The heavier drill rod should bear the periodic stress of stretching, compression, torsion and bending. Stretching and bending are the most dangerous stresses. At present, the main reason for the fatigue failure of the drill rod is the periodic stress caused by the rotation of the drill rod in the curved hole. In a curved well hole, even if the drill collar has enough thickness, it may still have fatigue damage, and the location of the damage is not necessarily. When the drill rod is bent, each rotation of the drill rod is rotated, and its corresponding position will produce periodic stress due to repeated tension and compression.

The drill rods near the drill collar are most likely to be bent because the drill collar has a large rigidity that resists bending, and the bending will occur over the drill collar. At the same time, the maximum stress on the drill rod occurs at the end of the drill rod thickening part, about 50 centimeters from the joint. As mentioned above, the joint can not be bent, bending can only happen on the drill pipe body with a thin wall. In the changing position of the section, it acts like a vise fixing, making it a fulcrum of bending force. If the drill rod is flexed evenly on its length, the stress on the drill rod will become lower, and the number of stress cycles of the fatigue damage can be increased.

The values of fatigue failure are: drill pipe bending load, drill string tension load, drill hole bending severity, drill pipe bending stress at each drill point, drill pipe size and steel performance. Because the size of the drill rod's tension is the key to fatigue failure, in the deep well, the borehole bending in the upper part is often a factor of fatigue failure.

It should be pointed out that as long as the drill pipe passes through the bending point of the drill hole, even if the pulling force is eliminated, accumulation fatigue damage still exists. Therefore, if repeated several times through the bending point of drill hole, it may cause fatigue failure of drill pipe. It is also important to see that a drill pipe may have been damaged even if there is no accident in a well. So. When it is used in the same well or other well, though it is connected to the top of drill string, even if the bending stress acting on the drill pipe is in the rated value, it may also make it unbearable, resulting in an accident.

In addition, there are some other reasons that cause the bending stress of pure fatigue damage. For example, any curved drill pipe is often a potential factor in fatigue failure. The curved square drill rod will bend the first drill rod below the turntable. Once the stress is enough, it will cause fatigue damage. The misalignment of the car will also cause fatigue damage to the drill rod because it causes bending stress on the square drill rod and drill rod.