SUMMARYMicropolar theories offer a possibility to model size effects in the constitutive behaviour of materials. Typical feature of such models is that they deal with a microrotation, which is supposed to represent an independent state variable, and its space gradient. As a consequence, the stress tensor is no longer symmetric and couple stresses enter the theory. Accordingly, a micropolar plasticity law exhibiting kinematic hardening effects should account for both, a back-stress tensor and a back-couple stress tensor. This has been considered in the micropolar plasticity model developed by Grammenoudis and Tsakmakis. The purpose of the current paper is to specify some constitutive functions in this model, to elucidate the finite element implementation as well as to demonstrate its capabilities in describing size effects.
An energy equivalence method for modeling damage effects in material response is proposed. In the present article, the main issues of the method are discussed for the less complicated case of isotropic constitutive functions. Otherwise, the material response addressed is supposed to be (rate-independent) elasto-plastic exhibiting isotropic and kinematic hardening. In order to make clear the difference to other continuum damage models, it suffices to deal here with isotropic damage expressed in terms of a scalar state variable. Our approach is based on the concept of effective stress and effective strain combined with a principle of energy equivalence as explained in the article. As a result, both the yield function and the evolution equations governing the hardening response of the damaged material are obtained from a given undamaged model material. Characteristic properties of the damage theory proposed are illustrated by comparing predicted responses with those according to damage models based on the principle of strain equivalence.
Kinematic hardening rules are employed in classical plasticity to capture the so-called Bauschinger effect. They are important when describing the material response during reloading. In the framework of thermodynamically consistent gradient plasticity theories, kinematic hardening effects were first incorporated into a micropolar plasticity model by Grammenoudis and Tsakmakis. The aim of the present paper is to investigate this model by predicting size effects in torsional loading of circular cylinders. It is shown that kinematic hardening rules compared with isotropic hardening rules, as adopted in the paper, provide more possibilities for modelling size effects in the material response, even if only monotonous loading conditions are considered.
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