Numerical Simulation of Tee Pipe Failure Under Mixed Mode-I/III Loading

This study probes the brittle fracture mechanisms of an S32205 duplex stainless steel tee pipe (DN300×10 mm) subjected to mixed-mode I/III loading under internal pressures spanning 10 to 30 MPa, leveraging advanced finite element analysis to address reliability concerns in high-pressure oil and gas transport systems. Drawing on recent fracture mechanics insights, an energy-driven three-dimensional framework was formulated to forecast crack propagation, emphasizing metallurgical flaws such as micro-cracks and σ-phase precipitates across varied pressure conditions. By partitioning the crack tip into discrete zones, the model calculates energy release rates and stress intensity factors (<i>K<SUB>I</SUB>, K<SUB>III</SUB>, K<sub>eff</sub></i>) for crack morphologies, including trifurcation, symmetric branching, and lateral bifurcation. Findings reveal that Mode I stresses maximize at the crack’s deepest point (90°), with <I>K<SUB>I</SUB></I> and <i>K<sub>eff</sub></i> exhibiting nonlinear escalation as pressure, crack depth-to-length ratio (a/c), and crack half-length (c) increase, portending elevated risks of unstable crack advancement. Mode III stresses, peaking at 60° and 120°, induce localized tearing, with <I>K<SUB>III</SUB></I> displaying marked sensitivity to pressure fluctuations. Pressure-amplified stress concentrations at the branch neck and main pipe abdomen corroborate chevron fracture patterns observed in duplex stainless steel tests, affirming the model’s fidelity. The results elucidate the synergistic degradation of fracture toughness by micro-cracks, σ-phase, and pressure-induced stresses, precipitating brittle failure. Heat treatment at 1030°C with water quenching to mitigate σ-phase and periodic ultrasonic inspections are advocated to bolster tee pipe durability in rigorous oilfield settings.