Elastic-plastic Fracture Analysis of External Thread of Drive Shaft Shell of a Positive Displacement Motor
Keywords:positive displacement motor (PDM); threaded joint; surface crack; elastic-plastic fracture; helix angle
For a positive displacement motor (PDM), the threaded joint connecting drive shaft shell (DSS) and universal shaft shell that is close to the bit is inclined to fracture. In this paper, elastic-plastic fracture performance of the threaded connection is simulated under both make-up torque and bending moment. Firstly, an FE model, which includes a cracked external thread of the DSS and an engaging internal thread of the universal shaft shell, is established and validated. Secondly, influences of both plastic deformation and the helix angle on fracture properties of the cracked thread are evaluated quantitatively. Meanwhile interactions between two cracks are also discussed. Finally, under the two kinds of loading conditions, i.e. loaded by pre-load only and loaded by both pre-load and bending moment, explicit relationships between characteristic J-integrals and the crack depth are obtained for the DSS.