COLLECTIONS

Documents published in Scipedia

• Impact damage identification in composite laminates using vibration testing

  M. Perez, L. Gil, S. Oller

  Composite Structures (2014). Vol. 108, pp. 267-276

  
  

  Abstract

  Due to the problems arising from impact damage in composite laminates, there is a need to develop fast, accurate, cost-effective and non-destructive testing methods to identify this type of damage at an early stage and thus enhance the service life of composite structures. This paper presents the results of an extensive experimental campaign conducted to investigate the feasibility of using vibration-based methods to identify damages sustained by composite laminates due to low-velocity impacts. The experimental programme included an evaluation of impact damage resistance and tolerance according to ASTM test methods, characterisation of induced damage by ultrasonic testing and quantification of the effects on the vibration response. The damage identification involved the detection, localisation, quantification and estimation of the remaining bearing capacity. Four damage indicators based on modal parameters were assessed by comparing pristine and damaged states. The results allowed for conclusions to be drawn regarding the capability and suitability of each damage indicator, including its ability to detect impact-induced damage, its precision in determining the location of damage, its sensitivity regarding damage extent and pertinent correlations with residual bearing capacity.

  Abstract
  Due to the problems arising from impact damage in composite laminates, there is a need to develop fast, accurate, cost-effective and non-destructive testing methods to identify [...]

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• An Integrated Procedure for the Structural Design of a Composite Rotor-Hydrofoil of a Water Current Turbine (WCT)

  S. Oller, L. Nallim

  Applied Composite Materials (2013). Vol. 20 (6), pp. 1273-1288

  
  

  Abstract

  This paper shows an integrated structural design optimization of a composite rotor-hydrofoil of a water current turbine by means the finite elements method (FEM), using a Serial/Parallel mixing theory (Rastellini et al. Comput. Struct. 86:879–896, 2008, Martinez et al., 2007, Martinez and Oller Arch. Comput. Methods. 16(4):357–397, 2009, Martinez et al. Compos. Part B Eng. 42(2011):134–144, 2010) coupled with a fluid-dynamic formulation and multi-objective optimization algorithm (Gen and Cheng 1997, Lee et al. Compos. Struct. 99:181–192, 2013, Lee et al. Compos. Struct. 94(3):1087–1096, 2012). The composite hydrofoil of the turbine rotor has been design using a reinforced laminate composites, taking into account the optimization of the carbon fiber orientation to obtain the maximum strength and lower rotational-inertia. Also, these results have been compared with a steel hydrofoil remarking the different performance on both structures. The mechanical and geometrical parameters involved in the design of this fiber-reinforced composite material are the fiber orientation, number of layers, stacking sequence and laminate thickness. Water pressure in the rotor of the turbine is obtained from a coupled fluid-dynamic simulation (CFD), whose detail can be found in the reference Oller et al. (2012). The main purpose of this paper is to achieve a very low inertia rotor minimizing the start-stop effect, because it is applied in axial water flow turbine currently in design by the authors, in which is important to take the maximum advantage of the kinetic energy. The FEM simulation codes are engineered by CIMNE (International Center for Numerical Method in Engineering, Barcelona, Spain), COMPack for the solids problem application, KRATOS for fluid dynamic application and RMOP for the structural optimization. To validate the procedure here presented, many turbine rotors made of composite materials are analyzed and three of them are compared with the steel one.

  Abstract
  This paper shows an integrated structural design optimization of a composite rotor-hydrofoil of a water current turbine by means the finite elements method (FEM), using a [...]

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• Impact damage prediction in carbon fiber-reinforced laminated composite using the matrix-reinforced mixing theory

  M. Perez, X. Martinez, S. Oller, L. Gil, F. Rastellini, F. Flores

  Composite Structures (2013). Vol. 104, pp. 239-248

  
  

  Abstract

  The impact damage tolerance of fiber-reinforced laminated composite materials is a source of concern, mainly due to internal induced damage which causes large reductions on the strength and stability of the structure. This paper presents a procedure based on a finite element formulation that can be used to perform numerical predictions of the impact induced internal damage in composite laminates. The procedure is based on simulating the composite performance using a micro-mechanical approach named matrix-reinforced mixing theory, a simplified version of the serial/parallel mixing theory that does not require neither the iterative procedure nor the calculation of the tangent stiffness tensor. The numerical formulation uses continuum mechanics to simulate the phenomenon of initiation and propagation of interlaminar damage with no need to formulate interface elements, resulting in a computationally less demanding formulation. To demonstrate the capability of numerical procedure when applied to a low-velocity impact problem, numerical results are compared with the experimental ones obtained in a test campaign performed on 44 laminates specimens subjected to an out-of-plane and concentrated impact event, according to ASTM test method. Results are in good agreement with experimental data in terms of delamination onset and the internal spatial distribution of induced damage.

  Abstract
  The impact damage tolerance of fiber-reinforced laminated composite materials is a source of concern, mainly due to internal induced damage which causes large reductions [...]

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• Analytical and numerical analysis of human dental occlusal contact

  F. Bastos, E. Casas, S. Oller

  Computer Methods in Biomechanics and Biomedical Engineering (2013). Vol. 16 (5), pp. 495-503

  
  

  Abstract

  The knowledge of contact forces in teeth surfaces during mastication or para-functional movements can help to understand processes related to friction and wear of human dental enamel. The development of a numerical model for analysis of the occlusal contact between two antagonistic teeth is proposed, which includes three basic steps: the characterisation of the surface roughness, its homogenisation using an assumed distribution function and the numerical determination of the resulting forces. Finite element strain results for the main different asperities are statistically combined, deriving the predicted macroscopic behaviour of the interface. Axisymmetric and 3D numerical models with an elasto-plastic constitutive law are used to simulate micro-indentations and micro-contacts, respectively. The contact is allowed to occur locally in planes not necessarily parallel to the surface's mean plane, a problem for which there is no analytical solution. The three identified parameters, homogenised surface hardness (3.68 GPa), surface yield stress (3.08 GPa) and static friction coefficient (0.23), agree with the experimental values reported in the literature.

  Abstract
  The knowledge of contact forces in teeth surfaces during mastication or para-functional movements can help to understand processes related to friction and wear of human dental [...]

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• An inverse approach for the mechanical characterisation of vascular tissues via a generalised rule of mixtures

  F. Bellomo, S. Oller, L. Nallim

  Computer Methods in Biomechanics and Biomedical Engineering (2012). Vol. 15 (12), pp. 1257-1262

  
  

  Abstract

  Mechanical factors such as stresses and strains play a major role in the growth and remodelling of soft biological tissues. The main constituents of tissue undergo different processes reacting to mechanical stimulus. Thereby, the characterisation of growth and remodelling requires an accurate estimation of the stresses and strains of their main components. Many soft tissues can be considered as composite materials and can be analysed using an appropriate rule of mixtures. Particularly, arterial tissue can be modelled as an isotropic soft matrix reinforced with preferentially oriented collagen fibres. An inverse approach to obtain the mechanical characterisation of each main component is proposed in this work. The procedure is based on a rule of mixtures raised in a finite deformation framework and generalised to include kinematics and compatibility equations for serial–parallel behaviour. This methodology allows obtaining the stress–strain relationship of the components fitting experimental data.

  Abstract
  Mechanical factors such as stresses and strains play a major role in the growth and remodelling of soft biological tissues. The main constituents of tissue undergo different [...]

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• Study and prediction of the mechanical performance of a nanotube-reinforced composite

  F. Otero, X. Martinez, S. Oller, O. Salomón

  Composite Structures (2012). Vol. 94 (9), pp. 2920-2930

  
  

  Abstract

  Carbon nanotubes (CNTs) have been regarded as ideal reinforcements for high-performance composites. A key factor for the reinforcement efficiency is the interface bonding between the CNTs and the matrix. This paper presents a new constitutive model to predict the mechanical performance of composites made with CNTs. The model takes into account explicitly the performance of the interface between the matrix and the CNTs. The formulation developed is based in the mixing theory. It divides the composite in matrix and in a new material result of coupling the CNTs with the interface. The relation defined between interface and CNTs assumes that the load is transferred to the nanotubes along their ends and that in the central part the CNTs can develop their full strength. The composite non-linear behavior results from the non-linearities of its constituents. In case of interface damage, it also becomes non-linear the law defined to couple the interface with the CNTs. After describing the formulation, it is validated studying the elastic response of several composites made with different types of CNTs reinforcements. The non-linear behavior provided by the formulation is also studied. In both cases the numerical results are compared with experimental data showing good agreement.

  Abstract
  Carbon nanotubes (CNTs) have been regarded as ideal reinforcements for high-performance composites. A key factor for the reinforcement efficiency is the interface bonding [...]

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• A constitutive model for tissue adaptation: Necrosis and stress driven growth

  F. Bellomo, F. Armero, L. Nallim, S. Oller

  Mechanics Research Communications (2012). Vol. 42 (SI), pp. 51-59

  
  

  Abstract

  This paper presents a general constitutive model for tissue stress driven growth and necrotic remodeling. The general theoretical framework is given by a kinematics formulation in large strain. The model allows the analysis of tissue behaviour considering the coupling of biological and mechanical fields, through the implementation of an internal variable that represents the bioavailability, together with the growth induced by mechanical stimuli. To take into account the tissue necrosis phenomenon a reinterpretation of the classical theory of mixtures is proposed. The proposed formulation is based on the consideration of two components, normal tissue and necrotic tissue, each one with its own volumetric participation, which is treated in an evolutive way, allowing simulating the phenomena of ischemia. This article also presents a brief description of the numerical implementation of the model as well as the obtained results, which shows the process of necrosis of part of the tissue, its influence on the surrounding healthy tissue, and the interaction between the nutrients availability and the stress driven growth and necrotic remodeling process.

  Abstract
  This paper presents a general constitutive model for tissue stress driven growth and necrotic remodeling. The general theoretical framework is given by a kinematics formulation [...]

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• Dynamical analysis of long fiber-reinforced laminated plates with elastically restrained edges

  F. Bellomo, R. Quinteros, S. Oller, L. Nallim

  Advances in Acoustic and Vibration (2012). Vol. 2012, pp. 1-16

  
  

  Abstract

  This paper presents a variational formulation for the free vibration analysis of unsymmetrically laminated composite plates with elastically restrained edges. The study includes a micromechanics approach that allows starting the study considering each layer as constituted by long unidirectional fibers in a continuous matrix. The Mori-Tanaka method is used to predict the mechanical properties of each lamina as a function of the elastic properties of the components and of the fiber volume fraction. The resulting mechanical properties for each lamina are included in a general Ritz formulation developed to analyze the free vibration response of thick laminated anisotropic plates resting on elastic supports. Comprehensive numerical examples are computed to validate the present method, and the effects of the different mechanical and geometrical parameters on the dynamical behavior of different laminated plates are shown. New results for general unsymmetrical laminates with elastically restrained edges are also presented. The analytical approximate solution obtained in this paper can also be useful as a basis to deal with optimization problems under, for instance, frequency constraints.

  Abstract
  This paper presents a variational formulation for the free vibration analysis of unsymmetrically laminated composite plates with elastically restrained edges. The study includes [...]

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• Nonlinear homogenization techniques to solve masonry structures problems

  R. Quinteros, S. Oller, L. Nallim

  Composite Structures (2012). Vol 94 (2), pp. 724-730

  
  

  Abstract

  The behaviour of masonry material subjected to different in-plane loading combination is studied in this work. The masonry is considered as a periodic composite material composed by a regular distribution of brick and mortar and it is analyzed using a homogenization technique. The mechanical properties of the masonry, as an orthotropic homogeneous material, depend on the geometrical and mechanical properties of the components based on the study of the equilibrium and compatibility of a basic cell. The masonry is a frictional material and its behaviour depends on the loading direction, for these reasons, a unilateral damage model is chosen for the analysis. This model describes the behaviour of brittle materials subjected to tension–compression cyclic loads based on the introduction of two damage variables and it assumes that the damage is due to the beginning and growth of cracks only in the mortar joints. It is considered that the bricks have a linear elastic constitutive relationship. Numerical applications are performed with a nonlinear finite element code in order to test the proposed procedure by comparing the results with those available in the literature and also with experimental data.

  Abstract
  The behaviour of masonry material subjected to different in-plane loading combination is studied in this work. The masonry is considered as a periodic composite material composed [...]

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• Dimensionado sísmico de edificios porticados de hormigón armado mediante factores de amplificación con base en energía

  J. Vielma-Perez, H. Barbat, S. Oller

  Hormigón y Acero (2012). Vol. 63 (263), pp. 83-96

  
  

  Abstract

  El proyecto sismorresistente de estructuras actual se realiza mediante una serie de prescripciones normativas que persiguen garantizar su adecuado comportamiento. Los procedimientos actuales de análisis se realizan utilizando el comportamiento elástico, aplicando los factores de reducción de respuesta para obtener fuerzas sísmicas equivalentes, representativas del comportamiento inelástico que pueden alcanzar las estructuras al ser sometidas a la acción de sismos fuertes. A partir de los resultados del análisis elástico se obtienen los desplazamientos inelásticos, amplificados mediante factores que son función de los factores de reducción; sin embargo, se ha observado una amplia variación en los valores de los factores de amplificación prescritos en las normas a nivel mundial, por lo que se justifica profundizar en el estudio de dichos factores. En este trabajo se presenta un procedimiento de dimensionado sísmico con base en la respuesta no lineal. El procedimiento es aplicado a un conjunto de edificios regulares de hormigón armado de diferentes alturas, proyectados para niveles altos de amenaza sísmica y es evaluado aplicando análisis dinámico no lineal para tres Estados Límite, correlacionado con tres umbrales de daño que se definen a través de las derivas de planta. Los resultados demuestran que el procedimiento es adecuado para realizar el dimensionado sísmico de edificios, proporcionando al proyectista criterios de evaluación más cónsonos con las características de los edificios y de la amenaza sísmica.

  Abstract
  El proyecto sismorresistente de estructuras actual se realiza mediante una serie de prescripciones normativas que persiguen garantizar su adecuado comportamiento. Los procedimientos [...]

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