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给出了抽油杆预紧力、工作载荷及以螺纹段处应力比的表达式，公式表明：初始预紧力和相对拧入螺距β＿ｔ有关，此值同公接头相对接箍的周向位移成正比。利用动态监测技术、共振频率法有机地结合一般降载勾线技术，可以对裂纹近槛区到失稳扩展阶段的全部过程实行监控．作者利用降载勾线法研究裂纹深度α的变化规律。采用动态监测技术，解决抽油杆杆头螺纹段在近门槛区、高应力比的疲劳裂纹扩展降开勾线问题，研究了耦合法施加预紧力及预紧力的释放规律。
The formulas of the preload, the working load and the stress ratio at the threaded section are given. The formulas show that the initial preload is relative to the screw pitch β_t, which is the same as the circumferential displacement Is proportional. Using the dynamic monitoring technology and the resonance frequency method in combination with the common down-loading hook-line technique, the whole process of the near-sliver cracking zone to the instable expansion stage can be monitored. The authors study the variation of crack depth α by the method of loading and downlining. The dynamic monitoring technology is adopted to solve the problem that the thread length of the rod head of the sucker rod rod descends and opens at the near-sill zone and the high-stress ratio, and the releasing law of the pre-tightening force and the pre-tightening force by the coupling method is studied.