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

## TL;DR

This study shows that pre-strain from dents in X80 pipelines significantly reduces their fatigue life and suggests ways to improve pipeline safety under pressure.

## Contribution

The study introduces a parametric P-S-N model incorporating pre-strain and reliability for predicting fatigue life in dented pipelines.

## Key findings

- Pre-strain significantly reduces fatigue life of X80 steel.
- Fatigue cracks initiate at the dent periphery with critical planes perpendicular to the circumferential direction.
- Maximum operating pressure is the dominant factor affecting fatigue life in dented pipelines.

## Abstract

What are the main findings?
Higher pre-strain significantly reduces fatigue life of X80 steel.Pre-strain increases local strain concentration at the shoulder region.Crack initiation is accelerated under higher Δp loading.

Higher pre-strain significantly reduces fatigue life of X80 steel.

Pre-strain increases local strain concentration at the shoulder region.

Crack initiation is accelerated under higher Δp loading.

What are the implications of the main findings?
Pre-strain history must be considered in fatigue assessment.Residual deformation reduces resistance to cyclic loading.Findings support safer pipeline design under internal pressure.

Pre-strain history must be considered in fatigue assessment.

Residual deformation reduces resistance to cyclic loading.

Findings support safer pipeline design under internal pressure.

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 P-S-N 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.