V. Livescu

Los Alamos National Laboratory, Los Alamos, California, United States

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Publications (17)10.67 Total impact

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    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.
    Journal of Physics Conference Series 05/2014; 500(11):112031. DOI:10.1088/1742-6596/500/11/112031
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    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.
    Materials Science and Engineering A 03/2013; 565:450–458. DOI:10.1016/j.msea.2012.12.007 · 2.41 Impact Factor
  • V. Livescu, J. F. Bingert, T.A. Mason
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    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.
    Materials Science and Engineering A 10/2012; 556:155. DOI:10.1016/j.msea.2012.06.071 · 2.41 Impact Factor
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    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.
    The European Physical Journal Conferences 08/2012; 26:02004-. DOI:10.1051/epjconf/20122602004
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    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.
    The European Physical Journal Conferences 01/2011; 10. DOI:10.1051/epjconf/20101000006
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    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.
    The Review of scientific instruments 06/2010; 81(6):063903. DOI:10.1063/1.3436637 · 1.58 Impact Factor
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    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.
    Materials Science Forum 06/2010; 654-656:2297-2302. DOI:10.4028/www.scientific.net/MSF.654-656.2297
  • John F. Bingert, Veronica Livescu, Ellen K. Cerreta
    03/2010: pages 301-315;
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    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.
    12/2009; 1195(1). DOI:10.1063/1.3294989
  • Davis Tonks, John Bingert, Veronica Livescu
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    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.
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    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.
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    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.
    International Journal of Refractory Metals and Hard Materials 01/2006; 24:122-128. DOI:10.1016/j.ijrmhm.2005.06.005 · 1.86 Impact Factor
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    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.
    Materials Science and Engineering A 06/2005; 399(1):134-140. DOI:10.1016/j.msea.2005.02.024 · 2.41 Impact Factor
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    ABSTRACT: When performed in conjunction with neutron diffraction, in situ loading offers unique insights on microstructural deformation mechanisms. This is by virtue of the penetration and phase sensitivity of neutrons. At Los Alamos National Laboratory room and high temperature (up to 1500°C) polycrystalline constitutive response is modeled using finite element and self-consistent models. The models are compared to neutron diffraction measurements. In doing so the implications of slip and creep to microstructural response have been explored. Recently we have been considering low temperature phenomena. This includes changes in deformation mechanisms such as the increased predilection for twinning over slip. Since this is associated with measurable texture changes as well as microstructural strain effects, it is well suited for study using neutron diffraction. This paper outlines the design and rationale for a cryogenic loading capability that will be used on the Spectrometer for MAterials Research at Temperature and Stress (SMARTS) at the Los Alamos Neutron Science Center (LANSCE).
    06/2004; 711(1). DOI:10.1063/1.1774555
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    ABSTRACT: This work quantitatively assesses residual strains and stresses associated with the weld repair process used to repair cracks on NASA's space shuttle flow liners. The coupons used in this investigation were made of the same INCONEL 718 alloy used for the flow liners. They were subjected to identical welding and certification procedures that were carried out on the space shuttle. Neutron diffraction measurements at Los Alamos National Laboratory determined residual strains at selected locations in a welded coupon at 293 K and 135 K. The weld repair process introduced Mises effective residual stresses of up to 555 MPa. On comparing the measurements at 293 K and 135 K, no significant change to the residual strain profile was noted at the low temperature. This indicated minimal mismatch in the coefficients of thermal expansion between the base metal and the weld.
    06/2004; 711(1). DOI:10.1063/1.1774566
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    ABSTRACT: Shockwave shape can influence dynamic damage evolution. Features such as rise time, pulse duration, peak shock pressure, pull back, and release rate are influenced as wave shape changes. However, their individual influence on dynamic damage evolution is not well understood. Specifically, changing from a square to triangular or Taylor wave loading profile can alter the release kinetics from peak shock pressure and the volume of material sampled during release. This creates a spatial influence. In high purity metals, because damage is often linked to boundaries within the microstructure (grain or twin), changing the volume of material sampled during release, can have a drastic influence on dynamic damage evolution as the number of boundaries or defects sampled is altered. In this study, model-driven dynamic experiments have been conducted on eu with four different grain sizes to examine, for a given shockwave shape, how the spatial effect of boundary distribution influences dynamic damage evolution. Both two and three dimensional damage characterization techniques have been utilized. This study shows the critical influence of spatial effects, in this case boundary density, on dynamic damage evolution. As the boundary density decreases, the damage evolution transitions from nucleation controlled to growth controlled. It also shows that specific boundaries, those with high Schmid factor orientations on either side, maybe a necessary condition for void formation.