A new ultra-low cycle fatigue model applied to the X60 piping steel

Abstract Fatigue damage under extreme cyclic plastic loading conditions (ultra-low-cycle fatigue (ULCF)) has been increasingly investigated motivated by several applications involving structures and mechanical components, such as pipelines, that may be, during operation, likely exposed to severe cyclic loading regimes (e.g. accidental loads, earthquakes, pipeline reeling). ULCF corresponds to a transition damage mechanism between the monotonic ductile damage and the low-cycle-fatigue (LCF), both widely investigated in the literature, using independent approaches. Investigation in this transition damage mechanism is still scarce covering a reduced number of materials and few models are available. The main goal of this paper is to investigate the cyclic behavior of the X60 piping steel under cyclic extreme loading conditions, also covering the respective monotonic ductile and low-cycle fatigue behaviors. A unified model to describe the three damage regimes will be also proposed. This investigation is supported by an experimental program covering tests of smooth and notched specimens to derive monotonic and elastoplastic cyclic/fatigue data under a diversity of multiaxial stress conditions. Data reduction schemes based on each individual test simulation, by non-linear elastoplastic finite element models, is performed. In detail, the monotonic fracture strain, the average stress triaxiality and the average Lode angle parameters were obtained and considered for the calibration of 3D ductile fracture locus in accordance with Bai and Wierzbicki formulation, a satisfactory agreement being found. Afterwards, cyclic test data is used to calibrate a modified Xue model that is made explicitly sensitive to the stress triaxiality and Lode angle parameters. This model relates the equivalent plastic strain range, normalized by the fracture strain with the number of cycles to failure. Aiming the determination of the strain fracture of each specimen, two distinct methods are proposed. The first one consists of a direct method, based on the simulation of each ULCF specimen under monotonic conditions and in the other one is based on the use of the previously generated 3D monotonic ductile fracture locus.