A thermodynamics-based constitutive model is derived which predicts the nonlinear strain and magnetization response that magnetic shape memory alloys (MSMAs) exhibit when subjected to mechanical and magnetic loads. The model development is conducted on the basis of an extended thermo-magneto-mechanical framework. A novel free energy function for MSMAs is proposed, from which the constitutive equations are derived in a thermodynamically-consistent manner. The nonlinear and hysteretic nature of the macroscopic material behavior is captured through the evolution of internal state variables which are motivated by the crystallographic and magnetic microstructures of MSMAs. Model predictions are presented for different relevant loading cases and analyzed in detail. Finally, magnetostatic boundary value problems for MSMAs are considered and numerically solved using the finite element method. For these computations the developed constitutive model provides the nonlinear magnetic properties of the MSMA. The knowledge of the magnetic field distribution in the computational domain as a function of the applied field, which results from this magnetostatic analysis, is useful for the proper interpretation of experimental results as well as the design of experiments and applications.

No description available.

<p align="justify">The conservation of structures of the historical heritage is an increasing concern nowadays for public authorities. The technical design phase of repair operations for these structures is of prime importance. Such operations usually require an estimation of the residual strength and of the potential structural failure modes of structures to optimize the choice of the repairing techniques.</p> <p align="justify">Although rules of thumb and codes are widely used, numerical simulations now start to emerge as valuable tools. Such alternative methods may be useful in this respect only if they are able to account realistically for the possibly complex failure modes of masonry in structural applications.</p> <p align="justify">The mechanical behaviour of masonry is characterized by the properties of its constituents (bricks and mortar joints) and their stacking mode. Structural failure mechanisms are strongly connected to the mesostructure of the material, with strong localization and damage-induced anisotropy.</p> <p align="justify">The currently available numerical tools for this material are mostly based on approaches incorporating only one scale of representation. Mesoscopic models are used in order to study structural details with an explicit representation of the constituents and of their behaviour. The range of applicability of these descriptions is however restricted by computational costs. At the other end of the spectrum, macroscopic descriptions used in structural computations rely on phenomenological constitutive laws representing the collective behaviour of the constituents. As a result, these macroscopic models are difficult to identify and sometimes lead to wrong failure mode predictions.</p> <p align="justify">The purpose of this study is to bridge the gap between mesoscopic and macroscopic representations and to propose a computational methodology for the analysis of plane masonry walls. To overcome the drawbacks of existing approaches, a multi-scale framework is used which allows to include mesoscopic behaviour features in macroscopic descriptions, without the need for an a priori postulated macroscopic constitutive law. First, a mesoscopic constitutive description is defined for the quasi-brittle constituents of the masonry material, the failure of which mainly occurs through stiffness degradation. The mesoscopic description is therefore based on a scalar damage model. Plane stress and generalized plane state assumptions are used at the mesoscopic scale, leading to two-dimensional macroscopic continuum descriptions. Based on periodic homogenization techniques and unit cell computations, it is shown that the identified mesoscopic constitutive setting allows to reproduce the characteristic shape of (anisotropic) failure envelopes observed experimentally. The failure modes corresponding to various macroscopic loading directions are also shown to be correctly captured. The in-plane failure mechanisms are correctly represented by a plane stress description, while the generalized plane state assumption, introducing simplified three-dimensional effects, is shown to be needed to represent out-of-plane failure under biaxial compressive loading. Macroscopic damage-induced anisotropy resulting from the constituents' stacking mode in the material, which is complex to represent properly using macroscopic phenomenological constitutive equations, is here obtained in a natural fashion. The identified mesoscopic description is introduced in a scale transition procedure to infer the macroscopic response of the material. The first-order computational homogenization technique is used for this purpose to extract this response from unit cells. Damage localization eventually appears as a natural outcome of the quasi-brittle nature of the constituents. The onset of macroscopic localization is treated as a material bifurcation phenomenon and is detected from an eigenvalue analysis of the homogenized acoustic tensor obtained from the scale transition procedure together with a limit point criterion. The macroscopic localization orientations obtained with this type of detection are shown to be strongly related to the underlying mesostructural failure modes in the unit cells.</p> <p align="justify">A well-posed macroscopic description is preserved by embedding localization bands at the macroscopic localization onset, with a width directly deduced from the initial periodicity of the mesostructure of the material. This allows to take into account the finite size of the fracturing zone in the macroscopic description. As a result of mesoscopic damage localization in narrow zones of the order of a mortar joint, the material response computationally deduced from unit cells may exhibit a snap-back behaviour. This precludes the use of such a response in the standard strain-driven multi-scale scheme.</p> <p align="justify">Adaptations of the multi-scale framework required to treat the mesostructural response snap-back are proposed. This multi-scale framework is finally applied for a typical confined shear wall problem, which allows to verify its ability to represent complex structural failure modes.</p>

This work investigates the use of shape memory alloys (SMAs) for vibration isolation and damping of mechanical systems. The first part of this work evaluates the nonlinear dynamics of a passive vibration isolation and damping (PVID) device through numerical simulations and experimental correlations. The device, a mass connected to a frame through two SMA wires, is subjected to a series of continuous acceleration functions in the form of a sine sweep. Frequency responses and transmissibility of the device as well as temperature variations of the SMA wires are analyzed for the case where the SMA wires are pre-strained at 4.0% of their original length. Numerical simulations of a one-degree of freedom (1-DOF) SMA oscillator are also conducted to corroborate the experimental results. The configuration of the SMA oscillator is based on the PVID device. A modified version of the constitutive model proposed by Boyd and Lagoudas, which considers the thermomechanical coupling, is used to predict the behavior of the SMA elements of the oscillator. The second part of this work numerically investigates chaotic responses of a 1- DOF SMA oscillator composed of a mass and a SMA element. The restitution force of the oscillator is provided by an SMA element described by a rate-independent, hysteretic, thermomechanical constitutive model. This model, which is a new version of the model presented in the first part of this work, allows smooth transitions between the austenitic and the martensitic phases. Chaotic responses of the SMA oscillator are evaluated through the estimation of the Lyapunov exponents. The Lyapunov exponent estimation of the SMA system is done by adapting the algorithm by Wolf and co-workers. The main issue of using this algorithm for nonlinear, rateindependent, hysteretic systems is related to the procedure of linearization of the equations of motion. The present work establishes a procedure of linearization that allows the use of the classical algorithm. Two different modeling cases are considered for isothermal and non-isothermal heat transfer conditions. The evaluation of the Lyapunov exponents shows that the proposed procedure is capable of quantifying chaos in rate-independent, hysteretic dynamical systems.

Characterization of multiaxial mechanical properties of cardiovascular soft tissue is essential in order to better understand their growth and remodeling in homeostatic conditions and in response to injury or pathological conditions. Though numerous phenomenological models have been proposed to characterize such multiaxial mechanical behavior, the approach has certain drawbacks regarding experimental determination of the model coefficients. We propose a method that aims to overcome these drawbacks. The approach makes use of orthogonal polynomials to fit the biaxial test data and suggests a way to derive the strain energy function from these analytical fits by way of minimizing the deviation of the behavior from hyperelastic ideal. Using the proposed method, a strain energy function for a lymphatic vessel is derived and the method is compared with traditional ones that used non-orthogonal polynomials as independent variables in the functional form for strain energy. The unique coefficient values obtained using the proposed method, for the first time gives us an opportunity to attribute a physical characteristic of the material to the coefficient values. The method also provides a way to assess two different material behaviors by way of comparing their deviation from the hyperelastic behavior when a similar test protocol is used to collect the data, over a similar deformation range and the order of polynomial function is chosen so as to give a similar error of fit. The behavior of mesenteric lymph vessels from normal cows, cows subjected to sham surgery and those subjected to 3 days of edematous conditions by venous occlusion are compared using this method. To be able to better understand the changes in mechanical behavior, morphological analysis of the vessels was carried out and the geometric and structural changes in these vessels were studied. We found that the behavior of bovine mesenteric lymph vessels subjected to a high flow condition shows a small difference in their mechanical behavior as compared to the vessels from normal a cow and a cow subjected to sham surgery. The geometry and structure of these vessels also showed marked differences from the other two. The thickness to radius ratio increased and a rise in percentage of area occupied by smooth muscle cells and medial collagen was observed. Though not all the differences were statistically significant, we conclude that the behavior and the morphology are suggestive of the remodeling of the vessel in response to altered hemodynamic conditions and require further investigation.

Accurately simulating material systems in a virtual environment requires the synthesis and utilization of all relevant information regarding performance mechanisms for the material occurring over a range of length and time scales. Multiscale modeling is the basis for the Integrated Computational Materials Engineering (ICME) Paradigm and is a powerful tool for accurate material simulations. However, while ICME has experienced adoption among those in the metals community, it has not gained traction in polymer research. This thesis seeks establish a hierarchical multiscale modeling methodology for simulating polymers containing secondary phases. The investigation laid out in the chapters below uses mesoscopic Finite Element Analysis (FEA) as a foundation to build a multiscale modeling methodology for polymer material systems. At the mesoscale a Design of Experiments (DOE) parametric study utilizing FEA of polymers containing defects compared the relative impacts of a selection of parameters on damage growth and coalescence in polymers. Of the parameters considered, the applied stress state proved to be the most crucial parameter affecting damage growth and coalescence. At the macroscale, the significant influence of the applied stress state on damage growth and coalescence in polymers (upscaled from the mesoscale) motivated an expansion of the Bouvard Internal State Variable (ISV) (Bouvard et al. 2013) polymer model stress state sensitivity. Deviatoric stress invariants were utilized to modify the Bouvard ISV model to account for asymmetry in polymer mechanical performance across different stress states (tension, compression, torsion). Lastly, this work implements a hierarchical multiscale modeling methodology to examine parametric effects of heterogeneities on Polymer/Clay Nanocomposite’s (PCNs) mechanical performance under uncertainty. A Virtual Composite Structure Generator (VCSG) built three-dimensional periodic Representative Volume Elements (RVEs) coupled to the Bouvard ISV model and a Cohesive Zone Model (CZM) which featured a Traction-Separation (T-S) rule calibrated to results upscaled from Molecular Dynamics (MD) simulations. A DOE parametric examination utilized the RVEs to determine the relative effects of a selection of parameters on the mechanical performance of PCNs. DOE results determined that nanoclay particle orientation was the most influential parameter affecting PCN elastic modulus while intercalated interlamellar gallery strength had the greatest influence on PCN yield stress

No description available.

Polymer matrix composites are seeing increasing use in structural systems (e.g. ships, bridges) and require a quantitative basis for describing their performance under combined mechanical load and fire. Although much work has been performed to characterize the flammability, fire resistance and toxicity of these composite systems, an understanding of the structural response of sandwich type structures and laminate panels under combined mechanical and thermal loads (simulating fire conditions) is still largely unavailable. Therefore a research effort to develop a model to describe the structural response of these glass/vinyl esters systems under fire loading conditions is relevant to the continuing and future application of polymer matrix composites aboard naval ships. The main goal of the effort presented here is to develop analytical models and finite element analysis methods and tools to predict limit states such as local compression failures due to micro-buckling, residual strength and times to failure for composite laminates at temperatures in the vicinity of the glass transition where failure is controlled by viscoelastic effects. Given the importance of compression loading to a structure subject to fire exposure, the goals of this work are succinctly stated as the:(a)Characterization of the non-linear viscoelastic and viscoplastic response of the E-glass/vinyl ester composite above Tg. (b)Description of the laminate compression mechanics as a function of stress and temperature including viscoelasticity.(c)Viscoelastic stress analysis of a laminated panel ([0/+45/90/-45/0]S) using classical lamination theory (CLT). Three manuscripts constitute this dissertation which is representative of the three steps listed above. First, a detailed characterization of the nonlinear thermoviscoelastic response of Vetrotex 324/Derakane 510A - 40 through Tg was conducted using the Time - Temperature - Stress - Superposition Principle (TTSSP) and Zapas - Crissman model. Second, the modeling approach and viscoelastic relaxation mechanism is validated by substituting the shear relaxation modulus into a compression strength model to predict lifetimes for isothermal and one sided heating of unidirectional laminates. Finally, viscoelastic stress analysis using CLT is performed for a general laminated panel to predict lifetimes under one sided heating. Results indicate that when temperatures remain in the vicinity of Tg, the laminate behavior is controlled by thermoviscoelasticity. / Ph. D.

abstract: Weevils are among the most diverse and evolutionarily successful animal lineages on Earth. Their success is driven in part by a structure called the rostrum, which gives weevil heads a characteristic "snout-like" appearance. Nut weevils in the genus Curculio use the rostrum to drill holes into developing fruits and nuts, wherein they deposit their eggs. During oviposition this exceedingly slender structure is bent into a straightened configuration - in some species up to 90° - but does not suffer any damage during this process. The performance of the snout is explained in terms of cuticle biomechanics and rostral curvature, as presented in a series of four interconnected studies. First, a micromechanical constitutive model of the cuticle is defined to predict and reconstruct the mechanical behavior of each region in the exoskeleton. Second, the effect of increased endocuticle thickness on the stiffness and fracture strength of the rostrum is assessed using force-controlled tensile testing. In the third chapter, these studies are integrated into finite element models of the snout, demonstrating that the Curculio rostrum is only able to withstand repeated, extreme bending because of modifications to the composite structure of the cuticle in the rostral apex. Finally, interspecific differences in the differential geometry of the snout are characterized to elucidate the role of biomechanical constraint in the evolution of rostral morphology for both males and females. Together these studies highlight the significance of cuticle biomechanics - heretofore unconsidered by others - as a source of constraint on the evolution of the rostrum and the mechanobiology of the genus Curculio. / Dissertation/Thesis / Doctoral Dissertation Evolutionary Biology 2009

Les microtubules sont des composants structuraux de cellules et gouvernent des fonctions cellulaires essentielles telles que les mitoses et le transport des vésicules. Ils sont composés de deux sous-unités non identiques (tubulines α et β), formant un dimère, et sont arrangés de sorte à former une structure tubulaire de 20nm de diamètre. Généralement, ils sont constitués de 13 ou 14 protofilaments arrangés en spirale. Les liaisons longitudinales entre dimères sont plus rigides et fortes que les liaisons latérales. Aussi, les microtubules sont des structures fortement anisotropes. Dans ces travaux de thèse, nous avons pour but de définir l'ensemble des coefficients élastique qui permet de reproduire leur comportement atomistique ainsi que de rendre compte de leur réponse mécanique selon des chemins de chargement variés. En négligeant la discontinuité hélicoïdale souvent observée, un microtubule est représenté par une structure triangulaire de dimères à partir desquels un volume élémentaire représentatif est défini. Un potentiel harmonique est utilisé pour décrire les interactions entre dimères voisins. A partir de l'estimation des constantes élastiques et de l'utilisation de la méthode proposée par Arslan et Boyce (2006) -alors pour analyser le comportement mécanique d'un réseau triangulaire de spectrines composant les membranes des globules rouges-, un modèle continu de comportement mécanique est présenté pour reproduire le comportement des parois des microtubules. Un modèle numérique éléments finis est ensuite créé pour modéliser le comportement d'un microtubule dans sa globalité. Des éléments coques sont utilisés pour reproduire les fines parois des microtubules. Les propriétés du modèle éléments finis sont ajustées à partir des résultats du modèle présenté ainsi qu'aux données expérimentales provenant de la littérature. La rigidité de flexion calculée au cours de simulation des tests de flexion 3 points est en accord avec les valeurs de la littérature. Ces tests révèlent les mécanismes de déformation en fonction de la longueur utile du tube utilisé: Flexion et cisaillement locaux de la paroi gouvernent la déformation pour de "petits" tubes. Pour des longueurs "moyennes" le cisaillement et la flexion du tube prédominent. Enfin, dans le cas de tubes "longs", la déformation est uniquement associée aux effets de flexion. Ces résultats témoignent de l'influence de l'anisotropie du tube sur la réponse observée selon différents mode de sollicitation. Ils permettent également d'expliquer l'évolution de la rigidité de flexion avec la longueur utile du tube, comme reportée dans la littérature. Enfin, des micrographes montrent la propension des extrémités des microtubules à diverger radialement -"à boucler"-. Une telle géométrie est causée par des instabilités propres aux microtubules et implique un état précontraint. Un «modèle d'interactions» est alors proposé de manière à considérer un état précontraint et ainsi reproduire la cinétique des instabilités des microtubules au cours de la polymérisation/dépolymérisation.