1,720,990 research outputs found
Vertebral prosthesis
No full textIl brevetto descrive una nuova protesi vertebrale custom made altamente porosa e leggera, in metamateriale auxetico biocompatibile, realizzata in titanio tramite un processo di manifattura additiva ottimizzato, ed impiegabile come elemento di sostituzione ossea in ambito oncologico a seguito di un intervento di vertebrectomia totale
Analisi efficiente delle tensioni in giunzioni incollate mediante tecniche agli elementi finiti
No full textPeriodic tetrahedral auxetic metamaterial
No full textIn this work, we introduce a novel three-dimensional auxetic mechanical metamaterial consisting of rotating tetrahedra connected by ideal hinges at their vertices, arranged to form a periodic framework structure. Through analytical and kinematic approaches, we evaluated the deformation behavior of the proposed idealized model, revealing Poisson’s ratios ranging between -0.36 and -2.72 and a transverse isotropic response as a result of its geometry. A specimen of the proposed metamaterial concept is designed by introducing deformable ribs between the solid units, and fabricated via additive manufacturing in polymeric material. Auxetic behavior of the prototype was assessed through a compression test and accurately predicted by a full-scale Finite Element model. We envisage that this new metamaterial design can have a significant impact on a wide range of engineering applications, particularly as bone substitute biomaterials
A design oriented multiaxial stress-based criterion for the strength assessment of adhesive layers
Full text linkAdhesively bonded joints are becoming widespread in the composites industry and therefore there is a need for quantitative information on the mechanical strength of the material used. The great strength and stiffness of a composites structure may be strongly undermined by their weakest part, the bonded joint. Unfortunately, the testing of adhesives in bulk form may not be representative of their behaviour in a layered state, typically quite thin, because of differences in the polymerization process and lack of adhesive-adherend interfaces. The drawback of the test in thin layer is the stress concentration at the edges, typical in the single lap or t-peel joints, and also the chance of having the adhesive subjected both to a shear and predominant peel stress. This work deals with the characterization of adhesives in thin film under uniform distributions of multi-axial stresses, which is the typical application condition. The test exploits a tubular butt-bonded specimen, previously investigated by the authors, which guarantees a non-singular stress field over the adhesive layer both in shear and normal directions. According to the analytical prediction, in addition to the direct normal stress, both radial and circumferential secondary stresses arise in the adhesive, due to the constrained lateral contraction imposed by the adherends (Poisson's effect). The test campaign investigates two chemically different, commercial adhesives, an acrylic and an epoxy resin. By means of a biaxial testing machine, we applied to the specimens eight different combinations of normal and shear loads ranging from pure tensile to a shear-compressive stress state. As expected, both the pure shear stress and the compressive stresses lead to better performances of the adhesive layer with respect to tensile loading. The authors compare a variety of failure criteria from the literature and propose a simple multiaxial criterion to obtain a failure envelop of the experimental data. The applicability of the criterion is also assessed on experimental tests found in literature on different configurations and gives fairly good results. The outcome of study is a simple stress based, failure criterion, which can be used to predict the failure of several adhesive bonded joints, relying only on monoaxial experimental data
A Comparison between Rotating Squares and Anti-Tetrachiral Systems: Influence of Ligaments on the Multi-Axial Mechanical Response
Full text linkRotating unit systems are one of the most important and well-known classes of auxetic
mechanical metamaterials. As their name implies, when loaded, these systems deform
primarily via rotation of blocks of material, which may be connected together either directly
through joints (or ‘joint-like’ connections made by overlapping vertices of the rotating units)
as in the case of rotating rigid polygonal-unit systems or by ligaments/ribs as in the case of
chiral honeycombs. In this work, we used Finite Element Analysis to investigate the effect
which the presence/absence of ligaments has on the on-axis and off-axis mechanical
properties of these systems by analysing two of the most well-known structures which
characterise these two cases: the rotating square system and the anti-tetrachiral honeycomb. It
was found that while the presence of ligaments has a negligible effect on the on-axis
Poisson’s ratio of these systems, it has a profound influence on nearly all other mechanical
properties as well as on the off-axis loading behaviour. Systems with ligaments were found to
exhibit a high level of anisotropy and also a severely reduced level of stiffness in comparison
to their non-ligamented counterparts. On the other hand, the rotating square system suffers
from high localized stress-intensities and has a very low strain-tolerance threshold. In
addition, an optimized ‘hybrid’ geometry which is specifically designed to capture the best
features of both the anti-tetrachiral and rotating square system, was also analysed. This work
shows the main differences between ligament-based and non-ligament-based auxetic
structures and also highlights the importance of considering the off-axis mechanical response
in addition to the on-axis properties when investigating such systems
Electrostatic pull‐in instability for tweezer architectures
Full text linkThe work investigates the static pull-in instability of electrostatically actuated tweezers with tubular electrodes. At a critical voltage, named pull-in voltage, the attraction force between the two electrodes causes the unexpected pull-in of the tubular cantilevers, which defines the limit of the elastic region of the system, especially in the case of carbon-nano tubes (CNTs) applications. The work aims to evaluate the lower and upper bounds of pull-in parameters of a tweezer device with the use of an accurate analytical model which allows to calculate the critical voltage and deflection values of the system. In order to assess the accuracy of the analytical model, we built a prototype and measured the critical pull-in voltage for different geometrical configurations of the device. The experimental results confirm the analytical predictions, with a maximum relative difference between the experimental and analytical values of the pull-in voltage lower than 13%
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
A biomimetic chiral auxetic vertebral meta-shell
Full text linkThe work presents a novel thin-walled biomimetic auxetic meta-shell for patient-specific vertebral orthopedic implants. The proposed design stemmed from the concept of an intrinsically multiple curved auxetic meta-structure, which is created by folding a two-dimensional bio-inspired chiral geometry according to the morphology of human vertebral cortical bones. Through a multi-view stereo digital image correlation system, we investigated the mechanical response of a bio-grade titanium (Ti6Al4V ELI) additively manufactured prototype of the meta-structure under compressive loadings. In addition, we analyzed the morphology of the prototype using a scanning electron microscopy and an optical image dimension measurement system both before and after compressive tests. An accurate Finite Element model, which exactly reproduced the geometry of the three-dimensional printed meta-shell, was implemented and calibrated against experimental results, obtaining a precise prediction tool of its mechanical response. The findings of this work demonstrate that the designed meta-shell shows a peculiar auxetic behavior, a targeted stiffness matching to that of human vertebral bone tissues and a higher global elastic strain capability compared to those of monolithic traditional vertebral body replacements
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