Experimental Study on the Fatigue Crack Growth and Overload Effect in Medium Density Polyethylene
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JMEPEG https://doi.org/10.1007/s11665-020-05128-7
Experimental Study on the Fatigue Crack Growth and Overload Effect in Medium Density Polyethylene Yang-fan Qian, Feng-peng Yang, Hou-feng He, and He-yang Du (Submitted March 20, 2020; in revised form July 5, 2020; Accepted: 9 September 2020) The fatigue crack growth behavior of medium-density polyethylene (MDPE) under constant amplitude load was experimented on plate specimens. The effect of additional imposed single overload was also researched. An optimized crack front marking method was developed to obtain the actual crack front shape of polymer materials in fatigue crack growth, which was successfully applied to the studied MDPE. The results showed that the retardation of fatigue crack growth evidently appeared after the tensile overload was applied, which was similar to metallic materials. The passivation of the wedge-shaped plastic zone was a primary cause to crack retardation. The theory based on residual stress was introduced to describe the retardation effect, and the plastic zone size of crack tip was calculated by the Dugdale model. The Wheeler model was successfully used to fit the decreased crack growth rate. Keywords
fatigue crack growth, fracture surface, overload, polyethylene, wheeler model
1. Introduction In recent decades, polyethylene materials are widely used to manufacture the internal pressure pipes for water and gas supply with higher requirement on safety and service life. The main failure mode of PE pipe subjected to internal pressure is slow crack growth caused by quasi-brittle failure (Ref 1, 2), so it is important to research on crack growth and service life of PE pipe. Based on the linear elastic fracture mechanics (LEFM) method, a series of experimental studies on the creep crack growth (CCG) of polyethylene under static loading were conducted in the early stage (Ref 3). Recently, the method that characterizes the slow crack growth resistance of PE by its fatigue crack growth (FCG) behavior under cyclic loading has been developed (Ref 3-9), and it has been standardized in ISO 18489 (Ref 8). Parison et al. (Ref 4, 5, 7) studied the influence of different temperature, cycle ratios, and different loading history on the fatigue failure behavior of polyethylene. Frank et al. (Ref 9) successfully characterized the fatigue crack length and obtained its propagation law in the circumferential crack round bar by the compliance method. To obtain the actual crack tip front shape of polymer materials during fatigue crack growth, the beach marking method (Ref 10, 11), where the loading waveform was adjusted for marking, was widely used in metals. While in polymers, few or no crack front marking methods were recorded. In metals, the retardation in fatigue crack propagation caused by single or multiple overloads has been concerned for a Yang-fan Qian, Feng-peng Yang, Hou-feng He, and He-yang Du, State Key Laboratory of Ocean Engineering, School of Naval Architecture, Ocean and Civil Engineering, Shanghai Jiao Tong University, Shanghai 200240, China.
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