Characterization of NiTiNOL for Smart Structural Systems using ACES Methodology
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Characterization of NiTiNOL for Smart Structural Systems using ACES Methodology Shivananda P. Mizar and Ryszard J. Pryputniewicz Department of Mechanical Engineering Worcester Polytechnic Institute, 100 Institute Road Worcester MA 01609-2280 ABSTRACT SMAs are engineering alloys that recover extensive amounts of deformation when subjected to a temperature change. The recovery of deformation is as result of a phase transformation from martensite to austenite. For example, for NiTiNOL, a very popular SMA, this transformation occurs at 60ºC. Actuation is a type of shape memory process that utilizes deformation recovery that is used in smart structural systems. These systems are designed to detect and control structural deformations due to external time varying loads encountered during functional operation. In this paper, the actuating properties of NiTiNOL are characterized using an analytical, computational, and experimental solution (ACES) methodology. Analytical models are utilized to calculate the deformation under dynamic loads. Finite element method (FEM) is used for computational investigations. Optoelectronic holography (OEH) is the experimental method utilized to obtain temperature dependent behavior of NiTiNOL. As a result of the phase transformation, NiTiNOL undergoes a stiffness change indicated by a change in the modulus of elasticity from 33 GPa to 75 GPa. Deformations of NiTiNOL samples subjected to static loads at different temperatures were measured indicating recovery of 85% due to the phase transformation. In addition, amplitudes of vibration were measured as a function of excitation frequency and temperature to determine the effects of phase transformation. The results show effectiveness of NiTiNOL in development of smart structural systems INTRODUCTION Shape memory effect (SME) is a property of certain bi and tri metallic alloys exhibiting martensitic transformation. The alloy is deformed in the low temperature phase, and will recover its original undeformed shape upon heating to a critical temperature known as the reverse transformation temperature. The effect was first found in an Au-47.5 at% Cd alloy by Chang and Read in 1951. SME was later publicized by its discovery in Ni-Ti alloys by Buehler et al in 1963[1]. SME is closely related to the martensitic transformation (MT). The MT is a solid to solid diffusionless phase transformation, in which atoms move cooperatively, and by a shear like mechanism. The MT occurs between a high temperature phase (parent phase, austenite) and a low temperature phase (martensite). Austenite is cubic and martensite has a crystal structure of lower symmetry. When the temperature is increased, martensite becomes unstable, and the reverse transformation (RT) from martensite to austenite takes place, and the martensite reverts to the parent phase in the original undeformed orientation. The existence of equiatomic NiTi was first reported by Laves, and he suggested the structure to be ordered and of the CsCl type [2]. Equiatomic NiTi is popularly known as NiTiNO
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