Effect of Inclusion Size and Distribution on the Corrosion Behavior of Medical-Device Grade Nitinol Tubing

Nonmetallic inclusions in Nitinol, such as carbides (TiC) and intermetallic oxides (Ti₄Ni₂O _x ), are known to be triggers for fatigue failure of Nitinol medical devices. These mechanically brittle inclusions are introduced during the melting process. As a result of hot and cold working in the production of Nitinol tubing inclusions are fractionalized due to the mechanical deformation imposed. While the role of inclusions regarding Nitinol fatigue performance has been studied extensively in the past, their effect on Nitinol corrosion behavior was investigated in only a limited number of studies. The focus of the present work was to understand the effect of inclusion size and distribution on the corrosion behavior of medical-device grade Nitinol tubing made from three different ingot sources during different manufacturing stages: (i) for the initial stage (hollow: round bar with centric hole), (ii) after hot drawing, and (iii) after the final drawing step (final tubing dimensions: outer diameter 0.3 mm, wall thickness 0.1 mm). For one ingot source, two different material qualities were investigated. Potentiodynamic polarization tests were performed for electropolished samples of the above-mentioned stages. Results indicate that inclusion size rather than inclusion quantity affects the susceptibility of electropolished Nitinol to pitting corrosion.     

Non‐destructive detection of corrosion applied to steel and galvanized steel coated with organic paints by the pulsed phase thermography

Organic coatings in the automobile industry have to resist corrosion and mechanical damage from stone chipping. Currently, no tool is established in industrial non‐destructive applications for analyzing the damage of stone‐impacts and the following corrosion after accelerated corrosion tests. Measurement methods such as the scanning Kelvin probe can analyze the corrosion progress in a detailed manner, but with a long measurement time. The pulsed phase thermography (PPT) is a non‐destructive tool to analyze inhomogeneities and defects in materials, with a huge field of applications existing. The present work shows advances in using the PPT to detect propagation of corrosion under coatings. Physical principles of the mechanism of the corrosion detection under coatings are described. Results of measurements of organic coatings on carbon steel as well as of organic coated galvanized steel show the corrosion propagation. Influencing factors to the measurement such as the thickness of the coatings are investigated, but no significant effect on the quality of the analysis was found. The corrosion progress can be monitored by the PPT fast and reliably. The achieved results correlate with the theoretical basis and the test results after surface characterization and destructive analysis of samples.     

Effect of crystal habit and superstructure on modulus of elasticity of isotactic polypropylene by AFM nanoindentation

In this study the effects of crystal habit, crystallinity and superstructure on the modulus of elasticity of isotactic polypropylene films were analyzed by atomic force microscope (AFM) nanoindentation. The modulus of elasticity, evaluated on the nanometer scale by AFM, showed a qualitatively similar dependence on the crystal habit, crystallinity and superstructure as the modulus of elasticity measured by dynamic-mechanical analysis and tensile testing. The observed values of both the surface stiffness measured by AFM and the macroscopic/bulk stiffness were distinctly larger in the presence of non-isometric lamellae organized in spherulite than in the presence of isometric nodular crystals, not organized in a spherulitic superstructure. The experimental data showed that the modulus of elasticity is not primarily influenced by the presence or absence of spherulite but by the molecular-deformation constraint associated to the crystal habit.     

Analysis of New Nitinol Ingot Qualities

New ingot qualities, processed by optimized vacuum arc remelting (VAR), optimized vacuum induction melting followed by VAR and VAR followed by electron beam remelting, were compared with standard quality. Finished components as well as diamond-shaped samples representing a typical dimension of self-expanding stents were produced using Nitinol tubing drawn from the new ingot qualities. Metallographic longitudinal sections were prepared and analyzed to determine inclusion size and distributions of the various ingot qualities. Radial force and uniaxial tensile tests were used to determine the mechanical properties of fully processed material and tubing, respectively. Transformation temperatures of tubing as delivered from supplier and processed stents were measured by differential scanning calorimetry and deformation-and-free-recovery testing. Finally, fatigue tests were performed on diamond-shaped samples to evaluate the strain-life characteristics of the new ingot qualities. Results of this study are compared to ADMEDES historical data from standard Nitinol materials to gain an assessment of the new improved ingot qualities with regard to the production of Nitinol vascular implants. The latest developments in Nitinol ingot quality are highlighted and the results of the comparison from technical point of view are shown.     

A Combined First and Second Order Variational Approach for Image Reconstruction

In this paper we study a variational problem in the space of functions of bounded Hessian. Our model constitutes a straightforward higher-order extension of the well known ROF functional (total variation minimisation) to which we add a non-smooth second order regulariser. It combines convex functions of the total variation and the total variation of the first derivatives. In what follows, we prove existence and uniqueness of minimisers of the combined model and present the numerical solution of the corresponding discretised problem by employing the split Bregman method. The paper is furnished with applications of our model to image denoising, deblurring as well as image inpainting. The obtained numerical results are compared with results obtained from total generalised variation (TGV), infimal convolution and Euler’s elastica, three other state of the art higher-order models. The numerical discussion confirms that the proposed higher-order model competes with models of its kind in avoiding the creation of undesirable artifacts and blocky-like structures in the reconstructed images—a known disadvantage of the ROF model—while being simple and efficiently numerically solvable.