Typical Material Mechanical Behavior In Whole Life Process Under Tension-torsion Proportional Loading

The subject comes from the "twelfth-five" important support project "pressureequipment design and manufacturing method based on failure mode of the whole lifeprocess". The plastic deformation capacity of material under different stress states hasalways been an important content to study, so in this article a series of studies aboutthe material mechanical behavior under different stress states in the whole life processwere launched.Although the tension-torsion fixed proportion loading, which is consistent withsimple load theorem, is an effective way to build complex stress state, but in the largeplastic deformation process, the basic conditions for conform with the simple loadtheorem is Tension-torsion stress proportional loading, which bring the problem forthe loading progress and control of laboratory equipment, and has been a bottleneckfor complex stress state experiments for a long time. Therefore, a new combinedtensile torsion testing platform was built in the cooperation with the equipmentcompany, to meet the requirement of fixed proportional tension-torsion loading, bythe supporting jig, tension and torsion combined control program and so on. At thesame time, taking into account the rationality of plasticity theory and sampleprocessing operability, the equivalent stress and strain theory for the treatment of solidcylindrical specimens test data in the whole process were explored. Comparing withthe traditional thin-walled cylinder tensile torsion test, it is a breakthrough anddifficulty in this paper.On the basis of the new tension-torsion test platform, the experiments andtheoretical study of28specimens of16MnR and304stainless steel, which are typicaltypes of materials used in special equipment, were conducted. By two differentcharacterization methods of stress states, the effect of two different stress states on theplastic ultimate stress and strain were shown by the ways of mathematical model andgeometric graphic respectively. Not only the mathematical model of the stresscharacterization parameters (tension-torsion proportional parameter C or stresstriaxiaty TS) and plastic ultimate stress and strain were build, but also the geometricgraphic method about above innovative relationship was given in the principal stressspace, which based on the condition of isotropic hardening model assumptions, whichprovided the basic experimental evidence for the evaluation of the service plant life.The conclusion was that:(1) In the aspect of ultimate strain, there is a significant inverse relationshipbetween ultimate strain and the stress triaxiaty TS value, also between the ultimatestrain and the proportional parameter C.(2) In the aspect of ultimate stress, the position of ultimate stress stationary point was found. When the tension-torsion proportion C is in the range of0<C<1, theequivalent stress decreased with the proportion of torsion and tension (σ/τ) increases;when the tension-torsion proportion C is in the range of1<C<∞，the equivalent stressincreased with the proportion of torsion and tension (σ/τ) increases; when theproportion of torsion and tension is1(the TS value is0.167at the same time), theultimate stress is minimum. This point was defined as stress stationary point.(3) The effect of different stress state on the plastic ultimate stress and strain ofthe two materials can be analyzed in two ways of mathematical model andgeometrical diagrams. In engineering applications, when the stress state of one pointin engineering structures is certain, the stress characterization parameter TS value iscertain, the ultimate stress and strain at this point could not only be estimated bymathematical model quantitatively, but also could be shown in the principal stressspace base on the assumption of isotropic hardening model, which provided the basicexperimental evidence for the evaluation of the service plant life.At last, the stainless steel and carbon steel materials，which represented by304stainless steel and16MnR respectively, were compared in the aspects of the yieldproportion, the plastic range, the comprehensive evaluation of ultimate stress andstrain under different stress states. A conclusion was conducted as that austeniticstainless steel has excellent plasticity, stable ultimate strain level and better ultimatebearing capacity, which has the potential and advantages over carbon steel. There is abroad prospect for austenitic stainless steel in the strain hardening design approach ofpressure equipment.