| The sucker rods are important mechanic parts of beam pumping unit. The fatigue fracture of sucker rod is caused by the cyclic load, corrosion by well liquid, raising and dropping of well construction and using period during its service life, which leads to increasing times of well construction, further has severely influenced on normal production. Quantity-based evaluation of fatigue threshold values of different type defects. In order to establish the reasonable criterion for judging the rod to be unqualified, to guide the reasonable use and to control fracture accident of the rod, it is a problem to be resolved with urgency to evaluate quantitatively the fatigue threshold values of different type defects. Base on the research of load and cyclic stress properties and theoretical and room experimental study, an approach is established to evaluate quantitatively the fatigue threshold values of different type defects, using which the fatigue threshold values of using period and different type defects of the rod are established in the paper as well.The simulation model of load and cyclic stress properties of the rod is established, and relevant software is developed. In the simulation the influence of some parameters on maximum and minimum polished rod load and using factor of stress on the top of the rod are analyzed. The parameters include 2 well parameters, including water cut and dynamic fluid level, and 3 swabbing parameters including stroke, frequency and pump diameter.Based upon the fundamental theory of material mechanics, calculation of the fatigue strength of the rod is carried out, and the fatigue threshold values of the rod without defects are studied in room experiments.Based upon the fundamental theory of fracture mechanics, it is studied including the stress intensity factor, fracture criterion, crack growth threshold, propagation rules of fatigue crack and fatigue threshold values of the rod surfacecrack. The mathematic expression is established that concern with the residual life of rod with original crack size greater than threshold size. And relevant software is developed to calculate the critical crack size, threshold crack size and residual life of the rod. The room experiments for fatigue threshold values of rod surface crack are carried out to determine the critical crack depth.An approach calculating fatigue strength is established to determine quantitatively the fatigue threshold values of different type defects, in which maximum equivalent stress caused by the defect is got by the FEA software of ANSYS and the stress concentration factor is calculated, the critical size of fatigue source is determined based on theory of fatigue strength. |
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