arXiv subjects

J. Segurado

Publications and source records attributed to J. Segurado.

23 records · Page 2Linked to original sources

arXiv2018

Crystal plasticity simulation of the effect of grain size on the fatigue behavior of polycrystalline Inconel 718

A microstructure-based model that accounts for the effect of grain size has been developed to study the effect of grain size on the fatigue life of Inconel 718 alloys. The fatigue behavior of two alloys with different grain size was determined by means of uniaxial cyclic deformation tests under fully-reversed deformation ($R_\varepsilon$ = -1) at 400$^\circ$C in the low cycle fatigue regime. The model was based in the determination of the fatigue indicator parameter (based on the local crystallographic strain energy dissipated per cycle) by means of computational homogenization of a representative volume element of the microstructure. The mechanical response of the single crystal within the polycrystal was modelled through a phenomenological crystal plasticity model which was modified to account for the effect of grain size on the monotonic and cyclic hardening/softening mechanisms. The microstructure-based crack initiation model parameters were calibrated from the experimental tests of the material with fine grain size. The results of the fatigue simulations were in good agreement with the experimental results in terms of the cyclic stress-strain curves and of the number of cycles for fatigue crack initiation. The model did not show any grain size effect on the fatigue life for the largest cyclic strain ranges while the predicted fatigue life predicted was considerably longer in the case of the microstructure with fine grain size for the lowest strain ranges, in quantitative agreement with experimental data. These differences were attributed to changes in the deformation modes between homogeneous plastic deformation at large cyclic strain ranges and localized plasticity in a few grains at low cyclic strain ranges.

cond-mat.mtrl-sci↗

arXiv2018

An analysis of the influence of grain size on the strength of FCC polycrystals by means of computational homogenization

The effect of grain size on the flow stress of FCC polycrystals is analyzed by means of a multiscale strategy based on computational homogenization of the polycrystal aggregate. The mechanical behavior of each crystal is given by a dislocation-based crystal plasticity model in which the critical resolved shear stress follows the Taylor model. The generation and annihilation of dislocations in each slip system during deformation is given by the Kocks-Mecking model, which was modified to account for the dislocation storage at the grain boundaries. Polycrystalline Cu is selected to validate the simulation strategy and all the model parameters are obtained from dislocation dynamics simulations or experiments at lower length scales and the simulation results were in good agreement with experimental data in the literature. The model is applied to explore the influence of different microstructural factors (initial dislocation density, width of the grain size distribution, texture) on the grain size effect. It is found that the initial dislocation density, $ρ_i$, plays a dominant role in the magnitude of the grain size effect and that dependence of flow stress with an inverse power of grain size ($σ_ y -σ_\infty \propto d_g^{-x}$) breaks down for large initial dislocation densities ($> 10^{14}$ m$^{-2}$) and grain sizes $d_g > $ 40 $μ$m in FCC metals. However, it was found that the grain size contribution to the strength followed a power-law function of the dimensionless parameter $d_g\sqrt{ρ_i}$ for small values of the applied strain ($<$ 2 \%), in agreement with previous theoretical considerations for size effects in plasticity.

cond-mat.mtrl-sci↗

arXiv2017

Microstructure-based fatigue life model of metallic alloys with bilinear Coffin-Manson behavior

A microstructure-based model is presented to predict the fatigue life of polycrystalline metallic alloys which present a bilinear Coffin-Manson relationship. The model is based in the determination of the maximum value of a fatigue indicator parameter obtained from the plastic energy dissipated by cycle in the microstructure. The fatigue indicator parameter was obtained by means of the computational homogenization of a representative volume element of the microstructure using a crystal-plasticity finite element model. The microstructure-based model was applied to predict the low cyclic fatigue behavior of IN718 at 400$^\circ$C which exhibits a bilinear Coffin-Manson relationship under the assumption that this behavior is triggered by a transition from highly localized plasticity at low cyclic strain amplitudes to more homogeneous deformation at high cyclic strain amplitudes. The model predictions were in very good agreement with the experimental results for a wide range of cyclic strain amplitudes and two strain ratios ($R_ε$ = 0 and -1) and corroborated the initial hypothesis. Moreover, they provided a micromechanical explanation for the influence of the strain ratio on the fatigue life at low cyclic strain amplitudes.

cond-mat.mtrl-sci↗

arXiv2016

A 3D dislocation dynamics analysis of the size effect on the strength of [111] LiF micropillars at 300K and 600K

The mechanical behavior in compression of [111] LiF micropillars with diameters in the range 0.5 $μ$m to 2.0 $μ$m was analyzed by means of discrete dislocation dynamics at ambient and elevated temperature. The dislocation velocity was obtained from the Peach-Koehler force acting on the dislocation segments from a thermally-activated model that accounted for the influence of temperature on the lattice resistance. A size effect of the type "smaller is stronger" was predicted by the simulations, which was in quantitative agreement with previous experimental results by the authors \cite{SWC14}. The contribution of the different physical deformation mechanisms to the size effect (namely, nucleation of dislocations, dislocation exhaustion and forest hardening) could be ascertained from the simulations and the dominant deformation mode could be assessed as a function of the specimen size and temperature. These results shed light into the complex interaction among size, lattice resistance and dislocation mobility in the mechanical behavior of $μ$m-sized single crystals.

cond-mat.mtrl-sci↗

arXiv2016

Three-dimensional investigation of void growth leading to fracture in commercially pure titanium

The fracture process of commercially pure titanium was visualized in model materials containing artificial holes. These model materials were fabricated using a femtosecond laser coupled with a diffusion bonding technique to obtain voids in the interior of titanium samples. Changes in void dimensions during in-situ straining were recorded in three dimensions using x-ray computed tomography. Void growth obtained experimentally was compared with the Rice and Tracey model which predicted well the average void growth. A large scatter in void growth data was explained by differences in grain orientation which was confirmed by crystal plasticity simulations. It was also shown that grain orientation has a stronger effect on void growth than intervoid spacing and material strength. Intervoid spacing, however, appears to control whether the intervoid ligament failure is ductile or brittle.

cond-mat.mtrl-sci↗