A Study on the Evolution Patterns and Mechanisms of Crack Stress Intensity Factors Under Gas-Liquid Two-Phase Flow in Pipelines of Different Configurations

Characterizing the stress intensity factor (SIF) at the crack tip is difficult when pipe vibrations are induced by two-phase gas-liquid flow. To solve the issue, a three-dimensional dynamic SIF calculation framework was developed. The framework integrates fluid-structure interaction (FSI) with the extended finite element method (XFEM). Mesh refinement in the crack area and Mortar non-matching interface coupling technology were used to achieve transient fluid-structure interaction load transfer. The dynamic SIF was derived using the interaction integral method. Using the proposed framework, the influence of various parameters—including pipe bending angle (straight pipe, 30°, 60°, 90° bends), mixed-phase flow velocity, and gas-phase volume fraction—on the time-varying response of the dynamic SIF was investigated. The results indicate that high-frequency SIF oscillations are caused by transient pressure pulsations. As the bend angle increases, the relative volatility of SIF types rises monotonically, with the most pronounced increase occurring between 30° and 60°, and the maximum fracture volatility sensitivity is seen at 90°. Increasing flow velocity leads to higher average amplitude and frequency of SIF response, thus increasing fatigue propagation risk. The gas-phase volume fraction shows a non-monotonic effect on SIF amplitude, first increasing and then decreasing, with peak values between 0.15 and 0.2; the cause is attributed to transitions in the flow regime. These findings provide numerical methods and engineering design references for dynamic fracture assessment and fatigue life prediction in multiphase flow systems.