Search

Three-dimensional metamaterials endowed with two-dimensional in-plane periodicity exhibit peculiar thermoelastic behaviour when heated or cooled. By proper design of the unit cell, the equivalent thermal expansion coefficient can be programmed and can also reach negative values. The heterogeneity in the third direction of such metamaterials also causes, in general, a thermal-induced deflection. The prediction of the equivalent thermal properties is important to design the metamaterial suitable for a specific application. Under the hypothesis of small thickness with respect to the global in-plane dimensions, we make use of asymptotic homogenization to describe the thermoelastic behaviour of these metamaterials as that of an equivalent homogenous plate. The method provides explicit expressions for the effective thermal properties, which allow for a cost-effective prediction of the thermoelastic response of these metaplates. [ABSTRACT FROM AUTHOR]

This experimental study aims to assess the effect of external sulfate attack on structural concrete made with a common Portland-limestone cement. The experimental campaign, which ran for approximately three years, was performed on standard-sized cylinders (015 cm x 30 cm) subject to different exposure conditions. The overall swelling of the specimens due to chemical reactions was monitored over time. Scanning electron microscopy and energy dispersive X-ray spectrometry (SEM-EDS) observations and X-ray diffraction (XRD) tests were also carried out. Results showed the presence of secondary ettringite causing swelling and microcrack formation. The porosity increase due to sulfate attack of the outer skin of the specimens was measured. Ultrasonic measurements and compression tests on cores drilled from the cylinders immersed in different solutions allowed to confirm and quantify the consequent degradation of macroscopic elastic properties. Key words: concrete, sulfate attack, delayed ettringite formation, damage. Cette etude experimentale vise a evaluer l'effect de l'attaque externe de sulfates sur le beton structurel fabrique avec un ciment Portland au calcaire commun. La campagne experimentale, qui a dure environ 3 ans, a ete realisee en utilisant des eprouvettes de taille standard (15 cm x 30 cm) soumises a diverses conditions d'exposition. Le gonflement global des specimens en raison de reactions chimiques a ete surveille au cours de la periode. Des observations au moyen de microscopie electronique a balayage-spectroscopie des rayons X par dispersion d'energie (<< SEM-EDS >>) et des tests par spectrometrie de rayons X (<< XRD >>) ont egalement ete effectues. Les resultats ont montre la presence d'ettringite secondaire causant du gonflement et la formation de microfissures. L'augmentation de la porosite par l'attaque de sulfates sur la peau exterieure des specimens a ete mesuree. Des mesures par ultrasons et des essais de compression sur des carottes percees a partir des eprouvettes immergees dans diverses solutions ont permis de confirmer et de quantifier la degradation des proprietes elastiques macroscopiques qui en decoule. [Traduit par la Redaction] Mots-cles : beton, attaque de sulfates, formation retardee d'ettringite, dommages., 1. Introduction The sulfate attack (SA) is one of the phenomena that can affect the durability of concrete structures in contact with sulfate-rich soils or water, see e.g., Alexander et [...]

The evaluation of loss mechanisms in microscale mechanical resonators is addressed. Among various dissipation causes, thermoelastic loss is considered as a fundamental dissipation mechanism in microbeam resonators packed in a near-vacuum environment. However, the standard thermoelastic analysis is unable to interpret the size effect experimentally evidenced in resonators when the dimensions become very small, below several microns. In this paper we propose an enhanced nonlocal thermoelastic model, based on a thermodynamical formulation, which incorporates internal characteristic material lengths. Analytical results obtained with this nonlocal theory are compared with experimental results reported in the literature. It is shown how nonlocality can better interpret the observed behavior, at least in a certain range of resonators dimensions. CE Database subject headings: Structural analysis; Micromechanics; Thermoelasticity; Constitutive models; Damping.

Two new types of on-chip tests have been designed in order to evaluate the elastic Young modulus and the fracture strength of polysilicon used in microelectromechanical systems (MEMS). The former is a pure tension test, while the latter is a single-edge-notched tension test. The actuation in both tests is obtained by means of an ad hoc designed layout of parallel plates capacitors applying sufficiently high forces to reach significant strains in the tensile specimens and complete failure of the notched specimens. The pure tension tests on 20 specimens showed a low dispersion and gave a Young modulus for the polysilicon of 143 GPa. A total of 92 notched specimens were tested up to failure. The experimental results, supported by finite-element simulations, gave a value of the maximum stress for the notched specimens in the range 4144-4568 MPa. [984] Index Terms--Fracture strength, on-chip tests, polysilicon, Weibull statistics, Young modulus.

In this paper, we provide a comparison between two indirect experimental methodologies to assess ductile damage in P91 steel tested at 1100℃: the micro-tomography and the ultrasonic wave propagation techniques. To understand the effect of the strain rate on damage or porosity evolution, two series of tensile tests were carried out: a first (quasi-static) one featuring a velocity of the testing machine track of 0.06 mm/s and a second (high strain rate) one featuring a track velocity of 50 mm/s. Each set consisted of tests conducted up to increasing values of the maximum track displacement, so as to provide pictures of damage distribution at increasing severity. A simplified micromechanical analysis is developed for data elaboration of the ultrasonic measurements. We show that the estimates of overall damage, provided by the two experimental techniques for the necked region of the specimens, agree fairly well in most of the cases. [ABSTRACT FROM PUBLISHER]

An anisotropic two-phase coupled chemo-thermo-damage model is proposed, for the simulation of the behavior of concrete affected by the alkali-aggregate reaction, which may create significant damage in existing concrete structures. The chemical reaction produces a gel expanding in the concrete pores, leading to macroscopic strength and stiffness deterioration in the concrete skeleton. The model is capable to account for the anisotropic damage development and consequent directional degradation of material properties. The model is validated against experimental tests taken from the literature. [ABSTRACT FROM PUBLISHER]

The paper analyzes the nonlinear dynamic response of microresonators. A simple physical model of a single span beam axially constrained at both end, possibly with elastic constraints, is considered as representative of different common layout of microresonators. An analytical model, based on the Hamilton's principle, accounting for large displacements is presented and validated thanks to numerical finite element analyses. In order to widen the linear operation range of the device an optimal geometry of the resonator is proposed. [ABSTRACT FROM AUTHOR]