Ultra low cycle fatigue of steel under cyclic high-strain loading conditions (ULCF): final report

Pipelines and piping components when subjected to extreme loading conditions (e.g. earthquakes, hurricanes, support settlements, industrial plant shutdown) undergo large plastic deformations, associated with widespread yielding, leading to fracture, either due to monotonic loading or ultra-low-cycle fatigue (ULCF). ULCF is neither satisfactorily understood nor conveniently investigated compared to the monotonic ductile or low-cycle fatigue damage mechanisms. The ULCF project aimed the development of innovative computational methodologies for the simulation of steel material fracture under both monotonic and ultra-low-cycle fatigue. The main contributions of the project are summarised as follows: I) Database of small-scale testing data covering the X52, X60, X65, X70 and X80 piping steel grades. Smooth and special notched specimens were tested under both monotonic and cyclic (LCF & ULCF) loading. Dedicated monotonic tests were performed to assess the anisotropic behaviour of some materials (X70/X80). Combined multiaxial loading conditions were also investigated. II) Database of full-scale testing results covering same materials also tested under small-scale conditions was generated. Monotonic and ULCF tests of pipe components were performed. ULCF tests included buckled and dented pipes, elbows and straight pipes. III) constitutive models for both monotonic and ULCF loading were proposed. Besides the Barcelona model, alternative approaches were investigated such as the combined Bai-Wierzbicki-Ohata-Toyoda model. Further constitutive refinements of non-linear monotonic damage and enhanced anisotropic models, were also studied. IV) constitutive models were calibrated and validated using experimentally derived testing data. Guidelines for damage simulation were proposed and current code-based rules for monotonic and ULCF damage were assessed and new procedures recommended.