Large-scale experimental investigation of the effect of biaxial loading on the deformation capacity of pipes with defects

This paper presents results from large-scale four-point bending tests of 12″ X65 seamless pipes with circumferential defects subjected to different levels of internal pressure. The aim of the tests has been to investigate the effect of biaxial loading on the strain capacity of the pipes. The results from the tests show a significant effect of the biaxial loading. For cases without internal pressure, the pipes fail due to local buckling on the compression side of the pipe. Upon application of internal pressure the failure mode shifts to fracture from the defect on the tension side. The failure bending strain levels for these cases were 1.5–2.25%, whereas the local buckling occurred at strain levels in the range of 3.5–4%. The main reason for this appears to be that the biaxial loading increases the crack driving force for a given applied strain level. No significant effect of the biaxial loading on the ductile tearing resistance was observed. The results are of great importance for fracture assessment of pressurised pipelines loaded into the plastic region, as the biaxial loading effect observed is not accounted for in current fracture assessment procedures.