Daniel Crespo - Profile on Academia.edu (original) (raw)

Papers by Daniel Crespo

The current prototype’s key contribution to the field of light-weight structures is that it is th... more The current prototype’s key contribution to the field of light-weight structures is that it is the first time that a pure tensegrity ring has been used in place of a compression ring. This design features a cladding structure for a sports arena, which consist of a ring-shaped outer section and a central roof structure. The ring-shaped outer section of the stadium consists of a tensegrity structure, which uses textile membranes in a place of conventional tension cables to bear the tensile forces occurring between the pressure elements. The supporting framework and spatial enclosure therefore become one an extension to the tension integrity principle. The central area of the roof is covered over by a Geiger dome, which in turn is a specific version of the tensegrity principle.

Materials, 2019

In this work, experimental and numerical analyses of repairs on carbon fiber reinforced epoxy (CF... more In this work, experimental and numerical analyses of repairs on carbon fiber reinforced epoxy (CFRE) substrates, with CFRE and aluminum alloy doublers typical of aircraft structures, are presented. The substrates have a bridge gap of 12.7 mm (simulated crack), repaired with twin doublers joined with riveted, adhesive bonded, and hybrid joints. The performance of the repairs using different doubler materials and joining techniques are compared under static loading. The experimental results show that riveted joints have the lowest strength, while adhesive bonded joints have the highest strength, irrespective of the doubler material. Finite element analysis (FEA) of the studied joints is also performed using commercial FEA tool Abaqus. In the FEA model, point-based fasteners are used for the rivets, and a cohesive zone contact model is used to simulate the adhesive bond. The FEA results indicate that the riveted joints have higher tensile stresses on the metal doublers compared to the ...

Journal of Alloys and Compounds, 2018

behavior of a novel aluminum metal matrix composite and comparison with pure aluminum, aluminum a... more behavior of a novel aluminum metal matrix composite and comparison with pure aluminum, aluminum alloys, and a composite made of Al-Mg-Si alloy reinforced with SiC particles,

Metals, 2016

The viscoelastic response of pure Al and 7075 (AlZnMg) and 2024 (AlCuMg) alloys, obtained with a ... more The viscoelastic response of pure Al and 7075 (AlZnMg) and 2024 (AlCuMg) alloys, obtained with a dynamic-mechanical analyzer (DMA), is studied. The purpose is to identify relationships between viscoelasticity and fatigue response of these materials, of great interest for structural applications, in view of their mutual dependence on intrinsic microstructural effects associated to internal friction. The objective is to investigate the influence of dynamic loading frequency and temperature on fatigue, based on their effect on the viscoelastic behavior. This research suggests that the decrease of yield and fatigue behavior reported for Al alloys as temperature increases may be associated to the increase of internal friction. Also, materials subjected to dynamic loading below a given threshold frequency exhibit a static-like response, such that creep mechanisms dominate and fatigue effects are negligible. In this work, an alternative procedure to the time-consuming fatigue tests is proposed to estimate this threshold frequency, based on the frequency dependence of the initial decrease of storage modulus with temperature, obtained from the relatively short DMA tests. This allows for a fast estimation of the threshold frequency. The frequencies obtained for pure Al and 2024 and 7075 alloys are 0.001-0.005, 0.006 and 0.075-0.350 Hz, respectively.

459 Microstructure description from the differential Avrami formulation MICROSTRUCTURE DESCRIPTION FROM THE DIFFERENTIAL AVRAMI FORMULATION

A model of microstructural evolution, based only on the original assumptions of the Kolmogorov, J... more A model of microstructural evolution, based only on the original assumptions of the Kolmogorov, Johnson & Mehl, and Avrami model of nucleation and growth kinetics, is presented. It is based on the computation of the average grain size of all grains born at the same time along the transformation, neglecting the dispersion of the grain size around this value. The model is computationally simple and inexpensive, and gives a good description of the grain size distribution along the transformation as well as all its distinctive features.

Metallurgical and Materials Transactions A, 2012

The viscoelastic response of commercial aluminum alloys 7075-T6 and 2024-T3 as a function of temp... more The viscoelastic response of commercial aluminum alloys 7075-T6 and 2024-T3 as a function of temperature is presented. Experimental data are obtained with a dynamic-mechanical analyzer (DMA) at different loading frequencies and compared with the available transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) data. The effect of successive microstructural transformations (particle precipitation and redissolution) is revealed. An analytical model is developed, which fits the mechanical response up to 573 K (300°C). The model takes into account the concentration of Guinier-Preston Zones (GPZ) and metastable precipitates (g¢ in AA 7075-T6 and h¢/S¢ in AA 2024-T3), allowing us to determine the kinetic parameters of these transformations. The activation energies were previously obtained by several authors from DSC measurements and other techniques, showing considerable dispersion. The presented data, obtained with a completely different technique, allow us to reduce the uncertainty on these data and show the potential of DMA measurements in the study of microstructural transformations.

Advances in Materials Science and Engineering, 2017

The viscoelastic response of commercial Al–Zn–Mg and Al–Cu–Mg alloys was measured with a dynamic-... more The viscoelastic response of commercial Al–Zn–Mg and Al–Cu–Mg alloys was measured with a dynamic-mechanical analyzer (DMA) as a function of the temperature (from 30 to 425°C) and the loading frequency (from 0.01 to 150 Hz). The time-temperature superposition (TTS) principle has proven to be useful in studying mechanical relaxations and obtaining master curves for amorphous materials. In this work, the TTS principle is applied to the measured viscoelastic data (i.e., the storage and loss moduli) to obtain the corresponding master curves and to analyze the mechanical relaxations responsible for the viscoelastic behavior of the studied alloys. For the storage modulus it was possible to identify a master curve for a low-temperature region (from room temperature to 150°C) and, for the storage and loss moduli, another master curve for a high-temperature region (from 320 to 375°C). These temperature regions are coincidental with the stable intervals where no phase transformations occur. Th...

Phase-field modelling of microstructural evolution in primary crystallization

One of the main routes to obtain nanostructured materials is through the primary crystallization ... more One of the main routes to obtain nanostructured materials is through the primary crystallization of metallic glasses. In such transformations, crystallites with a different composition than the amorphous precursor grow with a diffusion-controlled regime. Particle growth is slowed and eventually halted by the impingement between the concentration gradients of surrounding particles. Primary crystallization kinetics is not well described by the KJMA equation, and this fact was generally ascribed to both the soft-impingement effect and the non-random nucleation. However, recent phase-field simulations showed that the underlying physical reason is the change in the local diffusion properties of the amorphous precursor due to the variation of the composition during the transformation. The kinetics of primary crystallization is thus well described by considering a diffusion coefficient of the slowest diffusing species dependent on the local concentration. The nanostructure developed in suc...

The structural relaxation, glass transition and crystallization processes of Mg65Cu25Y10 metallic... more The structural relaxation, glass transition and crystallization processes of Mg65Cu25Y10 metallic glass are studied by Differential Scanning Calorimetry (DSC) and Mechanical Spectroscopy. The relaxation model derived from the mechanical measurements is compared with the kinetics of these transformations obtained from the DSC curves. The structural relaxation kinetics is found to be controlled by the glassy dynamics following an Adams-GibbsVogel function. The glass transition and crystallization kinetics are controlled by the dynamics of the supercooled melt following a Vogel-Fulcher-Tammann behaviour. The results suggest that the microscopic processes responsible of structural relaxation and aging below the glass transition correspond to the same processes generating the -relaxation peak.

The influence of shear in the atomic structure of ternary Cu46Zr46Al8 metallic glass was studied ... more The influence of shear in the atomic structure of ternary Cu46Zr46Al8 metallic glass was studied at different temperatures by molecular dynamics (MD) simulation. At temperatures above and below the glass transition temperature the system was subjected to a shear deformation cycle; the shear deformation was carried in the [100] direction and then the original geometry was recovered. The system was analyzed in three states: initial state (before deformation), deformed state (sheared) and final state (recovery). The different states obtained by the atomistic simulations were examined by computing the directional pair distribution functions (dPDF) in the coordinate planes. The results showed by dPDFs of the planes perpendicular to the coordinate axis are apparently isotropic. However, the dPDF disengage when computed perpendicular to rotated axis. This reveals that the anisotropy introduced during the deformation cycles appears in directions [110] and [1 0], tilted 45 with respect to t...

Science China Physics, Mechanics & Astronomy

Dynamic mechanical relaxation is a fundamental tool to understand the mechanical and physical pro... more Dynamic mechanical relaxation is a fundamental tool to understand the mechanical and physical properties of viscoelastic materials like glasses. Mechanical spectroscopy shows that the high-entropy bulk metallic glass (La 30 Ce 30 Ni 10 Al 20 Co 10) exhibits a distinct β-relaxation feature. In the present research, dynamic mechanical analysis and thermal creep were performed using this bulk metallic glass material at a temperature domain around the β relaxation. The components of total strain, including ideal elastic strain, anelastic strain, and viscous-plastic strain, were analyzed based on the model of shear transformation zones (STZs). The stochastic activation of STZ contributes to the anelastic strain. When the temperature or external stress is high enough or the timescale is long enough, the interaction between STZs induces viscous-plastic strain. When all the spectrum of STZs is activated, the quasi-steady-state creep is achieved.

The viscoelastic response of a novel composite (A356 aluminum alloy matrix with ceramic reinforce... more The viscoelastic response of a novel composite (A356 aluminum alloy matrix with ceramic reinforcement particles developed from colliery shale waste) is measured with dynamic-mechanical analyzer, and is compared to pure aluminum, aluminum alloys A356, 7075 and 2024, and another composite (6061 aluminum alloy matrix reinforced with SiC particles). The studied materials show some common features but the novel composite is one of the most stable (a rapid decrease in stiffness starts only at very high temperature). Moreover, compared to the A356 alloy, the composite shows higher stiffness (since the reinforcement particles are stiffer than the A356 matrix and may foster precipitation hardening) and higher mechanical damping/internal friction (likely due to relaxations associated with the reinforcement particles and to the larger grain size for the A356 alloy). A typical relaxation peak in aluminum attributed to grain boundary sliding is suppressed in the composite because the reinforceme...

Materials Characterization, 2017

The microstructural evolution of an Al–Zn–Mg–Cu alloy under continuous heating over the temperatu... more The microstructural evolution of an Al–Zn–Mg–Cu alloy under continuous heating over the temperature range of 298 to 648 K (25 to 375 °C) is characterized by focused ion beam-scanning electron microscopy, transmission electron microscopy, and atom probe tomography. The chemical composition, dimensions, number density, and volume fraction of precipitates are measured comprehensively. Quantitative measurements of Guinier-Preston zones volume fraction and η′/η phase volume fraction are correlated to the viscoelastic response of the Al–Zn–Mg–Cu alloy with temperature in the studied range. This provides valuable evidence indicating that observed variations in the viscoelastic behavior of the Al–Zn–Mg–Cu alloy may be associated with phase transformations involving Guinier-Preston zones and phases η′ and η.

Journal of Non-Crystalline Solids

The current view of structural relaxation in metallic glasses assumes the presence of primary and... more The current view of structural relaxation in metallic glasses assumes the presence of primary and secondary processes with different activation energies. While the faster, secondary process can be well characterized in the out-of-equilibrium state below the glass transition temperature T g , the experimental direct determination of the primary process in this temperature region is more difficult due to the long relaxation times. In this work, we merge new and literature data to analyze the temperature behavior of the primary relaxation *Manuscript Click here to view linked References 2 time below T g as observed by mechanical spectroscopy and stress relaxation of metallic glasses of different fragility. We compare these results with the microscopic structural relaxation times previously measured with X-ray photon correlation spectroscopy. The coincidence between the macroscopic and microscopic relaxation times allows us to discuss the underlying mechanisms responsible of primary relaxation over different length scales, as well as to propose an overall picture of the primary relaxation behavior in the glassy regime near T g .

Physical Review B, 2016

Experimental data on the phase sound speed of metallic glasses show anomalies in the Terahertz ra... more Experimental data on the phase sound speed of metallic glasses show anomalies in the Terahertz range, reflecting an underlying complex behavior of their phonon dispersion spectrum not yet explained. We determine the phonon dispersion curve of metallic glasses by means of massive molecular dynamics simulations, allowing us to obtain the low-q region behavior with unprecedented detail. Results confirm that the sound speed is constant below the THZ range, down to the macroscopic limit. On the contrary, a hardening of the sound speed, more notable in the transverse case, is found in the THZ range. This behavior is modeled in terms of a relaxation model. The model gives quantitative agreement, and allows us to determine a new threshold frequency ω h , at the end of the boson peak region. Above ω h the shear modulus increases dramatically, reflecting the end of the amorphous-like acoustic propagation region characterized by the excess density of vibrational states.

Metals, 2015

Relaxation phenomena in glasses are a subject of utmost interest, as they are deeply connected wi... more Relaxation phenomena in glasses are a subject of utmost interest, as they are deeply connected with their structure and dynamics. From a theoretical point of view, mechanical relaxation allows one to get insight into the different atomic-scale processes taking place in the glassy state. Focusing on their possible applications, relaxation behavior influences the mechanical properties of metallic glasses. This paper reviews the present knowledge on mechanical relaxation of metallic glasses. The features of primary and secondary relaxations are reviewed. Experimental data in the time and frequency domain is presented, as well as the different models used to describe the measured relaxation spectra. Extended attention is paid to dynamic mechanical analysis, as it is the most important technique allowing one to access the mechanical relaxation behavior. Finally, the relevance of the relaxation behavior in the mechanical properties of metallic glasses is discussed.

Metals

The mechanical relaxation behavior of the (La0.5Ce0.5)65Al10(CoxCu1−x)25 at% (x = 0, 0.2, 0.4, 0.... more The mechanical relaxation behavior of the (La0.5Ce0.5)65Al10(CoxCu1−x)25 at% (x = 0, 0.2, 0.4, 0.6, and 0.8) metallic glasses was probed by dynamic mechanical analysis. The intensity of the secondary β relaxation increases along with the Co/Cu ratio, as has been reported in metallic glasses where the enthalpy of mixing for all pairs of atoms is negative. Furthermore, the intensity of the secondary β relaxation decreases after physical aging below the glass transition temperature, which is probably due to the reduction of the atomic mobility induced by physical aging.

Journal of Alloys and Compounds

The atomic structure of a Cu13Ni34Pd53 metallic glass was studied by molecular dynamics simulatio... more The atomic structure of a Cu13Ni34Pd53 metallic glass was studied by molecular dynamics simulation at different temperatures along a shear deformation cycle. A simulation box of 1 million atoms was deformed in the x axis and then the original orthogonal shape was recovered. Directional pair distribution functions were computed in the coordinate planes and some significant directions along the shear deformation cycle. No anisotropy was found in the initial state, while post-deformation anisotropy was revealed by significant differences of the partial pair distribution functions. The analysis of atomic environments concluded that the remnant anisotropy remains constrained into the shear plane. Low temperature samples showed remnant anisotropy after the full shear sample, while close to the glass transition the anisotropy induced by the initial shear process is removed by the subsequent recovery process.

Journal of Applied Crystallography, 2003

The computation of small-angle scattering (SAS) curves is still an open problem in densely packed... more The computation of small-angle scattering (SAS) curves is still an open problem in densely packed systems. This fact reduces the applicability of SAS largely, in particular to solid-state densely packed systems. Solid-state transformations often follow nucleation and growth kinetics. In this case, even for isotropic growth the spherical symmetry of the precipitates is broken due to impingement, and their actual shape becomes unknown. However, in these systems the developed microstructure is given by the transformation kinetics. Because the SAS curve is given by the microstructure, the SAS curve should also be determined by the nucleation and growth protocol. In this work this link is revealed and the kinetics of the system is connected with the average radial density function. Statistical considerations allow us to determine the adequate interparticle correlation function, enabling the computation of the SAS curve along the transformation. The formalism is applied to a system where grains appear randomly at initial time and grow with a constant growth rate (p-cell system). The resulting SAS curve at different times are computed and compared to Monte Carlo simulations, showing quantitative agreement for transformed fractions up to 95%.

M�ssbauer characterization of an amorphous steel with optimal Mo content

J Non Cryst Solids, 2008