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Graca-E-Costa R.,ICIST | Graca-E-Costa R.,FARO | Alfaiate J.,ICIST | Alfaiate J.,University of Lisbon | And 3 more authors.
International Journal of Fracture | Year: 2012

Due to the softening behaviour of quasi-brittle materials, in particular the localisation of initially diffused cracking, convergence problems are often found using an iterative procedure, such as the Newton-Raphson method. This is why a new non-iterative procedure is adopted in this paper, which is inspired by the sequentially linear approach(SLA) (Rots et al. in Eng Fract Mech 75(3-4):590-614, 2008). However, several important differences between the present approach and the SLA are presented. In the present model, multi-linear material laws are adopted such that non-linearities occur only due to changes in loading/unloading states. An incremental solution is obtained until non-convergence occurs, upon which a secant approach is used in a corresponding step. The update of the stiffness in the secant approach is based on information obtained from the previous incremental solution. This method is applied to: (i) softening materials, within the scope of the discrete crack approach, and to (ii) hardening materials. As a consequence, conversely to the smeared crack approach adopted in the SLA, no mesh size sensitivity problems are obtained and there is no need to adjust material parameters. Several numerical examples are shown in order to illustrate the proposed formulation. © 2012 Springer Science+Business Media Dordrecht.

Areias P.,University of Évora | Garcao J.,University of Évora | Pires E.B.,ICIST | Pires E.B.,University of Lisbon | Barbosa J.I.,University of Évora
Computational Mechanics | Year: 2011

The corotational method for frame-invariant elements is generalized to obtain a consistent large-strain shell element incorporating thickness extensibility. The resulting element allows arbitrary in-plane deformations and is distinct from the traditional corotational methods (either quadrature-based or element-based) in the sense that the corotational frame is exact. The polar decomposition operation is performed in two parts, greatly simplifying the linearization calculations. Expressions for the strain-degrees-of-freedom matrices are given for the first time. The symbolic calculations are performed with a well-known algebraic system with a code generation package. Classical linear benchmarks are shown with excellent results. Applications with hyperelasticity and finite strain plasticity are presented, with asymptotically quadratic convergence and very good benchmark results. An example of finite strain plasticity with fracture is solved successfully, showing remarkable robustness without the need of enrichment techniques. © 2011 Springer-Verlag.

Areias P.,University of Évora | Dias-Da-Costa D.,INESC Coimbra | Dias-Da-Costa D.,University of Coimbra | Pires E.B.,ICIST | Van Goethem N.,University of Lisbon
Computational Mechanics | Year: 2013

Very good results in infinitesimal and finite strain analysis of shells are achieved by combining either the enhanced-metric technique or the selective-reduced integration for the in-plane shear energy and an assumed natural strain technique (ANS) in a non-symmetric Petrov-Galerkin arrangement which complies with the patch-test. A recovery of the original Wilson incompatible mode element is shown for the trial functions in the in-plane components. As a beneficial side-effect, Newton-Raphson convergence behavior for non-linear problems is improved with respect to symmetric formulations. Transverse-shear and in-plane patch tests are satisfied while distorted-mesh accuracy is higher than with symmetric formulations. Classical test functions with assumed-metric components are required for compatibility reasons. Verification tests are performed with advantageous comparisons being observed in all of them. Applications to large displacement elasticity and finite strain plasticity are shown with both low sensitivity to mesh distortion and (relatively) high accuracy. A equilibrium-consistent (and consistently linearized) updated-Lagrangian algorithm is proposed and tested. Concerning the time-step dependency, it was found that the consistent updated-Lagrangian algorithm is nearly time-step independent and can replace the multiplicative plasticity approach if only moderate elastic strains are present, as is the case of most metals. © 2012 Springer-Verlag Berlin Heidelberg.

Valenca J.,ICIST | Valenca J.,Polytechnic Institute of Coimbra | Dias-Da-Costa D.,INESC Coimbra | Dias-Da-Costa D.,University of Coimbra | And 5 more authors.
Measurement: Journal of the International Measurement Confederation | Year: 2013

This manuscript presents an integrated approach for automatic crack monitoring combining photogrammetry and image processing. In summary, the strain field obtained from photogrammetric data is used to map the cracked areas where image processing is applied. All processing is completely automatic since only a threshold value, related to the width of the crack, needs to be provided. Direct Shear Tests (DSTs) have been selected for calibration, validation and also as an experimental example. In conclusion, critical areas, the corresponding crack pattern and all related measures (e.g. crack width, length, area or path) could be provided for any stage of loading, until the complete failure of the specimens. Furthermore, all outputs require low computational cost, thus allowing monitoring vast campaigns of laboratorial tests. © 2012 Elsevier Ltd. All rights reserved.

Saldanha R.,ICIST | Saldanha R.,University of Coimbra | Julio E.,ICIST | Julio E.,University of Lisbon | And 4 more authors.
Construction and Building Materials | Year: 2013

Presently, there are several tests available to evaluate the adhesion between concrete layers cast at different times. Among these, the Slant Shear Test (SST) has a wide spread use, mainly for being sensitive to the roughness of the interface surface, one of the most influencing parameters in this scope, but also because of the simplicity of the experimental setup. However, the major drawback of this test is the fact of being often obtained cohesive (monolithic) failures, therefore providing a lower bound of the interface strength, which is related with the concrete compressive strength. To definitely solve this issue, i.e., to enforce adhesive (interface debonding) failures in all situations, the 'Modified Slant Shear Test' (M-SST) was herein designed and developed. Both numerical and experimental tests have been conducted to validate this new method. All relevant conclusions are presented. In summary, it can be stated that the M-SST always enforces adhesive failure, therefore allowing to accurately assessing the adhesion of concrete-to-concrete interfaces. © 2013 Elsevier Ltd. All rights reserved.

Valenca J.,ICIST | Valenca J.,Polytechnic Institute of Coimbra | Goncalves L.M.S.,INESC Coimbra | Goncalves L.M.S.,Polytechnic Institute of Leiria | And 2 more authors.
Construction and Building Materials | Year: 2013

To monitor the state of conservation of structures is extremely important to plan maintenance interventions, thus allowing minimizing associated costs. The aim of this paper is to investigate if Very High Spatial Resolution (VHSR) multispectral images can be used to detect, analyse and measure areas with biological colonization, stains and/or repair materials on exposed concrete surfaces. With this aim, the authors present a method, SurfCrete-Multi-spectral image analysis of concrete surfaces, which was tested to classify surface images of exposed concrete buildings where biological colonization, exposed aggregates, repairing mortars and cracks were previously detected on a visual inspection. This technique was performed using four multi-spectral bands (blue, green, red and near infrared). A vegetation index was also considered, as additional band information, to evaluate its usefulness in this kind of applications. SurfCrete allowed to achieve a global classification accuracy of 94%, which indicates that the identification of different materials, as well as damage, can be obtained from VHSR multispectral images using the developed methodology with a high accuracy. Surface mapping and statistical results are automatically obtained and stored in digital format which can be valuable when applied to large exposed concrete areas. Furthermore, results demonstrate that most accurate outputs can be derived using the near infrared band. © 2012 Elsevier Ltd. All rights reserved.

Dias-da-Costa D.,INESC Coimbra | Dias-da-Costa D.,University of Coimbra | Alfaiate J.,ICIST | Alfaiate J.,University of Lisbon | And 5 more authors.
International Journal for Numerical Methods in Engineering | Year: 2013

The embedding of discontinuities into finite elements has become a powerful technique for the simulation of fracture in a wide variety of mechanical problems. However, existing formulations still use non-conforming finite elements. In this manuscript, a new conforming formulation is proposed. The main properties of this formulation are as follows: (i)variational consistency; (ii) no limitations on the choice of the parent finite element; (iii)comprehensive kinematics of the discontinuity, including both rigid body motion and stretching; (iv) fully compatible enhanced kinematic field; (v) additional global DOFs located at the discontinuity; (vi) continuity of both jumps and tractions across element boundaries; and (vii) stress locking free. The performance of the proposed formulation is tested by means of academic and structural examples. The numerical results are compared with available experimental results and other numerical approaches, namely the generalized strong discontinuity approach and the generalized FEM/Extended FEM. © 2012 John Wiley & Sons, Ltd.

Hassanein M.F.,Tanta University | Silvestre N.,ICIST
Research and Applications in Structural Engineering, Mechanics and Computation - Proceedings of the 5th International Conference on Structural Engineering, Mechanics and Computation, SEMC 2013 | Year: 2013

In this paper, the Lateral-Distortional Buckling (LDB) of Hollow Tubular Flange Plate Girders (HTFPGs) with slender unstiffened webs is investigated. Firstly, shell finite strip analyses are performed to have a first glance on the elastic buckling behaviour of simply supported girders under uniform bending. The numerical results clearly show the girder length range in which LDB is relevant. After these analyses, analytical models to calculate the critical LDB moment are proposed and validated by comparison with numerical values. The formulae for lateral-distortional buckling of HTFPGs are shown to be accurate and they provide the means to obtain Mcr values analytically without resorting to numerical tools. © 2013 Taylor & Francis Group, London, UK.

Loret B.,Ecole Polytechnique - Palaiseau | Simoes F.M.F.,ICIST
Biomechanics and Modeling in Mechanobiology | Year: 2010

Articular cartilages swell and shrink depending on the ionic strength of the electrolyte they are in contact with. This electro-chemo-mechanical coupling is due to the presence of fixed electrical charges on proteoglycans (PGs). In addition, at nonphysiological pH, collagen fibers become charged. Therefore, variation of the pH of the electrolyte has strong implications on the electrical charge of cartilages and, by the same token, on their transport and mechanical properties. Articular cartilages are viewed as three-phase multi-species porous media. The constitutive framework is phrased in the theory of thermodynamics of porous media. Acid-base reactions, as well as calcium binding, are embedded in this framework. Although macroscopic in nature, the model accounts for a number of biochemical details defining collagen and PGs. The change of the electrical charge is due to the binding of hydrogen ions on specific sites of PGs and collagen. Simulations are performed mimicking laboratory experiments where either the ionic strength or the pH of the bath, the cartilage piece is in contact with, is varied. They provide the evolutions of the chemical compositions of mobile ions, of the sites of acid-base reactions and calcium binding, and of the charges of collagen and glycosaminoglycans, at constant volume fraction of water. Emphasis is laid on the effects of pH, ionic strength and calcium binding on the transport properties of cartilages, and, in particular, on the electrical conductivity and electro-osmotic coefficient. © 2009 Springer-Verlag.

Carmo R.N.F.,ICIST | Carmo R.N.F.,Polytechnic Institute of Coimbra | Dias-da-Costa D.,University of Sydney
Engineering Structures | Year: 2015

This paper presents a detailed study concerning the behaviour of Lightweight Aggregate Concrete (LWAC) members subjected to short-term tensile or flexural loading. For this purpose, a comprehensive experimental programme was carried out covering a wide range of LWAC specimens with different reinforcement ratios and classes of LWAC concrete, with compressive strength ranging from 40. MPa up to 70. MPa. The performance of Eurocode-2 (EC2) and Model Code (MC) guidelines were checked against experimental results, namely in what concerns deformation and cracking prediction for typical service stress levels. In the case of pure tensile loads, it was observed that MC adequately predicts the experimental deformations, whereas EC2 estimates are more adequate for higher reinforcement ratios. In the case of flexural loading, EC2 shows good agreement with experimental results. Furthermore, safe estimates were obtained when using EC2 to predict the crack widths in LWAC members under pure tensile load, whereas MC provided estimates in agreement with the experimental results. Finally, the paper addresses the viability of proposing a new parameter for quantifying deflections and cracking without relying on the tensile strength of concrete, thus avoiding the high variability of this experimental parameter. © 2015 Elsevier Ltd.

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