Effect of Residual Plastic Strain on the Fatigue Failure Mechanism and Service Life Prediction of Dented X80 Pipelines

In the field of oil and gas transportation, X80 pipelines are susceptible to localized plastic deformation caused by mechanical impact or geological activity. This leads to the formation of dents and the introduction of pre-strain, thereby affecting the structural integrity and fatigue life. This study systematically investigates the influence mechanism of pre-strain on the high-cycle fatigue performance of dented regions in X80 steel. Fatigue tests conducted across pre-strain levels of 1%, 2%, and 3% revealed that the induced plastic strain significantly degrades fatigue performance. Under constant stress amplitude, fatigue life decreases markedly with increasing pre-strain, a trend driven by the accumulation of micro-damage. Furthermore, a parametric <i>P-S-N</i> curve model that incorporates both pre-plastic strain and reliability was developed, providing a basis for quantitatively assessing the impact of pre-strain. By combining finite element analysis with the Smith-Watson-Topper (SWT) critical plane method, it was predicted that fatigue cracks in unconstrained dent primarily initiate at the dent periphery, with the critical plane orientation perpendicular to the circumferential direction, which aligns well with field observations. Parametric analysis indicates that the maximum operating pressure is the dominant factor affecting the fatigue life of the dented pipelines. This research elucidates the material-level fatigue failure characteristics of dented X80 pipelines and provides theoretical insights for life prediction and engineering protection.