Very low-cycle fatigue failure behaviours of pipe elbows under displacement-controlled cyclic loading

In this study, quasi-static very low-cycle fatigue tests were conducted on pipe elbows at room temperature to investigate the effects of the pipe thickness, internal pressure, and material on the failure behaviours under large-amplitude cyclic loads. Three types of elbow specimens comprising two different materials and two different pipe thicknesses were used in the tests. Elbow specimens were subjected to displacement-controlled, fully reversed, in-plane mode cyclic bending with and without internal pressure. All specimens failed due to through-wall cracking of which the location and orientation depend only on the pipe thickness regardless of the internal pressure and elbow material. For thin-walled elbows of Schedule 40, the axial crack at flank was developed, whereas for thick-walled elbows of Schedule 160, the circumferential crack at intrados. The number of cycles to failure decreased with increasing internal pressure in thin-walled elbows. However, the effect of internal pressure on the number of cycles to failure was negligible in thick-walled elbows. Regardless of internal pressure level, stainless steel elbows were more resistant to failure, compared with carbon steel elbows of identical dimensions. Finally, the strain-based evaluation model considering the void growth and shrinkage effect was applied to predict fatigue failure location and cycles. Two parameters in the model, the multi-axial fracture strain locus and void shrinkage ratio, were determined from monotonic tensile test data and the fatigue life curve of the material. The predicted crack initiation site and orientation were consistent with experimental finding. For all cases, the predicted failure cycles were less but overall close to experimental data.