S.P. Gadaj

Characteristics of energy storage and dissipation in TiNi shape memory alloy

Abstract The characteristics of energy storage and dissipation in TiNi shape memory alloys were investigated experimentally based on the superelastic properties under various thermomechanical loading conditions. The influence of strain rate, cyclic loading and temperature-controlled condition on the characteristics of energy storage and dissipation of the material was investigated. Temperature on the surface of the material was observed and the influence of variation in temperature on the characteristics was clarified. The results obtained can be summarized as follows. (1) In the case of low strain rate, the stress plateaus appear on the stress-strain curves due to the martensitic transformation and the reverse transformation during loading and unloading. In the case of high strain rate, the slopes of the stress–strain curves are steep in the phase-transformation regions during loading and unloading. The recoverable strain energy per unit volume increases in proportion to temperature, but the dissipated work per unit volume depends slightly on temperature. In the case of low strain rate, the recoverable strain energy and dissipated work do not depend on both strain rate and the temperature-controlledcondition. (2) In the case of high strain rate, while the recoverable strain energy density decreases and dissipated work density increases in proportion to strain rate under the temperature-controlled condition, the recoverable strain energy density increases and dissipated work density decreases under the temperature-uncontrolled condition. In the case of the temperature-uncontrolled condition, temperature varies significantly due to the martensitic transformation and therefore thecharacteristics of energy storage and dissipation differ from these under the temperature-controlled condition. (3) In thecase of cyclic loading, both the recoverable strain energy and dissipated work decrease in the early 20 cycles, but change slightly thereafter. (4) The influence of strain rate, cyclic loading and the environment on the characteristics of energy storage and dissipation is important to be considered in the design of shape memory alloy elements.

Energy storage during the tensile deformation of Armco iron and austenitic steel

Abstract A modification of the single-step method based on continuous detection of IR radiation emitted directly from a strained sample was employed to study energy storage during the initial stage of plastic deformation of Armco iron and austenitic stainless steel. The existence of a maximum in the dependence of the ratio of stored energy to expended energy on strain was confirmed for both materials in the range of homogeneous deformation and was interpreted in terms of variations in the contributions of endo-energetic and exo-energetic processes; the contribution of endo-energetic processes is related to the increase in the dislocation density due to the generation of dislocations and the contribution of exo-energetic processes is related to energy dissipation due to movement, rearrangement and annihilation of dislocations.

Investigation of nucleation and propagation of phase transitions in TiNi SMA

The attention of the present paper is focused on the aid provided by infrared thermography, for spectacular investigation of nucleation and further development of the stress induced phase transitions in TiNi shape memory alloy. This is a qualitative analysis aimed to verify the feasibility of further study in the application of IR for studying change phenomena. To this end, the material stress-strain curves, obtained during tension test with various strain rates, were completed by the temperature characteristics. The temperature distributions on the specimens surface were determined by using an infrared camera. A heterogeneous temperature distribution, related to the nucleation and development of the new martensite phase, were registered and analyzed. A significant temperature increase, up to 30 K was registered during the martensite transformation. Similar effects of the heterogeneous temperature distribution were observed during unloading, while the reverse transformation, austenite into martensite, tooks place. The reverse transformation was accompanied, in turn, by a temperature decrease, of up to 10 K.

Thermoelastic and thermoplastic effects investigated in steel, polyamide, and shape memory alloys

This research is an experimental confirmation of the theoretical model, presented by M.G. Beghi, C.E. Bottani and G. Cagliotti, realized due to the contact less method of temperature measurement and conversion. The effects of thermomechanical coupling occurring in austenitic steel, polyamide and shape memory alloys were examined. The change of character of the samples temperature was applied as a criterion for the limit between the elastic and plastic regimes. The thermal effects concomitant with thermoelastic unloading were also quite precisely evaluated. The obtained results indicate on both qualitative and quantitative differences between thermomechanical behaviors of the materials subjected to deformation. TiNi shape memory alloy was tested both at room and at elevated temperatures. The temperature distribution on the surface of specimen examined at ambient temperature was uniform, while for the specimen deformed at elevated temperature the areas of higher temperature were registered. There are probably the regions of creation and developing of the new - martensite phase.