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Silva, F. G. A., M. F. S. F. de Moura, N. Dourado, J. Xavier, F. A. M. Pereira, J. J. L. Morais, M. I. R. Dias, P. J. Lourenço, and F. M. Judas. "Fracture characterization of human cortical bone under mode II loading using the end-notched flexure test." Medical {&} Biological Engineering {&} Computing. 55 (2017): 1249-1260. AbstractWebsite

Fracture characterization of human cortical bone under mode II loading was analyzed using a miniaturized version of the end-notched flexure test. A data reduction scheme based on crack equivalent concept was employed to overcome uncertainties on crack length monitoring during the test. The crack tip shear displacement was experimentally measured using digital image correlation technique to determine the cohesive law that mimics bone fracture behavior under mode II loading. The developed procedure was validated by finite element analysis using cohesive zone modeling considering a trapezoidal with bilinear softening relationship. Experimental load-displacement curves, resistance curves and crack tip shear displacement versus applied displacement were used to validate the numerical procedure. The excellent agreement observed between the numerical and experimental results reveals the appropriateness of the proposed test and procedure to characterize human cortical bone fracture under mode II loading. The proposed methodology can be viewed as a novel valuable tool to be used in parametric and methodical clinical studies regarding features (e.g., age, diseases, drugs) influencing bone shear fracture under mode II loading.

Silva, F. G. A., M. F. S. F. de Moura, N. Dourado, J. Xavier, F. A. M. Pereira, J. J. L. Morais, and M. I. R. Dias. "Mixed-mode I+II fracture characterization of human cortical bone using the Single Leg Bending test." Journal of the Mechanical Behavior of Biomedical Materials. 54 (2016): 72-81. AbstractWebsite

Abstract Mixed-mode I+II fracture characterization of human cortical bone was analyzed in this work. A miniaturized version of the Single Leg Bending test (SLB) was used owing to its simplicity. A power law criterion was verified to accurately describe the material fracture envelop under mixed-mode I+II loading. The crack tip opening displacements measured by digital image correlation were used in a direct method to determine the cohesive law mimicking fracture behavior of cortical bone. Cohesive zone modeling was used for the sake of validation. Several fracture quantities were compared with the experimental results and the good agreement observed proves the appropriateness of the proposed procedure for fracture characterization of human bone under mixed-mode I+II loading.

Silva, F., M. de Moura, N. Dourado, J. Xavier, F. Pereira, J. Morais, M. Dias, P. Lourenço, and F. Judas. "Fracture Characterization of Human Cortical Bone Under Mode I Loading." Journal of Biomechanical Engineering. 137 (2015): 121004. AbstractWebsite

A miniaturized version of the double cantilever beam (DCB) test is used to determine the fracture energy in human cortical bone under pure mode I loading. An equivalent crack length based data-reduction scheme is used with remarkable advantages relative to classical methods. Digital image correlation (DIC) technique is employed to determine crack opening displacement at the crack tip being correlated with the evolution of fracture energy. A method is presented to obtain the cohesive law (trapezoidal bilinear softening) mimicking the mechanical behavior observed in bone. Cohesive zone modeling (CZM) (finite-element method) was performed to validate the procedure showing excellent agreement.

Silva, F., J. Xavier, F. A. M. Pereira, J. Morais, N. Dourado, and M. F. S. F. de Moura. "Determination of cohesive laws in wood bonded joints under mode I loading using the DCB test." Holzforschung. 67 (2013): 835-959. AbstractWebsite
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Silva, A. L. L., A. M. P. de Jesus, J. Xavier, J. A. F. O. Correia, and A. A. Fernandes. "Combined analytical-numerical methodologies for the evaluation of mixed-mode (I+II) fatigue crack growth rates in structural steels." Engineering Fracture Mechanics. 185 (2017): 124-138. AbstractWebsite

Abstract This paper proposes an experimental study aiming to evaluate stress intensity factors (SIFs) for fatigue cracks propagating under pure mode I and mixed-mode I+II for a S235 structural steel. Compact tension (CT) specimens with a side hole were manufactured in order to generate a stress field, ahead of the crack tip, resulting in mixed-mode fatigue crack propagation. Specimens with distinct side hole locations were submitted to fatigue tests under stress controlled conditions for a stress rati

Silva, F. G. A., J. J. L. Morais, N. Dourado, J. Xavier, F. A. M. Pereira, and M. F. S. F. de Moura. "Determination of cohesive laws in wood bonded joints under mode II loading using the ENF test." International Journal of Adhesion and Adhesives. 51 (2014): 54-61. AbstractWebsite

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Silva, Patrícia, Pedro Fernandes, José Sena-Cruz, José Xavier, Fernando Castro, Delfim Soares, and Vítor Carneiro. "Effects of different environmental conditions on the mechanical characteristics of a structural epoxy." Composites Part B: Engineering. 88 (2016): 55-63. AbstractWebsite

Abstract With the aim of characterising a commercially available epoxy adhesive used for fibre-reinforced polymers strengthening applications, when submitted to different environmental conditions, mainly thermal (TC), freeze–thaw (FT), and wet–dry (WD) cycles and immersion in pure (PW) and water with chlorides (CW) for periods of exposure that lasted up to 16 months, an experimental program was carried out. Several methodologies were used in its characterization, mainly the scanning electron microscope (SEM), dynamic mechanical analysis (DMA), standard tensile tests (STT) coupled with digital image correlation (DIC). In general the results revealed that the chemical composition was not affected by the environmental conditions. Nevertheless, it was verified through \{DMA\} and \{STT\} that the modulus of elasticity and tensile strength of the epoxy adhesive increased in the TC, while the specimens submitted to \{PW\} and \{CW\} faced a high degradation in terms of its mechanical properties. Eventually, the glass transition temperature (Tg) was not affected by the environmental conditions, apart from the specimens subjected to \{TC\} and FT, presenting a higher and lower Tg, respectively, when compared with the reference specimens.

Silva, TEF, S. Gain, D. Pinto, A. M. P. de Jesus, J. Xavier, A. Reis, and P. A. R. Rosa. "Fracture characterization of a cast aluminum alloy aiming machining simulation." Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications. 233 (2019): 402-412. AbstractWebsite

Despite extensive research regarding metal cutting simulation, the current industrial practice very often relies on empirical data when it comes to tool design. In order accurately simulate the cutting process it is not only important to have robust numerical models that closely portray the phenomenon, but also to properly characterize the material taking into account the cutting conditions. The goal of this investigation focuses on the mechanical characterization of the cast aluminum alloy AlSi9Cu3 by conducting both compression and fracture tests. Due to its very good castability, machinability, and attractive mechanical properties, this alloy is widely used in casting industry for the manufacture of automotive components, among others. Besides the experimental characterization, a numerical methodology is proposed for the modeling of the cast alloy, making use of the Johnson–Cook constitutive material model, in Abaqus/CAE. The material model is calibrated based on compression tests at multiple conditions (quasi-static, incremental dynamic and high temperatures). The identified model is then validated by simulation of the ductile fracture tests of notched specimens. The obtained numerical results were consistent with the experimentally obtained, contributing to the validity of the presented characterization technique.

Sousa, A. M. R., J. Xavier, M. Vaz, J. J. L. Morais, and V. M. J. Filipe. "Cross-correlation and differential technique combination to determine displacement fields." Strain. 47 (2011): 87-98. Abstract

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Sousa, A. M. R., J. Xavier, J. J. L. Morais, V. M. J. Filipe, and M. Vaz. "Processing discontinuous displacement fields by a spatio-temporal derivative technique." Optics and Lasers in Engineering. 49 (2011): 1402-1412. Abstract
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Subramani, P., Sohel Rana, Bahman Ghiassi, Raul Fangueiro, Daniel V. Oliveira, Paulo B. Lourenco, and Jose Xavier. "Development and characterization of novel auxetic structures based on re-entrant hexagon design produced from braided composites." Composites Part B: Engineering. 93 (2016): 132-142. AbstractWebsite

Abstract This paper reports the first attempt of developing macro-scale auxetic structures based on re-entrant hexagon design from braided composite materials for civil engineering applications. Braided composite rods (BCRs) were produced and arranged as longitudinal and horizontal elements to produce three types of auxetic structures: (1) basic re-entrant hexagon structure, (2) basic structure modified by adding straight longitudinal elements and (3): structure-2 modified by changing structural angle. The influence of various material and structural parameters as well as structure type on Poisson's ratio and tensile properties was thoroughly investigated. The auxetic behaviour was found to strongly depend on the structural angle and straight elements, resulting in lower auxeticity with lower angles and in presence of straight elements. Material parameters influenced the auxetic behaviour to a lesser extent and a decrease in auxetic behaviour was noticed with increase in core fibre linear density and using stiffer fibres such as carbon. The reverse effect was observed in case of tensile strength and work of rupture. Among these structures, structure-3 exhibited good auxetic behaviour, balanced tensile properties, and high energy absorption capacity and their auxetic behaviour could be well predicted with the developed analytical model. Therefore, these novel structures present good potential for strengthening of civil structures.

Subramani, P., S. Rana, D. V. Oliveira, R. Fangueiro, and J. Xavier. "Development of novel auxetic structures based on braided composites." Materials & Design. 61 (2014): 286-295. AbstractWebsite
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Xavier, J., M. Oliveira, P. Monteiro, J. J. L. Morais, and M. F. S. F. Moura. "Direct Evaluation of Cohesive Law in Mode I of Pinus pinaster by Digital Image Correlation." Experimental Mechanics. 54 (2014): 829-840. AbstractWebsite
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Xavier, J., M. Oliveira, J. J. L. Morais, and M. F. S. F. de Moura. "Determining mode İI\} cohesive law of Pinus pinaster by combining the end-notched flexure test with digital image correlation." Construction and Building Materials. 71 (2014): 109-115. Abstract

Abstract This work addresses the direct identification of cohesive law of Pinus pinaster in mode II. The end-notched flexure (ENF) test is selected for mode İI\} loading. The strain energy release rate in mode İI\} (GII) is determined according to the compliance-based beam method (CBBM) by processing the global load�displacement curve, without requirements to monitor the crack length during test. The fracture test is coupled with digital image correlation (DIC) for the local measurement of the crack tip opening displacement in mode İI\} (wII). Using a direct method, the cohesive law in mode İI\} (sII�wII) is determined by differentiating the GII�wII relationship. The procedure is validated from both numerical, using finite element analyses including cohesive zone modelling, and experimental approaches. The methodology and accuracy on this reconstruction is discussed. It is concluded that the proposed data reduction scheme is suitable for assessing the cohesive law of P. pinaster in mode II.

Xavier, J., U. Belini, F. Pierron, J. Morais, J. Lousada, and M. Tomazello. "Characterisation of the bending stiffness components of MDF panels from full-field slope measurements." Wood Science and Technology. 47 (2013): 423-441. Abstract
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Xavier, J. C., N. M. Garrido, M. Oliveira, J. L. Morais, P. P. Camanho, and F. Pierron. "A comparison between the Iosipescu and off-axis shear test methods for the characterization of ıt Pinus pinaster Ait." Composites Part A: Applied Science and Manufacturing. 35 (2004): 827-840. Abstract

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Xavier, J., M. Oliveira, J. Morais, and J. Pinto. "Measurement of the shear properties of clear wood by the Arcan test." Holzforschung. 63 (2009): 217-225. Abstract
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Xavier, J., J. R. A. Fernandes, J. J. L. Morais, and O. Frazão. "Fracture behaviour of wood bonded joints under modes I and II by digital image correlation and fibre Bragg grating sensors." Ciência & Tecnologia dos Materiais. 27 (2015): 27-35. AbstractWebsite

Abstract Direct identification of cohesive laws in modes I and İI\} of wood bonded joints is addressed by the double cantilever beam (DCB) and end-notched flexure (ENF) tests, respectively. Moreover, the development and extension of fracture process zone (FPZ) ahead of the initial crack tip, is analysed by means of digital image correlation (DIC) and embedded fibre Bragg grating (FBG) sensors. From \{FBG\} spectral response, the spectrum geometric mean is determined and the strain induced by wavelength variation employed to identify the initial and final stages of the FPZ. These stages are used to consistently define the cohesive laws in both modes I and II. Resistance-curves are determined from the compliance-based beam method (CBBM). Besides, the crack tip opening displacements (CTOD) are determined by post-processing displacement field provided by \{DIC\} around the initial crack tip. The strain energy release rate as a function of the \{CTOD\} are then determined for both mode I and mode II. The respective cohesive laws are reconstructed by numerical approximation and differentiation. It is concluded that the proposed data reduction scheme is effective to determine both the \{FPZ\} development phase and the corresponding cohesive laws of wood bonded joints in both mode I and mode II.

Xavier, J., J. Morais, N. Dourado, and M. F. S. F. de Moura. "Measurement of mode I and mode II fracture properties of wood-bonded joints." Journal of Adhesion Science and Technology. 25 (2011): 2881-2895. Abstract
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Xavier, J., A. Majano-Majano, and J. Fernandez-Cabo. "On the identifiability of stiffness components of clear wood from a 3D off-axes prismatic specimen: angle orientation and friction effects." European Journal of Wood and Wood Products. 74 (2016): 285-290. AbstractWebsite

The robustness of the test method based on a single 3D off-axis prismatic specimen for the simultaneous identification of the orthotropic stiffness components of clear wood is addressed. In this method, the specimen is consecutively submitted to uniaxial compression tests along its three orthogonal axes. A data reduction based on anisotropic elasticity is applied to extract active material parameters from 3D full-field deformation measurements provided by stereo-correlation over adjacent faces. Two major limitations of this test method, directly affecting the parameter identification, are analysed and discussed: (1) off-axes angle orientation; (2) friction effects. A numerical study pointed out that radial and tangential rotations of about 29° and 9°, respectively, balances out the strain components in the specimen response. Moreover, friction can be reduced by using mass lubricant or soft material in the contact interface, realising transverse shear deformation.

Xavier, J., A. M. R. Sousa, J. J. L. Morais, V. M. J. Filipe, and M. Vaz. "Measuring displacement fields by cross-correlation and a differential technique: experimental validation." Optical Engineering. 51 (2012): 043602. Abstract
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Xavier, J., P. Monteiro, J. J. L. Morais, N. Dourado, and M. F. S. F. de Moura. "Moisture content effect on the fracture characterisation of Pinus pinaster under mode I." Journal of Materials Science. 49 (2014): 7371-7381. AbstractWebsite
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Xavier, J., M. Oliveira, J. J. L. Morais, and M. F. S. F. de Moura. "Determining mode I cohesive law of Pinus pinaster by coupling double cantilever beam test with digital image correlation." Fracture and Structural Integrity. 31 (2015): 13-22. AbstractWebsite

The direct identification of the cohesive law in pure mode I of Pinus pinaster is addressed. The approach couples the double cantilever beam (DCB) test with digital image correlation (DIC). Wooden beam specimens loaded in the radial-longitudinal (RL) fracture propagation system are used. The strain energy release rate in mode I (GI ) is uniquely determined from the load-displacement ( P ?? ) curve by means of the compliance-based beam method (CBBM). This method relies on the concept of equivalent elastic crack length ( eq a ) and therefore does not require the monitoring of crack propagation during test. The crack tip opening displacement in mode I ? ? I w is determined from the displacement field at the initial crack tip. The cohesive law in mode I I I (? ? w ) is then identified by numerical differentiation of the I I G ? w relationship. Moreover, the proposed procedure is validated by finite element analyses including cohesive zone modelling. It is concluded that the proposed data reduction scheme is adequate for assessing the cohesive law in pure mode I of P. pinaster.

Xavier, J., S. Avril, F. Pierron, and J. Morais. "Variation of transverse and shear stiffness properties of wood in a tree." Composites Part A: Applied Science and Manufacturing. 40 (2009): 1953-1960. Abstract
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Xavier, J., J. Morais, and F. Pereira. "Non-linear shear behaviour of bovine cortical bone by coupling the Arcan test with digital image correlation." Optics and Lasers in Engineering. 110 (2018): 462-470. AbstractWebsite

The non-linear shear behaviour of bovine cortical bone is measured by coupling the Arcan test with digital image correlation. The experimental study is carried out on specimens taken from bovine femur and oriented along the longitudinal-tangential material plane. An ad hoc Arcan fixture is built to transfer the cross-head displacement of the testing machine into a shear loading at the centre of the V-notched section. A validation of predominant shear behaviour at the gauge section is shown from full-field deformation measurements. Moreover, direct evaluation of the shear modulus is obtained by integrating the shear strain component along the V-notches, avoiding numerical correction factors required in the classical data reduction scheme. The shear modulus of bovine cortical bone is found in good agreement with references from literature. Moreover, the shear stress at maximum load is understood as a suitable estimation of the shear strength. Furthermore, the Ramberg�Osgood model is found to provide an accurate description of the non-linear shear behaviour of bone tissue.