Florin Andrei Nae

Constitutive model of shape memory alloys for unidirectional loading considering inner hysteresis loops

A specimen-based macroscopic constitutive model of shape memory alloys for unidirectional loading, which is simple yet accurate and has a physical background, was derived from a grain-based microscopic model. To consider the inner hysteresis loops of a stress?strain?temperature relationship, a new inner loop model called the shift and skip model was proposed. This model is based on microscopic aspects and includes the memory effect of deformation history. Stress?strain relationships were simulated for some representative strain cycles. Numerical results showed that the proposed simple model could capture corresponding experimental results accurately enough to be applied for smart structural design. Comparison with major specimen-based macroscopic models was also discussed, under a unifying approach based on the driving energy and required transformation energy pair.

Micromechanical modeling of polycrystalline shape-memory alloys including thermo-mechanical coupling

The main objective of this paper is to derive a simple, engineering model for a NiTi-based shape memory alloy (SMA) that can be used as a first-step, inexpensive computational tool in designing components including SMAs. The model is based on a Reuss approximation in which the stress in every grain is considered the same. A random and a simple texture distribution for the grain orientations as well as a normal distribution for the transformation force are used in the calculations so that a round shape of stress–strain curve and transformation start and finish temperatures can be considered. A new algorithm based on minimizing the energy at each transformation step is provided that is simple, fast and accurate. Thermo-mechanical coupling is taken into account, therefore various strain-rate regimes can be modeled. Both superelastic and shape memory effect (SME) are analyzed. The model can also replicate complex behavior encountered in real materials such as small strain–amplitude hysteresis cycles, ratio of lateral to longitudinal strain during transformation and asymmetric behavior in tension compared with compression, while keeping the number of modeling parameters small. Numerical simulations show excellent agreement with available experimental results by applying the adequate grain orientation and the transformation force distribution.

The active tuning of a shape memory alloy pseudoelastic property

To improve the damping performance of a shape memory alloy (SMA), we propose a concept of active tuning of its pseudoelastic property where the shape of the stress–strain curve is modified as desired by utilizing the materials strong thermo-mechanical coupling characteristic. Simulated results are presented for a structural system in which SMA wires are used to suppress the vibration of a cantilevered beam by controlling the wire temperature using an electric current. This control increases the area of inherent hysteresis loop associated with the pseudoelastic effect and a large damping is obtained. In the system discussed, airflow is applied for cooling. It is shown that under forced heat convection the wire temperature can be changed fast enough to control a structure vibrating at more than 5 Hz. Simulated results seem to promise that the active tuning of SMA characteristics can increase its already high inherent damping by a factor of more than two. Experiments are also performed to validate the SMA model used in the numerical simulations.

Macroscopic constitutive model of shape memory alloys for partial transformation cycles

A simple yet accurate specimen-based macroscopic constitutive model of shape memory alloys (SMA) was derived from a grain-based micromechanical model, to understand the complicated thermo-mechanical behavior of SMA and to design structural elements with SMA components optimally. This model was composed of a phase transformation energy criterion, a strain equation, and a heat and energy flow equation. New features are that (1) a partial transformation cycle model was proposed, which is called the shift-skip model, and that (2) required energy for phase transformation was found to be well approximated by a sum of two exponential functions in terms of martensite volume fraction. In the shift-skip model, the energy required for the partial transformation was obtained by shifting and skipping the energy required for the complete transformation, based on a microscopic transformation rule. Comparison of the calculated stress-strain loops for the complete and partial transformation cycles with experimental data and with other often used models was carried out. Result showed that the proposed model could capture the measured stress-strain loops well and much better than the other models.

Specific Damping Capacity of Multiphase Unidirectional Hybrid Fiber Composites

Modeling and analysis on the specific damping capacity (SDC) of a unidirectional hybrid fiber composite lamina have been presented in this paper. In an attempt to refine the SDC analysis for the case of hybrid composites a new unit cell has been identified. Idealization of the proposed unit cell introduces a fiber-matrix interphase into the hybrid composite damping model. Explicit lamina level SDC equations are derived using the strain energy method. The validity and the advantages of the present method have also been demonstrated by performing numerical simulations for conventional as well as shape memory alloy (SMA) hybrid fiber composites.

Analytical modeling for passive damping in smart composites using a multicell method

The hysteresis type of material specific damping capacity (SDC) of a unidirectional hybrid fibre reinforced smart composite has been studied in the present work using a multi-cell method.To do this, as a first step, we reviewed various micromechanics modelling for the mechanical properties in general and material damping in particular in order to compare the theoretical capabilities and limitations of the existing analytical models. A new refined unit cell featuring a more realistic fibre-matrix domain has then been proposed for the present modelling. SDC equations corresponding to all the six directions were derived using the strain energy concept within the framework of mechanics of material approach. The generality of the present model in terms of the range of fibre volume ratio, different combinations of fibre-matrix systems etc., has been verified by comparing the present results with the literature including available experimental results. An important merit of the present theory that has to be emphasized over other available theories is the accurate prediction of the transverse and shear directions SDC for composites having a high fibre/matrix modulus ratio. Further, the scope of the present model to the practical applications of a typical shape memory alloy hybrid composite has also been demonstrated through numerical simulations.