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In this paper, shape memory alloy (SMA) helical springs are produced by shape setting two sets of NiTi (Ti-55.87 at% Ni) wires, one of which showing shape memory effect and another one showing pseudoelasticity at the ambient temperature. Different pitches as well as annealing temperatures are tried to investigate the effect of such parameters on the thermomechanical characteristics of the fabricated springs. Phase transformation temperatures of the products are measured by differential scanning calorimetry and are compared with those of the original wires. Compression tests are also carried out, and stiffness of each spring is determined. The desired pitches are so that a group of springs experiences phase transition during loading while the other does not. The former shows a varying stiffness upon the application of compression, but the latter acts as passive springs with a predetermined stiffness. Based on the von-Mises effective stress and strain, an enhanced one-dimensional constitutive model is further proposed to describe the shear stress-strain response within the coils of an SMA spring. The theoretically predicted force-displacement responses of the produced springs are shown to be in a reasonable agreement with the experimental results. Finally, effects of variations in geometric parameters on the axial force-displacement response of an SMA spring are investigated.

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The behaviors of shape memory alloys (SMAs) strongly depend on the presence of different phases: austenite, thermally-induced martensite and stress-induced martensite. Consequently, it is important to know the phase volume fraction of each phases and their evolution during thermomechanical loadings. In this work, a three-phase proportioning method based on electric resistivity variation of a CuAlBe SMA is proposed. Simple thermomechanical loadings (i. e. pseudoplasticity and pseudoelasticity), one-way shape memory effect, recovery stress, assisted two-way memory effect at different level of stress and cyclic pseudoelasticity tests are investigated. Based on the electric resistivity results, during each loading path, evolution of the microstructure is determined. The origin of residual strain observed during the considered thermomechanical loadings is discussed. A special attention is paid to two-way shape memory effect generated after considered cyclic loadings and its relation with the developed residual strain. These results permit to identify and to validate the macroscopic models of SMAs behaviors.

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Fatigue in shape memory alloys is one of the crucial aspects of their behavior; however, the current knowledge is mainly focused on uniaxial fatigue and is inadequate for engineering purposes. In this article, a fatigue criterion based on the stabilized dissipated energy has been presented to investigate the torsional low-cycle fatigue of superelastic shape memory alloys. To this aim, a one-dimensional torsional constitutive model in addition to a modified fully coupled thermomechanical model has been utilized so that the torsional cyclic responses especially in relatively high loading frequencies, which contribute to remarkable temperature variations and consequent response changes, could be taken into account. The calculated stabilized dissipated energy, then, has been used in an energy approach fatigue criterion in order to predict the fatigue life; hence, an explicit relation, which is capable of determining the number of cycles to failure for different loading conditions at a given loading frequency, has been obtained. The numerical results have been appraised for NiTi specimens, and they have been shown to be in a good agreement with the experimental data. Finally, using the proposed approach, the effect of fatigue test parameters on the fatigue life has been studied.

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