Evaluation of fatigue life prediction models and investigation of fracture mechanisms of mild steel

Fatigue is one of the dilemmas of coiled tubing (CT) in oil and gas engineering. This study investigated the fatigue behavior of CT employing mild steel, which is commonly utilized in the CT110. Two fatigue life prediction methods based on cyclic strain-life principles, the Manson–Coffin and Landgraf models, as well as the Smith and Ellyin models based on energy-life principles, were evaluated. The Ellyin model demonstrated superior accuracy in predicting the low-cycle fatigue life of CT110, displaying dispersion within the 1.5 scatter band. This research integrated mechanical property analysis and microscopic deformation assessments for CT110. Fatigue mechanics studies show cyclic softening behavior in the CT110 of mild steel. The results of the microscopic analysis show that the fatigue fracture of CT110 has typical fatigue striations and equiaxial dimples as a plastic fracture. Combined with TEM and EBSD analysis, fatigue deformation increases grain deformation and dislocation density. In addition, the increase in dislocation density results from the combination of cycles and strain amplitude. The results of in situ EBSD further support this view. At 8% strain amplitude, the dislocation density increases to 0.196B when the cycle increases from 0 number to 10 number. Furthermore, the ferrite grain slip becomes accessible, but the 〈111〉 // Z0 direction is still the main texture. These are fundamental reasons for the cyclic softening of the CT110 mild steel, supported by a decrease in stress amplitude.