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| Strnadel, B., Kursa, M. |
| MODELLING RESIDUAL STRAINS DURING THE CYCLING OF TI-NI AND TI-NI-CU SHAPE MEMORY ALLOYS UNDER PSEUDOELASTICITY CONDITIONS |
Abstract
The effect of three types Ti-Ni shape memory alloys in pseudoelasticity state on mechanical cycling has been studied under conditions of hard and soft loading cycling. Experimental results proved that the residual deformation after unloading increases with increasing number of cycles, however the critical stress for the induction of martensite and the energy dissipated in one cycle decline in the course cycling. A higher critical stress for slip, and more intense cyclic dislocation hardening promoted by greater maximum deformation and greater maximum applied stresses, in general reduce the rate at which residual elongation grows with the number of cycles, and tend to stabilize the cyclic stress-elongation diagrams. The small magnitude of critical stress for slip in low nickel alloys, as well as cyclic strain hardening induce greater internal stresses and a more marked decrease of the critical stress for the induction of martensite as cycling progresses. The fact that hysteresis diminishes in the course of cycling is due to the restriction of transformation strain, which is caused in hard cycling by the cumulation of residual strain and in soft cycling chiefly by the increasing resistance of the dislocation structure against movements of the phase boundary. Lower nickel alloys display lesser slip stresses and more residual plastic strain, and therefore their hysteresis drops off more rapidly in hard than in soft cycling. Detailed analysis of plastic deformation propagation in cyclically loaded specimen helped to work out a model of the dependence of residual elongation on the number of cycles. This model enables to identify the three main factors that govern the magnitude of the residual elongation. One is the residual plastic elongation caused by dislocation hardening after the alloy is heat treated, and the other two are cyclic strain hardening parameters describing how residual elongation grows with the number of cycles, and how this residual elongation is reduced, as cycles increase, by the rising critical stress level for slip. The model has proved to yield very close agreement with experimental findings. This results can be employed as a design tool of structural parts in mechatronics and robotics systems.
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| Keywords: shape memory alloys|Ti-Ni alloy|Ti-Ni-Cu alloy|pseudoelasticity|critical stress for inducing martensite|plastic strain propagation| |
| No 4 (2006), p. 366-378 |
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