Research Center Cientifica Of Yucatan Ac

Mérida, Mexico

Research Center Cientifica Of Yucatan Ac

Mérida, Mexico
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Hernandez-Stefanoni J.L.,Research Center Cientifica Of Yucatan Ac | Alberto Gallardo-Cruz J.,National Autonomous University of Mexico | Meave J.A.,National Autonomous University of Mexico | Dupuy J.M.,Research Center Cientifica Of Yucatan Ac
Ecological Indicators | Year: 2011

Information on the spatial distribution and composition of biological communities is essential in designing effective strategies for biodiversity conservation and management. Reliable maps of species richness across the landscape can be useful tools for these purposes. Acquiring such information through traditional survey techniques is costly and logistically difficult. The kriging interpolation method has been widely used as an alternative to predict spatial distributions of species richness, as long as the data are spatially dependent. However, even when this requirement is met, researchers often have few sampled sites in relation to the area to be mapped. Remote sensing provides an inexpensive means to derive complete spatial coverage for large areas and can be extremely useful for estimating biodiversity. The aim of this study was to combine remotely sensed data with kriging estimates (hybrid procedures) to evaluate the possibility of improving the accuracy of tree species richness maps. We did this through the comparison of the predictive performance of three hybrid geostatistical procedures, based on tree species density recorded in 141 sampling quadrats: co-kriging (COK), kriging with external drift (KED), and regression kriging (RK). Reflectance values of spectral bands, computed NDVI and texture measurements of Landsat 7 TM imagery were used as ancillary variables in all methods. The R2 values of the models increased from 0.35 for ordinary kriging to 0.41 for COK, and from 0.39 for simple regression estimates to 0.52 and 0.53 when using simple KED and RK, respectively. The R2 values of the models also increased from 0.60 for multiple regression estimates to 0.62 and 0.66 when using multiple KED and RK, respectively. Overall, our results demonstrate that these procedures are capable of greatly improving estimation accuracy, with multivariate RK being clearly superior, because it produces the most accurate predictions, and because of its flexibility in modeling multivariate relationships between tree richness and remotely sensed data. We conclude that this is a valuable tool for guiding future efforts aimed at conservation and management of highly diverse tropical forests. © 2010 Elsevier Ltd.


Ku-Herrera J.J.,Research Center Cientifica Of Yucatan Ac | Aviles F.,Research Center Cientifica Of Yucatan Ac | Seidel G.D.,Virginia Polytechnic Institute and State University
Smart Materials and Structures | Year: 2013

The piezoresistive response of multiwalled carbon nanotube/vinyl ester composites containing 0.3, 0.5 and 1% w/w carbon nanotubes (CNTs) loaded in tension and compression is investigated. The change in electrical resistance (ΔR) under tension loading was positive and showed a linear relationship with the applied strain up to failure, with slightly increased sensitivity for decreased CNT content. In compression, a nonlinear and non-monotonic piezoresistive behavior was observed, with ΔR initially decreasing in the elastic regime, leveling off at the onset of yielding and increasing after matrix yielding. The piezoresistive response of the composite is more sensitive to the CNT content for compression than for tension, and the calculated gage factors are higher in the compressive plastic regime. The results show that the piezoresistive signal is dependent on the CNT concentration, loading type and material elastoplastic behavior, and that recording ΔR during mechanical loading can allow self-identification of the elastic and plastic regimes of the composite. © 2013 IOP Publishing Ltd.


Zozulya V.V.,Research Center Cientifica Of Yucatan Ac
Engineering Analysis with Boundary Elements | Year: 2014

This article addresses weakly singular, hypersingular integrals, which arise when the boundary integral equation (BIE) methods are used for 3-D potential theory problem solutions. An approach based on the theory of distributions and the application of the second Green theorem has been explored for the calculation of such divergent integrals. The divergent integrals have been transformed to a form that allows easy and uniform calculation of weakly singular and hypersingular integrals. For flat boundary elements (BE), piecewise constants and piecewise linear approximations, only regular integrals over the contour of the BE have to be evaluated. Furthermore, all calculations can be done analytically, so no numerical integration is required. In the case of 3-D, rectangular and triangular BE have been considered. The behavior of divergent integrals has been studied in the context that the collocation point moves to the contour of the BE. © 2013 Elsevier Ltd.


Zozulya V.V.,Research Center Cientifica Of Yucatan Ac
Computer Methods in Applied Mechanics and Engineering | Year: 2011

A mathematical model of an elastodynamic contact problem for a body with a crack with unilateral restrictions and friction on the crack faces is presented in classical and weak forms. Different variational formulations of unilateral contact problems with friction based on the principles of Hamilton-Ostrogradskii and Tupin, and boundary variational principles are considered. In particular, boundary variational functionals that are used with boundary integral equations are established. Nonsmooth optimization algorithms of Udzawa type for the solution of these unilateral contact problems with friction are developed. The convergence of the proposed algorithms is studied numerically. © 2010 Elsevier B.V.


Zozulya V.V.,Research Center Cientifica Of Yucatan Ac
Engineering Analysis with Boundary Elements | Year: 2010

In this article the hypersingular integrals that arise when boundary integral equation (BIE) methods are used to solve fracture mechanics problems are considered. An approach for hypersingular integral regularization is based on the theory of distribution and Green's theorems. This approach is applied for regularization of the hypersingular integrals over triangular boundary elements (BEs) for the case of piecewise-constant and piecewise-linear approximations. The hypersingular integrals are transformed into regular contour integrals that can be easily calculated analytically. © 2009 Elsevier Ltd. All rights reserved.


Bautista-Quijano J.R.,Research Center Cientifica Of Yucatan Ac | Aviles F.,Research Center Cientifica Of Yucatan Ac | Cauich-Rodriguez J.V.,Research Center Cientifica Of Yucatan Ac
Journal of Applied Polymer Science | Year: 2013

Multiwall carbon nanotube (MWCNT)/elastomeric composite films were fabricated using two segmented polyurethanes: an in-house synthesized one (SPU) and a commercial medical grade one (Tecoflex®, TF). Electrical, mechanical, and electromechanical (piezoresistive) properties of both composites were evaluated as a function of the MWCNT weight concentration (1-10 wt %). An increase in electrical conductivity for both types of polymers was observed for MWCNT concentrations as low as 1 wt %. The electrical conductivity of MWCNT/TF composites was higher than that achieved for MWCNT/SPU composites. Mechanical properties of 8 wt % MWCNT/SPU composites showed a threefold increase in stiffness compared to neat SPU. The changes in electrical resistance of the composites showed higher sensitivity to strain for lower MWCNT concentrations. The piezoresistive signal of the composites allows to measure strains up to ∼400% before electrical depercolation occurs. The strain at which electrical depercolation occurs depends on the conductivity of the composite in its unloaded state. This kind of composites may find sensing applications in prosthetics, biomedical devices, and smart textiles. © 2013 Wiley Periodicals, Inc.


Hernandez-Stefanoni J.L.,Research Center Cientifica Of Yucatan Ac | Dupuy J.M.,Research Center Cientifica Of Yucatan Ac | Tun-Dzul F.,Research Center Cientifica Of Yucatan Ac | May-Pat F.,Research Center Cientifica Of Yucatan Ac
Landscape Ecology | Year: 2011

Three central related issues in ecology are to identify spatial variation of ecological processes, to understand the relative influence of environmental and spatial variables, and to investigate the response of environmental variables at different spatial scales. These issues are particularly important for tropical dry forests, which have been comparatively less studied and are more threatened than other terrestrial ecosystems. This study aims to characterize relationships between community structure and landscape configuration and habitat type (stand age) considering different spatial scales for a tropical dry forest in Yucatan. Species density and above ground biomass were calculated from 276 sampling sites, while land cover classes were obtained from multi-spectral classification of a Spot 5 satellite imagery. Species density and biomass were related to stand age, landscape metrics of patch types (area, edge, shape, similarity and contrast) and principal coordinate of neighbor matrices (PCNM) variables using regression analysis. PCNM analysis was performed to interpret results in terms of spatial scales as well as to decompose variation into spatial, stand age and landscape structure components. Stand age was the most important variable for biomass, whereas landscape structure and spatial dependence had a comparable or even stronger influence on species density than stand age. At the very broad scale (8,000-10,500 m), stand age contributed most to biomass and landscape structure to species density. At the broad scale (2,000-8,000 m), stand age was the most important variable predicting both species density and biomass. Our results shed light on which landscape configurations could enhance plant diversity and above ground biomass. © 2010 Springer Science+Business Media B.V.


Zozulya V.V.,Research Center Cientifica Of Yucatan Ac
Engineering Analysis with Boundary Elements | Year: 2013

Two different boundary element methods (BEM) for crack analysis in two dimensional (2-D) antiplane, homogeneous, isotropic and linear elastic solids by considering frictional contact of the crack edges are presented. Hypersingular boundary integral equations (BIE) in time-domain (TD) and frequency domain (FD), with corresponding elastodynamic fundamental solutions are applied for this purpose. For evaluation of the hypersingular integrals involved in BIEs a special regularization process that converts the hypersingular integrals to regular integrals is applied. Simple regular formulas for their calculation are presented. For the problems solution while considering frictional contact of the crack edges a special iterative algorithm of Udzava's type is elaborated and used. Numerical results for crack opening, frictional contact forces and dynamic stress intensity factors (SIFs) are presented and discussed for a finite III-mode crack in an infinite domain subjected to a harmonic crack-face loading and considering crack edges frictional contact interaction using the TD and FD approaches. © 2013 Elsevier Ltd.


Zozulya V.V.,Research Center Cientifica Of Yucatan Ac
CMES - Computer Modeling in Engineering and Sciences | Year: 2010

In this article the divergent integrals, which arise when the boundary integral equation (BIE) methods are used for solution of the 3-D elastostatic problems is considered. The same approach for weakly singular, singular and hypersingular integral regularization is developed. The approach is based on theory of distribution and Green's theorems. This approach is applied for regularization of the divergent integrals over convex polygonal boundary elements (BE) in the case of piecewise constant approximation and over rectangular and triangular BE for piecewise linear approximation. The divergent integrals are transformed into the regular contour integrals that can be easily calculated analytically. Proposed methodology easy can be extended to other problems: elastodynamics, analytical calculation of the regular integrals, when collocation point situated outside the BE. Calculations of the divergent and regular integrals for square and triangle of the unit side are presented. Copyright © 2010 Tech Science Press.


Ku-Herrera J.J.,Research Center Cientifica Of Yucatan Ac | Aviles F.,Research Center Cientifica Of Yucatan Ac
Carbon | Year: 2012

The piezoresistive behavior of multi-walled carbon nanotube/vinyl ester composites tested under a low number of cycles and different levels of applied strain is investigated for specimens loaded in axial tension and compression. Since the vinyl ester thermosetting matrix is brittle in tension but ductile in compression, the piezoresistive behavior of the composite material is linear and reversible in tension for all levels of applied strain, but it is nonlinear and non-monotonic in compression. In compression, the reversibility of the electrical resistance upon unloading depends on the level of strain attained during loading, and the permanent changes of electrical resistance which occur after matrix yielding are correlated to matrix irreversible damage. © 2012 Elsevier Ltd. All rights reserved.

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