X-Ray Diffraction Study of Microstructural Changes During Fatigue Damage in Steel Pipelines

Steel pipes used for oil and gas exploitation undergo the action of cyclic loads that can cause their failure by fatigue. A consistent evaluation of the fatigue behavior should take into account the micromechanisms of fatigue damage initiation, which precede macroscopic cracking and macrocrack propagation. In this work, microstructural changes in terms of variations in microdeformations and residual stresses (macrostresses) are evaluated by X-ray diffraction in real time during alternating bending fatigue tests performed on samples taken from an API 5L X60 grade steel pipe. Three stages of microstructural changes are detected. It is found that their amplitudes and durations are proportional to the level of alternating stress applied. Changes in density and distribution of dislocations are observed by transmission electron microscopy combined with the technique of focused ion beam. To understand the role of the initial dislocation structure, fatigue tests on annealed samples are performed under the same test conditions. Again, three stages of changes are observed, but with an increase in microdeformations during the first stage instead of a decrease as found for as-machined samples, suggesting the influence of the initial state of the dislocation network. The results obtained are very encouraging for the consideration of microstructural evolutions in the development of an indicator of fatigue damage initiation in steel pipes.