Fatigue Life Prediction of Steel Pipelines Based on X-ray Diffraction Analyses

Pipelines are the most used and safest equipment to transport oil and its liquid or gaseous derivatives. Steel pipes are usually manufactured of API 5L X60, X65, X70 or X80 grade steels. x-ray diffraction technique was used to evaluate the microstructural behavior of API X65 steel samples through the analysis of the full width at half maximum (FWHM) values of the diffraction peak during the fatigue damage process. Based on the obtained results, it was possible to identify three stages of fatigue damage. Stage I is associated with crack nucleation, and a deeper understanding of this stage is one of the most relevant goals of this study. Stage II of microcracking is the longest stage, representing most of the fatigue life of the material. Stage III is related to macrocracking, when the material is already structurally harmed and driven to abrupt failure, leaving the operator with little time to act and prevent it. Another important result observed was that the major changes in the FWHM values occurred around 21% of the fatigue life of the material. This point was taken as a critical point and used as a reference to calculate the number of cycles up to the failure of the material. The results obtained show that the x-ray technique can be used to study the beginning of the fatigue life of metallic material with reliably once the FWHM results were compared with earlier indentation results that showed the same trend.