Chapter 2 In situ Observation of NiTi Transformation Behaviour: A Micro–Macro Approach Kasun S. Wickramasinghe, Rachel A. Tomlinson, and Jem A. Rongong Abstract A novel experimental investigation is presented of thermally and stress induced transformation behaviour of a Polycrystalline NiTi Shape Memory Alloy (SMA) plate for flexural-type applications: In situ techniques are employed to allow simultaneous macroscopic and microstructural observation of the SMA in a 4-point flexural test. Forming part of a wider research towards realising a NiTi SMA Variable Stator Vane assembly for the gas turbine engine, the study explores variables critical to flexural-type morphing NiTi structures: (1) temperature; (2) strain; and (3) cyclic loading. It builds a relationship between the macro and micro response of the SMA under these key variables and lends critical implications for the future understanding and modelling of shape memory alloy behaviour for all morphing applications. This paper presents the methodological aspects of this study. Keywords Shape memory • NiTi • In situ • Phase transformations • Micro–macro approach • Cyclic loading 2.1 Introduction Gas turbine performance development has been governed traditionally by the “worst case” deterioration and operating condition [1]. This leads to severe compromises and large safety margins. Active control of the engine operation using smart materials could potentially improve engine efficiency. Shape Memory Alloys (SMAs), a class of smart materials, exhibit several desirable characteristics exploitable for this purpose. NiTi, based on an equiatomic compound of nickel and titanium is the most widely used SMA in commercial applications [2]. Besides the ability of tolerating relatively large amounts of shape memory strain, NiTi shows high stability in cyclic applications, possesses an elevated electrical resistivity, and is corrosion resistant [3]. An exciting possibility is the incorporation of SMA in plate form into blade structures of the Gas Turbine compressor. Airflow control is introduced into compressor designs through the use of Variable Inlet Guide Vanes and a number of stages incorporating Variable Stator Vanes. They operate by progressively closing as the compressor speed is reduced from the original design value to maintain an acceptable air angle value into the following rotor blades. Traditionally, such a system encompasses a complex structure employing control levers that are actuated through an electrical or bleed air system. Switching this system to a NiTi plate based actuation mechanism is intrinsically very attractive due to its high power density, solid-state actuation, high damping capacity, durability and fatigue resistance. A concept actuator design using NiTi plates to form a solid-state actuator that replicates the behaviour of the VSV system is depicted in Fig. 2.1. In this concept, the activation of each NiTi plate translates to a deflection at the tip of the actuator. To achieve this, the system exploits a flexural type strain application/recovery using the Shape Memory Effect. K.S. Wickramasinghe • R.A. Tomlinson (*) • J.A. Rongong Department of Mechanical Engineering, University of Sheffield, Mappin St., Sheffield S11 3JD, UK e-mail: r.a.tomlinson@sheffield.ac.uk N. Sottos et al. (eds.), Experimental and Applied Mechanics, Volume 6: Proceedings of the 2014 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-06989-0_2, #The Society for Experimental Mechanics, Inc. 2015 13
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