied by Plexco Inc. (Franklin Park, Illinois, USA). The pipes were especially extruded from unpigmented (natural) resin specified as PE 2306-IIC (natural). This code designates that the material is extruded medium- density polyethylene with melt flow index in the range 0.4 to 1.5 [10]. Each pipe has a minimum wall thick- ness of 1|.1 mm and an average outside diameter of 113.7ram. 2.2. Specimen preparation Rectangular strips were cut from the pipe wall. The preparation employs an assembly of two circular saws mounted on a Bridgeport milling machine (Fig. I). Each saw is 69.85mm diameter and 1.04ram thick. The specimen thickness was set by appropriate spacers between the saws. Cutting was performed at the lowest speed of the machine (325 r.p.m.). Through this operation, pressurized air was used as a cooling medium to minimize possible material softening due to frictional heat generation. The specimens were cut to the dimensions 152.4mm x 2mm x ll.lmm. Two identical specimens were notched from the outer side of the pipe wall (specimen A) and the other from the inner side (specimen B). A sketch showing both specimens is presented in Fig. 2. Prior to notch- ing, the specimens were placed in liquid nitrogen to ensure minimum damage at the notch-tip. A notching press was employed to control the notch direction as well as the notch depth (1 mm). 2.3. Loading conditions The specimens were subjected to a constant load of 6.75MNm -2 ("~0.30"y). This stress level was chosen based on the results of fatigue crack propagation experiments [11] to ensure slow propagation within a reasonable time. Some of the axial residual stresses were relieved upon preparation causing some curva- ture in the specimens (Fig. 2). This amount was restored by applying the appropriate bending moment to straighten the specimens before final clamping to the creep fixture. Pure Igepal CO-630, an environ- mental stress cracking agent supplied by General Aniline and Film Co. (GAF), was sprayed into the notch-mouth to accelerate initiation. When the cracks were 1 mm ahead of the notch-tip, no Igepal was subsequently used. Crack propagation was observed under transmitted light. The crack tip was followed at x 10 magnification. The entire experiment was con- ducted at ambient conditions. 0022-2461/87 $03.00 + .12 © 1987 Chapman and Hall Ltd. 3873