J. Gracio

University of Aveiro, Aveiro, Aveiro, Portugal

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Publications (332)625.96 Total impact

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    ABSTRACT: Strong increase in electrical conductivity of graphene oxide (GO) (I ~10^-9 A) is found by addition of Ni nanoparticles (NiNPs) preliminarily solved by HCl (Nisol) (I ~ 10^-4 A) or powder (Nipow) obtained from this solution (I ~ 10^-6 A), while simply mixing GO with NiNPs an insulator similar to pure GO is obtained. Thus, Nisol and Nipow can be used to transform GO from insulator to semiconductor. One of the transformation mechanisms is Ni as spillover. At the same time, different kinds of the magnetic response are obtained on GO and reduced GO (rGO) samples with and without Ni. Weak paramagnetic response is detected in pure GO. Stronger paramagnetic behavior is observed for GO and rGO mixed with Nisol or Nipow. Pure rGO sample shows weak ferromagnetism represented by slim but visible hysteresis with remnant magnetization Mr of 0.05 emu/g. GO with NiNPs presents clear hysteresis with Mr of 2.8 emu/g, while sample prepared by addition of NiNPs to rGO presents the largest hysteresis with Mr as high as 11.8 emu/g. Thus, the optimal procedure to obtain the magnetic response requested for particular application can be chosen.
    Journal of Materials Science 05/2015; 50(9):3425. DOI:10.1007/s10853-015-8901-8 · 2.31 Impact Factor
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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.
    IOP Conference Series Materials Science and Engineering 04/2015; 82(1). DOI:10.1088/1757-899X/82/1/012089
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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.
    Journal of Materials Processing Technology 04/2015; 224. DOI:10.1016/j.jmatprotec.2015.04.021 · 2.04 Impact Factor
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    ABSTRACT: TWIP steels are materials with very high strength and exceptional strain hardening capability, parameters leading to large energy absorption before failure. However, TWIP steels also exhibit reduced (often negative) strain rate sensitivity (SRS) which limits the post-necking deformation. In this study we demonstrate for an austenitic TWIP steel with 18% Mn a strong dependence of the twinning rate on the strain rate, which results in negative strain hardening rate sensitivity (SHRS). The instantaneous component of SHRS is large and negative, while its transient is close to zero. The SRS is observed to decrease with strain, becoming negative for larger strains. Direct observations of the strain rate dependence of the twinning rate are made using electron microscopy and electron backscatter diffraction, which substantiate the proposed mechanism for the observed negative SHRS.
    Materials Science and Engineering A 04/2015; 629. DOI:10.1016/j.msea.2015.01.080 · 2.41 Impact Factor
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    ABSTRACT: The mechanical behavior under uniaxial tension of Al-Mg alloy 5182 pre-deformed in conventional rolling (CR), asymmetric rolling-continuous (ASRC), and asymmetric rolling-reversed (ASRR) was investigated and modeled with a rate dependent crystal plasticity finite element method and VPSC (Visco-Plastic Self Consistent) model. M-K theory combined with Yld2000 model by Barlat et al. (Int. J. Plasticity 2003, 19, 1297) was used to predict the strain-based and stress-based formability for AA 5182 material. It was concluded that the new ASRR process has very compatible formability with improved strength compared to CR process. These merits can be directly applied for clam-shell resistant design in rigid-packaging industry.
    Steel Research International 03/2015; DOI:10.1002/srin.201500024 · 1.02 Impact Factor
  • X. Xue, J. Liao, G. Vincze, J.J. Gracio
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    ABSTRACT: Twist springback evaluation of asymmetric thin-walled bent tube is proposed considering section property and plasticity deformation theory.•The developed FE model with surface-based coupling HINGE constraint and anisotropic constitutive models has higher precision of twist springback prediction in mandrel rotary draw bending.•Numerical inverse method for interfacial friction coefficient identification is applied to obtain a high desirability of twist and springback responses.•The interfacial frictions have significant effects on twist springback of asymmetric thin-walled tube.
    Journal of Materials Processing Technology 02/2015; 216. DOI:10.1016/j.jmatprotec.2014.10.007 · 1.95 Impact Factor
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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.
    Procedia Engineering 12/2014; 81. DOI:10.1016/j.proeng.2014.10.305
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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.
    Procedia Engineering 12/2014; 81. DOI:10.1016/j.proeng.2014.10.147
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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.
    Procedia Engineering 12/2014; 81. DOI:10.1016/j.proeng.2014.10.102
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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.
    International Journal of Mechanical Sciences 12/2014; 89:311–322. DOI:10.1016/j.ijmecsci.2014.09.016 · 2.06 Impact Factor
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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.
    Scripta Materialia 11/2014; 74:76. DOI:10.1016/j.scriptamat.2013.11.003 · 2.97 Impact Factor
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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.
    Scientific Reports 10/2014; 4. DOI:10.1038/srep06735 · 5.58 Impact Factor
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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
    Polymer Composites 10/2014; DOI:10.1002/pc.23275 · 1.46 Impact Factor
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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.
    09/2014; 3(3). DOI:10.1166/jon.2014.1108
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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.
    07/2014; 2(8). DOI:10.1002/adhm.201300023
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    ABSTRACT: The fundamental objective of the present work consists of the improvement of the plastic strain ratio (R-values) of AA-5182 aluminium alloy through improving the crystallographic texture by asymmetric rolling (ASR). The ASR process imposes intense shear deformation across the thickness of the sheet samples, leading to shear texture development. Finite element (FE) simulations of the ASR were first performed in order to investigate the impact of process parameters on the onset and growth of shear deformation throughout the thickness of sheet samples. In the present work, polycrystal simulations were also done to predict the texture evolutions and the mechanical response of the sheets deformed by different rolling processes. Afterwards, experimental studies were conducted. Conventional rolling (CR) as well as two types of ASR processes was carried out to reduce the thickness of the sheet samples. After each process, the rolled sheet samples were annealed. The crystallographic textures achieved in CR and ASR processes and annealing were measured. Furthermore, uniaxial tensile tests on the rolled sheets were also carried out in order to consider the effects of crystallographic texture on the macroscopic anisotropy. In agreement with the FE predictions, the experimental results showed that the shear strain spreads throughout the thickness of sheet samples during ASR and develops the shear texture. The mechanical behaviour and texture evolution predicted by numerical models are in good agreement with the experimental results. The modified texture leads to an increase of normal anisotropy as well as an increase of absolute value of planar anisotropy.
    Materials Science and Engineering A 05/2014; 603:150–159. DOI:10.1016/j.msea.2014.02.048 · 2.41 Impact Factor
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    ABSTRACT: Experimental tests involving shear stresses allow material to be deformed to very high plastic strain by overcoming localization phenomena. The simple shear texture development, which is also common near the surface of rolled parts, is important to study since it is directly connected to the metal anisotropy. Crystal plasticity models are used to simulate large deformation plasticity and texture evolution. The main insufficiency of most existing models is that they are, in certain cases, unable to predict all type of experimentally observed textures as well as texture transitions. In this paper, we show that the polycrystalline φφ -model can be used to compute simple shear crystallographic texture transition for face-centered cubic metals (fcc) at large strains. This model takes into account the grains interaction effects but without the Eshelby inclusion theory. Predicted results are compared to experimental shear textures for medium stacking fault energy (SFE) metals (i.e. copper) and low SFE metals (i.e. silver). We show that the φφ -model is able to predict a clear shear texture transition characterizing a range of fcc metals having high/medium to low SFE. The twinning mechanism is included in the φφ -model in order to improve the predicted shear textures for low SFE metals. The effect of twinning on the ideal shear texture components is shown and is consistent with experimental results from the literature.
    International Journal of Plasticity 05/2014; DOI:10.1016/j.ijplas.2014.03.020 · 5.97 Impact Factor
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    ABSTRACT: A novel method for the rapid hydrogenation of graphene oxide (GO) is established using nickel nanoparticles (Ni NPs) and through electrochemical reaction. In-situ generated hydrogen and enhanced spillover on graphene sheet makes the hydrogenated graphene oxide (HGO) easily at room temperature. Initially, a well intercalated Ni-GO species was produced due to the nanosize effect of nickel nanoparticles and promoted the homogenous hydrogenation of GO sheets. The structure, composition and morphology of GO, Ni-GO and HGO were fully characterized. I-V characteristics of HGO showed very interesting properties such as hysteresis, resistive switching and rectifier properties.
    International journal of electrochemical science 04/2014; · 1.96 Impact Factor
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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.
    04/2014; 85(4). DOI:10.1002/srin.201300216
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Publication Stats

3k Citations
625.96 Total Impact Points

Institutions

  • 1994–2015
    • University of Aveiro
      • • Division of Mechanical Engineering
      • • Department of Mechanical Engineering
      • • Centre for Mechanical Technology and Automation
      Aveiro, Aveiro, Portugal
  • 2011
    • Carnegie Mellon University
      • Department of Mechanical Engineering
      Pittsburgh, Pennsylvania, United States
  • 2010
    • University of Central Lancashire
      • School of Computing, Engineering and Physical Sciences
      Preston, ENG, United Kingdom
  • 1990–1994
    • University of Coimbra
      • Department of Mechanical Engineering
      Coímbra, Coimbra, Portugal