Papers by Author: Edward G. Little

Abstract: 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.

Abstract: Thermoelastic Stress Analysis (TSA) is a non-contacting technique that provides full field stress information and can record high-resolution measurements from small structures. The work presented in this paper summarises the application of TSA to two types of small medical devices that are used to treat diseased arteries; angioplasty balloons and vascular stents. The use of high resolution optics is described along with a calibration methodology that allows quantitative stress measurements to be taken from the balloon structure. A brief account of a study undertaken to characterise the thermoelastic response from Nitinol is also included and it is demonstrated that thermoelastic data can be obtained from a stent at high resolutions.

Abstract: Self-expanding stents are small medical devices used to treat vascular disease and are typically fabricated from a super-elastic, shape memory alloy known as Nitinol and have a fine mesh structure. This paper describes preliminary work on the application of Thermoelastic Stress Analysis (TSA) to Nitinol stents. Uniaxial tensile tests were conducted on thin tubes of Nitinol to characterise the material mechanical properties. TSA calibration exercises were conducted, which showed that Nitinol exhibits a non-uniform thermoelastic response through its elastic region that corresponded to the superelastic behaviour. Initial TSA demonstrated that a viable thermoelastic signal could be obtained from the stents. In high resolution tests the effect of motion and noise were considerable but it was still possible to obtain a readable thermoelastic signal.