Factors Influencing the Suitability of Thermal Methods for Stress Analysis of NiTi Shape Memory Alloys

The material assumptions made to facilitate Thermoelastic Stress Analysis (TSA) are linear elasticity, material homogeneity and isotropy, and mechanical properties that are independent of temperature. The unusual shape memory and superelastic properties of near equiatomic NiTi alloys complicate the application of any experimental stress analysis technique, and in the case of TSA, make these assumptions invalid. This paper describes a detailed analysis conducted to characterise the material properties of NiTi shape memory alloys and to identify loading conditions suitable for quantitative stress analysis using TSA. The mechanical behaviour of the material in three distinct regions is considered and the suitability of each region for TSA is discussed. It is shown that the thermoelastic response is dependent on the mean stress when tested at room temperature in the pre-martensitic phase, due the presence of an intermediate R-phase. Theoretical calculations are used to confirm that this effect is related to the high temperature dependence of the material’s Young’s modulus. Introduction In Thermoelastic Stress Analysis (TSA) [1] it is assumed that the test material is homogenous and isotropic, that quasi-adiabatic conditions can be achieved by cyclically loading the specimen, the loading remains within the linear elastic range of the material, the mechanical properties of the material are not dependent on temperature and that the ambient temperature remains constant during testing [2, 3]. These assumptions are necessary for an accurate analysis; however an irregularity in the thermoelastic response that results from a failure to comply with one of the assumed conditions can be used to infer additional information about the loaded component. For example, it has been shown [4] that areas of residual stresses can be identified by examining variations in the thermoelastic constant or, non-adiabatic behaviour can be used to locate and quantify hidden damage [5]. The unusual mechanical behaviour of near equiatomic NiTi alloys is well reported in the literature, e.g. [6]; however, quantitative experimental stress analysis of components manufactured from this category of material has not yet been demonstrated. The material’s complex stress-straintemperature relationship and non-linear mechanical response make some techniques unsuitable and adds significant complexity to the task of interpreting data from other techniques [7]. In the present paper, the feasibility of using TSA to obtain quantitative stress data from a NiTi alloy is considered. The suitability of three loading regions is examined and affect of an intermediate R-phase on the thermoelastic response is investigated in detail. The possibility of using the thermoelastic response to infer information about the material phase change behaviour is investigated and the best approach for experimental analysis of NiTi alloy specimens is discussed. Applied Mechanics and Materials Online: 2008-07-11 ISSN: 1662-7482, Vols. 13-14, pp 225-230 doi:10.4028/www.scientific.net/AMM.13-14.225 © 2008 Trans Tech Publications, Switzerland This is an open access article under the CC-BY 4.0 license (https://creativecommons.org/licenses/by/4.0/) Mechanical Behaviour The mechanical behaviour of NiTi alloys is strongly dependent on the alloy composition, heat treatment history and mechanical work. It is therefore important to experimentally characterise the as-received test material. Fig. 1 shows a plot of stress versus strain obtained from a tensile test conducted on a Ni55.8Ti44.2 tube. The tubes were 3 mm in diameter, with a wall thickness of 0.25 mm, and were cut into specimens of length of 150 mm. The specimen was first loaded and unloaded at a strain rate of 0.025 /s, and then loaded from zero stress to failure at the same rate. Testing was conducted using an Instron (Bucks, UK) 8802 static test machine and strain readings were obtained using an extensometer with a 12.5 mm gauge length. Fig. 1 represents measurements from a single specimen; however, the data are in good agreement with other published work, e.g. [8]. 0 200 400 600 80