James Eaton-Evans

AAA Stent–Grafts: Past Problems and Future Prospects

Endovascular aneurysm repair (EVAR) has quickly gained popularity for infrarenal abdominal aortic aneurysm repair during the last two decades. The improvement of available EVAR devices is critical for the advancement of patient care in vascular surgery. Problems are still associated with the grafts, many of which can necessitate the conversion of the patient to open repair, or even result in rupture of the aneurysm. This review attempts to address these problems, by highlighting why they occur and what the failings of the currently available stent grafts are, respectively. In addition, the review gives critical appraisal as to the novel methods required for dealing with these problems and identifies the new generation of stent grafts that are being or need to be designed and constructed in order to overcome the issues that are associated with the existing first- and second-generation devices.

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.

Thermoelastic assessment of plastic deformation

Thermoelastic stress analysis (TSA) is used to assess the effect of plastic deformation in small-diameter stainless steel pipework that has been subjected to multiple deformation cycles. In theory the residual stress resulting from the plastic deformation cannot be detected as the thermoelastic response is only a function of the stress change. However, it has been shown that plastic deformation may modify the thermoelastic constant in steel and aluminium. This change can be used to estimate the level of plastic strain that a component has experienced and has potential for TSA to be used as the basis for non-contact non-destructive full-field residual stress assessment. In this paper it is shown that strain hardening during the deformation process plays an important role in modifying the thermoelastic constant. X-ray computed tomography is used on the pipework to verify the estimates of geometry change that are used in determining the applied stress and estimating the residual stress levels. It is shown that TSA is not sensitive to residual stress; however, some interesting anomalies appear in the experimental results that provide a basis for discussion and open avenues of further work. Finally the non-adiabatic thermoelastic response is used to assess whether any damage has occurred in the pipework.

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.

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.

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.

A New Approach to Stress Analysis of Vascular Devices Using High Resolution Thermoelastic Stress Analysis

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.