Fracture failure analysis and research on thread joint of drive shaft shell of positive displacement motor

•The specimens were investigated by mechanical property measurements.•Carried out the three-dimension simulation of the shell thread.•Completed the make-up torque testing of the shell thread. Microstructures and mechanical properties of the fractured shell are investigated by scanning electron microscopy and tensile tests in this paper. Tensile test shows that yield strength of the drive shaft shell are low. It can be seen that the fatigue arc lines with small spacing of striations in the fracture surface. It is found that there are deformation characteristics in the local dimple and the quasi cleavage characteristics of specimen. The finite element analysis shows that the stress on the first four teeth of the external thread will gradually increase greatly and the rupture risk will increase with the increase of the torque. When the torque reaches the upper limit, the maximum stress on the external thread is close to the material yield limit. In the torque test, the torque of the thread connection part exceeds the standard of torque value, resulting in too much preload on the thread connection part. When the drive shaft shell bears a large combined load in the wellbore, it will cause the crack propagates from inside to outside gradually, and finally lead to the fracture of the shell.