On the importance of prismatic/basal interfaces in the growth of (-1012) twins in hexagonal close-packed crystals

The growth process of of (-1012) twins is studied in Magnesium using atomistic simulations. Two twin seeds are considered and both cases, a specific interface, which places face-to-face prismatic and basal planes, plays an important role. This interface has a low energy corresponding to a cusp in the orientation-dependent interface energy of a twinned bicrystal. This interface appears in several published twin structures and for instance accommodates the large deviations of twin interfaces from (-1012) planes reported recently [Zhang et al., Scr. Mater. 67 (2012) 862].Comment: 11 pages, 4 figures, submitted to Scripta Materiali

Homogénéisation des matériaux hétérogènes élastoviscoplastiques basée sur la technique des « champs translatés » : extension « affine » au cas non linéaire pour des composites biphasés

Dans cette contribution, on passe tout d’abord en revue les principales étapes de la technique à « champs translatés » pour déterminer le comportement effectif de matériaux élasto-viscoplastiques dont les phases suivent un couplage spatio-temporel de type Maxwellien et dont le comportement est supposé dans un premier temps linéaire. L’application de l’approche à « champs translatés » au problème de l’inclusion d’Eshelby viscoélastique linéaire est également présentée dans cette première partie. Le traitement de cas particuliers montre la pertinence de l’approche en viscoélasticité linéaire par rapport aux solutions obtenues classiquement par transformées de Laplace-Carson. Ensuite, l’extension de la méthode à « champs translatés » au comportement local élasto-viscoplastique avec une viscoplasticité non linéaire est résolue par le biais d’une linéarisation du comportement viscoplastique des phases de type « affine ». Cette extension couplée à un schéma d’homogénéisation à champs moyens (Mori-Tanaka ou schéma autocohérent) pour le problème hétérogène élastoviscoplastique donne une nouvelle loi d’interaction qui contient les interactions mécaniques entre les champs moyens par phase et les grandeurs macroscopiques. Dans le but de situer la validité de l’approximation à « champs moyens », les réponses mécaniques du modèle sont reportées pour des composites biphasés et sont comparées aux résultats d’autres approches d’homogénéisation de type analytique ou numériques existants dans la littérature

Stress influence on high temperature oxide scale growth: modeling and investigation on a thermal barrier coating system.

International audienceIn thermal barrier coating (TBC) systems, an oxide layer develops at high temperature below the ceramic coating, leading at long term to the mechanical failure of the structure upon cooling. This study investigates a mechanism of stress-affected oxidation likely to induce the growth of a non-uniform oxide scale detrimental to the TBC lifetime. A continuum thermodynamics formulation is derived accounting for the influence of the stress and strain situation at the sharp metal/oxide phase boundary on the local oxidation kinetics. It specially includes the contributions of the large volumetric strain and the mass consumption associated with metal oxidation. A continuum mechanics/mass diffusion framework is used along with the developed formulation for the interface evolution to study the growth of an oxide layer coupled with local stress development. The implementation of the model has required the development of a specific simulation tool, based on a finite element method completed with an external routine for the phase boundary propagation. Results on an electron-beam physical vapor deposited (EB-PVD) TBC case are presented. The processes resulting in a non-uniform oxide scale growth are analyzed and the main influences are discussed

Beyond Status: Exploring the Psychological and Physiological Effects of Trait Dominance and Prestige

Social status is associated with benefits that include reduced stress and better health. The route by which social status is earned &ndash; i.e., the trait-like reliance on dominance versus prestige &ndash; may modulate known relationships. These personality traits might be related to stress and health outcomes, but few studies have explored the relationship based on the routes by which they are achieved. Dominant individuals are driven to seek power by being assertive, intimidating, and coercive. Because of the association of trait dominance with these relatively anti-social processes, it is expected that trait dominance is affiliated with more robust indicators of stress responses which include higher basal glucocorticoids, higher heart rate, higher sympathetic activity, and lower parasympathetic activity, all while correlated with reduced mental and physical health. Prestigious individuals try to earn respect and admiration from others through knowledge, a particular skill set, or success in life&rsquo;s endeavors. On the other hand, due to the association trait prestige individuals have, it is expected they are affiliated with attenuated stress responses which include lower basal glucocorticoids, lower heart rate, lower sympathetic activity, and higher parasympathetic activity, together with better mental and physical health. This research aims to fill the gaps on how one&rsquo;s status-relevant personality is associated with the autonomic nervous system, cardiovascular system, glucocorticoids, testosterone, and self- reported mental and physical health. The study contained 129 (N=129) college-aged students who were given surveys (Dominance and Prestige Scale (DPS), the Perceived Stress Scale (PSS), Lifestyle and Habits Questionnaire-Brief (LHQB), and a Positive and Negative Affect Schedule (PANAS)), and had physiology recorded at baseline, stressor, and recovery during a social-evaluated cold pressor task. Relevant biomarkers to stress and health were collected and assayed in saliva, including basal glucocorticoids (cortisol, cortisone, and corticosterone), and testosterone; cardiovascular physiology measured during the stress task included heart rate, high-frequency heart rate variability (an index of parasympathetic functioning), and pre-ejection period (an index of sympathetic functioning). The study found that trait dominance was associated with worse overall health, substance abuse, poor nutritional care, low social integration, positive affect, and high-frequency heart rate variability; trait prestige was associated with better overall health, a higher sense of purpose, high social integration, and positive affect. This study puts forward information on the effects of social status necessary for understanding the link between status, health, and stress.&nbsp; </div

A Fast Fourier Transform-based approach for Generalized Disclination Mechanics within a Couple Stress theory

International audienceRecently, a small-distortion theory of coupled plasticity and phase transformation accounting for the kinematics and thermodynamics of generalized defects called generalized disclinations (abbreviated g- disclinations) has been proposed. Then, a first numerical spectral approach has been developed to solve the elasto-static equations of field dislocation and g-disclination mechanics set out in this theory for periodic media and for linear elastic media using the classic Hooke’s law. Here, given a spatial distribution of generalized disclination density tensors in a homogenous linear higher order elastic media described, a couple stress theory with elastic incompatibilities of first and second orders is developed. The incompatible and compatible elastic second and first distortions are obtained from the solution of Poisson and Navier-type equations in the Fourier space. The efficient Fast Fourier Transform (FFT) algorithm is used based on intrinsic Discrete Fourier Transforms (DFT) that are well adapted to the discrete grid to compute higher order partial derivatives in the Fourier space. Therefore, stress and couple stress fields can be calculated using the inverse FFT. The numerical examples are given for straight wedge disclinations and associated wedge disclination dipoles which are of importance to geometrically describe tilt grain boundaries at fine scales in polycrystalline solids

Photoluminescence-Based Techniques for the Detection of Micro- and Nanoplastics

The growing numbers related to plastic pollution are impressive, with ca. 70 % of produced plastic (&gt;350 tonnes/year) being indiscriminately wasted in the environment. The most dangerous forms of plastic pollution for biota and human health are micro- and nano-plastics (MNPs), which are ubiquitous and more bioavailable. Their elimination is extremely difficult, but the first challenge is their detection since existing protocols are unsatisfactory for microplastics and mostly absent for nanoplastics. After a discussion of the state of the art for MNPs detection, we specifically revise the techniques based on photoluminescence that represent very promising solutions for this problem. In this context, Nile Red staining is the most used strategy and we show here its pros and limitations, but we also discuss other more recent approaches, such as the use of fluorogenic probes based on perylene-bisimide and on fluorogenic hyaluronan nanogels, with the added values of biocompatibility and water solubility

Simulative Evaluation of Intragrain Precipitate Influence on the Material Nonlinearity using Nonlinear Ultrasound

Nondestructive evaluation using ultrasonic waves is commonly used to experimentally probe for the presence of defects (i.e. dislocations, precipitates, cracks) in complex metallic microstructures. Such defects and abnormalities are evidenced by monitoring the acoustic nonlinearity parameter β. However, from the mathematical standpoint, the correlation between the microstructural behavior and the measured acoustic nonlinearity parameter is not explicit yet. The present work aims to assess the existence of statistical correlations between microstructural defects and material nonlinearity. The effects of defect geometry, density, and geometrical arrangements (i.e. relative position) on material nonlinearity are studied. To do so, the acoustic response of Fe-Cu single crystals containing 1 % Cu precipitates with radii on the order of 10 nm is simulated by means of finite element analysis. Several thousand initial microstructures with random arrangement of precipitates are virtually tested using statistical methods, such as principal component analysis. Therefore, it is expected that a causal link can be made between the acoustic nonlinearity parameter and the precipitates-induced microstructural behavior via the proposed numerical analysis