J. Gracio

University of Aveiro, Aveiro, Aveiro, Portugal

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Publications (239)361.06 Total impact

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    ABSTRACT: Twist springback induced by unbalance elastic deformation and residual stresses in a sheet after a forming is of particular importance and complexity. The twist springback can be influenced by several factors, such as blank shape geometry, material anisotropy and sheet piercing. A sensitivity study of these parameters provides insight into the twist springback control of rail parts. In the present work, a more reasonable evaluation of twist springback of a long member with respect to the central principal inertia axes of the longitudinal cross-section was proposed. In order to explore the source of twist springback, the analysis of the relationship between torsion moment and twist angle was introduced. Mechanical tests including uniaxial tension, forward-reverse shear and hydraulic bulge tests were conducted to determine the material constitutive parameters of a DP500 steel. Several key modelling techniques including the friction coefficient identification and digital image correlation were performed for improving the robustness of twist springback prediction of a typical design benchmark of a curved rail channel formed by deep drawing. Finally, the sensitivity of the constitutive model, material direction and blank piercing on twist springback was analysed in-depth and discussed based on experimental and numerical results.
    No preview · Article · Jan 2016 · Materials and Design
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    ABSTRACT: Thymine (2-oxy-4-oxy-5 methyl pyrimidine) is one of the four nucleobases of deoxyribonucleic acid (DNA). In the DNA molecule, thymine binds to adenine via two hydrogen bonds, thus stabilizing the nucleic acid structure and is involved in pairing and replication. Here, we show that synthetic thymine microcrystals grown from the solution exhibit local piezoelectricity and apparent ferroelectricity, as evidenced by nanoscale electromechanical measurements via Piezoresponse Force Microscopy. Our experimental results demonstrate significant electromechanical activity and polarization switchability of thymine, thus opening a pathway for piezoelectric and ferroelectric-based applications of thymine and, perhaps, of other DNA nucleobase materials. The results are supported by molecular modeling of polarization switching under an external electric field.
    No preview · Article · Aug 2015 · Journal of Applied Physics
  • W Wen · M Borodachenkova · A Pereira · F Barlat · J Gracio
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    ABSTRACT: Polycrystal materials exhibit large changes in the flow stress and hardening behavior during the strain path change. Such changes are related with the crystallographic texture anisotropy and the rearrangement of dislocation structure during the pre-loading. These effects have been captured by a dislocation hardening model embedded in the visco-plastic selfconsistent (VPSC) model. In this work, the texture evolution and mechanical behavior of TWIP steel during the strain path change are investigated. The experimental studies are carried out on rolled TWIP steel sheet. The mechanical responses are obtained under tensile tests along rolling direction, followed by tension along the directions with 0° and 90° from the pre-loading direction. The simulated results of strain-stress curves and the texture evolution are in good agreement with the experimental data.
    No preview · Article · Apr 2015 · IOP Conference Series Materials Science and Engineering
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    ABSTRACT: A microstructure-based hardening model that accounts for the dislocation reversal-related mechanisms and the cut-through effect is extended to HCP metals. This model, which is embedded in the visco-plastic self-consistent (VPSC) framework, is applied in this work to predict the mechanical response of Zn alloy during forward-reverse simple shear loading. The predicted mechanical behavior and texture evolution during pre-loading and reloading are in good agreement with experimental observations. The change in hardening behavior after reloading is well reproduced by this model. The contributions of the different mechanisms are also analyzed.
    No preview · Article · Apr 2015 · Journal of Materials Processing Technology
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    Juan Liao · Xin Xue · Frederic Barlat · Jose Gracio
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    ABSTRACT: The aim of this paper is to compare several hardening models and to show their relevance for the prediction of springback and deformation of an asymmetric aluminium alloy tube in multi-stage rotary draw bending process. A three-dimensional finite-element model of the process is developed using the ABAQUS code. For material modelling, the newly developed homogeneous anisotropic hardening model is adopted to capture the Bauschinger effect and transient hardening behaviour of the aluminium alloy tube subjected to non-proportional loading. The material parameters of the hardening model are obtained from uniaxial tension and forward-reverse shear test results of tube specimens. This work shows that this approach reproduces the transient Bauschinger behaviour of the material reasonably well. However, a curve-crossing phenomenon observed for this material cannot be captured by the homogeneous anisotropic hardening model. For comparison purpose, the isotropic and combined isotropic-kinematic hardening models are also adopted for the analysis of the same problem. The predictions of springback and cross-section deformation based on these models are discussed.
    Preview · Article · Dec 2014 · Procedia Engineering
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    ABSTRACT: The microstructure-based hardening model (Beyerlein and Tomé, 2007), that accounts for the dislocation reversal-related mechanisms and the cut-through effect, is extended to HCP metals. This model, which is embedded in the visco-plastic self-consistent framework, is applied in this work to predict the mechanical response of Zn alloy during strain path change. The predicted mechanical behavior and texture evolution during pre-loading and reloading is in good agreement with experimental observations. The change in hardening behavior after reloading is well reproduced by this model. The contributions of the different mechanisms are also analyzed.
    Full-text · Article · Dec 2014 · Procedia Engineering
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    Xin Xue · Juan Liao · Gabriela Vincze · Jose Gracio
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    ABSTRACT: This paper aims to develop an effective numerical model and analyse the twist springback behaviour of asymmetric thin-walled tube in mandrel rotary draw bending. Yld2000-2d anisotropic yield criterion integrated with mixed isotropic and kinematic hardening model was used to describe the material properties including anisotropy and Bauschinger effect. The corresponding mechanical experiments such as uniaxial tension, monotonic and forward-reverse shear tests were performed to obtain the material parameters. A three-dimensional elastic-plastic finite element model was developed, and its validity was assessed by comparing the predicted twist springback with experiment one. Based on the present FE model, the tangential stress distribution during different bending steps were analysed to explore the source of twist springback. The results indicate that the torsion moment of cross sections caused by the non-homogenous stress states play a considerable role in twist springback prediction.
    Preview · Article · Dec 2014 · Procedia Engineering
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    ABSTRACT: Biomedical engineering tools can be harnessed to address some of the world's most challenging nondisease-focused problems. Copyright © 2014, American Association for the Advancement of Science.
    Preview · Article · Dec 2014 · Science translational medicine
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    ABSTRACT: Springback and twist deformation of asymmetric AA6060-T4 aluminum tube in rotary draw bending process are studied experimentally and numerically. Of particular interest is the influence of constitutive model on the twist springback prediction results. The whole forming and springback process of this aluminum tube is performed using the finite element code ABAQUS. Several material models are analyzed, all considering isotropic and kinematic hardening combined with one of the following plasticity criterion: von Mises, Hill׳48 and Yld2000-2d. The material parameters of these constitutive models are determined from the tensile and forward-reversal shear tests of the tube. The material tests show that transient Bauschinger effect and curve crossing phenomena are observed for this tube subjected to reversal loading. The capability of two hardening model, naming isotropic and combined isotropic/ kinematic hardening model, to capture these behaviors are discussed. Comparison between the wist springback prediction results by different constitutive models shows that the springback angle is more sensitive to the hardening model while the twist deformation is more sensitive to the yield criterion. The stress distributions of the tube during different forming stages are analyzed and some explanations concerning their influence on springback mechanism are given. A detailed study on the tangent and hoop stress distributions of the tube also explains some source of the twist deformation for this asymmetric tube.
    No preview · Article · Dec 2014 · International Journal of Mechanical Sciences
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    ABSTRACT: Nanoindentation-induced polar properties in (001)-oriented SrTiO3 single crystals were analysed using piezoresponse force microscopy and local Raman spectroscopy measurements. The area directly beneath the indenter showed a strong piezoelectric response, together with an enhanced response in the area along the direction tangential to the residual indent. Stress-induced stable polarization states near the crack areas were also observed. Local poling performed on the strained areas suggests a possibility of polarization reversal and the stability of field-induced polar states.
    Full-text · Article · Nov 2014 · Scripta Materialia
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    ABSTRACT: Nano-graphene oxide (nano-GO) is a new class of carbon based materials being proposed for biomedical applications due to its small size, intrinsic optical properties, large specific surface area, and easy to functionalize. To fully exploit nano-GO properties, a reproducible method for its production is of utmost importance. Herein we report, the study of the sequential fracture of GO sheets onto nano-GO with controllable lateral width, by a simple, and reproducible method based on a mechanism that we describe as a confined hot spot atomic fragmentation/reduction of GO promoted by ultrasonication. The chemical and structural changes on GO structure during the breakage were monitored by XPS, FTIR, Raman and HRTEM. We found that GO sheets starts breaking from the defects region and in a second phase through the disruption of carbon bonds while still maintaining crystalline carbon domains. The breaking of GO is accompanied by its own reduction, essentially by the elimination of carboxylic and carbonyl functional groups. Photoluminescence and photothermal studies using this nano-GO are also presented highlighting the potential of this nanomaterial as a unique imaging/therapy platform.
    Full-text · Article · Oct 2014 · Scientific Reports
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    ABSTRACT: Ultra high molecular weight polyethylene (UHMWPE) composites reinforced with multiwalled carbon nanotubes (MWCNT) were produced using planetary ball milling. The aim was to develop a more wear resistant composite with improved mechanical properties to be used in stress bearing joints. The effect of manufacturing parameters such as the effect of ball milling time and rotational speed on the final composite was analyzed by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), particle size distribution, and contact angle measurements. Ball milling as a mixing technique for UHMWPE based composites is not a new approach but yet, the effect of time, rotational speed, loading of milling jar, and type of ball mill has not been reported properly for UHMWPE. Composites with 0.5 and 1.0 wt% UHMWPE/MWCNTs were manufactured with different rotational speed and mixing times. The results indicate that rotational speed rather than mixing time is important for dispersing MWCNTs using planetary ball milling. Tensile test showed a slight decrease for the MWCNT concentration of 1 wt% suggesting that this amount is the threshold for a satisfactory distribution of the fillers in the matrix. POLYM. COMPOS., 2014. © 2014 Society of Plastics Engineers
    Full-text · Article · Oct 2014 · Polymer Composites
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    ABSTRACT: In this paper, nanofluids were prepared by dispersing ferromagnetic particles (Fe3O4) in the base fluids such as 20:80% and 40:60% of propylene glycol and water mixture (by weight). Thermal conductivity and viscosity was measured experimentally as a function of particle concentrations and temperatures. Results indicate that thermal conductivity of nanofluids increases with increase of particle concentrations and temperatures. Viscosity of nanofluids increases with increase of particle concentrations, but decreases with increase of temperatures. Thermal conductivity enhancement of 20.53% and viscosity enhancement of 1.23-times was obtained at 0.5% volume concentration in 20:80% PG/W based nanofluid at a temperature of 60 °C. At same particle concentrations, temperatures and base fluid, viscosity enhancement is more compared to thermal conductivity enhancement. Based on the experimental data, correlations were proposed to predict thermal conductivity and viscosity of nanofluids. A systematic analysis was performed in order to use the nanofluids as a heat transfer fluids. It is noticed that nanofluids prepared by considering 20:80% and 40:60% PG/W are beneficial heat transfer fluids than its base fluid.
    Full-text · Article · Sep 2014
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    ABSTRACT: Nano-GO is a graphene derivative with a 2D atomic layer of sp(2) bonded carbon atoms in hexagonal conformation together with sp(3) domains with carbon atoms linked to oxygen functional groups. The supremacy of nano-GO resides essentially in its own intrinsic chemical and physical structure, which confers an extraordinary chemical versatility, high aspect ratio and unusual physical properties. The chemical versatility of nano-GO arises from the oxygen functional groups on the carbon structure that make possible its relatively easy functionalization, under mild conditions, with organic molecules or biological structures in covalent or non-covalent linkage. The synergistic effects resulting from the assembly of well-defined structures at nano-GO surface, in addition to its intrinsic optical, mechanical and electronic properties, allow the development of new multifunctional hybrid materials with a high potential in multimodal cancer therapy. Herein, a comprehensive review of the fundamental properties of nano-GO requirements for cancer therapy and the first developments of nano-GO as a platform for this purpose is presented.
    Full-text · Article · Jul 2014
  • Juan Liao · Xin Xue · Chi Zhou · Frederic Barlat · Jose Gracio
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    ABSTRACT: With the increase of using aluminum alloys and high strength steels to replace traditional steels, precise prediction of sheet springback and corresponding compensation methods become more and more important for die design because these material' s higher ratio of yield strength to elastic modulus causes more serious springback problems. In this paper, a semi-analytic springback prediction and compensation model based on in-process measurement is proposed. For springback prediction, measured strain and curvature data from the actual stamping process are incorporated in a semi-analytic model, and then springback is predicted based on the elastic unloading from the residual differential stress during sheet metal forming. In the next stage of die design, a procedure to automatically compensate the tool geometry, including the fundamental mechanics analysis and the reconstruction of the tool surface, is presented. For validation purpose, a case study was carried out for the tool optimization of a double curved panel.
    No preview · Article · Apr 2014
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    Full-text · Dataset · Mar 2014
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    ABSTRACT: Large-area polycrystalline diamond (PCD) coatings are important for fields such as thermal management, optical windows, tribological moving mechanical assemblies, harsh chemical environments, biological sensors, etc. Microwave plasma chemical vapor deposition (MPCVD) is a standard technique to grow high-quality PCD films over large area due to the absence of contact between the reactive species and the filament or the chamber wall. However, the existence of temperature gradients during growth may compromise the desired uniformity of the final diamond coatings. In the present work, a thick PCD coating was deposited on a 100-mm silicon substrate inside a 915-MHz reactor; the temperature gradient resulted in a non-uniform diamond coating. An attempt was made to relate the local temperature variation during deposition and the different properties of the final coating. It was found that there was large instability inside the system, in terms of substrate temperature (as high as ΔT = 212 °C), that resulted in a large dispersion of the diamond coating’s final properties: residual stress (∼15.8 GPa to +6.2 GPa), surface morphology (octahedral pyramids with (111) planes to cubo-octahedrals with (100) flat top surfaces), thickness (190 μm to 245 μm), columnar growth of diamond (with appearance of variety of nanostructures), nucleation side hardness (17 GPa to 48 GPa), quality (Raman peak FWHM varying from 5.1 cm−1 to 12.4 cm−1 with occasional splitting). This random variation in properties over large-area PCD coating may hamper reproducible diamond growth for any meaningful technological application.
    Full-text · Article · Mar 2014
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    ABSTRACT: We report a new type of magnetic nanofluids, which is based on a hybrid composite of nanodiamond and nickel (ND-Ni) nanoparticles. We prepared the nanoparticles by an in-situ method involving the dispersion of caboxylated nanodiamond (c-ND) nanoparticles in ethylene glycol (EG) followed by mixing of nickel chloride and, at the reaction temperature of 140°C, the use of sodium borohydrate as the reducing agent to form the ND-Ni nanoparticles. We performed their detailed surface and magnetic characterization by X-ray diffraction, micro-Raman, high-resolution transmission electron microscopy, and vibrating sample magnetometer. We prepared stable magnetic nanofluids by dispersing ND-Ni nanoparticles in a mixture of water and EG; we conducted measurements to determine the thermal conductivity and viscosity of the nanofluid with different nanoparticles loadings. The nanofluid for a 3.03% wt. of ND-Ni nanoparticles dispersed in water and EG exhibits a maximum thermal conductivity enhancement of 21% and 13%, respectively. For the same particle loading of 3.03% wt., the viscosity enhancement is 2-fold and 1.5-fold for water and EG nanofluids. This particular magnetic nanofluid, beyond its obvious usage in heat transfer equipment, may find potential applications in such diverse fields as optics and magnetic resonance imaging.
    Full-text · Article · Feb 2014 · Scientific Reports
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    ABSTRACT: Mussel-inspired synthetic poly(dopamine) thin films from dihydroxyphenylalanine (DOPA) and lysine, structurally similar to natural melanin, have drawn extensive interest as a versatile surface functionalization and coating material for use in a broad range of applications. In order to gain a better understanding of its complex and heterogeneous polymeric structure and mechanical properties, we report a computational model of poly(dopamine) by mimicking the polymerization process of the intermediate oxidized product of dopamine, 5,6-dihydroxyindole (DHI), via controlled in silico covalent cross-linking under the two most possible reaction schemes proposed in experiments. To validate our results using experiment, we synthesize poly(dopamine) thin films and perform experimental nanoindentations on the film. We observe an overall linear behavior for Young's modulus as a function of the degree of cross-linking, demonstrating the possibility of enhancing the mechanical robustness of poly(dopamine) materials by increasing the extent of polymerization. At the highest degree of polymerization considered (70%), the model mimics the linear tetrameric model for poly(dopamine) and melanin. At this degree of polymerization, we find a Young's modulus of 4.1-4.4 GPa, in agreement with our nanoindentation results of 4.3-10.5 GPa, previous experiments for natural melanin, as well as simulation results for the cyclic tetrameric melanin model (Chen et al., ACS Nano, 2013). Our results suggest that the non-covalent DHI aggregate model might not be appropriate to represent the structure of poly(dopamine) and melanin-like materials, since it gives a much smaller Young's modulus than the experimental lower bound. Our model not only nicely complements the previous computational work, but also provides new computational tools to study the heterogeneous structural and physicochemical properties of poly(dopamine) and melanin, as well as their formation pathways.
    Full-text · Article · Dec 2013 · Soft Matter
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    ABSTRACT: Although commercially-available poly(methyl methacrylate) bone cement is widely used in total joint replacements, it has many shortcomings, a major one being that it does not osseointegrate with the contiguous structures. We report on the in vitro evaluation of the biocompatibility of modified formulations of the cement in which a high loading of hydroxyapatite (67 wt/wt%), an extra amount of benzoyl peroxide, and either 0.1 wt/wt% functionalized carbon nanotubes or 0.5 wt/wt% graphene oxide was added to the cement powder and an extra amount of dimethyl-p-toluidiene was added to the cement's liquid monomer. This evaluation was done using mouse L929 fibroblasts and human Saos-2 osteoblasts. For each combination of cement formulation and cell type, there was high cell viability, low apoptosis, and extensive spread on disc surfaces. Thus, these two cement formulations may have potential for use in the clinical setting.
    Full-text · Article · Aug 2013 · Journal of Materials Science Materials in Medicine

Publication Stats

2k Citations
361.06 Total Impact Points

Institutions

  • 1997-2016
    • University of Aveiro
      • • Division of Mechanical Engineering
      • • Centre for Mechanical Technology and Automation
      • • Department of Mechanical Engineering
      Aveiro, Aveiro, Portugal
  • 2011
    • Carnegie Mellon University
      • Department of Mechanical Engineering
      Pittsburgh, Pennsylvania, United States
  • 2000
    • Manchester Metropolitan University
      Manchester, England, United Kingdom
  • 1999
    • INSTITUTO DE ENGENHARIA MECÂNICA E GESTÃO INDUSTRIAL
      Oporto, Porto, Portugal