Numerical Simulation Of Quasi-brittle Fracture Behavior Of Polyethylene Pipe Material

As the innovation of chemical engineering technique,the property of Polyethylene(PE)pipe material consistently increased and was adapted to the more critical work environment,which promoted the widespread application of PE pressure pipe in water supply and gas transportation engineering.PE pressure pipes majorly are subjected to intermediate level of stresses,their main failure mode is quasi-brittle failure and responding failure mechanism is the slow crack growth(SCG).The research on quasi-brittle fracture behavior of PE pipe material is very important for the safe operation of the pipeline.At present,there are few reliable models to predict the delayed failure behavior and the creep lifetime of polymer pressure pipes under various stress levels.In particular,the research on theory and simulation of discontinuous SCG is still in its infancy.Therefore,the research on simulation modeling of discontinuous SCG has important application value and scientific meaning.In this paper,a modified crack-tip process zone model was tried to be established based on current research,which was used to simulate the discontinuous SCG process of PE pipe.The research contents and conclusions are listed as follows:(1)A crack-tip process zone model considering crazing damage and simulating the discontinuous SCG process of PE pipe material was established.To achieve this end,the crazing damage caused by stress concentration at crack tip of PE material is taken into account,the stress on the bulk/craze interface layer is supposed to be uniformly distributed due to the micro necking mechanism of the layer,and a linear crack growth kinetics is used.In order to describe the delay characteristics of the fracture process of PE materials,the fracture surface energy is considered as an exponential decay function of time,which is attributed to the creep damage of the craze fibrils.And the characteristic time,which was used to describe the craze fibril degradation speed,is considered as a linear function of the crack tip stress intension factor.In addition,the model was equivalent to the superposition of two subsystems,which respectively described the process of craze zone and crack propagation.And based on Green function,the governing equations of crack-tip process zone model used for SCG process of single edge notched specimen were deduced.(2)Using the numerical iteration method,the governing equations of the crack-tip process zone model was resolved.In this paper,the numerical algorithm of the iteration method was introduced in details.And in the iteration cycles,the method of determining the time of craze fibril degradation process was elaborated.As a simulation example,the numerical simulation for describing the SCG process of Pennsylvania Edge Notch(PENT)PE specimen based on the crack-tip process zone model is programmed by self-programming.According to the comparison between experimental and simulated results,they are in good agreement,which demonstrated that the modified crack-tip process zone model could describe the discontinuous SCG behavior of PE pipe material.Besides,it also verifies the hypothesis of the exponential function of fracture surface energy and time,as well as the linear relationship between characteristic time and SIF.(3)The influence on the numerical simulation of SCG process through changing these parameters of the crack-tip process zone was analyzed.The results showed that the simulation results basically are in agreement with the power function of Paris-Erdogan,which increasingly reflects the capacity of the crack-tip process zone model to describe SCG process.