Fernando Ramirez

Discrete Layer Solution to Free Vibrations of Functionally Graded Magneto-Electro-Elastic Plates

Natural frequencies of orthotropic magneto-electro-elastic graded composite plates are determined using a discrete layer model with two different approaches. In the first, the functions describing the gradation of the materials properties through the thickness of the plate are incorporated into the governing equations. In the second approach, the plate is divided into a finite number of homogeneous layers. The model is validated by comparing the natural frequencies of a simply supported Al/ZrO2 graded square plate with the exact solution. Excellent agreement is obtained. Rectangular plates with different boundary conditions, aspect ratios, and made of different types of composite materials are also considered: Al/ZrO2 and BaTiO3/CoFe2O4 plates for which the volume fraction of the phases change as a function of the z coordinate, graphite/epoxy plates with the orientation of the fibers changing through the thickness of the plate, and plates having an exponential variation of the material properties. Applicability of the proposed model is not limited to specific boundary conditions and gradation functions.

Two-Dimensional Static Fields in Magnetoelectroelastic Laminates:

The two-dimensional behavior of laminated magnetoelectroelastic plates is investigated for two specific geometries: laminates under conditions of cylindrical bending and homogeneous plates under traction-free conditions. These plates are composed of a collection of elastic, piezoelectric, and magnetostrictive layers with perfect bonding between each interface. We investigate the through-thickness behavior of the five primary unknowns (the three displacements and the electric and magnetic potentials) under a variety of boundary conditions and aspect ratios using a discrete-layer theory. Results are compared with exact solutions for the case of cylindrical bending and finite element models for traction-free deformation. Excellent agreement is found between the approaches, and generalizations regarding global plate behavior are summarized.

Dowel-Bearing Strength Behavior of Glued Laminated Guadua Bamboo

AbstractBamboo is an excellent eco-friendly construction material because of its high renewable rate, embodied energy, reduction of pollution, high strength-to-weight ratio, and low cost. However, the use of round bamboo is limited because of its variations in dimensions, properties, and composition and the difficulty of making connections. Laminated bamboo has the potential to overcome these difficulties. Therefore, its mechanical properties and the behavior of the connections need to be established. In this research, the dowel-bearing strength of glued laminated Guadua angustifolia Kunth bamboo is experimentally determined under nail and threaded bar fasteners with different diameters and in different loading directions. A three-dimensional FEM computational model is developed with excellent agreement with experimental results. It was found that, similar to wood, the bearing strength depends on both the diameter and the specimen width-to-diameter ratio. The local behavior of the zone under the fastener ...

Cyclic Performance of Glued Laminated Guadua Bamboo-Sheathed Shear Walls

AbstractThis paper presents a study aimed to determine the cyclic performance of wood-frame shear walls sheathed with glued laminated Guadua bamboo (GLG) panels as a first step to evaluate their potential application. GLG-sheathed walls having 2:1 and 1:1 aspect ratios and three different sheathing nail spacing were tested under cyclic conditions. Parameters, such as peak shear strength, shear modulus, displacement ductility demand, and energy dissipation of the walls, were examined. Values for peak shear strength were then compared with those of walls sheathed with wood or wood-based panels. The wall aspect ratio was found to be a parameter with lesser impact than nail spacing. It was also found that the reduction in nail spacing increases to some extent the strength, stiffness, and energy dissipation, but lowers the ductility of the wall. The peak cyclic shear strength of shear walls sheathed with GLG panels was found to be within the shear strength reported for wood or wood-based panels and the tested ...

Arco Axilar de Langer (Músculo Axilopectoral): Variante Supernumeraria Inusual del Músculo Latísimo del Dorso. Reporte de Tres Casos

El arco axilar de Langer o musculo axilopectoral es una variacion relativamente rara de la insercion del musculo latisimo del dorso. Se identifica en aproximadamente el 1.7 a 7% de las disecciones axilares y corresponde a un complejo musculofascial, la parte muscular junto con el tendon del musculo pectoral mayor se inserta dentro del labio lateral del surco intertubercular del humero, mientras que, la parte fascial esta formada por bandas fibrosas que se extienden en la parte profunda del musculo pectoral mayor para insertarse en el proceso coracoide entre la union de los musculos coracobraquial y pectoral menor. El reconocimiento de esta anomalia es importante para los medicos clinicos, cirujanos y fisioterapeutas, ya que obliga a plantearse el diagnostico diferencial de masas axilares, historia de obstruccion venosa axilar intermitente, o su aparicion inesperada en el transcurso de la diseccion axilar del linfonodo centinela o linfadenectomias. Se presentan tres casos de arco axilar de Langer diagnosticados en la Unidad de Patologia Mamaria del Instituto Autonomo Hospital Universitario de los Andes, Merida, Venezuela, en el periodo 1999-2006, en el transcurso de 210 disecciones axilares practicadas en pacientes con cancer de mama para identificacion del linfonodo centinela o en linfadenectomias.

Vibrational modes of free nanoparticles: From atomic to continuum scales

Vibration analysis of free standing silicon nanoparticles, with sizes ranging from 0.732 to 4.223 nm, are calculated using two different methods: molecular mechanics and classical continuum elasticity. Three different geometries are studied: cubes, spheres, and tetrahedrons. Continuum mechanics methods provide good estimates of the lowest natural frequency of particles having at least 836 (R>1.5 nm) and 800 (R>1.28 nm) atoms for cube- and tetrahedron-shaped nanostructures, respectively. Equations for vibrational frequencies of smaller particles as a function of size are proposed. The vibrational modes obtained from both methods were practically the same for the sphere- and tetrahedron-shaped particles with a large number of atoms. However, for the cube geometry only the shape of the modes corresponding to the lowest couple of frequencies occur in the same order. In general, vibrational modes shapes obtained using both methods are the same although the order in which they appear may be shifted.

The Effective Continuum Properties of Carbon and Inorganic Nanotubes

The effective mechanical continuum properties of carbon and inorganic nanotubes and their molecular sheet precursors are studied using combinations of molecular dynamics, spring-force models, finite element continuum approximations, and beam elastica. The primary features of interest are the change in properties when moving from molecular sheet to nanotube and the relative difference in these properties for different atomic structures. The three primary types of nanotube of interest in this study are carbon, boron nitride, and molybdenum sulfide.

Optimum Green Concrete Using Different High Volume Fly Ash Activated Systems

Environmental issues related to CO2 emissions have become a key focus for many different industries, including the cement and concrete industry. An environmentally optimized ‘green’ concrete can provide a much needed alternative to conventional concrete to reduce the carbon footprint of the construction industry. This can be achieved through high Portland cement replacement by fly ash and with the inclusion of activators to enhance the rate of development of strength and other properties. This study evaluates different fly ashes and different activators (Na2SO4, lime and quicklime) that are added to enhance the reaction of the fly ash to achieve a comparable performance to that of standard Portland cement in mixes of much lower CO2 emissions. TGA, XRD and SEM are used to determine the development of hydration products and the consumption of portlandite by the fly ash. It is found that the amorphous content of the fly ash is an important parameter influencing compressive strength evolution. Based on the results, Na2SO4 as an activator, and a fly ash with high reactive SiO2 and Al2O3 contents and low Fe2O3 are found to provide the best options for producing a high volume fly ash matrix with the potential to show comparable behavior to a Portland cement control mix.

Activated Hybrid Cementitious System Using Portland Cement and Fly Ash with Na 2 SO 4

A number of alternatives have been explored by the cement industry in recent years to reduce CO2 emissions. One of the alternatives, the subject of this chapter, is an intermediate system between a high volume fly ash concrete and a geopolymer concrete. This concrete includes a hybrid system of 50% OPC–50% fly ash, and an activator. In this study, the long-term durability was studied for laboratory and outdoor cured concretes. It was found that chloride diffusion coefficient was reduced significantly at 90 days and beyond for the activated system compared to control samples (100% OPC and 80% OPC–20% fly ash) of the same water to cementitious content ratio (W/CM). This behavior was exhibited by samples cured under laboratory controlled curing conditions (100% RH and 23 °C). On the other hand, outdoor curing increased concrete permeability for all concretes. Long-term carbonation was also explored and samples under outdoor curing had a significant carbonation depth. Alkali silica reaction problems were mitigated with this activated hybrid system. In order to improve the carbonation resistance of this concrete, a reduction in W/CM seems necessary. Based on these results, activated high volume fly ash systems provide a low CO2 concrete alternative; however, more studies are needed for establishing specifications and service life prediction models.

Comportamiento Viscoelástico del Tejido Corneal

The biomechanical study of the corneal tissue has been primarily focused on the cornea computational simulation under different stress-strain conditions. However, there is still very little information regarding the mechanical properties and response of the corneal tissue, information required to perform adequate numerical simulations. Taking this issue into account, stress relaxation displacement controlled tests and creep stress controlled tests were performed in order to characterize the time–dependent, viscoelastic behavior of corneal tissue. Vertical strips were obtained from porcine corneas and were subjected to uniaxial tensile tests under different stress-strain levels. A time dependent, viscoelastic behavior was initially observed for high stress levels and an empirical constitutive model is proposed based on the experimental data. Creep and relaxation tests at low stress levels, within physiological conditions, were also performed. Results indicate that the corneal tissue viscoelastic behavior may be neglected for low stress levels which considerably simplifies the constitutive model to be implemented in computational simulations.