Limit Load Analysis Of Typical Materials Based On Effect Of Enhanced Plasticity

The subject attaches to the "Eleventh Five-Year" plan to support national science and technology issues, "Key security technology research and demonstration project of complete equipments based on large-scale high parameters and high-risk in long-term running." Aiming at plastic collapse failure and local failure of bearing structure, Strain-hardening effect of metallic materials is considered, and the maximum and allowable loads are analyzed, in order to extend the safe life of pressure equipment and save energy, the potential carrying capacity of pressure equipment in service need to be excavated.The research, in which test method and FEM are used to analyze the mechanical property and constitutive relation of typical materials, starts from plasticity theory, and three major problems are solved:First of all, in this article, the constitutive relation is obtained from the experimental of large deformation torsion of solid rods and unidirectional tension, and the problem about stress calculation of large deformation torsion is figure out in terms of theory. Taking low alloy steel 16MnR, mild steel Q235 and austenitic stainless steel 304 for example, not only the mathematic model of constitutive relation, but also the scope of the formula adapted is given.Second, the equivalent constitutive of typical material in a multi-state stress is established, and the effect of stress state is analyzed. The method is that according to total strain theory, combinatorial tests of torsion and tension, in varying proportions, are carried out to attain the equivalent constitutive mathematic modelσ-ε, and the results are compared and analyzed to find the rule of effect of stress state. Then yield criterion is validated and subsequent yield is studied, from this work, the relation between maximal equivalent strain and triaxial stress coefficient which is a characterization of stress state is changed into mathematic modelε_max-TS.Finally, in a multi-state stress, the FEM is applied to calculate limit load of typical structure according to strain control criterion, meanwhile the microscopic feature is observed to represent the limit load. The method of combining macroscopic calculation and microscopic test is first used to find the relation between strain-hardening of material and allowable load of structure, further more, projects adaptation of ASME local strain failure control factors is validated.The results show that:①Comparing with the unidirectional tension constitutive relation, large deformation torsion equivalent constitutive extend to a greater scope, and they tally with each other very well.②Under the combined effects of torsion and tension, modes of material failure and constitutive relation are associate with triaxial stress coefficient, for example, in the loading way of first tension and then torsion, the plastic deformation capacity of material cut down along with the increase of triaxial stress coefficient. In principal stress space, all the test points fall over Mises yield locus, when strain is strengthen between 15% and 20%, test points almost tally with subsequent yield locus, but they deviate after 20%, the greater the triaxial stress coefficient, the further the test points deviate.③Plastic deformation is greatly effected by stress state, and the incidence changes with different materials. Test results demonstrate that: plastic deformation capacity of low alloy steel (16MnR) is sensitive to triaxial stress coefficient, and the fall of maximal equivalent strain can be 1000%; but austenitic stainless steel 304 reverses, only 25%.④Test results coincide well with FEM results calculated on the basis of ASME local strain control criterion. For instance, the errors of 16MnR specimens with defect radius of 0.06mm and 5mm are only 1% and 3%. On one hand, local destruction of structure is associated with materials maximal allowable strain, on the other hand, the way of destruction is affected by stress state.⑤There are different reduces on impact absorbing energy of three typical materials with the increase of plastic deformation, and the rule vary with materials. For example, basically Q235 with pre-strain do not have the capacity to bear impact load, so it is not adapt to the work in plastic state.