Description of Deformation of Shape-Memory Materials by Thermomechanical and Alternating Loads
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DESCRIPTION OF DEFORMATION OF SHAPE-MEMORY MATERIALS BY THERMOMECHANICAL AND ALTERNATING LOADS I. M. Holyboroda
UDC 539.373
We develop a phenomenological model of irreversible deformation of dislocation nature for shape-memory materials based on iron. The abnormal decrease in the yield stress in the vicinity of the temperature of onset of the direct martensitic transformation and residual microstresses depending on the history of force and temperature loads are taken into account. The model is applied to the results of thermomechanical and alternating cyclic tests. The loading surface is constructed and its transformations are described. The numerical results are compared with the experimental data for a shape-memory alloy based on iron with 9% Cr, 5% Ni, 14% Mn, and 6% Si. It is shown that the proposed model is applicable to the description of the deformation behavior of the material. The obtained results can be used for the description of the thermomechanical behavior of elements of shape-memory mechanisms and structures.
Under typical operating conditions (complex loads and temperature drops), the contemporary shape-memory (SM) materials exhibit specific properties connected with irreversible deformations of defect nature. Thus, in particular, the yield stress abnormally decreases in the vicinity of the temperature of onset of the direct martensitic transformation (MT). At the same time, these materials are deformed according to the microstructural mechanism fundamentally different from the dislocation mechanism typical of the reversible MT. Moreover, we observe the multiple-factor mutual influence of these phenomena. The observed picture of deformation is quite complicated, first of all, for shape-memory materials based on iron in which irreversible deformation of defect nature is especially intense under the conditions of thermomechanical cycling and repeated shape memory [1]. Therefore, it is necessary to develop a theory capable of the efficient mathematical description and prediction of the thermomechanical behavior of contemporary SM materials based on iron, including the irreversible deformation of defect nature [2]. Among the existing approaches to the solution of this problem, we especially mention the physically substantiated concept of sliding [3] based on the assumption of shear character of deformation on the microlevel. This concept can be applied to the description of the elastic, defect, and martensitic deformation. In this connection, one can also mention the synthetic theory of plasticity [4] combining the theory of sliding with the concept of yield with singular loading surface. The determining relations of the synthetic theory are simpler than in the theory of sliding. The process of deformation of SM materials (in particular, the cases of reversible deformation of martensitic nature, irreversible plastic deformation of defect nature, elastic deformation, and thermal expansion) was studied in [5–9]. In what follows, we study the process of irreversible deformation of dislocation nature (in
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