Finite element simulation and experimental investigation on the effect of temperature on pseudoelastic behavior of perforated Ni–Ti shape memory alloy strips

In the present study, the temperature-dependent pseudoelastic behavior of shape memory alloy sheets is studied experimentally and by finite element modeling. For this purpose, temperature-dependent mechanical properties for Ni-Ti alloy materials are first obtained by using direct tensile and three-point bending experiments at 23, 50, and 80 °C temperatures, respectively. The structure of these materials is examined at different temperatures using SEM images and the XRD test. Furthermore, using the finite element model, the pseudoelastic behavior and the effect of temperature on the residual deflection of the prose-shape memory strips with a circular hole under three-point bending loads are studied. After validating the results of the finite element model with the results of experimental tests, the effects of various parameters such as the diameter and number of holes on residual deformation and residual strains are investigated. The results show that with increasing temperature, the mechanical properties including the tensile strength, Young's modulus, yield stress, and flexural strength of SMA strips increase significantly. For solid strips, although increasing the temperature increases the maximum flexural force, in contrast, it reduces the flexural stiffness. In solid strips, flexural stiffness decreases by 5.5% with increasing temperature from 23 °C to 80 °C.

Modeling the Stress-Induced Transformation Behavior of Shape Memory Alloys under Multiaxial Loading Conditions

A large number of criteria to model the onset of plasticity for ductile metals have been proposed by researchers in the last century. Strangely, very few researchers have tried to model the stress-induced crystalline phase transformation of Shape Memory Alloys (SMAs) according to yield criteria. This paper focuses on the question: is a yield criterion originally proposed for describing the plastic behavior of metals suitable to model the “pseudoelastic” behavior of SMAs? To answer this question, two yield criteria originally proposed by the present author are used to predict the initial surface of transformation onset of two different SMAs: Cu-Al-Be and Ni-Ti alloy. The predicted initial transformation onset surfaces of the two SMAs are compared with experimental results and existing theories reported in the literature and some significant conclusions and recommendations are given.

A mechanical contact model for superelastic shape memory alloys

Shape memory alloys (SMA) are nowadays widely used in different industries. The two extraordinary behaviors of superelasticity and shape memory effect make these alloys a super wear-resistant material. In a range of SMA applications, contact between adjacent surfaces occurs. In this research, a formerly-developed contact model, which individually considers each asperity, is extended to cases where superelastic shape memory alloys are used. Since constitutive equations of SMAs are based on stress and strain, to establish a relationship between classical contact models and the main arguments of these constitutive equations, a representative strain based on the pseudoelastic behavior of SMAs was defined. Experiments were conducted to verify the model’s predictions. In these experiments, a NiTi wire was pressed against a Steel plate; then, the measured penetration in the test and the values predicted by the contact model were compared. The reported results show an acceptable agreement between theory and experiment.

Modeling and Experimental Verification of the Aeroelastic Behavior of a Typical Airfoil Section With Shape Memory Alloy Springs

This paper presents the modeling, simulation and wind tunnel experimental verification of the aeroelastic behavior of a two-degree-of-freedom (pitch and plunge) typical airfoil section with superelastic shape memory alloy helical springs in the pitch degree-of-freedom. A linearly elastic spring is considered in the plunge degree-of-freedom. Although viscous damping is considered in both degrees-of-freedom, hysteretic damping simultaneously takes place in the pitch degree-of-freedom due to the (stress-induced) pseudoelastic behavior of the shape memory alloy springs. The shape memory alloy phase transformation kinetics and constitutive modeling are based on Brinsons model and the shape memory alloy helical spring behavior is based on classical spring design. The nonlinear effects of shape memory alloy phase transformation are included in the shape memory alloy spring modeling for the representation of hysteretic force-displacement behavior. A two-state linear aerodynamic model is employed to determine the unsteady pitching moment and lift. The aeroelastic behavior of the typical section is numerically and experimentally investigated for different preload levels applied to the shape memory alloys. Numerical predictions and experimental results show that for large enough preload levels (such that shape memory alloy phase transformations take place at small pitch angles) unstable post-flutter regime is replaced by stable limit-cycle oscillations. Moreover, the amplitudes of aeroelastic oscillations decrease with increasing preload levels since more expressive phase transformations are achieved at small pitch angles. Although the amplitudes of the post-flutter limit-cycle oscillations increase with increasing airflow speed (since aerodynamic loads increase with the square of the airflow speed), they remain bounded within acceptable levels over a range of airflow speeds due to hysteretic damping. Moreover, the cutoff airflow speed increases with increasing preload. The experimentally verified results show that the pseudoelastic behavior of shape memory alloy elements can passively enhance the aeroelastic behavior of a typical section.

Complex transformation field created by geometrical gradient design of NiTi shape memory alloy

Owing to geometrical non-uniformity, geometrically graded shape memory alloy (SMA) structures by design have the ability to exhibit different and novel thermal and mechanical behaviors compared to geometrically uniform conventional SMAs. This paper reports a study of the pseudoelastic behavior of geometrically graded NiTi plates. This geometrical gradient creates partial stress gradient over stress-induced martensitic transformation, providing enlarged stress controlling interval for shape memory actuation. Finite element modeling framework has been established to predict the deformation behavior of such structures in tensile loading cycles, which was validated by experiments. The modeling results show that the transformation mostly propagates along the gradient direction as the loading level increases.