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Modeling of Shape Memory Alloys
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Proceedings Papers
SMST 2022, SMST 2022: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 61-62, May 16–20, 2022,
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Strain-based criteria is most often used for characterizing the fatigue reliability of Nitinol cardiovascular devices. Fatigue testing of Nitinol specimens has also relied on finite element analysis (FEA) to compute cyclic strain amplitudes and mean strains. Recently, the digital image correlation (DIC) technique has been shown to have high resolution to experimentally determine the local material strains of Nitinol fatigue specimens. In this study, the authors explored the feasibility of alignment between DIC strain measurement, and the strain calculated by the continuum mechanics approach used in the FEA technique. The agreements and discrepancies are discussed with their implications on fatigue reliability assessment of Nitinol cardiovascular devices.
Proceedings Papers
SMST 2022, SMST 2022: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 63-64, May 16–20, 2022,
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Passive vibration isolation is a key element to achieve precise results in milling processes and to increase tool durability. Damping of vibrations near to the cutting edge is considered highly effective as well as hard to implement because of the limited damping properties of conventional materials in the available space. The use of damping elements made of NiTi shape memory alloys (SMA) represents an innovative approach. Their use is based on the ability to convert mechanical energy into thermal energy through the pseudoelastic effect, whereby the pronounced conversion hysteresis of the material provides information about the usable damping potential. Studies on the properties of pseudoelastic SMA under compressive loading are only sporadically available in comparison to tensile loading. In this paper, the stress-compression curves and the hysteresis energy of tests results are compared with the results of finite element simulations. The simulation results based on the material model used so far is a good basis for the further development of damping elements.
Proceedings Papers
SMST 2022, SMST 2022: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 65-66, May 16–20, 2022,
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This paper presents an extension of a well-established constitutive model for NiTi covering both reversible (elastic, martensitic transformation, martensite reorientation) and irreversible (plastic) deformation mechanisms. Besides the inclusion of mechanisms of plastic deformation in both austenitic and martensitic phases in an independent manner, the model also newly captures more complex coupled phenomena of martensitic transformation and plastic deformation, such as transformation-induced plasticity, stabilization of martensite by plastic deformation, or plasticity-induced microstrain heterogeneity leading to functional fatigue. Despite a large number of different mechanisms involved in the model, which is reflected by a considerable number of internal parameters introduced for the description of the evolving microstructure of the material, the model still brings a basic, simple phenomenological understanding of the coupled transformation-plasticity proceeding in NiTi. After successful implementation to FEM software, the model provides new possibilities for simulations of NiTi components' behavior and processing.
Proceedings Papers
SMST 2022, SMST 2022: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 67-68, May 16–20, 2022,
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Specific heat treatment or addition of a ternary element may induce a two-stage transformation sequence in NiTi shape memory alloys (SMA). A typical example of intermediating phases is so-called R-phase, a rhombohedral distortion of the cubic austenitic phase, which exhibits much lower transformation strain and thermal hysteresis than the subsequent transition to monoclinic martensite. In specific alloys, e.g., in NiTi slightly enriched by iron, the temperature and stress intervals in which R-phase is stable are quite broad. Hence, the influence of R-phase on the macroscopic (thermo)mechanical response should be considered when developing and designing products from these alloys. Within this context, tailored constitutive models allowing to reproduce the response in complex loading scenarios without additional experimental effort can be extremely beneficial. This paper presents an enhanced constitutive model for NiTi SMA featuring the R-phase transition. The model recognizes R-phase as a distinct phase, respects the coupled influence of stress and temperature on any phase transformation, and covers reorientation (reconfiguration) of both martensite and R-phase with applied stress. The core of the model consists of two material functions: one captures the energy stored in the material at a given thermodynamic state, the other defines the energy released during dissipative processes, which are considered rate-independent. The model was validated through a direct comparison of experimental tests (isothermal tensile tests, isobaric thermal tests, recovery stress tests) with simulated counterparts.
Proceedings Papers
SMST 2022, SMST 2022: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 69-70, May 16–20, 2022,
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A new thermomechanical constitutive modeling approach for shape memory alloys (SMAs) that undergo a martensite to austenite phase transformation and the associated pseudoelastic and shape memory responses is presented. The novelty of this new formulation is that a single transformation surface is implemented in order to capture the forward and reverse phase transformations, as well as the reorientation and detwinning of the martensite phase. The framework is akin to the usual flow theory plasticity with kinematic hardening, however in addition to the transformation strain there is also a transformation entropy that is directly related to the martensite volume fraction appearing in prior theories. A transformation surface in effective stress and effective temperature space is introduced and an associated flow rule governs the evolution of the transformation strain and entropy. In order to capture the multitude of SMA behaviors, a transformation potential function is introduced in transformation strain and entropy space for the derivation of the back stresses and back temperatures that define the kinematic hardening behavior. It is this potential function that governs all of the important behaviors within the model. After the description of the general theory, specific forms for the transformation surface and the transformation potential are devised and results for the behaviors captured by the model are provided for a range of thermomechanical loadings. The model is then implemented in finite element calculations to investigate the structural response of shape memory alloy tubes, bars, and beams.
Proceedings Papers
SMST 2022, SMST 2022: Extended Abstracts from the International Conference on Shape Memory and Superelastic Technologies, 71-72, May 16–20, 2022,
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Thermal expansion (TE) is inherent material property and a critical design parameter in applications in which dimensional stability and/or thermal fatigue resistance over a wide range of temperatures are required. Examples include high precision instruments, satellite antennas, and optical instruments. Metallic materials that undergo martensitic transformation have been recently shown to exhibit tailorable bulk TE due to the TE anisotropy of the low-crystallographic-symmetry martensite lattice. In these materials, the TE anisotropy of bulk polycrystals can be exploited through martensite variant "orientation" upon deformation processing. This work proposes a constitutive model for tailoring the anisotropic CTE in shape memory alloys (SMAs) during martensite variant texturing, which is validated against experimental data from NiTiPd. A description of the evolution of the anisotropic macroscopic thermal expansion (TE) tensor of bulk shape memory alloys (SMAs) during phase transformation and martensite (re)orientation is proposed. Given that the tailorability of the TE of SMAs originates from the crystallographic TE anisotropy of the low-crystallographic-symmetry martensite, the TE tensor is approximated by a function of the oriented martensite volume fraction and the orientation direction unitary tensor. The proposed model is validated against recent experiments on tailoring TE through martensite orientation in a NiTiPd high temperature SMA. In those experiments, the TE tensor component in the loading direction of NiTiPd in the martensite state was shown to decrease with increasing inelastic strain induced by uniaxial tensile loading. According to the model, the TE tensor components in the transverse to the loading directions decrease with increasing tensile inelastic strain.