Monotonic, Low-Cycle Fatigue, and Ultralow-Cycle Fatigue Behaviors of the X52, X60, and X65 Piping Steel Grades

Seismic actions, soil settlements and landslides, fluctuations in permafrost layers, accidental loads, and reeling are responsible for large plastic deformations and widespread yielding of pipelines, which may lead to damage or failure, either due to monotonic loading or cyclic plastic strain fluctuations of high amplitude and short duration (e.g., Ni < ∼100 cycles). The damage associated to high intensity cyclic plasticity shows a combination of distinct mechanisms typical of both monotonic and low-cycle fatigue (LCF) (∼102 < Ni < ∼104 cycles) damage regimes. This fatigue domain is often called ultralow-cycle fatigue (ULCF) or extreme-low-cycle fatigue, in order to distinguish it from LCF. Despite monotonic ductile fracture and LCF have been subjected to significant research efforts and a satisfactory level of understanding of these phenomena has been already established, ULCF is neither sufficiently investigated nor understood. Consequently, further advances should be done since the data available in literature is scarce for this fatigue regime. In addition, ULCF tests are very challenging and there are no specific standards available in literature providing guidance. In this work, the performances of the X52, X60, and X65 API steel grades under monotonic, LCF, and ULCF loading conditions are investigated by means of an experimental program. Smooth specimens are susceptive to instability under ULCF tests. To overcome or minimize this shortcoming, antibuckling devices may be used in the ULCF tests. The use of notched specimens facilitates the deformation localization and therefore contributes to overcome the instability problems. However, the nonuniform stress/strain states raise difficulties concerning the analysis of the experimental data, requiring the use of multiaxial stress/strain parameters. Optical methods and nonlinear finite-element models were used to assess the strain and stress histories at critical locations, which were used to assess some existing damage models.