V. Livescu

Los Alamos National Laboratory, Лос-Аламос, California, United States

Are you V. Livescu?

Claim your profile

Publications (24)20.77 Total impact

  • Source
    Gray, G.T., III · K.S. Vecchio · V. Livescu
    [Show abstract] [Hide abstract]
    ABSTRACT: A new specimen geometry, the compact forced-simple-shear specimen (CFSS), has been developed as a means to achieve simple shear testing of materials over a range of temperatures and strain rates. The stress and strain state in the gage section is designed to produce essentially "pure" simple shear, mode II in-plane shear, in a compact-sample geometry. The 2-D plane of shear can be directly aligned along specified directional aspects of a material's microstructure of interest; i.e., systematic shear loading parallel, at 45°, and orthogonal to anisotropic microstructural features in a material such as the pancake-shaped grains typical in many rolled structural metals, or to specified directions in fiber-reinforced composites. The shear-stress shear-strain response and the damage evolution parallel and orthogonal to the pancake grain morphology in 7039-Al are shown to vary significantly as a function of orientation to the microstructure. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    Full-text · Article · Jan 2016 · Acta Materialia
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The microstructural changes and mechanical response of an HT-9 sample shock loaded to a peak pressure of 11. GPa have been investigated by TEM, XRD, microhardness and EBSD techniques. Dislocation densities obtained by both direct measurements (via TEM) and indirect calculations (by XRD and hardness) indicate that shock loading results in ~2-3 fold increase in dislocation density. TEM analyses show that the shape, and density of the dislocations change after shock loading. In addition, shock loading causes local plastic deformation of the continuous parallel lath structure in some regions, together with an overall decrease in the aspect ratio of laths due to local plastic deformation and lath fragmentation. As a result of XRD analyses, the fraction of edge dislocations is determined to increase by ~24% after shock loading. Furthermore, hardness increases by ~40 HV after shock loading due to the increased dislocation density. EBSD analyses show that the fraction of CSL boundaries decreases by ~5-10% as a result of shock loading.
    Full-text · Article · Oct 2015 · Materials Science and Engineering A
  • [Show abstract] [Hide abstract]
    ABSTRACT: The convergence of multiple characterization tools has been applied to investigate the relationship of microstructure on damage evolution in high purity aluminum. The extremely coarse grain size of the disc-shaped sample provided a quasi-two dimensional structure from which the location of surface-measured features could be inferred. In particular, the role of pre-existing defects on damage growth was accessible due to the presence of casting porosity in the aluminum. Micro tomography, electron backscatter diffraction, and digital image correlation were applied to interrogate the sample in three dimensions. A recently developed micro-bulge testing apparatus was used to deform the pre-characterized disc of aluminum in biaxial tension, and related analysis techniques were applied to map local strain fields. Subsequent post-mortem characterization of the failed sample was performed to correlate structure to damaged regions. It was determined that strain localization and associated damage was most strongly correlated with grain boundary intersections and plastic anisotropy gradients between grains. Pre-existing voids played less of an apparent role than was perhaps initially expected. These combined techniques provide insight to the mechanism of damage initiation, propagation, and failure, along with a test bed for predictive damage models incorporating anisotropic microstructural effects.
    No preview · Article · Sep 2015 · Materials Science and Engineering A
  • Shraddha J. Vachhani · Nathan Mara · Veronica Livescu · Ellen Cerreta

    No preview · Article · Jan 2015 · The European Physical Journal Conferences

  • No preview · Article · Jan 2015 · The European Physical Journal Conferences
  • [Show abstract] [Hide abstract]
    ABSTRACT: Spallation is well known to be a complex process strongly influenced by microstructure, loading path, and the loading profile yet often a singular "spall strength" is utilized in hydrocodes to quantify the dynamic fracture behavior of a material. In the current study, the influence of loading path on the "spall strength" and damage evolution in high-purity Ta is presented. Tantalum samples where shock loaded to three different peak shock stresses using both symmetric impact, and two different composite flyer plate configurations such that upon unloading the three samples displayed nearly identical "pull-back" signals as measured via rear-surface velocimetry. While the "pull-back" signals observed are similar in magnitude, the highest peak stressed sample resulted in complete spall scab separation while the two lower peak stresses resulted in incipient spall. The damage evolution in the "soft" recovered Ta samples was quantified using optical metallography, electron-back-scatter diffraction, and tomography. The effect of loading path on spallation and its ramifications for the stress and kinetic dependency of dynamic damage evolution is discussed.
    No preview · Article · May 2014 · Journal of Physics Conference Series
  • V. Livescu · C.A. Bronkhorst · J.F. Bingert
    [Show abstract] [Hide abstract]
    ABSTRACT: The dynamic deformation of metallic polycrystalline materials leading to ductile damage and failure events involves a complex series of physical processes which are poorly understood. This lack of understanding prevents us from properly formulating and offering the appropriate physically based theories for accurate and robust representation of the ductile damage and failure response of ductile materials. This paper briefly describes and illustrates a coupled experimental and computational methodology to develop greater physical insight linking the structural details of the material to its formation of damage sites. Results from examinations of both tantalum and copper are presented to illustrate the types of methodologies that will be needed in the future to better understand the critical physical processes occurring in polycrystalline metallic materials leading to their catastrophic failure.
    No preview · Article · Jan 2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Two alloys of alumina dispersion-strengthened copper were subjected to 1-4 passes of equal channel angular extrusion (ECAE) by route BC. Microstructures before and after deformation were characterized by scanning electron microscopy, scanning ion microscopy and electron backscatter diffraction. Mechanical and electrical properties were evaluated using uniaxial tensile testing and the four point probe method, respectively. The initial microstructure consisted of cylindrical grains, elongated in the extrusion direction and highly textured in a <100>/<111> orientation. Following four ECAE passes, the average major axis of grains in both alloys decreased by over 50%, and the microstructure approached an equiaxed morphology. The texture decreased in intensity and shifted to a <112> orientation after one ECAE pass, followed by a transition to a <101> orientation by the fourth pass. Flow stress of AL-25 and AL-60 increased only 48 MPa (10%) and 24 MPa (5%), respectively, while the conductivity of both alloys remained essentially unchanged. A combination of Hall–Petch strengthening and texture softening are used to explain the observed changes in mechanical behavior.
    Full-text · Article · Mar 2013 · Materials Science and Engineering A
  • V. Livescu · J. F. Bingert · T.A. Mason
    [Show abstract] [Hide abstract]
    ABSTRACT: Deformation twinning resulting from high explosive-driven shock and associated plasticity was investigated in high-purity b.c.c. tantalum. Post mortem characterization of samples shocked at relatively higher and lower pressures showed significant {112}〈111〉 twin activity. Further analysis of the lower shock pressure sample showed twins to be spatially clustered at the mesoscale, indicating the role of twin termination at grain boundaries to produce requisite twin initiation stresses in neighbor grains. In addition, analysis of electron backscatter diffraction data suggests that twin propagation across boundaries does not require minimal misorientations between the active variants of the twins in adjacent parent grains. A minimum threshold grain size of approximately 25 μm was determined for both samples, below which twinning was suppressed. Finally, the observation of spall voids at twin intersections implied that twinning increases the density of preferred damage initiation sites during the shock deformation process. Overall, twinning was shown to play a significant role in the deformation and damage evolution of shock-loaded tantalum.
    No preview · Article · Oct 2012 · Materials Science and Engineering A
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Widespread research over the past five decades has provided a wealth of experimental data and insight concerning the shock hardening, damage evolution, and the spallation response of materials subjected to square-topped shock-wave loading profiles. However, fewer quantitative studies have been conducted on the effect of direct, in-contact, high explosive (HE)-driven Taylor wave (unsupported shocks) loading on the shock hardening, damage evolution, or spallation response of materials. Systematic studies quantifying the effect of sweeping-detonation wave loading are yet sparser. In this study, the shock hardening and spallation response of Ta is shown to be critically dependent on the peak shock stress and the shock obliquity during sweeping-detonation-wave shock loading. Sweeping-wave loading is observed to: a) yield a lower spall strength than previously documented for 1-D supported-shock-wave loading, b) exhibit increased shock hardening as a function of increasing obliquity, and c) lead to an increased incidence of deformation twin formation with increasing shock obliquity.
    Preview · Article · Aug 2012 · The European Physical Journal Conferences
  • D. L. Tonks · J. Bingert · V. Livescu · S. Luo · C. Bronkhorst
    [Show abstract] [Hide abstract]
    ABSTRACT: The goal of this project is to produce a damage model forspallation in metals informed by the polycrystalline grain structure at themesoscale. Earlier damage models addressed the continuum macroscale in whichthese effects were averaged out. In this work we focus on cross sectionsfrom recovered samples examined with EBSD (electron backscattereddiffraction), which reveal crystal grain orientations and voids. We seek tounderstand the loading histories of specific sample regions by meshing upthe crystal grain structure of these regions and simulating the stress,strain, and damage histories in our hydrocode, FLAG. The stresses and strainhistories are the fundamental drivers of damage and must be calculated. Thecalculated final damage structures are compared with those from therecovered samples to validate the simulations.
    No preview · Article · Jan 2011 · The European Physical Journal Conferences
  • George T. Gray III · Veronica Livescu · Ellen K. Cerreta
    [Show abstract] [Hide abstract]
    ABSTRACT: Orientation-imaging microscopy offers unique capabilities to quantify the defects and damage evolution occurring in metals following dynamic and shock loading. Examples of the quantification of the types of deformation twins activated, volume fraction of twinning, and damage evolution as a function of shock loading in Ta are presented. Electron back-scatter diffraction (EBSD) examination of the damage evolution in sweeping-detonation-wave shock loading to study spallation in Cu is also presented.
    No preview · Article · Jun 2010 · Materials Science Forum
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A novel capability was designed, implemented, and tested for in situ neutron diffraction measurements during loading at cryogenic temperatures on the spectrometer for materials research at temperature and stress at Los Alamos National Laboratory. This capability allowed for the application of dynamic compressive forces of up to 250 kN on standard samples controlled at temperatures between 300 and 90 K. The approach comprised of cooling thermally isolated compression platens that in turn conductively cooled the sample in an aluminum vacuum chamber which was nominally transparent to the incident and diffracted neutrons. The cooling/heat rate and final temperature were controlled by regulating the flow of liquid nitrogen in channels inside the platens that were connected through bellows to the mechanical actuator of the load frame and by heaters placed on the platens. Various performance parameters of this system are reported here. The system was used to investigate deformation in Ni-Ti-Fe shape memory alloys at cryogenic temperatures and preliminary results are presented.
    Full-text · Article · Jun 2010 · The Review of scientific instruments
  • John F. Bingert · Veronica Livescu · Ellen K. Cerreta
    [Show abstract] [Hide abstract]
    ABSTRACT: This chapter provides examples of the application of EBSD characterization to microstructures influenced by two conditions: (1) shear localization, and (2) dynamic deformation and damage from shock loading. © Springer Science+Business Media, LLC 2009. All rights reserved.
    No preview · Chapter · Mar 2010
  • Davis L Tonks · John F Bingert · Veronica Livescu · P. Peralta
    [Show abstract] [Hide abstract]
    ABSTRACT: Cavities of coalesced voids have been found in recovered samples of Tantalum in high‐explosive‐driven experiments. The boundaries of these cavities are imprinted with details of the coalescence and void growth processes. One way of quantifying these details is to measure the roughness of the surfaces. In this work, we calculate the roughness of 2D cross sections of such cavity surfaces from micrographs by analyzing the images with the box counting technique. Spall plane damage driven by flyer plates in Copper samples is also analyzed. The different length scale regimes found will be discussed.
    No preview · Article · Dec 2009
  • Davis Tonks · John Bingert · Veronica Livescu
    [Show abstract] [Hide abstract]
    ABSTRACT: Cavities of coalesced voids have been found in recovered samples of Tantalum in gas gun and high-explosive-driven experiments. The boundaries of these cavities are imprinted with details of the coalescence and void growth processes. One way of quantifying these details is to measure the roughness of the surfaces. In this work, we calculate the roughness of 2D cross sections of such cavity surfaces from micrographs by analyzing the images with the box counting technique. Both gas gun samples and explosively driven samples are treated. The cavities in the explosively driven samples appear rougher than those in the gas gun samples so we expect a larger roughness exponent for them. Possible reasons for the roughness differences will be discussed.
    No preview · Article · Jun 2009
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Energetic loading subjects a material to a “Taylor wave” (triangular wave) loading profile that experiences an evolving balance of hydrostatic (spherical) and deviatoric stresses. While much has been learned over the past five decades concerning the propensity of deformation twinning in samples shock-loaded using “square-topped” profiles as a function of peak stress, achieved most commonly via flyer plate loading, less is known concerning twinning propensity during non-1-dimensional sweeping detonation wave loading. Systematic small-scale energetically-driven shock loading experiments were conducted on Ta samples shock-loaded with PETN that was edge detonated. Deformation twinning was quantified in post-mortem samples as a function of detonation geometry and radial position. In the edge detonated loading geometry examined in this paper, the average volume fraction of deformation twins was observed to increase with increasing shock obliquity. The results of this study are discussed in light of the formation mechanisms of deformation twins, previous literature studies of twinning in shocked materials, and modeling of the effects of shock obliquity on the stress tensor during shock loading.
    Full-text · Article · Jan 2009
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The strain response of WC and Ni in WC–Ni cemented carbide composites (5, 10 and 20 wt.% Ni) was studied under uniaxial compressive load to −2000 MPa using neutron diffraction. Measurements of elastic strain were made simultaneously in the axial and transverse directions of the samples, for both phases. Thermal residual stresses (TRS) were also measured, before and after loading. Ni plasticity was observed from the earliest load levels. The superposition of tensile Poisson strain (in the transverse direction) on pre-existing tensile Ni strain due to TRS produces anisotropic yielding in binder regions. Yielding is progressive with applied strain, leading to a reversal of transverse binder strain, and anisotropic relaxation of the TRS. The effect is greatest for 20 wt.% Ni, where Ni constraint is much less than for 5 wt.% Ni. These results provide a quantitative basis for the mechanical origins of the toughness of cemented carbide composites.
    Full-text · Article · Mar 2006 · International Journal of Refractory Metals and Hard Materials
  • [Show abstract] [Hide abstract]
    ABSTRACT: The deformation in compression of pure magnesium and AZ31B magnesium alloy, both with a strong basal pole texture, has been investigated as a function of temperature, strain rate, and specimen orientation. The mechanical response of both metals is highly dependent upon the orientation of loading direction with respect to the basal pole. Specimens compressed along the basal pole direction have a high sensitivity to strain rate and temperature and display a concave down work hardening behavior. Specimens loaded perpendicularly to the basal pole have a yield stress that is relatively insensitive to strain rate and temperature and a work hardening behavior that is parabolic and then linearly upwards. Both specimen orientations display a mechanical response that is sensitive to temperature and strain rate. Post mortem characterization of the pure magnesium was conducted on a subset of specimens to determine the microstructural and textural evolution during deformation and these results are correlated with the observed work hardening behavior and strain rate sensitivities were calculated.
    No preview · Article · Jan 2006
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In situ neutron diffraction measurements were performed on a tungsten carbide (WC)–10wt.% cobalt (Co) cemented carbide composite subjected to compressive loading. The sample was subjected to consecutive load/unload cycles to −500, −1000, −2000 and −2100MPa. Thermal residual stresses measured before loading reflected large hydrostatic tensile stresses in the binder phase and compressive stresses in the carbide phase. The carbide phase behaved elastically at all but the highest load levels, whereas plasticity was present in the binder phase from values of applied stress as low as −500MPa. A finite element simulation utilizing an interpenetrating microstructure model showed remarkable agreement with the complex mean phase strain response during the loading cycles despite its under-prediction of thermal residual strains.
    Full-text · Article · Jun 2005 · Materials Science and Engineering A