Deformation Characteristics of Several Types of Steel in Cyclic Plasticity and Their Modelling

Abstract

There have been numerous discussions on several aspects of cyclic plasticity and their constitutive modelling, e.g., the cyclic strain hardening and softening under proportional or non-proprtional loading [1-4], the strainor stress-rate dependence [5-8], the effect of temperature and its variation on the material behaviour [9-11], etc., Especially in the last decade, a great deal of effort has been made to model the ratchetting at roomand elevated temperatures [12-17]. Although it is already known that the above-mentioned material behaviour strongly depends on types of metal, the pevious work have not addressed this problem. As a result, even when a particular constitutive model is able to properly describe the cyclic behaviour for a certain material, we do not have an assurrance that the model is enough accurate for the other materials. The present paper provides two examples which prove that various types of steel posesses different deformation characteristics in cyclic plasticity. One of them is the behaviour in uniaxial cyclic straining (or stressing), and the other is the problem of multiaxially strain-controlled ratchetting. In the discussion of the uniaxial cyclic plasticity, a special emphasis is plased on the difference in cyclic hardening (or softening) characteristics between two types of steel (304 stainless steel [SUS304] and a mild steel). Moreover, the work addreses the differences in the yielding behaviour — an apparent yield-point and the subsequent abrupt yield-drop is observed in the mild steel while it does not appear in SUS304. In the case of multiaxial ratchetting, the difference in shakedown phenomenon between the two types of steel (SUS304 steel and a 0.25-%-C steel [STPG38]) is discussed. The present author introduces a new constitutive model of cyclic viscoplsticity in the framework of the overstress-type modelling for which the physical background lies in the rapid dislocation multiplication and the stress-dependence of dislocation velocity. The model describes both the cyclic hardening and softening characteristics, as well as the yield-drop phenomenon. In order to predict both the shakedown and non-shakedown characterestics in the multiaxial ratchetting, two different types of kinematic hardening rule are introduced.