Study On Yield Stress Prediction Model Based On The Deformation History Of Pipeline

Significant working hardening and Bauschinger effect exist in pipeline steel during plastic forming. The yield strength of the materials increases with work hardening and decreases with Bauschinger effect after they undergo repeated plastic deformation. The yield strength of line pipe is the result of combined effects of both. Generally, these two effects can not be completely offset, which results in the yield strength difference between blank plate and line pipe body. The yield strength difference has a certain impact on line pipe manufacturing and the corresponding selection for blank plate. If the relationship of the yield strength between blank plate and line pipe body based on the deformation history can be built, the prediction model for the yield strength of the line pipe based on the deformation history can be established, the prediction model for the reverse yield strength can be embedded into FEM, and the reverse yield strength can be calculated under different deformations with different initial yield strengths by using the finite element simulation model, it will provide a simple and effective method of calculation on the prediction for the reverse yield strength and provide the basis for selecting blank plate with a appropriate initial yield strength according to the ultimate yield strength of the line pipe body and different pre-deformations.In the present article, pipeline was as the research object. With the method of pre-stretching in the direction of forward, unloading, and reverse compressing, yield strength variation regulations of X90 pipeline steel produced in some factory during the reverse loading process were studied. And determined were variation relationships of the reverse yield strength with prestrain. Then, based on the Hodge combined hardening theory, utilizing plastic deformation volume invariance principle, deducted was the simplified model of a uniaxial loading condition, and derived was variation formula of uniaxial reverse yield strength with pre-strain. Corresponding hardening model parameters were then determined with the use of experimental data. Then the reverse yield strength prediction model was created.With User Material Subroutine UMAT, the reverse yield strength prediction model was embedded into ABAQUS, and the goal was achieved to simulate yield strength variation of the material when bearing forward loading and reverse loading. By simulation and experimental verification, the created reverse yield strength prediction model was verified. Verification results show that under different pre-deformation conditions, the maximum error and the average error between simulation and calculation results were 1.37% and 0.69%; the maximum error and the average error between simulation and experimental results were 2.87% and 1.39%.The created reverse yield strength prediction model was applied to thick plate bending model, to calculate different reverse yield strength on different layers, which was divided according the thickness. On the basis of plane assumption, and we assuming that there is no change on the thickness in the thickness direction, the composite reverse yield strength calculation method on thick plate bending model was defined. Application scope was extended and correctness was validated of the created reverse yield strength prediction model.