Nuestros investigadores

Javier Aldazábal Mensa


Publicaciones científicas más recientes (desde 2010)

Autores: Cicero, S., ; Garcia, T., ; Alvarez, J., ; et al.
ISSN 0141-0296  Vol. 111  2016  págs. 152 - 161
Thermal cutting is commonly used in engineering practice to obtain the final shape of structural components, and includes oxy-fuel, plasma and laser cut technologies. The characteristics of the cut surface and the material transformations caused by these cutting methods determine the corresponding fatigue behaviour. However, in the case of thermally cut straight edges, design codes, including the Eurocode 3, provide fatigue design curves for oxy-fuel cuts, whereas emergent technologies such as plasma and laser cutting are not associated to any design curve, limiting their use in many engineering applications. This paper analyses the effect of oxy-fuel cutting, plasma cutting and laser cutting on the fatigue behaviour of cut straight edges performed on structural steels S355M, S460M, S690Q and S890Q An experimental programme composed of 150 fatigue specimens has been completed, combining the four steels, the three thermal cutting methods and two different thicknesses (15 mm and 25 mm). The obtained S-N results have been used to estimate the corresponding Eurocode 3 FAT classes, which have been finally validated by comparing them to numerous experimental data found in the literature. (C) 2015 Elsevier Ltd. All rights reserved.
Autores: Andres, D., ; Garcia, T., ; Cicero, S., ; et al.
ISSN 1044-5803  Vol. 119  2016  págs. 55 - 64
Thermal cutting processes introduce changes in the heat affected zone (HAZ), which can lead to a significant reduction of the service life of components. In order to assess their influence, different cutting processes have been analysed on a structural steel. The characterization of the reduced volumes of HAZ posed a major challenge, since conventional techniques require greater pieces of material. Alternative miniature techniques had to be applied, such as Small Punch tests and microhardness measurements, from which the material tensile properties and fracture toughness values have been obtained. Results show that oxyfuel HAZ exhibit minor alterations of the material, while plasma cutting seems to improve the material tensile properties and fracture toughness. Besides, the suitability and accuracy of the Small Punch technique for similar applications can be derived from this work, turning it into a promising candidate to perform integrity assessments of actual components. (C) 2016 Elsevier Inc All rights reserved.
Autores: Cicero, S., ; Garcia, T., ; Alvarez, J., ; et al.
ISSN 0142-1123  Vol. 87  2016  págs. 50 - 58
When the fatigue behaviour of structural components containing holes is analysed, Eurocode 3 only considers the fatigue performance of drilled holes, limiting the use of thermal cutting processes to produce, for example, bolt holes. This paper studies the fatigue performance of structural steel plates containing thermally cut holes. The research covers three thermal cutting methods: the traditional one (oxy-fuel cutting) and two more modern processes (plasma and laser cutting). An experimental program composed of 150 fatigue specimens has been completed, combining four steels (S355M, 5460M, S690Q and S890Q), the three thermal cutting methods and two different thicknesses (15 mm and 25 mm). The S-N results obtained have been used to estimate the corresponding Eurocode 3 FAT classes, which have finally been validated by comparing them to additional experimental data found in the literature. (C) 2016 Elsevier Ltd. All rights reserved.
Autores: Capdevila, C., ; Aranda, M., ; Rementeria, R., ; et al.
ISSN 1359-6454  Vol. 107  2016  págs. 27 - 37
The strengthening mechanism observed during ageing at temperatures of 435 and 475 degrees C in the oxide dispersion strengthened (ODS) Fe-Cr-Al-Ti system has been investigated. Atom probe tomography (APT) and high-resolution transmission electron microscopy (HRTEM) analyses determined that the alloy undergoes simultaneous precipitation of Cr-rich (alpha' phase) and nanoscale precipitation of TiAl-rich intermetallic particles (beta' phase). APT indicated that the composition of the intermetallic beta' phase is Fe2AlTi0.6Cr0.4, and the evolving composition of alpha' phase with ageing time was also determined. The results obtained from HRTEM analyses allow us to confirm that the beta' precipitates exhibit a cubic structure and hence their crystallography is related to the Heusler-type Fe2AlTi (L2(1)) structure. The strengthening could be explained on the basis of two hardening effects that occur simultaneously: the first is due to the alpha-alpha' phase separation through the modulus effect, and the second mechanism is due to the interaction of nanoscale beta' particles with dislocations. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Autores: Gil Sevillano, Javier; Aldazabal, I., ; Luque, A., ; et al.
ISSN 0025-6455  Vol. 51  Nº 2  2016  págs. 401 - 413
The tensile elongation of an < 011 > oriented columnar nanocrystalline pure iron structure at a temperature of 300 K has been simulated by molecular dynamics (MD). The simulated sample contains 4.3 x 10(6) atoms and has been subject to free elongation along the < 011 > axis common to the grains. Periodic boundary conditions have been assumed. The grains are randomly oriented around their common < 011 > and the size of their cross section is about 10 nm. The stress-strain curve has been calculated up to 0.5 true strain. After elastic deformation and heterogeneous dislocation nucleation from the grain boundaries, it shows a peak stress of 8 GPa followed by a remarkably stable steady state with a flow stress of 5.15 GPa, where neither the crystallographic texture nor the grain structure show any important change despite the large plastic deformation imparted. Upon a strain reversal, a pronounced Bauschinger effect is then observed (-3.3 GPa compressive yield stress), followed by a hardening transient until the absolute level of the flow stress in compression reaches near the same value it had in tension when the unloading took place. The results of the MD simulation are discussed by comparison with experimental values of the strength and structural evolution of heavily drawn iron wires available in the bibliography.
Autores: Garcia, T., ; Cicero, S., ; Ibañéz, F.T., ; et al.
ISSN 1877-7058  Vol. 133  2015  págs. 590 - 602
Current fatigue codes only consider the fatigue performance of drilled and punched holes, limiting the use of thermal cutting processes to produce bolt holes. This paper studies the fatigue performance of structural steel S460 M plates containing thermally cut bolt holes. The research covers three thermal cutting methods: the traditional one (oxy-fuel cutting) and two more modern processes (plasma and laser cutting). Specimen geometry is defined by a rectangular cross section with a cut hole in the middle. All the specimens were conducted to failure by applying fatigue cycles, the stress ratio (R) being 0.1. The corresponding S-N curve and fatigue limit were obtained for each cutting method. Fatigue results have been compared with previous researches on fatigue performance of drilled and punched holes, and with the predictions provided by current fatigue standards, analyzing the possibility to extrapolate their S-N curves, focused on drilled and punched holes, to thermally cut holes.
Autores: Atkinson, C., ; Coman, C., ; Aldazábal Mensa, Javier
ISSN 1364-503X  Vol. 373  Nº 2038  2015 
An assessment is made here of the role played by the micropolar continuum theory on the cracked Brazilian disc test used for determining rock fracture toughness. By analytically solving the corresponding mixed boundary-value problems and employing singular-perturbation arguments, we provide closed-form expressions for the energy release rate and the corresponding stress-intensity factors for both mode I and mode II loading. These theoretical results are augmented by a set of fracture toughness experiments on both sandstone and marble rocks. It is further shown that the morphology of the fracturing process in our centrally pre-cracked circular samples correlates very well with discrete element simulations.
Autores: Pimentel, G., ; Capdevila, C., ; Bartolome, M., ; et al.
ISSN 0034-8570  Vol. 48  Nº 4  2012  págs. 303 - 316
Technologies and means for developing biomass plant with higher energy conversion efficiencies are essential in order to implement the commitment to renewable biomass energy generation. Advanced, indirect Combined Cycle Gas Turbine (CCGT) systems offer overall biomass energy conversion efficiencies of 45 % and above, compared with the 35 % efficiency of conventional biomass steam plant. However to attain this efficiency in CCGT operation it will be necessary to develop a heat exchanger capable of gas operating temperatures and pressures of around 1100 degrees C and 15-30 bar, respectively, for entry heating the gas turbine working fluid. ODS ferritic steels is the kind of advance material to deal with this challenge, however work to optimize the coarse grain microstructure to improve creep hoop strength needs to be done. In this sense, this paper reports the recrystallisation behaviour of PM 2000 oxide dispersion strengthened ferritic alloy which was cold deformed after hot-rolling and extrusion. The results can be interpreted if it is assumed that anything which makes the microstructure heterogeneous, stimulates recrystallisation. In this sense, larger strain gradients lead to more refined and more isotropic grain structures. The combination of these results with finite element modeling are used to interpret the role of residual shear stresses on the development of recrystallized grain structure.
Autores: Luque Gómez, Aitor; Aldazábal Mensa, Javier; Martínez Esnaola, José Manuel; et al.
ISSN 0378-4754  Vol. 81  Nº 11  2011  págs. 2564 - 2580
Liquid-phase sintering (LPS) is a consolidation process for metallic and ceramic powders. At given temperature conditions, the process occurs with constant amount of liquid phase. However, the evolution of solid-particle shape is observed, namely, the rounding of particles and the growth of big particles at the expense of the small ones, which is known as Ostwald ripening. In this work, we propose a Monte Carlo (MC) model to simulate the microstructural evolution during LPS. The model considers the change of state of the discretising elements, namely voxels, of the system. The microstructural evolution proceeds accounting for both the geometrical characteristics of the particles, such as the number of solid neighbours, and the amount of solute contained in or surrounding a randomly chosen voxel. This has been implemented in terms of two probability distribution functions (PDFs). The diffusion of solute has also been considered by means of the implementation of a three-dimensional finite-difference algorithm. The diffusional MC model that we present is able to reproduce the Ostwald ripening behaviour and, in particular, results match the case in which the process is limited by the diffusion of the solute in the liquid phase. (C) 2011 IMACS. Published by Elsevier B.V. All rights reserved.
Autores: Luque Gómez, Aitor; Aldazábal Mensa, Javier; Martínez Esnaola, José Manuel; et al.
ISSN 1478-6435  Vol. 90  Nº 27-28  2010  págs. 3743 - 3756
We present molecular dynamics (MD) simulations of the shear-coupled migration (SCM) behaviour of symmetrical tilt boundaries perturbed by the presence of nano-cracks or nano-precipitates lying on the boundary plane. The simulations have been performed for copper bicrystals at room temperature (300 K). The tilt boundary gets pinned by the crack tip or precipitates; shear-coupled migration occurs only ahead of the pinning points. Bulging of the tilt boundary reduces the shear stress on the boundary surface near the pinning points. In the case of cracks, the local deviation of the boundary from the crack plane close to the crack tip hinders mode II crack propagation; in fact, crack healing is observed in some cases. The applied stress grows until depinning of the boundary takes place by SCM bulging or by the combined action of SCM with another deformation mechanism (emission of dislocations from the pinning point vicinity, grain boundary sliding).
Autores: Luque Gómez, Aitor; Aldazábal Mensa, Javier; Martínez Esnaola, José Manuel; et al.
ISSN 0378-4754  Vol. 80  Nº 7  2010  págs. 1469 - 1486
Liquid-phase sintering (LPS) is an industrial process used to consolidate materials composed of two different kinds of metallic and/or ceramic powders. At constant temperature, the amount of the present liquid-phase is constant However, the shape of particles of solid phase changes over time In general, the rounding of particles and the growth of big particles at the expense of the small ones are observed This process is known as Ostwald ripening. In this work, we propose a Monte Carlo (MC) model to simulate the microstructural evolution during LPS The discretizing elements of the system. namely the voxels. change state between solid and liquid. according to previously defined melting and solidification probability distribution functions (PDFs) The generated PDFs take into account the geometrical characteristics of the system particles in terms of number of solid neighbours that surround a randomly chosen voxel The geometrical MC model that we present is able to reproduce the Ostwald ripening behaviour and, in particular, matches the case in which the process occurs limited by the attachment/detachment of the solid phase to/from the surface of the particle (C) 2009 IMACS Published by Elsevier B V All rights reserved.
Autores: Sancho Erkizia, Ana; Aldazábal Mensa, Javier; Rainer, A., ; et al.
Libro:  Tissue engineering: computer modeling, biofabrication and cell behavio
2014  págs. 129 - 147
Tissue Engineering is a promising emerging field that studies the intrinsic regenerative potential of the human body and uses it to restore functionality of damaged organs or tissues unable of self-healing due to illness or ageing. In order to achieve regeneration using Tissue Engineering strategies, it is first necessary to study the properties of the native tissue and determine the cause of tissue failure; second, to identify an optimum population of cells capable of restoring its functionality; and third, to design and manufacture a cellular microenvironment in which those specific cells are directed towards the desired cellular functions. The design of the artificial cellular niche has a tremendous importance, because cells will feel and respond to both its biochemical and biophysical properties very differently. In particular, the artificial niche will act as a physical scaffold for the cells, allowing their three-dimensional spatial organization; also, it will provide mechanical stability to the artificial construct; and finally, it will supply biochemical and mechanical cues to control cellular growth, migration, differentiation and synthesis of natural extracellular matrix. During the last decades, many scientists have made great contributions to the field of Tissue Engineering. Even though this research has frequently been accompanied by vast investments during extended periods of time, yet too often these efforts have not been enough to translate the advances into new clinical therapies. More and more scientists in this field are aware of the need of rational experimental designs before carrying out complex, expensive and time-consuming in vitro and in vivo trials. This review highlights the importance of computer modeling and novel biofabrication techniques as critical key players for a rational design of artificial cellular niches in Tissue Engineering.
Autores: Salvatori, I., ; Guarnaschelli, C., ; Coppola, T., ; et al.
In order to meet the increasing needs from economic and social developments in future, the research on new generation steels with higher strength and longer duration, has become a worldwide issue. It is well known that various mechanisms to strengthen the steels exist, but grain refinement is the only method to improve both strength and toughness simultaneously. A ferrite grain size in the range of 1÷4 ¿m and a steel microstructure characterised by a mixture of ferrite-pearlite and/or martensite, bainite microstructure, could give a very good combination of mechanical (strength, ductility, toughness, fatigue) and technological properties (machinability, cold/warm metal forming, etc.) for final application to automotive components.
Autores: Paul, G., ; Grobterlinden, R., ; Aldazábal Mensa, Javier; et al.
Bainite is nowadays playing a major role in the microstructure and mechanical properties in a variety of industrially-produced steel grades. Multiphase transformation-induced plasticity (TRIP) steels for automotive applications, ultra-low carbon bainitic (ULCB) steels for pipeline applications are some examples of the increasing involvement of bainite as a microstructural constituent in steels. Nevertheless, optimization of the production and exploitation of bainite-involving steel grades and, more importantly, optimization of the design of new alloy compositions and/or processing routes, necessitate the clarification of the effect of chemical composition and heat-treatment conditions on the evolution of the bainitic transformation. This in turn creates a necessity for the development of appropriate models, with the highest possible degree of accuracy and applicability. However, because of the great morphological variety under which bainite can exist, and because of the complex situation of transforming into bainite not all controlling processes of the bainitic transformation are understood and ¿ until today ¿ are discussed with controversy and disagreement. As a consequence of these difficulties associated with the bainitic transformation, such modelling tools of practical generality and applicability have not been developed. The projects major objective was to achieve the development of a physically-based approach, or of a combination of appropriate physical approaches to describe the overall, macroscopic kinetics of the bainitic transformation in dual-phase, TRIP and complex-phase steels, with the highest possible degree of qualitative and quantitative accuracy, and with the widest possible field of applicability with respect to alloy compositions and cooling conditions.