Characterization of the fatigue crack behavior of pipe grade polyethylene using circular notched specimens

Several standard tests have been widely used for evaluating the pipe grade polyethylene with respect to toughness and lifetime. However, some of these tests turn to be not adequate for new generation of high performance pipe grade polyethylene: the testing takes extremely long time, which makes it impractical. Recently, it has been proposed to use the circular notched specimen (CNS) for studying the crack growth resistance of pipe grade polyethylene. In CNS the stress intensity factor (SIF) increases with crack size much faster than in commonly used test specimens like compact tension (CT) specimen for instance. Thus, CNS may be a good candidate for an accelerated testing as long as it allows reproducing the mechanisms of slow crack growth (SCG) in field conditions. The objectives of the present studies are twofold: (1) to compare pipe grade polyethylene materials with respect to fracture resistance using CNS in order to complete the program in a relatively short time; and (2) to evaluate the applicability of CNS for studies of slow crack growth kinetics. Fatigue crack growth resistance of four PE resins is evaluated in this work. The fracture surfaces after CNS failure, are analyzed by means of optical and scanning electron microscopy (OM and SEM) in order to determine the mechanism of SCG. The effect of load level, stress ratio (R) and notch depth is also studied using CNS. In addition, some technical issues associated with CNS testing are discussed.