Effect of Inhomogeneous Mechanical Properties on Local Stress and Crack Growth Path in Friction-Welded Drill Pipe Joints

Abstract Under the complex downhole conditions of torque and pressure, the friction-welded joint of a drill pipe becomes the critical region for fracture failure due to inherent stress concentration and welding residual stress. Traditional homogeneous material models fail to accurately predict the crack propagation behavior of friction-welded drill pipe joints. This study investigates the crack-tip stress–strain fields and propagation paths at various locations of a friction-welded drill pipe joint through a computational model that accounts for its inherent mechanical inhomogeneity across the tool joint, pipe body, and heat-affected zone. First, the mechanical properties of the friction-welded drill pipe joint were characterized in detail using a microhardness tester. Then, a numerical model with continuously varying material properties along spatial positions was developed using the USDFLD user subroutine, ensuring the continuous transition of mechanical properties across the joint region. Finally, the extended finite element method was employed to analyze the influence of inhomogeneous mechanical properties on the crack propagation paths in the tool joint, pipe body, and heat-affected zone. The results show that the inhomogeneous mechanical properties of the friction-welded joint cause the crack propagation path to deflect toward the region with lower material strength, and the total crack propagation length is affected by the mechanical strength near the crack tip.