Issue 43

F. Majid et alii, Frattura ed Integrità Strutturale, 43 (2018) 97-105; DOI: 10.3221/IGF-ESIS.43.07 98 Several researchers have studied the behavior of PPR pipes. Zgoul et al [3] evaluated the convenience of two types of thermoplastic pipes to transport industrial and domestic hot water by a comparative study of Polypropylene random copolymer (PPR) and Cross-Linked Polyethylene (PEX) pipes in terms of melting temperature and mechanical strength. Authors used differential scanning calorimetry (DSC), uniaxial tensile tests and hydrostatic pressure. Poduska et al were interested in the determination of residual stresses, and the fusion temperature of PPR pipes [4]. Geertz et al have put PPR pipes under hydrostatic pressure for a long time, which the purpose was to analyze an antioxidant diffusion named Irganox1010 (phenolic stabilizer) using infrared microscopy technique [5]. Litvinov and Soliman reported the effect of storage under hydrostatic pressure and high temperature on morphology, molecular mobility and behavior at the fracture of PPR tubes using the differential analyses of X-Ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC) [6]. In the framework of damage mechanics, researchers developed theoretical expressions of damage like Miner, Starkey and Shanly. Bui-Quoc has developed the unified theory of metal fatigue and suggested expressions of damage that allows a quantification of the damage according to the initial and present fatigue limits [7]. In order to use this model for polymers’ characterize, through static tests, needs a judicious adaptation taking into account different parameters. However, Makadir et al [8] used the normalized damage to characterize an Acrylonitrile-butadiene styrene (ABS) polymer plate under uniaxial loading. To highlight the influence of the notch on the behavior of Polyvinyl chloride (PVC) pipes, Arid et al [9] used normalized damage formulation on notched plat specimens. In this paper, we have used the normalized damage to define the different stages of the damage development within defected PPR pipes subjected to burst pressure tests. Furthermore, the calculation of the reliability allows the determination of the critical depth (βc) of a defect modeled as an external longitudinal groove on the PPR pipe. M ETHOD AND MATERIAL he chosen pipes for the study are manufactured by extrusion from PPR material. They were prepared according to the ASTM D1599 standard [10], which requires samples of 450 mm length. The Fig. 1 shows dimensions of the used pipes in this work. In order to study the notch effect on the strength of the PPR pipes under pressure, eight pipes were notched using a universal milling machine with grooves of 6 mm width, 100 mm length and a depth from 2.42 mm to 14.5 mm. The life fraction β is defined as the ratio between the notch depth (a) and the total thickness of the pipe (t). Fig. 2 and Tab. 1 show the notched samples dimensions. The experimental test bench is constituted of a tank filled with water and a hydraulic pump for the pressurization. The pump is equipped with a screen to display the pressure inside the pipe. In order to prevent any leakage, caps were fixed at the ends of pipes and strongly tightened through bolts; those caps are connected to a hydraulic pump through a pressurizing hose. After fixing the caps, pipes were immersed in the tank filled with water. Then, the hydraulic pump applies a gradient of pressure until their burst. The goal of this test is the determination of the residual ultimate pressures according to each life fraction β. Figure 1 : Specimen dimensions. T Diameter: 90 mm Length: 450 mm Thickness: 15 mm