ULCF assessment of X52 piping steel by means of cyclic bending tests

Pipelines and piping components may experience large cyclic deformations during a reduced number of cycles (Ni = 1-100 cycles), when subjected to extreme cyclic loading events (e.g. hurricanes, support settlements, earthquakes). In accordance with these loading scenarios, a lateral movement can be applied to the pipeline inducing bending stresses that gradually promotes strain localization, due to progressive plastic instabilities (buckling), damage evolution and final failure. This work aims at characterizing ultra-low cycle fatigue (ULCF) behaviour of the X52 piping steel under bending and local buckling state. An experimental program was carried out to derive ULCF data for smooth, notched and flat-grooved specimen geometries under cyclic bending. Furthermore, the small-scale tests were simulated in ABAQUS® with the objective of computing the parameters governing the fatigue damage models. The classical Coffin-Manson strain-life relation commonly used in the low cycle fatigue (LCF) regime is proposed to model the fatigue lives. In addition, the Xue model, particularly dependent of the monotonic fracture strain was also used for the prediction of the number of cycles until the crack initiation. The numerical data obtained with these models are compared, being achieved similar fatigue lives predictions for notched plane specimens. For the case of flat-grooved specimens which provides plain strain conditions, an overestimation from the Coffin-Manson relation was observed while the Xue model reproduces very good results for both specimens' series.