Effect of bioprosthetic leaflet anisotropy on stent dynamics of Transcatheter Aortic Valve Replacement devices.

The assessment of stent fatigue in Transcatheter Aortic Valve Replacement (TAVR) systems is critical for the design of next-generation devices, both in vitro and in vivo. The mechanical properties of the bioprosthetic heart valves (BHVs) have a significant impact on the fatigue life of the metallic stent and thus must be taken into consideration when evaluating new TAVR device designs. This study aims to investigate the relationship between BHV anisotropic behaviour and the asymmetric deflections of the stent frame observed during in vitro testing. An explicit dynamics finite element model of the nitinol stent with attached bioprosthetic valve leaflets was developed to evaluate the deflections of the TAVR device under haemodynamic loading. Our results demonstrate that pericardium behaviour plays a dominant role in determining stent frame deflection. The anisotropic behaviour of the leaflets, resulting from collagen fibre orientation, affects the extent of deflection encountered by each commissure of the frame. This leads to asymmetric variation in frame deflection that can influence the overall fatigue life of the nitinol stent. This study highlights the importance of considering both the flexible nature of the metallic stent as well as the leaflet anisotropic behaviour in the design and fatigue assessment of TAVR systems.

Residual Stress Enhancement by Laser Shock Treatment in Chromium-Alloyed Steam Turbine Blades.

In-service turbine blade failures remain a source of concern and research interest for engineers and industry professionals with attendant safety and economic implications. Very high-pressure shock impacts from laser shots represent an evolving technique currently gaining traction for surface improvement and failure mitigation in engineering components. However, the physical characteristics and effects of parameter variations on a wide range of materials are still not fully understood and adequately researched, especially from a computational point of view. Using the commercial finite element code ABAQUS©, this paper explores the application of laser shock peening (LSP) in the enhancement of residual stresses in Chromium-based steel alloyed turbine blade material. Results of the numerically developed and experimentally validated LSP model show that peak compressive residual stresses (CRS) of up to 700 MPa can be induced on the surface and sub-surface layers, while the informed varying of input parameters can be used to achieve an increase in the magnitude of CRS imparted in the peened material. Analysis of the hierarchy of influence of the five input parameters under investigation on residual stress enhancement reveals the laser shock intensity as the most influential, followed in descending order of influence by the exposure time, shot size, degree of overlaps, and the angle of shot impact.