A Finite Element Based Approach for Nonlocal Stress Analysis for Multi-Phase Materials and Composites

© 2024 The Author(s). This is an open access article distributed under the Creative Commons Attribution License, to view a copy of the license, see: https://creativecommons.org/licenses/by/4.0/This study proposes a numerical method for calculating the stress fields in nano-scale multi-phase/composite materials, where the classical continuum theory is inadequate due to the small-scale effects, including intermolecular spaces. The method focuses on weakly nonlocal and inhomogeneous materials and involves post-processing the local stresses obtained using a conventional finite element approach, applying the classical continuum theory to calculate the nonlocal stresses. The capabilities of this method are demonstrated through some numerical examples, namely, a two-dimensional case with a circular inclusion and some commonly used scenarios to model nanocomposites. Representative volume elements of various nanocomposites, including epoxy-based materials reinforced with fumed silica, silica (Nanopox F700), and rubber (Albipox 1000) are subjected to uniaxial tensile deformation combined with periodic boundary conditions. The local and nonlocal stress fields are computed through numerical simulations and after post-processing are compared with each other. The results acquired through the nonlocal theory exhibit a softening effect, resulting in reduced stress concentration and less of a discontinuous behaviour. This research contributes to the literature by proposing an efficient and standardised numerical method for analysing the small-scale stress distribution in small-scale multi-phase materials, providing a method for more accurate design in the nano-scale regime. This proposed method is also easy to implement in standard finite element software that employs classical continuum theory.Peer reviewe

An advanced underplatform damper modelling approach based on a microslip contact model

International audienceHigh-cycle fatigue caused by large resonance stresses remains one of the most common causes of turbine blades failures. Friction dampers are one of the most effective and practical solutions to limit the vibration amplitude, and shift the resonance frequencies of the turbine assemblies far from operating speeds. However, predicting with good accuracy the effects of underplatform dampers on the blades dynamics, still represents a major challenge today, due to the complex nature of the nonlinear forces at the interface, characterised by transitions between stick, slip, and separation conditions. The most common modelling approaches developed recently are based on the explicit FE model for the damper, and on a dense grid of 3D contact elements comprised of Jenkins elements, or on a single 2D microslip element on each surface. In this paper, a combination of the two approaches is proposed. A 3D microslip element, based on a modified Valanis model is proposed and a series of these elements are used to describe the contact interface. The proposed model and its predicting capabilities are then evaluated against a simplified blade-damper model, based on an underplatform damper test rig recently developed by the authors. A comparison with a more simplistic modelling approach based on macroslip contact elements, highlights the improved accuracy of the new model to predict the experimental nonlinear response

Computation of quasi-periodic localised vibrations in nonlinear cyclic and symmetric structures using harmonic balance methods

In this paper we develop a fully numerical approach to compute quasi-periodic vibrations bifurcating from nonlinear periodic states in cyclic and symmetric structures. The focus is on localised oscillations arising from modulationally unstable travelling waves induced by strong external excitations. The computational strategy is based on the periodic and quasi-periodic harmonic balance methods together with an arc-length continuation scheme. Due to the presence of multiple localised states, a new method to switch from periodic to quasi-periodic states is proposed. The algorithm is applied to two different minimal models for bladed disks vibrating in large amplitudes regimes. In the first case, each sector of the bladed disk is modelled by a single degree of freedom, while in the second application a second degree of freedom is included to account for the disk inertia. In both cases the algorithm has identified and tracked multiple quasi-periodic localised states travelling around the structure in the form of dissipative soliton

Nonlinear modal analysis of frictional ring damper for compressor blisk

The use of integrally blisk is becoming popular because of the advantages in aerodynamic efficiency and mass reduction. However, in an integrally blisk, the lack of the contact interface leads to a low structural damping compared to an assembled bladed-disk. One emerging damping technique for the integrally blisk is based on the use of friction ring damper which exploits the contact interfaces at the underneath of the disk. In this paper, three different geometries of the ring dampers are investigated for damping enhancement of a blisk. A full-scale compressor blisk is considered as a case study where a node to node contact model is used to compute the contact forces. The dynamic behaviour of the blisk with the ring damper is investigated by using nonlinear modal analysis which allows a direct estimation of the damping generated by the friction interface. The damping performance for the different ring dampers are evaluated and compared. It appears that the damping efficiency as well as the shift in the resonant frequency for the different geometries are highly related to the nodal diameter and contact pressure/gap distributed within contact interface. The geometry of the ring damper has significant impact on the damping performance

Non-intrusive reduced order modelling for the dynamics of geometrically nonlinear flat structures using three-dimensional finite elements

Non-intrusive methods have been used since two decades to derive reduced-order models for geometrically nonlinear structures, with a particular emphasis on the so-called STiffness Evaluation Procedure (STEP), relying on the static application of prescribed displacements in a finite-element context. We show that a particularly slow convergence of the modal expansion is observed when applying the method with 3D elements, because of nonlinear couplings occurring with very high frequency modes involving 3D thickness deformations. Focusing on the case of flat structures, we first show by computing all the modes of the structure that a converged solution can be exhibited by using either static condensation or normal form theory. We then show that static modal derivatives provide the same solution with fewer calculations. Finally, we propose a modified STEP, where the prescribed displacements are imposed solely on specific degrees of freedom of the structure, and show that this adjustment also provides efficiently a converged solution

Strain-rate-dependent mechanics and impact performance of epoxy-based nanocomposites

Strain-rate-dependent mechanical properties and impact performance of manufactured epoxy-based nanocomposites are investigated. As reinforcements, fumed silica (FS) and halloysite nanotube (HNT) are used alongside Albipox 1000 and Nanopox F700. First, the internal structures of the composites are visualised using scanning electron microscopy (SEM). To identify the strain-rate-dependent mechanical properties, three-point bend tests are conducted at three different strain rate levels. For the impact resistance, Charpy impact tests are performed. For further investigations of the mechanical properties of the composites, mean-field homogenisation (MFH) and finite element (FE) analyses on the representative volume elements (RVE) are performed for each type of composite material. Overall, the modelling and experiments are in good agreement and account for the mechanical behaviour of these epoxy-based nanocomposites

Dissipative solitons in forced cyclic and symmetric structures

The emergence of localised vibrations in cyclic and symmetric rotating struc-tures, such as bladed disks of aircraft engines, has challenged engineers in thepast few decades. In the linear regime, localised states may arise due to alack of symmetry, as for example induced by inhomogeneities. However, whenstructures deviate from the linear behaviour, e.g. due to material nonlinearities,geometric nonlinearities like large deformations, or other nonlinear elements likejoints or friction interfaces, localised states may arise even in perfectly symmet-ric structures. In this paper, a system consisting of coupled Duffing oscillatorswith linear viscous damping is subjected to external travelling wave forcing.The system may be considered a minimal model for bladed disks in turboma-chinery operating in the nonlinear regime, where such excitation may arise dueto imbalance or aerodynamic excitation. We demonstrate that near the reso-nance, in this non-conservative regime, localised vibration states bifurcate fromthe travelling waves. Complex bifurcation diagrams result, comprising stableand unstable dissipative solitons. The localised solutions can also be continuednumerically to a conservative limit, where solitons bifurcate from the backbonecurves of the travelling waves at finite amplitudes

A French multicentric prospective prognostic cohort with epidemiological, clinical, biological and treatment information to improve knowledge on lymphoma patients: study protocol of the "REal world dAta in LYmphoma and survival in adults" (REALYSA) cohort.

BACKGROUND: Age-adjusted lymphoma incidence rates continue to rise in France since the early 80's, although rates have slowed since 2010 and vary across subtypes. Recent improvements in patient survival in major lymphoma subtypes at population level raise new questions about patient outcomes (i.e. quality of life, long-term sequelae). Epidemiological studies have investigated factors related to lymphoma risk, but few have addressed the extent to which socioeconomic status, social institutional context (i.e. healthcare system), social relationships, environmental context (exposures), individual behaviours (lifestyle) or genetic determinants influence lymphoma outcomes, especially in the general population. Moreover, the knowledge of the disease behaviour mainly obtained from clinical trials data is partly biased because of patient selection. METHODS: The REALYSA ("REal world dAta in LYmphoma and Survival in Adults") study is a real-life multicentric cohort set up in French areas covered by population-based cancer registries to study the prognostic value of epidemiological, clinical and biological factors with a prospective 9-year follow-up. We aim to include 6000 patients over 4 to 5 years. Adult patients without lymphoma history and newly diagnosed with one of the following 7 lymphoma subtypes (diffuse large B-cell, follicular, marginal zone, mantle cell, Burkitt, Hodgkin, mature T-cell) are invited to participate during a medical consultation with their hematologist. Exclusion criteria are: having already received anti-lymphoma treatment (except pre-phase) and having a documented HIV infection. Patients are treated according to the standard practice in their center. Clinical data, including treatment received, are extracted from patients' medical records. Patients' risk factors exposures and other epidemiological data are obtained at baseline by filling out a questionnaire during an interview led by a clinical research assistant. Biological samples are collected at baseline and during treatment. A virtual tumor biobank is constituted for baseline tumor samples. Follow-up data, both clinical and epidemiological, are collected every 6 months in the first 3 years and every year thereafter. DISCUSSION: This cohort constitutes an innovative platform for clinical, biological, epidemiological and socio-economic research projects and provides an opportunity to improve knowledge on factors associated to outcome of lymphoma patients in real life. TRIAL REGISTRATION: 2018-A01332-53, ClinicalTrials.gov identifier: NCT03869619

Etude de l'usure par fretting sous chargements dynamiques dans les interfaces frottantes : application aux pieds d'aubes de turbomachines

The rotating parts of aeronautical turbomachineries are made of bladed disks which enable the transfer of energy from the air to the rotor. These bladed disks are especially critical parts because their dimensioning has to meet strict requirements in terms of aerodynamical performance, aeroacoustics and mechanical resistance to rotation, temperature and aerodynamical loads. The frictional contact at the interface between blade and disk has an important influence on the vibratory levels.This work deals with the study of fretting-wear in frictional interfaces under dynamical loading. Indeed,the blade's vibrations can produce micro-slidings in blade's root which may entail fretting-wear. Wear under dynamical loading is a badly known phenomenon. Numerical tools exist for quasic-static conditions only. Here,methods are proposed to quantify the evolution of wear along dynamical loadingcycles based on a time-multiscale approach. The vibratory response of the structure is linked with a fasttime scale which is calculated by a harmonic balance method : displacements and forces are expressed through Fourier series. Different frequency-time approaches are presented to compute the Fourier coefficients of contact forces. Wear kinetics is linked with a slow time scale and different methods are proposed to integrate it.Worn geometries are taken into account in the finite elements model by a wear depth vector included in the penalty term of dynamic lagrangians. Academic examples validate and illustrate the proposed methods. These methods are then used to study fretting-wear in a fan's bladeroot. The numerical results highlight the coupling between vibration and fretting-wear in frictional interfaces.The modification of the global dynamical behaviour of the bladed disk is also observed.Les parties tournantes des turbomachines aéronautiques sont composées d'une succession de roues aubagées qui permettent le transfert de l'énergie entre l'air et le rotor. Ces roues aubagées constituent des pièces particulièrement sensibles car elles doivent répondre en termes de dimensionnement à des impératifs de performances aérodynamiques, d'aéroacoustique et de tenue mécanique à la rotation,à la température et à la charge aérodynamique. Le contact avec frottement existant au niveau des attaches aube-disque joue un rôle important sur les niveaux vibratoires.Ce travail porte sur l'étude de l'usure par fretting sous chargements dynamiques dans les interfaces frottantes. En effet, les vibrations de l'aube peuvent produire des micro-glissements en pied d'aubequi peuvent entraîner un phénomène d'usure par fretting. Les connaissances sur le comportement de l'usure sous sollicitations dynamiques sont faibles. Seuls existent des outils numériques pour modéliser l'usure dans le cas de sollicitations quasi-statiques. Nous proposons dans cette thèse des méthodes pour calculer l'évolution de l'usure au cours des cycles de chargement dynamique basées sur une approche multi-échelle en temps. La réponse vibratoire de la structure est liée à une échelle de temps rapide qui est calculée par une méthode d'équilibrage harmonique, dans laquelle les déplacements et les efforts sont projetés sur la base de Fourier. Différentes approches temps-fréquence de calcul des coefficients de Fourier des forces de contact sont présentées. La cinétique d'usure est liée à une échelle lente et différentes méthodes sont proposées pour l'intégrer. La prise en compte des géométries usées dans le modèle éléments finis se fait par l'ajout d'un vecteur des profondeurs d'usure dans le terme de pénalité des lagrangiens dynamiques. Des exemples académiques valident et illustrent les méthodes proposées. Ces méthodes sont ensuite appliquées à l'étude de l'usure par fretting en pied d'aube de soufflante. L'étude numérique met en lumière le couplage entre vibration et usure par fretting aux interfaces de contact. La modification du comportement dynamique global de la roue aubagée est aussi observée

Chapter 4- Taste and trigeminal perception; from detection to integration

International audienceTaste is crucial for the sensory evaluation of food. This sense allows the detection of essential nutrients and toxic compounds. Human beings are able to discriminate five primary different taste qualities, sweet, umami (the taste of amino acids), bitter, salty, and sour. Tasting compounds are remarkably diverse chemically. They activate specialized detectors expressed in the oral cavity. In this chapter, we review the main tasting molecules known to generate these five tastes and trigeminal compounds. This chapter describes also the current knowledge on the physiology of taste, the integration of taste perception, and how taste-taste interaction occurs at the brain level