This paper describes experimental and numerical studies on the application of the end loaded split test to mode II wood fracture characterization. A new data reduction scheme, based on the specimen compliance and on the equivalent crack... more
This paper describes experimental and numerical studies on the application of the end loaded split test to mode II wood fracture characterization. A new data reduction scheme, based on the specimen compliance and on the equivalent crack concept, is proposed. The method presents three main advantages relatively to the classical methodologies: it does not require crack measurement during propagation; it
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Research Interests: Civil Engineering, Aerospace Engineering, Finite Element, Damage Mechanics, Computer Simulation, and 15 moreCompressive Strength, Delamination, Composite Material, Cracking, Energy absorption, Crashworthiness, Composite Structure, Experimental Tests, Constitutive model, Epoxy Resin, Computer Modelling, Carbon Fiber, Elastic Properties, Experimental Data, and Failure Mechanism
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An experimental and numerical study on mode II fracture behaviour of European beech (Fagus sylvatica L.) in the RL and TL crack propagation systems is performed. It is a hardwood species that has attracted increasing interest for... more
An experimental and numerical study on mode II fracture behaviour of European beech (Fagus sylvatica L.) in the RL and TL crack propagation systems is performed. It is a hardwood species that has attracted increasing interest for structural use in Europe in recent years. Three-point end notched flexure tests are performed. The R-curves of both crack propagation systems are obtained, from which the critical strain energy release rate (GIIc) is derived by applying the compliance-based beam method. This data reduction scheme avoids crack length monitoring during its propagation, which is an advantage in wood. Using a direct method, the shear traction‐separation laws in mode II loading are determined. Full field displacements around the crack tip are monitored by 3D digital image correlation technique, and the crack tip shear displacements are analysed. The proposed method is numerically validated by finite element analysis. Cohesive zone models are developed implementing a shear tracti...
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In this work a numerical study has been performed to verify the adequacy of the End Notched Flexure (ENF) test to determine the fracture toughness under mode II loading of cortical bovine bone tissue. In this work a detailed numerical... more
In this work a numerical study has been performed to verify the adequacy of the End Notched Flexure (ENF) test to determine the fracture toughness under mode II loading of cortical bovine bone tissue. In this work a detailed numerical analysis using the finite element method and a cohesive damage model was performed in order to optimize the specimen geometry when applied to bone fracture characterization under mode II loading. A data reduction scheme based on specimen compliance and crack equivalent concept was used to overcome the difficulties inherent to crack monitoring during its growth. It was verified that a judicious selection of the geometry allows a rigorous estimation of toughness in mode II.
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For the past decades composite materials are increasing their importance as one of the most interesting group of materials, due to their unique properties of low weight, high strength and stiffness. Although earlier development was... more
For the past decades composite materials are increasing their importance as one of the most interesting group of materials, due to their unique properties of low weight, high strength and stiffness. Although earlier development was related with aerospace and aeronautical industries, recent years had seen the spread of their use in many others like automotive, transport, energy, construction and sport goods.
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Este trabajo esta dedicado a la elaboracion y diseno de un aparato inspirado en la solucionpropuesta por Fernlund y Spelt para caracterizar la fractura de las uniones adhesivas bajoaplicaciones de cargas en modo mixto I + II . El aparato... more
Este trabajo esta dedicado a la elaboracion y diseno de un aparato inspirado en la solucionpropuesta por Fernlund y Spelt para caracterizar la fractura de las uniones adhesivas bajoaplicaciones de cargas en modo mixto I + II . El aparato permite una facil variacion delmodo de mezcla y cubre todo el rango de combinaciones posibles (I y II).Ademas, se propone un procedimiento para tratamiento de datos en funcion del tamano degrieta, basado en la teoria de la viga y el concepto de grieta equivalente, superando variasdificultades inherentes a los ensayos realizados. El desarrollo se basa en una metodologiade diseno mediante la validacion numerica basada en datos experimentales obtenidos conun equipo de ensayo de doble accion en Virginia Tech. Esta metodologia se presenta acontinuacion, y esta validada con la aplicacion experimental del aparato.
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In this work a parametric study of the Edge Crack Torsion (ECT) specimen was performed in order to maximize the mode III component (GIII) of the strain energy release rate for carbon-epoxy laminates. A three-dimensional finite element... more
In this work a parametric study of the Edge Crack Torsion (ECT) specimen was performed in order to maximize the mode III component (GIII) of the strain energy release rate for carbon-epoxy laminates. A three-dimensional finite element analysis of the ECT test was conducted considering a (90/0/(+45/-45)2/(-45/+45)2/0/90)S lay-up. The main objective was to define an adequate geometry to obtain an almost pure mode III at crack front. The geometrical parameters studied were specimen dimensions, distance between pins and size of the initial crack. The numerical results demonstrated that the ratio between the specimen length and the initial crack length had a significant effect on the strain energy release rate distributions. In almost all of the tested configurations, a mode II component occurred near the edges but it did not interfere significantly with the dominant mode III state.
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O recurso ao Metodo de Elementos Finitos (MEF) para a simulacao e estudo damaquinagem tem vindo a merecer um interesse crescente. Neste trabalho comparam-se osresultados experimentais obtidos usando diferentes brocas helicoidais com uma... more
O recurso ao Metodo de Elementos Finitos (MEF) para a simulacao e estudo damaquinagem tem vindo a merecer um interesse crescente. Neste trabalho comparam-se osresultados experimentais obtidos usando diferentes brocas helicoidais com uma simulacaonumerica da furacao usando elementos solidos tridimensionais do software ABAQUS®Explicit. Os resultados obtidos mostram a aplicabilidade do modelo, nomeadamente no querespeita a variacao do principal parâmetro de corte no processo de furacao o avanco egeometria da ferramenta. O desenvolvimento das forcas axiais durante a furacao estadirectamente relacionado com a seleccao dos parâmetros de corte e geometria de ferramentae contribui para a existencia de danos na placa. Assim, a minimizacao destas forcas permitereduzir o risco de delaminacao. Os resultados relativos a forca necessaria para o inicio dadelaminacao sao comparados com modelos analiticos ja conhecidos. Os resultadosnumericos apresentam uma boa concordância com os obtidos experiment...
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The fracture behaviour of human cortical bone was analysed considering a miniaturized version of the Double Cantilever Beam (DCB) test. A specific data reduction scheme based on crack equivalent concept was used to obtain the resistance... more
The fracture behaviour of human cortical bone was analysed considering a miniaturized version of the Double Cantilever Beam (DCB) test. A specific data reduction scheme based on crack equivalent concept was used to obtain the resistance curves. The definition of the cohesive laws mimicking the fracture process was performed measuring the crack tip opening displacement by digital image correlation during the test. The differentiation of the relation between the strain energy release rate and crack tip opening displacement allows to define the experimental cohesive law. In order to validate the procedure, trapezoidal cohesive laws with bilinear softening were adjusted to the experimental ones. The DCB tests were simulated by finite element analysis including cohesive zone modelling with the adjusted laws. The resulting numerical load-displacement and resistance curves were compared with the numerical ones. Good agreement was obtained which validates the proposed procedure.
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This work describes the application of end loaded split and single-leg bending tests to the mixed-mode fracture characterization of wood. Experimental tests and numerical validation analyses were performed. A new data reduction scheme... more
This work describes the application of end loaded split and single-leg bending tests to the mixed-mode fracture characterization of wood. Experimental tests and numerical validation analyses were performed. A new data reduction scheme based on the crack equivalent concept is proposed. The method overcomes the difficulties inherent to these tests, such as crack length monitoring during propagation and influence of clamping conditions. The single-leg bending test is simpler to execute and provided accurate results. The obtained mixed-mode fracture energy is associated with the pure mode values and the obtained trend point to a linear fracture criterion as a candidate to describe the fracture behavior of the Pinus pinaster Ait. wood.
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The present work is dedicated to the development and design of a load jig inspired in the proposed solution by Fernlund and Spelt in order to characterize fracture of bonded joints under mixed-mode I+II loading. The jig allows for easy... more
The present work is dedicated to the development and design of a load jig inspired in the proposed solution by Fernlund and Spelt in order to characterize fracture of bonded joints under mixed-mode I+II loading. The jig allows for easy alteration of the mode-mixity and permits covering the full range of mixed-mode I+II combinations. A data reduction scheme based on specimen compliance, beam theory and crack equivalent concept is proposed to overcome several difficulties inherent to the test analysis. The development was based in a design methodology using numerical validation based on experimental data obtained with a Dual Loading Frame from Virginia Tech. This methodology is presented and then validated with the experimental application of the jig. INTRODUCTION Bonded joints are being increasingly applied in structures involving risk, as is the case of the aeronautical, automotive, and civil infrastructure industries. The classical strength prediction based on stress or strain anal...
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Mode I fracture characterization was induced in wood (Picea abies L.) using the single-edge-notched beamloaded in three-point-bending. A developed data reduction scheme based on the equivalent linear elasticfracture mechanics was used to... more
Mode I fracture characterization was induced in wood (Picea abies L.) using the single-edge-notched beamloaded in three-point-bending. A developed data reduction scheme based on the equivalent linear elasticfracture mechanics was used to evaluate the Resistance-curve instead of classical methods. The method isfound on beam theory and crack equivalent concept taking into account the triangular stress relief regionthat develops in the crack vicinity. The method dispenses crack length monitoring in the course of theloading process, providing a complete Resistance-curve which is essential for a clear identification of thefracture energy. The validation of the procedure has been performed numerically using a bilinear cohesivedamage model, thus allowing the simulation of both damage initiation and growth. The numerical modelalso provided the critical specimen dimensions that permit the attainment of accurate evaluation of thefracture toughness in wood
The single edge notched beam loaded in three-point-bending (SEN-TPB) was used in this study to induce mode I fracture in wood (Picea abeas L.). Numerical analyses of stress profiles along the specimen length revealed a stress relief... more
The single edge notched beam loaded in three-point-bending (SEN-TPB) was used in this study to induce mode I fracture in wood (Picea abeas L.). Numerical analyses of stress profiles along the specimen length revealed a stress relief region as crack grows in size. Based on this data, beam theory and crack equivalent concepts were employed to develop an expedite data reduction scheme to estimate the Resistance-curve, taking advantage of a simplification performed on the revealed stress relief region. Besides allowing the evaluation of wood fracture toughness without crack length monitoring during loading, the developed procedure provides a user-friendly method when compared to other sophisticated procedures. Experiments involving the SEN-TPB were performed to evaluate fracture toughness in wood. INTRODUCTION A significant interest for wood as a structural material is increasing in the developed countries, motivated by environmental concerns and energy needs. Hence, the improvement of ...
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The increasing use of Carbon-Fibre Reinforced Plastic (CFRP) laminates in high responsibility applications introduces an issue regarding their handling after damage. The availability of efficient repair methods is essential to restore the... more
The increasing use of Carbon-Fibre Reinforced Plastic (CFRP) laminates in high responsibility applications introduces an issue regarding their handling after damage. The availability of efficient repair methods is essential to restore the strength of the structure. The availability of accurate predictive tools for the repairs behaviour is also essential for the reduction of costs and time associated to extensive tests. This work reports on a numerical study of the tensile behaviour of three-dimensional (3D) adhesively-bonded scarf repairs in CFRP structures, using a ductile adhesive. The Finite Element (FE) analysis was performed in ABAQUS ® and Cohesive Zone Models (CZM's) was used for the simulation of damage in the adhesive layer. A parametric study was performed on two geometric parameters. The use of over-laminating plies covering the repaired region at the outer or both repair surfaces was also tested as an attempt to increase the repairs efficiency. The results allowed th...
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The interlaminar fracture toughness in pure mode II (GIIc) of a Carbon-Fibre Reinforced Plastic (CFRP) composite is characterized experimentally and numerically in this work, using the End-Notched Flexure (ENF) fracture characterization... more
The interlaminar fracture toughness in pure mode II (GIIc) of a Carbon-Fibre Reinforced Plastic (CFRP) composite is characterized experimentally and numerically in this work, using the End-Notched Flexure (ENF) fracture characterization test. The value of GIIc was extracted by a new data reduction scheme avoiding the crack length measurement, named Compliance-Based Beam Method (CBBM). This method eliminates the crack measurement errors, which can be non-negligible, and reflect on the accuracy of the fracture energy calculations. Moreover, it accounts for the Fracture Process Zone (FPZ) effects. A numerical study using the Finite Element Method (FEM) and a triangular cohesive damage model, implemented within interface finite elements and based on the indirect use of Fracture Mechanics, was performed to evaluate the suitability of the CBBM to obtain GIIc. This was performed comparing the input values of GIIc in the numerical models with the ones resulting from the application of the C...
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ABSTRACT The fracture behaviour of a quasibrittle material such as wood, characterized by the development of a large fracture process zone (FPZ), is nowadays well-known to be efficiently described by cohesive crack models. The most common... more
ABSTRACT The fracture behaviour of a quasibrittle material such as wood, characterized by the development of a large fracture process zone (FPZ), is nowadays well-known to be efficiently described by cohesive crack models. The most common applications of fictitious or cohesive crack models used to simulate nonlinear fracture mechanics of quasibrittle materials are considered as variations [1] of a model proposed by Hillerborg and co-workers [2]. These applications need to introduce a cohesive zone at the crack tip, i.e., a fictitious line crack transmitting normal stress dependent on the corresponding opening displacement w. Cohesive crack models are typically applied through finite elements calculations. Interface elements or springs [3] with prearranged strain-softening properties are integrated into the structural model along the most probable crack path. Since the pioneering application of cohesive crack models to wood due to Boström [4], a so called bilinear strain-softening model was applied by Stanzl-Tschegg et al. [5] to obtain wood load-deflection curves according to a developed wedge-splitting test protocol.
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This work reports on the experimental and numerical study of the bending behaviour of two-dimensional adhesively-bonded scarf repairs of carbon-epoxy laminates, bonded with the ductile adhesive Araldite 2015®. Scarf angles varying from 2... more
This work reports on the experimental and numerical study of the bending behaviour of two-dimensional adhesively-bonded scarf repairs of carbon-epoxy laminates, bonded with the ductile adhesive Araldite 2015®. Scarf angles varying from 2 to 45º were tested. The experimental work performed was used to validate a numerical Finite Element analysis using ABAQUS® and a methodology developed by the authors to predict the strength of bonded assemblies. This methodology consists on replacing the adhesive layer by cohesive elements, including mixed-mode criteria to deal with the mixed-mode behaviour usually observed in structures. Trapezoidal laws in pure modes I and II were used to account for the ductility of the adhesive used. The cohesive laws in pure modes I and II were determined with Double Cantilever Beam and End-Notched Flexure tests, respectively, using an inverse method. Since in the experiments interlaminar and transverse intralaminar failures of the carbon-epoxy components also ...
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Research Interests: Materials Engineering, Mechanical Engineering, Materials Science, Biomedical Engineering, Medicine, and 15 moreFracture and cohesive zone model, Animals, Data Reduction, Composite Material, Cattle, Cantilever, Bone fracture, Cortical Bone, Fracture Toughness, Materials Testing, Fracture Energy, Bone and Bones, Biomechanical Phenomena, double cantilever beam, and Fractures Bone
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An experimental and numerical study of the tensile behaviour of three-dimensional carbon-epoxy adhesively bonded strap repairs is presented. Experimentally, the failure mode, elastic stiffness and strength were evaluated for different... more
An experimental and numerical study of the tensile behaviour of three-dimensional carbon-epoxy adhesively bonded strap repairs is presented. Experimentally, the failure mode, elastic stiffness and strength were evaluated for different overlap lengths and patch thicknesses. The ...
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ABSTRACT The accurate prediction of failure of sandwich structures using cohesive mixed-mode damage models depends on the accurate characterization of the cohesive laws under pure mode loading. In this work, a numerical and experimental... more
ABSTRACT The accurate prediction of failure of sandwich structures using cohesive mixed-mode damage models depends on the accurate characterization of the cohesive laws under pure mode loading. In this work, a numerical and experimental study on the asymmetric double cantilever beam (DCB) sandwich specimen is presented with the objective to characterize the debonding fracture between the face sheet and the core under pure mode I. A data reduction method based on beam theory was formulated in such a way to incorporate the complex damaging phenomena of the debonding due to the material and geometric asymmetry of the specimen, via the consideration of an equivalent crack length (ae). Experimental DCB tests were performed and the proposed methodology was followed to obtain the debonding fracture energy (GIc). The experimental tests were numerically simulated and a cohesive damage model was employed to reproduce crack propagation. An inverse method was followed to obtain the local cohesive strength (σu,I) based on the fitting of the numerical and experimental load–displacement curves. With the value of fracture energy and cohesive strength defined, the cohesive law for interface mode I fracture is characterized. Good agreement between the numerical and the experimental R-curves validates the accuracy of the proposed data reduction procedure.
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The concept of the R-curve was first introduced in 1958 by Irwin [1] and more clearly in 1961 by Kraft, Sullivan and Boyle [2], stating that crack growth resistance is not invariable but increases with the crack length , frequently... more
The concept of the R-curve was first introduced in 1958 by Irwin [1] and more clearly in 1961 by Kraft, Sullivan and Boyle [2], stating that crack growth resistance is not invariable but increases with the crack length , frequently expressed through . Experimental and theoretical investigations in materials that exhibit toughening behaviour have proved however, that the R-curve is not an intrinsic material property but depends on many factors. These include test specimen geometry [3], relative crack size [4], mode of loading [5], and specimen size [6]. To the authors knowledge the influence of the specimen size on the R-curve is not well understood in wood. As a result, experiments in spruce (Picea abies L.) have been conducted inducing fracture (Mode I) in geometrically similar Single-Edge-Notched composite beams in Three Point-Bending (SEN-TPB) (Figure 1), under displacement control. The experiments involved 6 series of specimens resulting in a size range of 1:12 (Table 1) and a s...