Ultra or extreme low-cycle fatigue of steels has been deserving increasing interest by the researchers since it corresponds to a fatigue domain not fully understood nor explored. It has been recognized that fatigue damage under extreme loading conditions is representative of several practical applications (e.g. seismic actions, accidental loads) and pipelines are a type of components that could undergo such extreme loading conditions. In addition, concerning the pipelines, reeling could also contribute to significant plastic cycles. ULCF damage corresponds to a transition damage behavior between the LCF and monotonic ductile damage. Therefore studies on ULCF usually needs to cover those bounding damage processes. ULCF testing exploring large-scale specimens is rare. The aim of this paper is to investigate the ultra-low-cycle fatigue of large-scale straight pipes subjected to cyclic pure bending tests which were performed under the framework of the ULCF European/RFCS project. In detail, two steel grades used on pipelines manufacturing were investigated, namely the X60 and X65 piping steels, respectively with the following nominal diameters of 16” (w.t. 9.5 mm) and 8 5/8” (w.t. 5.59 mm). A specifically developed testing setup was used to perform the cyclic bending of the straight pipes, combined with internal pressure, until the pipes collapse. The failure was preceded by local plastic instability (buckling), motivating the concentration of cyclic plastic deformation leading to macroscopic crack initiation and propagation. In addition to the full-scale tests, the plain material was investigated under monotonic and ULCF conditions using both smooth and notched specimens. In order to assess the stress/strain fields in the straight pipes, finite element models of the straight pipes were developed and simulations were performed under the experimental displacement histories. Nonlinear plasticity models with kinematic hardening, inputted on finite element simulations, were calibrated by means of small-scale data. Moreover, the test data of small-scale tests was used on the identification of damage models constants (e.g. Coffin-Manson), which in turn were applied to simulate the failure cycles of the tested straight pipes. The ASME B&PVC VIII Div.2 procedures were also used to compute the failure cycles for the straight pipes to allow an assessment of these existing procedures.
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