1,721,097 research outputs found
A three-dimensional phenomenological model describing cyclic behavior of shape memory alloys
Full text linkNowadays, shape memory alloys, in particular Nickel–Titanium alloys (NiTi), are widely adopted in many fields (biomedical, aerospace, automotive, civil ...) for producing devices exploiting pseudo-elasticity or shape memory effect. These devices are often subjected to cyclic loadings and sometimes to large deformations that modify the material response inducing functional fatigue and/or plastic strains. Accordingly, the device effectiveness is limited or even completely compromised. In this paper, a model able to take into account both the phenomena is presented. Aiming to propose this model for the design and assessment of cyclically loaded devices by finite element simulations, a phenomenological approach was used, introducing internal variables able to describe the accumulation of inelastic strains due to fatigue and plasticity and assuming that their evolution law affects also the phase transformation domain amplitude. Two limit functions was introduced for defining the phase transformation domain and plastic region. The discrete formulation of the model is also presented. Peculiar attention is posed on the material model parameters and the procedure to calibrate them. Initially, a driver code was written in MATLAB for verifying the algorithm correctness and effectiveness; successively, the model was implemented by a user subroutine in a commercial finite element code. The results of several numerical tests are reported in order to show the model functionality and stability under different loading conditions. Finally, the model capability of reproducing 1D and 3D experimental tests is shown by comparison of numerical results with experimental tests published in the literature
A numerical investigation on multiaxial fatigue assessment of Nitinol peripheral endovascular devices with emphasis on load non-proportionality effects
No full textThis work first numerically investigates on the effect of the application of cyclic remote loads on the local stress/strain response of a peripheral stent. For this purpose, three stents of identical geometry, but different material properties (linear elastic, elastic-plastic and Nitinol material), were modelled and subjected to uniaxial and multiaxial proportional cyclic loads. The local response under uniaxial loads is non-proportional only for the case of Nitinol stent, while it happens for all the materials when multiaxial loads are applied. This paper also aims to evaluate how the use of different multiaxial fatigue criteria may affect the fatigue assessment for Nitinol peripheral stents. The same stent model was subjected to different proportional and non-proportional multiaxial cyclic loads and the results were interpreted using the standard von Mises and three approaches based on the concept of critical plane. The results showed that the von Mises is the most conservative criterion
Analytical methods for braided stents design and comparison with FEA
Full text linkBraiding technology is nowadays commonly adopted to build stent-like devices. Indeed, these endoprostheses, thanks to their typical great flexibility and kinking resistance, find several applications in mini-invasive treatments, involving but not limiting to the cardiovascular field. The design process usually involves many efforts and long trial and error processes before identifying the best combination of manufacturing parameters. This paper aims to provide analytical tools to support the design and optimization phases: the developed equations, based on few geometrical parameters commonly used for describing braided stents and material stiffness, are easily implementable in a worksheet and allow predicting the radial rigidity of braided stents, also involving complex features such as multiple twists and looped ends, and the diameter variation range. Finite element simulations, previously validated with respect to experimental tests, were used as a comparator to prove the reliability of the analytical results. The illustrated tools can assess the impact of each selected parameter modification and are intended to guide the optimal selection of geometrical and mechanical stent proprieties to obtain the desired radial rigidity, deliverability (minimum diameter), and, if forming processes are planned to modify the shape of the stent, the required diameter variations (maximum and minimum diameters)
A discussion about multi-axial fatigue criteria for NiTinol cardiovascular devices
No full textNickel-Titanium (NiTinol) alloys exploit a typical super-elastic behavior which makes them suitable for many biomedical applications, among which peripheral stenting, requiring the device being subjected to the high mobility of the lower limbs. Unfortunately, this complex environment can lead to the device fatigue fracture with likely other more severe complications, e.g. restenosis. Standards require to experimentally verify stent fatigue life behavior, without giving indications on how to select the loads to be applied for resembling most critical in-vivo conditions. Moreover, different multi-axial fatigue criteria have been originally developed for standard metals to predict the behavior under cyclic loads, but none of them is specifically formulated for NiTinol. This paper presents a numerical study having two aims: i) understanding how non-proportional loading conditions due to combination of axial compression, bending and torsion induced at each patient gait on the femoro-popliteal artery affects the implanted stent stress/strain distribution; ii) understanding how stent fatigue life prediction may be affected by the choice of the fatigue criteria. Accordingly, two different peripheral stent geometries, resembling commercial ones, were analysed under different sets of loading conditions. The cyclic deformations induced over the device structure by macroscopic loads are interpreted through four different fatigue approaches recently used in Nitinol fatigue analyses: Von Mises, Fatemi-Socie, Brown-Miller and Smith-Watson-Topper. The comparison between the outputs highlights that they are strongly influenced by the loading path, recognizing the major role in fatigue due to the combined torsional and bending actions. On the other hand, the choice of the fatigue criterion impacts on the fatigue life prediction
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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