Numerical Simulation Of Short Fatigue Crack Behavior At High Temperature

Since the 20th century,with the rapid development of industry,more and more workpieces need to work in high temperature and pressure environment and bear alternating loads.Fatigue damage accidents occur from time to time.When the workpiece enters the stage of long crack,the crack growth rate is accelerating continuously,and fatigue failure accidents often occur abruptly.It is found that about 4/5 of the whole fatigue life of the workpiece is in the short crack stage,and there is strong randomness and interaction in the process of initiation and subsequent evolution.Generally,the research in the field of long crack is not applicable.It is necessary to develop new research methods for micro-short crack behavior.The general law of crack initiation and propagation is obtained,the service life of the workpiece is predicted,and the reliability of the workpiece in service is improved.In this paper,20~#steel is selected as experimental material to carry out high temperature and low cycle fatigue experiments.The group evolution behavior images of short fatigue cracks under different strain amplitudes are obtained.The related experimental data are sorted out.According to the characteristics of crack initiation,propagation and fit interference process,the behavior model of short fatigue cracks is established.Considering the randomness of crack initiation,the random characteristics of crack initiation are considered.Initiation,propagation and interference between cracks are analyzed and numerically simulated.The research contents and relevant conclusions are as follows:(1)Low-cycle fatigue tests with different strain amplitudes were carried out at high temperatures to observe the behavior of crack initiation to failure,including prophase propagation and late fit interference.It is concluded that crack initiation occurs mainly in two parts,one is in ferrite grains,and the sliding band enlarges and deepens under cyclic loading,resulting in crack initiation.The other part is the unstable grain boundary between grains,in which ferrite and pearlite are the most prone to crack due to the difference of mechanical properties.The early stage of crack propagation is obviously hindered by pearlite,and the cracks do not germinate in the middle and late stages.The group evolution behavior between cracks is the main form of fatigue damage.The crack coalescence behavior will make the crack length increase suddenly and the growth rate accelerate.In addition,other cracks around larger cracks will be restrained by interference behavior.(2)Based on Monte Carlo analysis,the micro-structure of 20#steel specimens is constructed by using Tyson polygon to distinguish pearlite from ferrite,which makes the pearlite grains distribute in strips.Considering the difference of mechanical properties between pearlite and ferrite,the concept of structural energy is introduced,and the initiation and propagation of cracks are carried out in the form of energy numerical values.It is assumed that the behavior of crack fit is affected by the distance between the crack endpoints.The maximum distance of fit is related to the length of crack.Crack crack will produce a relaxation zone around it,and other crack growth in the range will be inhibited.(3)The crack behavior was simulated by MATLAB software.The crack initiation,propagation,interference and fit behavior of short fatigue crack at high temperature were reproduced.Compared with the experimental results,the crack behavior parameters were well fitted.