Margarita E. Evard

Simulation of Payload Vibration Protection by Shape Memory Alloy Parts

A system of vibroisolation under consideration consists of a payload connected to a vibrating housing by plane shape memory alloy (SMA) slotted elements. The calculation of the mechanical behavior of the SMA is based on a microstructural theory. Simulations of harmonic and of impact excitations are carried out. The results have shown that protective properties of this system depend on the SMA state. The maximum reduction of the acceleration amplitude for harmonic excitation is reached when the SMA is in the martensitic (pseudo-plastic) state or in the two-phase state. A variation of temperature allows changing the resonance frequency and thus escaping from the resonance and controlling a mode of vibration.

Modeling of strain accumulation and recovery due to fcc-hcp transformation at thermocycles

An account of crystallographic features of fcc

Computer simulation of the shape memory effects in Fe-Mn-type alloys accounting for the features of the FCC-HCP phase transformation

ARLR hcp transformations as well as of plastic accommodation of martensite has allowed to describe accumulation of strain under a constant stress at cyclic variations of temperature. The model also gives the recovery of this strain at subsequent thermal cycles when the stress is removed.

Simulation of Vibration Isolation by Shape Memory Alloy Springs Using a Microstructural Model of Shape Memory Alloy

An approach is presented to describe the mechanical behavior of shape memory alloys undergoing a fcc-hcp phase transformation. Previously encouraging results have been obtained within the frames of the structure-analytical theory. They were achieved by introducing specific limitations on the maximum allowable size of martensite and austenite crystals. The model described in this report differs by the averaging procedure which is organized in a way to account for the symmetry of fcc and hcp lattices as well as the symmetry of the transformation strain tensor. The results of modelling have shown that symmetrical considerations can explain the transformation plasticity (strain accumulation due to a transformation in a stressed specimen) while direct or reverse transformations, the incompleteness of the strain recovery in the subsequent reverse or direct transformation and the asymmetry of the mechanical behavior for these two transformations. A qualitative agreement with experiments on Fe-Mn alloys (except for the scale of the asymmetry between direct and reverse transformations) has been achieved, the errors of modeling being probably connected with the dislocation plasticity not accounted for in the model.

Model for self-accommodated groups of martensite

A vibroisolation of a payload connected to a vibrating housing by two helical shape memory alloy (SMA) elements is considered. A microstructural theory is used for the simulation of the mechanical behavior of the SMA. The simulations have shown that the resonant frequency and the mitigation of the external vibrations depend on the shape memory alloy state. The maximum reduction of the acceleration amplitude for harmonic excitation is reached when the SMA is in the martensitic pseudoplastic state or in the two-phase state. Variation of temperature allows changing the resonance frequency and thus escaping from resonance. The acceleration of the payload at impact is the smallest when the SMA elements are in the pseudoelastic state.

Peculiarities of strain and resistivity variations in TiNi

A microstructural model considering self-accommodated groups of martensite as independent formations has been developed. Equations describing the growth and shrinkage of the groups of martensite and martensite reorientation have been deduced from the hypotheses of the equilibrium of generalized thermodynamic forces, including the driving force, the force due to mixing of the phases and the friction force. This approach allows modeling of common deformation effects in shape memory alloys such as pseudoelasticity, ferroplasticity, transformation plasticity and strain recovery, and it also explains the less-known phenomenon of the oriented transformation deformation consisting in the spontaneous strain accumulation on cooling after the stress, which was applied at the beginning of the direct transformation, is removed.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Microstructural modelling of plastic deformation and defects accumulation in FeMn-based shape memory alloys

Abstract Simultaneous measurements of electrical resistivity and inelastic strain accumulation in Ti–49.5 at.% Ni undergoing a two-step B2→R→B19′ martensitic transformation were performed. To exclude the influence of specimen size changes during the deformation all experiments were made in torsion. It was detected that noticeable part of strain recovery after heating under a stress and active deformation in martensitic state takes place before the start of the reverse transformation and is obviously connected with twinning. At the same time the two-way shape memory effects are caused by the martensitic transformation. The obtained data may be useful for adequate modelling of mechanical properties of the shape memory alloys because allow to connect the macroscopic parameters with microscopic physical processes.