Vila Real de Santo António, Portugal
Vila Real de Santo António, Portugal

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Anjos O.,Polytechnic Institute of Castelo Branco | Anjos O.,University of Lisbon | Rodrigues C.,Polytechnic Institute of Castelo Branco | Morais J.,CITAB UTAD | Pereira H.,University of Lisbon
Materials and Design | Year: 2014

The compression properties of cork were studied for samples of different density. The densities were grouped into three classes: low density (0.13-0.15), mid density (0.15-0.19) and high density (0.19-0.25). The porosity of the cork samples increased from the low to the high density class, with porosity coefficients of 5.1%, 6.9% and 9.4%, respectively. The difference in the porosity was associated with structural features, namely the presence of thick walled cork cells and the presence of lignified cells lining the pores.The stress-strain curves were similar for all cases, showing an elastic compression up to a yield point of about 5% strain, followed by a plateau with a small slope. The cork strength was higher in the radial direction than in the other directions. The density influenced the compression such that the corks with high density presented higher stiffness in compression in three directions: Young's modulus was 17.4, 22.6 and 26.1MPa for low, mid and high density corks respectively. This density effect was more evident in the plateau region of the progressive buckling of the cell walls (σ30 was respectively 1.07, 1.29 and 1.54MPa for the three density classes).The recovery of dimensions after compression in each direction was also studied following compression to 50% strain. The recovery was on average 50% of the initial deformation on the first day, and almost total after 15. days. The recovery was higher for corks with low density and in non-radial directions. © 2013 Elsevier Ltd.

Optical Engineering | Year: 2012

A digital image correlation (DIC) algorithm for displacement measurements combining cross-correlation and a differential technique was validated through a set of experimental tests. These tests consisted of in-plane rigid-body translation and rotation tests, a tensile mechanical test, and a mode I fracture test. The fracture mechanical test, in particular, was intended to assess the accuracy of the method when dealing with discontinuous displacement fields, for which subset-based image correlation methods usually give unreliable results. The proposed algorithm was systematically compared with the Aramis DIC-2D commercial code by processing the same set of images. When processing images from rigid-body and tensile tests (associated with continuous displacement fields), the two methods provided equivalent results. When processing images from the fracture mechanical test, however, the proposed method obtained a better qualitative description of the discontinuous displacements. Moreover, the proposed method gave a more reliable estimation of both crack length and crack opening displacement of the fractured specimen. © 2012 Society of Photo-Optical Instrumentation Engineers.

Morel S.,University of Bordeaux 1 | Morel S.,French National Center for Scientific Research | Dourado N.,CITAB UTAD
International Journal of Solids and Structures | Year: 2011

The recent rewriting of the Bažant's size effect law (Morel, 2008) which has suggested the existence of an additional asymptotic regime for intermediate structure sizes is now compared to numerical simulations of fracture of geometrically similar notched structures of different sizes extending over 2.4 decades. The quasibrittle fracture behavior is simulated through cohesive zone model (bilinear softening) using a constant set of cohesive parameters whatever the specimen size D is. The R-curves resulting from the load-displacement responses are estimated and appear as size-independent. On this basis, the different asymptotic regimes expected for the size effect on fracture properties at peak load such as the relative crack length, the resistance to crack growth and the nominal strength are shown in fair agreement with the size effect observed on the results obtained from numerical simulations. © 2011 Elsevier Ltd. All rights reserved.

Dourado N.,CITAB UTAD | Pereira F.A.M.,CITAB UTAD | de Moura M.F.S.F.,University of Porto | Morais J.J.L.,CITAB UTAD
Engineering Structures | Year: 2012

Repair of wood damaged beams under bending using carbon-epoxy composites has been studied in this work. The influence of patch thickness and adhesive filleting was analyzed experimentally and numerically. The main goal was to assess the effect of these parameters on the ultimate load and failure type. Numerical analyses considering a mixed-mode cohesive damage model were performed in order to better understand the physical phenomena which explain the observed behavior. The main conclusion is that repairing leads to a remarkable gain on the bearing capacity of the damaged beam. Additionally, it was observed that a slight increase on the ultimate load can be obtained by diminishing the patch thickness. However, adhesive filleting did not induce a clear increase on the load bearing capacity, which was explained by the failure mode observed in this case. © 2011 Elsevier Ltd.

Costa M.R.,University of Aveiro | Calvao A.R.,ESTGA UA Escola Superior de Tecnologia e Gestao de Agueda | Aranha J.,CITAB UTAD
Applied Geochemistry | Year: 2014

Wildfires transform the landscape, leading to changes in surface cover and, potentially, in water quality. The purpose of this study was to assess changes in the chemical composition of soils and surface water as a result of a wildfire that burned in 2006 in the Marão Mountains, NE Portugal, by comparing pre- and post-fire hydrochemical data and burned/unburned soil data, and to examine the recovery of vegetation over time using Landsat TM imagery. Studies that have access to pre-fire data are rare and even fewer studies document changes in biomass as a result of fire and during the postfire recovery period. Samples of ash, soil and water, from within and outside the burned area, were collected 5. months, and one year after the fire, for chemical analyses. Landsat TM Images were downloaded and transformed into a vegetation index, in order to analyze landcover dynamics and to calculate biomass. The wildfire effects on the Marão River water quality, resulted in an increase in the total mineralization of water. Five months after the wildfire the electrical conductivity (E.C.) at the mainstem was about 56% higher than pre-fire values (E.C. increased from 25 to 39. μS/cm) and still higher one year after (36. μS/cm). Cations of Ca, Na, Mg and Mn showed the greatest increase. This increase was probably triggered by the movement of ash to the watercourses. This disturbance had already attenuated one year after wildfire to values closer to pre-fire data except for manganese. Manganese had anomalous concentrations in the water within the burned area. The concentration of Mn in ash samples reached values up to 5 times more than values found in underlying soils. One year after the wildfire, almost all the burned area had recovered with herbaceous vegetation and patches of shrub vegetation. The wildfire burned 1194.7. dry tons of biomass which means, on average, 4.9. dry ton/ha. Based on the mass of burned biomass, we calculated approximately 350. g/ha of Mn were released as a result of the fire. We suggest that this type of calculation can be conducted before a fire to help resource managers understand worst-case scenarios for changes in water quality that have the potential to affect aquatic biotic and the suitability of water for drinking water purposes and agriculture. © 2013 Elsevier Ltd.

Koerber H.,University of Porto | Xavier J.,CITAB UTAD | Camanho P.P.,University of Porto
Mechanics of Materials | Year: 2010

This paper presents an experimental investigation of strain rate effects on polymer-based composite materials. Quasi-static and dynamic experiments at strain rates up to 350 s-1 were performed with end-loaded, rectangular off-axis compression and transverse compression specimens. The dynamic tests were performed on a split-Hopkinson pressure bar, where pulse shaping ensured early dynamic equilibrium and near constant strain rates for all specimen types. The in-plane strain field of the specimen was obtained via digital image correlation. With the high speed camera used for the dynamic tests, the failure process of the specimen was monitored and the fracture angle was measured. The strain rate effect on modulus, yield, ultimate strength, strain to failure and on the in-plane shear properties was studied. The experimental failure envelope for combined transverse compression and in-plane shear loading was compared with the Puck failure criterion for matrix compression and excellent correlation between experimental and predicted failure envelopes was observed for both strain rate regimes. The quasi-static and dynamic yield envelopes for combined loading are also presented. © 2010 Elsevier Ltd. All rights reserved.

Dias G.F.,University of Porto | de Moura M.F.S.F.,University of Porto | Chousal J.A.G.,University of Porto | Xavier J.,CITAB UTAD
Composite Structures | Year: 2013

Mode I cohesive laws of carbon-epoxy composite bonded joints were obtained using the direct method applied to the double cantilever beam test. This method is based on the differentiation of the relation between the evolution of the strain energy release rate and the crack tip opening displacement (CTOD) during the test. An equivalent crack length method was used to obtain the strain energy release rate evolution. Following this procedure, several drawbacks inherent to crack length monitoring during the test are overcome. The CTOD was measured through digital image correlation technique and synchronized with the load-displacement data. Two different procedures were used to execute the pre-crack. It was verified that the use of a thin Teflon® film is more effective than a blade, although both procedures lead to consistent cohesive laws. It was concluded that the proposed methodology is adequate concerning the evaluation of the composite bonded joints cohesive laws. © 2013 Elsevier Ltd.

Dourado N.,CITAB UTAD | De Moura M.F.S.F.,University of Porto | Morais J.,CITAB UTAD
International Journal of Solids and Structures | Year: 2011

A numerical study involving the single edge notched-three point bending test was methodically performed to evaluate its adequacy for wood fracture characterization under mode I loading. Stress analysis along the ligament length using cohesive zone modeling allowed understating the influence of the compressive stresses induced by bending on the stress profile for geometrically similar specimens with different sizes. The stress relief region induced by the presence of the crack was identified by a stress analysis procedure along the beam length. A data reduction scheme based on the beam theory and on the crack equivalent concept was then developed considering an approach using a rectangular stress relief region. The method allows estimating the Resistance-curve without monitoring the crack length during propagation. Important remarks regarding the aspects which contribute to explain the size effect phenomenon in wood are highlighted in this study. Following the presented procedure the application of the single edge notched-three point bending test for wood fracture characterization under mode I has become considerably user-friendlier. © 2010 Elsevier Ltd. All rights reserved.

De Moura M.F.S.F.,University of Porto | Dourado N.,CITAB UTAD | Morais J.J.L.,CITAB UTAD | Pereira F.A.M.,CITAB UTAD
Fatigue and Fracture of Engineering Materials and Structures | Year: 2011

The objective of this work is to verify numerically the adequacy of the ENF and the ELS tests to determine the fracture toughness under mode II loading of cortical bovine bone tissue. A data-reduction scheme based on the specimen compliance and the equivalent crack concept is proposed to overcome the difficulties inherent to crack monitoring during its growth. A cohesive damage model was used to simulate damage initiation and growth, thus assessing the efficacy of the proposed data-reduction scheme. The influences of the initial crack length, local strength and toughness on the measured fracture energy were analysed, taking into account the specimen length restriction. Some limitations related to spurious influence on the fracture process zone of the central loading in the ENF test, and clamping conditions in the ELS test were identified. However, it was verified that a judicious selection of the geometry allows, in both cases, a rigorous estimation of bone toughness in mode II. © 2010 Blackwell Publishing Ltd.

Catalanotti G.,University of Porto | Camanho P.P.,University of Porto | Xavier J.,CITAB UTAD | Davila C.G.,NASA | Marques A.T.,University of Porto
Composites Science and Technology | Year: 2010

This paper presents a new methodology to measure the crack resistance curves associated with fiber-dominated failure modes in polymer-matrix composites. The crack resistance curves not only characterize the fracture toughness of the material, but are also the basis for the identification of the parameters of the softening laws used in the numerical simulation of fracture in composite materials. The proposed method is based on the identification of the crack tip location using Digital Image Correlation and the calculation of the J-integral directly from the test data using a simple expression derived for cross-ply composite laminates. It is shown that the results obtained using the proposed methodology yield crack resistance curves similar to those obtained using Finite Element based methods for compact tension carbon-epoxy specimens. However, it is also shown that, while the Digital Image Correlation based technique mitigates the problems resulting from Finite Element based data reduction schemes applied to compact compression tests, the delamination that accompanies the propagation of a kink-band renders compact compression test specimens unsuitable to measure resistance curves associated with fiber kinking. © 2010 Elsevier Ltd.

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