255 research outputs found

    Wave propagation in relaxed micromorphic continua: modelling metamaterials with frequency band-gaps

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    In this paper the relaxed micromorphic model proposed in [Patrizio Neff, Ionel-Dumitrel Ghiba, Angela Madeo, Luca Placidi, Giuseppe Rosi. A unifying perspective: the relaxed linear micromorphic continuum, submitted, 2013, arXiv:1308.3219; and Ionel-Dumitrel Ghiba, Patrizio Neff, Angela Madeo, Luca Placidi, Giuseppe Rosi. The relaxed linear micromorphic continuum: existence, uniqueness and continuous dependence in dynamics, submitted, 2013, arXiv:1308.3762] has been used to study wave propagation in unbounded continua with microstructure. By studying dispersion relations for the considered relaxed medium, we are able to disclose precise frequency ranges (band-gaps) for which propagation of waves cannot occur. These dispersion relations are strongly nonlinear so giving rise to a macroscopic dispersive behavior of the considered medium. We prove that the presence of band-gaps is related to a unique elastic coefficient, the so-called Cosserat couple modulus μc\mu_{c}, which is also responsible for the loss of symmetry of the Cauchy force stress tensor. This parameter can be seen as the trigger of a bifurcation phenomenon since the fact of slightly changing its value around a given threshold drastically changes the observed response of the material with respect to wave propagation. We finally show that band-gaps cannot be accounted for by classical micromorphic models as well as by Cosserat and second gradient ones. The potential fields of application of the proposed relaxed model are manifold, above all for what concerns the conception of new engineering materials to be used for vibration control and stealth technology

    A unifying perspective: the relaxed linear micromorphic continuum

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    We formulate a relaxed linear elastic micromorphic continuum model with symmetric Cauchy force-stresses and curvature contribution depending only on the micro-dislocation tensor. Our relaxed model is still able to fully describe rotation of the microstructure and to predict non-polar size-effects. It is intended for the homogenized description of highly heterogeneous, but non polar materials with microstructure liable to slip and fracture. In contrast to classical linear micromorphic models our free energy is not uniformly pointwise positive definite in the control of the independent constitutive variables. The new relaxed micromorphic model supports well-posedness results for the dynamic and static case. There, decisive use is made of new coercive inequalities recently proved by Neff, Pauly and Witsch and by Bauer, Neff, Pauly and Starke. The new relaxed micromorphic formulation can be related to dislocation dynamics, gradient plasticity and seismic processes of earthquakes. It unifies and simplifies the understanding of the linear micromorphic models

    Controlling the Limit-Cycle of the Ziegler Column via a Tuned Piezoelectric Damper

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    This paper is about the nonlinear analysis of a piezoelectric controlled Ziegler column. The piezoelectric controller, here referred to as Tuned Piezoelectric Damper (TPD), possesses evanescent characteristics and, moreover, it is tuned to the first natural frequency of the mechanical system, thus resembling the well-known Tuned Mass Damper. This means that the flow of energy between mechanical and electrical subsystems is driven by the resonance (Den Hartog principle) and magnified by the singularity of the evanescent electrical characteristics. Numerical simulations, showing how the proposed control strategy is effective in increasing the linear stability domain and decreasing the amplitude of the limit-cycles in the postcritical range, are presented

    Ex Vivo Evaluation of Cementless Acetabular Cup Stability Using Impact Analyses with a Hammer Instrumented with Strain Sensors

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    International audienceThe acetabular cup (AC) implant stability is determinant for the success of cementless hip arthroplasty. A method based on the analysis of the impact force applied during the press-fit insertion of the AC implant using a hammer instrumented with a force sensor was developed to assess the AC implant stability. The aim of the present study was to investigate the performance of a method using a hammer equipped with strain sensors to retrieve the AC implant stability. Different AC implants were inserted in five bovine samples with different stability conditions leading to 57 configurations. The AC implant was impacted 16 times by the two hammers consecutively. For each impact; an indicator I S (respectively I F) determined by analyzing the time variation of the signal corresponding to the averaged strain (respectively force) obtained with the stress (respectively strain) hammer was calculated. The pull-out force F was measured for each configuration. F was significantly correlated with I S (R 2 = 0.79) and I F (R 2 = 0.80). The present method has the advantage of not modifying the shape of the hammer that can be sterilized easily. This study opens new paths towards the development of a decision support system to assess the AC implant stability

    Smart drugs and neuroenhancement: what do we know?

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    Introduction: Smart drugs are among the most common drugs used by students. It is estimated that they are second in incidence after cannabis. Although they are usually used for diseases such as attention deficit hyperactivity disorder (ADHD) and dementia, in most cases the use of smart drugs is illegal and without a prescription. Methodological issues: A systematic review was conducted according to PRISMA guidelines. SCOPUS, Medline (using PubMed as a search engine), Embase, Web of Sciences, and Google Scholar were used as search engines from January 1, 1980 to June 1, 2021 to evaluate the association between smart drugs and neuro-enhancement. A total of 4715 articles were collected. Of these, 295 duplicates were removed. A total of 4380 articles did not meet the inclusion criteria. In conclusion, 48 articles were included in the present systematic review. Results: Most of the studies were survey studies, 1 was a prospective longitudinal study, 1 was a cross-over study, and 1 was an experimental study in an animal model (rats). The largest group of consumers was school or university students. The most frequent reasons for using smart drugs were: better concentration, neuro enhancement, stress reduction, time optimization, increased wake time, increased free time, and curiosity. There are conflicting opinions, in fact, regarding their actual functioning and benefit, it is not known whether the benefits reported by consumers are due to the drugs, the placebo effect or a combination of these. The real prevalence is underestimated: it is important that the scientific community focus on this issue with further studies on animal models to validate their efficacy

    Wave propagation in the framework of strain gradient continua: the example of hexachiral materials

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    Chirality, which means handedness, is a general property of asymmetry that is of prior importance in many fields of modern physics and biology. An object, or a system, is said to be chiral if it is not identical to its mirror image. Many materials exhibit chirality at different scales. At the microscopic scale chirality can be observed, for example, in chemistry, biological molecules (amino acids, protein, sugar), crystals, liquid crystals. Chiral organization is also observed at larger scales in bones, fiber-reinforced composites, mechanical structures. Effect of chirality on optical wave propagation is known since the first half of the 19th century with the work on crystal optical activity by, among others, Arago, Biot and Pasteur. But contrary to electromagnetism, the classical theory of elasticity is not chiral sensitive, although mechanical evidence of its importance has been reported in numerous and various situations. As a consequence, during the last few decades, different attempts have been made to extend the classical framework of elasticity in order to make it chirality-dependent. As shown in [1] a continuum description of chirality can be achieved using generalized continua theories

    Comparison between local and regional anesthesia in arteriovenous fistula creation.

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    PURPOSE: Assessment of the effectiveness of Brachial Plexus Block (BPB) via axillary approach compared to regional anesthesia for arteriovenous fistula surgery in patients affected by end-stage renal disease. METHODS: We compared forty patients randomly divided into two groups. Group A underwent BPB procedure with 15 mL ropivacaine 1% and 10 mL of saline (0.9% NaCl) via axillary approach. Group B received local anesthesia with lidocaine 2%. The forearm blood vessels were assessed by Doppler ultrasonography before and after the intervention. RESULTS: BPB performed on Group A was associated with a considerable venous dilation and a significant decrease (48.7%, P<.05) in pulsatility index (PI) measured by Doppler ultrasound. In Group B, PI and venous dilation remained unaltered in the postoperative phase. No complications such as thrombosis or occlusion were encountered among patients who underwent BPB. CONCLUSIONS: The axillary-approached BPB was more advantageous than local anesthesia. Its effectiveness was because of venous dilation and the decrease in the PI, consequent to the reduction in peripheral resistances and the increase in local blood flow, thus offering an ideal background for fistula creation and short-term patency
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