The yield stress of pipes is measured among purposes to assess the resistance of the pipe to the internal pressure of the fluid. Unfortunately, it is not possible to sample a straight specimen in the hoop direction, and therefore the pipe has to be flattened prior to testing. In the present investigation, different materials were tested in tensile–compression mode in order to provide data for a kinematic hardening model. Based on this experimental data set, a model was built to take into account several features of the material behaviour (presence of yield point elongation, strain hardening, etc…) and the processing route (longitudinal or spiral seam weld, expander, hydrotest…). Pipe production was also sampled at different moments (base material, after leveling, after pipe forming, after hydrotest). The testing program included tensile testing and ring expansion tests. The results show that the model gives a prediction in good correspondence with the experimental results. The model also reproduces several experimental facts, like for example the presence of a yield point elongation on the base material and its absence on the flattened pipe sample. Finally, the model is compared with an industrial database containing different steel grades (from grade B to X80) and different ratios of wall thickness over diameter (t/OD) ratios. The difference of yield stress between coil and pipe is predicted on this database with an accuracy of 20MPa.

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