E. G. Little

Observations during mechanical testing of Nitinol

Superelastic and shape memory capabilities of Nitinol are strongly dependent on the alloy composition, its heat treatment, and mechanical deformation history. The current article presents a review of the behaviour of Nitinol and describes a characterization study conducted to determine the mechanical properties of the material, both by means of differential scanning calorimetry (DSC) and by mechanical testing at a range of temperatures. Values for key transformation temperatures are found using both techniques. It is concluded that mechanical deformation during sample preparation for DSC measurements may have led to material property modifications and hence erroneous phase transformation temperature values. It is shown that mechanical testing can provide a means of benchmarking DSC data.

Thermoelastic studies on Nitinol stents

In this paper a methodology for applying thermoelastic stress analysis (TSA) to superelastic, nickel-titanium (Nitinol), shape memory alloy self-expanding stents is described. A test rig has been designed that allows the stents to be loaded under a pseudophysiological loading of internal pressure. High-resolution thermoelastic data obtained from a stent are presented, and features in the data are identified where high signal and potentially high stresses exist. The data are examined in detail and it is demonstrated that the thermoelastic signal is linearly related to internal pressure but is also dependent on mean stress. To gain further understanding of the thermoelastic response of Nitinol, a series of calibration-type experiments were conducted on thin-walled Nitinol cylinders so that the thermoelastic response of Nitinol was characterized throughout its non-linear elastic range. The effect of the mean stress on the signal was identified experimentally and compared with theoretical calculations. It is suggested that, if testing is conducted at elevated temperature (>37°C), these effects can be eliminated and quantitative analysis can be achieved. Other issues such as non-adiabatic behaviour and strain rate dependence are also discussed.

Three-dimensional embedded strain gauge analysis of the effect of collared versus collarless prostheses on cement mantle stresses in a femoral model of a total hip replacement

Abstract Experimental and finite element analyses have attributed considerable significance to the role of the prosthetic collar in load transfer to the proximal femur; however, some clinical evidence suggests that collar/calcar contact is unnecessary and detrimental. This study investigates the effects of collared versus collarless prostheses on cement mantle stresses in a model analysis of the proximal femur. Three-dimensional embedded strain gauging is used in a three-times full-size model of an implanted left femur. Tensor stresses in the cement mantle are derived for a collarless Exeter™ stem, a small and a large medial-collared stem and a full-collared prosthesis. Results show that the collar causes an increase in proximal-medial compressive longitudinal stresses, with other proximal stresses significantly reduced. Subsidence of the stem in the cement mantle is prevented, inducing unfavourable pivoting of the prosthesis about the calcar into varus. The collar type produces only minor additional effects in the mechanism of load transfer. The analysis suggests that collars may inhibit the prosthesis from attaining long-term secondary stability, by preventing the stem subsidence evident with cemented collarless implants. Derived stresses also illustrate the considerable variation in the mechanism of load transfer between collared and collarless prostheses in both the proximal and distal regions.

The effect of the transverse component of the hip reaction on cement mantle stresses in a model of an implanted Exeter™ prosthesis

Abstract Clinical studies have attributed considerable importance to the torsional loading of prosthetic femoral implants; however, the effect of this load has frequently been neglected in load simulations of the hip. The objective of this study, therefore, is to investigate the effects of the transverse joint load component in a model analysis of the proximal femur. Cement mantle stresses in a three-times full-size model of an Exeter™ total hip replacement were investigated using three-dimensional embedded strain transducers. Six sites were analysed for two separate loading configurations, namely the two-dimensionally loaded single-legged stance and the ‘toe off’ phase of gait which represents a three-dimensional hip reaction. Results showed a considerable variation in stem/cavity contact conditions due to the application of the transverse load component. Furthermore, large distal bending stresses are induced in the sagittal plane, with considerable shearing stresses due to torsion evident at all sites. The study highlights the significance of the transverse load and emphasizes the considerable limitations of finite element studies in modelling realistic load-dependent interface conditions.

Thermoelastic analysis of high pressure angioplasty balloons

Abstract Failures in angioplasty balloons are investigated using typical destructive techniques. The material properties of moulded balloons are derived from tensile tests and used to establish the reasons for failure of the balloons. Thermoelastic stress analysis is used to determine the stress distribution in the balloons, and a means of interpreting the data to derive actual stresses is described. The departure from linear elastic behaviour in the angioplasty balloons is identified using thermoelastic analysis. The results from the thermoelastic analysis are discussed and compared with those from the destructive tests, and the thermoelastic technique is shown to be a potential new means for non-destructive analysis of angioplasty balloons.

Modelling an orthopaedic knee prosthesis as a layered elastic system part 1: Experimental and theoretical analyses

Abstract The purpose of this investigation was to produce a simplified model of the plastic tibial plateau of a typical unicondylar knee prosthesis that would allow the parametric study of contact stresses experienced by the plastic component during relatively severe loading conditions. This involved the design, production and testing of a three-dimensional axisymmetric embedded strain gauge model of the tibial plateau and the application of a suitable theoretical analysis. The principal feature of the strain gauge model was the possibility of varying the thickness during the experimental procedure while keeping the maximum number of embedded gauges active. The Hertzian contact theory was used as a basis for the prediction of integration errors associated with placing strain gauges in locations subject to large strain gradients. A theoretical analysis that took the layered nature of the contact model into account was carried out which provided full field data for comparison with Hertzian and experimental results. Good agreement was obtained between theoretical and experimental values along the model axis, while at off-axis locations theoretical results based on the layered analysis compared reasonably with embedded strain gauge data. Very slight discrepancies between the experimental and idealized boundary conditions present in the initial stages of testing resulted in significant differences between embedded strain gauge and theoretical data.

Thermoelastic Stress Analysis of Nitinol Self-Expanding Stents

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.

Experimental Studies of NiTi Self-expanding Stent Designs

Preliminary investigations to apply thermoelastic stress analysis (TSA) to Nitinol self-expanding stents are described. Tests conducted at high resolution indicated that a viable thermoelastic signal can be obtained from the fine stent structure. It is shown that it is possible to digitally compensate for errors arising from motion at this high resolution. High variability in Nitinol’s material properties with stress and temperature result in a complex thermoelastic response. Correction strategies are proposed to account for variation in material properties and to minimize errors due to thermal variations in order to derive calibration factors for the austenite and martensite material phases. The greatest challenge is identified as calibrating the thermoelastic response from the radially loaded stent structure where it is likely the material is highly inhomogeneous

Thermoelastic Stress Analysis of Vascular Devices

The research described in this plenary paper deals with the application of experimental techniques to medical devices used in the treatment of vascular disease. Vascular disease is a medical condition where fatty material narrows the artery. It is reported that over 4 million deaths can be attributed to the disease each year in Europe alone. Vascular disease can occur at locations throughout the arterial network. A blockage located in the carotid artery can cause what is known as a stroke, or if located in one of coronary arteries may lead to a heart attack. A major advance in the treatment of the disease was made when the Percutaneous Transluminal Coronary Angioplasty (PTCA) was introduced by Gruntzig in 1977. The procedure offered a non-invasive, cost effective and rapid treatment for cardiovascular disease and was soon adapted to treat diseased vessels elsewhere in the body. During a typical procedure access to the vascular system is gained via the femoral artery at the groin. From this point a cardiologist uses endovascular techniques to navigate a catheter, with a polymer balloon deflated and tightly wrapped around its tip, through the artery network to the site of the blockage. The balloon is positioned across the blockage and is momentarily inflated. As the balloon inflates it exerts a radial force on the accumulated atherosclerotic plaque material, displacing it and thereby restoring patency to the vessel. Angioplasty balloons are used at locations throughout the arterial network and therefore range in size depending on the intended site of operation. They are typically constructed from rigid polymeric materials and are designed to exhibit low compliance at inflation high pressures; the balloon material is anisotropic. Fig. 1 shows a commercially available balloon. Studies have found the incidence of restenosis post angioplasty to be as high as 30 - 50%. To combat thisa fine mesh, metallic, cylindrical component known as a intravascular stent may be implanted as a follow up procedure to the angioplasty. The device is permanently implanted and acts as a scaffold within the artery (see Fig. 2). Two principal categories of stents exist: balloon expandable and self-expanding. The former are plastically deformed into position using an angioplasty balloon and are typically constructed from a stainless steel alloy. The latter, self-expanding stents (see Fig. 2) are deployed from the tip of a catheter and expand elastically into position, to provide support to the vessel wall. Self-expanding stents are constructed from a NiTi alloy commonly known as Nitinol. Nitinol is classed as a superelastic material as it can elastically recover strains in excess of 8% via a stress induced (reversible) transformation from a parent austenite microstructure to a martensitic phase. The stress-strain curve follows a non-linear path with significant thermal variations occurring as the material undergoes the phase change.

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.