| Since Elbert discovered crack closure effect, many researchers have studied the effect caused by plastic deformation. Guo and Huang brought forward two different models of crack closure based on the models of Newman and Budiansky-Hutchinson, in which the three dimensional effect of crack propagation was considerated. And their positive results had been applied at aerospace titanium alloys and some steel used in shipbuilding. This paper, through the experiment in which 20CrMo steel for oil drawing rods is the experiment material, gets the crack growth dates under different stress ratioes. Then analyze the dates to prove the approach of models of crack closure to be effective for 20CrMo steel. And depended on feasibility verification, this paper improved the method of crack growth rate. Analyzing the bearing force of the sucker rod and the axial distribution of load and stress, the root segment of stress groove was estimated to be the most dangerous place and the precrack was placed in it. With the above technology and methods, based on fracture mechanics theory, the basal line independent to stress ratio of crack growth rate and effective SIF range was drawed. According to the basal line, the relationship between crack growth rate and effective SIF range was established. We studied on the crack growth rate of thread segment under different pretightening forces to determine the correspondent effective SIF range. Then the correspondent SIF range and regular factor were got due to closed crack model. Comparing the regular factors of SIF adapted to aspect ratio, we can determine the optimum value of pretightening force.
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