Experimental and numerical investigations of shape memory alloy helical springs

Abstract

Shape memory alloys (SMAs) belong to the class of smart materials and have been used in numerous applications. Solid phase transformations induced either by stress or temperature are behind the remarkable properties of SMAs that motivate the concept of innovative smart actuators for different purposes. The SMA element used in these actuators can assume different forms and a spring is an element usually employed for this aim. This contribution deals with the modeling, simulation and experimental analysis of SMA helical springs. Basically, a one-dimensional constitutive model is assumed to describe the SMA thermomechanical shear behavior and, afterwards, helical springs are modeled by considering a classical approach for linear-elastic springs. A numerical method based on the operator split technique is developed. SMA helical spring thermomechanical behavior is investigated through experimental tests performed with different thermomechanical loadings. Shape memory and pseudoelastic effects are treated. Numerical simulations show that the model results are in close agreement with those obtained by experimental tests, revealing that the proposed model captures the general thermomechanical behavior of SMA springs.

A three-dimensional constitutive model for shape memory alloys

“…This hypothesis is based on experimental observations that show that both effects induce phase transformation. It is important to highlight experimental torsion tests that indicate that stressstrain curves are qualitatively similar to those obtained in tensile tests [1,8,15]. Under this assumption, the equivalent field Γ may be interpreted as a phase transformation inductor that defines what kind of martensitic variant is induced.…”

Factors Influencing Design of SMA Actuators

“…When a SMA wire is twisted under torsional loading, the phase transformation front gradually moves from the outer fibers towards the neutral axis. The shear strain tends to zero at the core of the specimen cross-section as the wire twists [2]. This implies that the possibility of having a fully transformed case can only be possible if the angle of twist asymptotically reaches infinity [3].…”

Experimental Study of Helical Shape Memory Alloy Actuators: Effects of Design and Operating Parameters on Thermal Transients and Stroke

“…At low deflections, the shear stress is due to direct and torsional shear, but predominantly torsional shear. Additionally, when SIM forms during its austenite phase or when martensitic de-twinning occurs, an SMA spring's internal stress has been shown to be non-linear [13,14,22,25], as opposed to the linear torsional shear stress observed in isotropic springs. Although Liang and Rogers developed a constitutive model for a spring that substituted the applied axial stress with the von Mises criterion under pure shear, i.e., σ = τ√3 [7], this cannot be directly substituted into Equations 6 and 7, as the internal stress is not constant throughout the SMA wire.…”

“…Although there have been numerous studies on the dynamic response of helical SMA actuators (see [4][5][6][7][8][9][10][11][12][13][14][15]), their mechanical behaviour is yet to be completely understood. Additionally, most of these studies involve using as-drawn SMA wire to be heat treated into a spring; as-drawn SMA wire is not as readily available than annealed SMA wire and is generally more costly.…”