Analysis and estimation of yield strength of API X80 linepipe steel pipe by low-cycle fatigue tests

In the present study, spiral piping was conducted on API X80 linepipe steel, and the outer and inner wall pipe yield strengths were measured. A low-cycle fatigue test was conducted on a leveled X80 steel sheet to simulate piping and flattening processes, and the strain hardening and Bauschinger effects, induced from different strain histories, were evaluated and combined using Swift's equation and the Bauschinger stress parameter, respectively. By analyzing the stress-strain curves obtained from the low-cycle fatigue test, the yield strengths of the outer and inner walls were estimated to be 592 MPa and 492 MPa, respectively, which are lower by 20-80 MPa than that of the actual pipe used. Possible reasons for measured and estimated yield strength differences could be the simulation determining procedure of the pre-strain and Bauschinger stress parameters, preposition of same strain hardening behavior depending on strain history, and pre-strain differences depending on thickness location in the steel sheet during piping.