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At least 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Transition, coexistence, and interaction of vector localized waves arising from higher-order effects

We study vector localized waves on continuous wave background with higher-order effects in a two-mode optical fiber. The striking properties of transition, coexistence, and interaction of these localized waves arising from higher-order effects are revealed in combination with corresponding modulation instability (MI) characteristics. It shows that these vector localized wave properties have no analogues in the case without higher-order effects. Specifically, compared to the scalar case, an intriguing transition between bright–dark rogue waves and w-shaped–anti-w-shaped solitons, which occurs as a result of the attenuation of MI growth rate to vanishing in the zero-frequency perturbation region, is exhibited with the relative background frequency. In particular, our results show that the w-shaped–anti-w-shaped solitons can coexist with breathers, coinciding with the MI analysis where the coexistence condition is a mixture of a modulation stability and MI region. It is interesting that their interaction is inelastic and describes a fusion process. In addition, we demonstrate an annihilation phenomenon for the interaction of two w-shaped solitons which is identified essentially as an inelastic collision in this system. -- Highlights: •Vector rogue wave properties induced by higher-order effects are studied. •A transition between vector rogue waves and solitons is obtained. •The link between the transition and modulation instability (MI) is demonstrated. •The coexistence of vector solitons and breathers coincides with the MI features. •An annihilation phenomenon for the vector two w-shaped solitons is presented.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Elastic and magnetic properties of Fe 3 P up to core pressures: Phosphorus in the Earth's core

Phosphorus (P) is considered a possible light element alloying with iron (Fe) in the Earth's core due to its siderophile nature and the ubiquity of P-bearing iron alloys in iron meteorites. The sequestration of P by liquid metals during the core formation possibly results in the relatively low concentration of P in the bulk silicate Earth. Here, we performed single-crystal and powder X-ray diffraction, synchrotron Mossbauer spectroscopy and nuclear resonant inelastic X-ray scattering measurements in diamond anvil cells to investigate the elastic and magnetic properties of Fe 3 P under high pressures. Our X-ray diffraction results suggest that there is no structural phase transition up to 111 GPa. However, a volume collapse was observed at 21.5 GPa in Fe 3 P, ascribed to a magnetic transition as evidenced by synchrotron Mhssbauer spectroscopy results. Fitting the volume-pressure data by the Birch-Murnaghan equation of state gives bulk modulus $K_{T0}$ = 162.4(7) GPa, its first pressure derivative $K'_{T0}$ = 4.0 (fixed) and zeropressure volume $V_0$ = 370.38(6) Å 3 for the magnetic phase and $K_{T0}$ = 220(7) GPa, $K'_{T0}$ = 4.0 (fixed) and $V_0$ = 357(1) Å 3 for the non-magnetic phase. Sound velocities of Fe 3 P were determined up to 152 GPa by nuclear resonant inelastic X-ray scattering, demonstrating that Fe 3 P bears a low shear velocity and high Poisson's ratio at core pressures compared to Fe and Fe 3 S. When forming a solid solution Fe 3 (S,P) with Fe 3 S at core pressures, Fe 3 P may favorably influence the elastic properties of Fe 3 (S,P) to match the seismic observations of the inner core.

58 GEOSCIENCES↗
DOE OSTI2016

Benchmark solution of the dynamic response of a spherical shell at finite strain

Our paper describes the development of high fidelity solutions for the study of homogeneous (elastic and inelastic) spherical shells subject to dynamic loading and undergoing finite deformations. The goal of the activity is to provide high accuracy results that can be used as benchmark solutions for the verification of computational physics codes. Furthermore, the equilibrium equations for the geometrically non-linear problem are solved through mode expansion of the displacement field and the boundary conditions are enforced in a strong form. Time integration is performed through high-order implicit Runge–Kutta schemes. Finally, we evaluate accuracy and convergence of the proposed method by means of numerical examples with finite deformations and material non-linearities and inelasticity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Measurement of leakage neutron spectra for zirconium with D-T neutrons and validation of evaluated nuclear data

Integral neutronics experiments have been a powerful tool and method for examining the reliability of evaluated nuclear data. In the present paper, the accuracy of evaluated nuclear data for Zirconium has been validated by comparing measured leakage neutron spectra with calculated ones. Leakage neutron spectra from the irradiation of D-T neutrons on Zirconium samples were experimentally measured at 60° and 120° by using a time-of-flight method. Theoretical calculations are carried out by Monte Carlo neutron transport code MCNP-4C with evaluated nuclear data of the ENDF/B-VII.0, ENDF/B-VIII.0, JENDL-4.0 and CENDL-3.1 libraries. From the comparisons, it is found that the calculations with JENDL-4.0 gives better agreements with the experiments at 60° while a large discrepancies are observed in the 4–13.5 MeV energy range at 120°. However, the calculated spectra using the data from ENDF/B-VII.0, ENDF/B-VIII.0 and CENDL-3.1 libraries showed some discrepancies with the measured ones, especially caused by the difference of the elastic and inelastic angular distributions and the secondary neutron energy distributions in the continuum inelastic scattering.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Reduced-order microstructure-sensitive protocols to rank-order the transition fatigue resistance of polycrystalline microstructures

We present that the transition fatigue regime between low cycle fatigue (LCF) and high cycle fatigue (HCF) is often addressed in the design and performance evaluation of load-bearing components used in many structural applications. Transition fatigue is characterized by elevated levels of local inelastic deformation in significant regions of the microstructure as compared to HCF. Typically, crystal plasticity finite element method (CPFEM) simulations are performed to model this phenomenon and to rank-order microstructures by their resistance to crack formation and early growth in the regime of transition fatigue. Unfortunately, these approaches require significant computational resources, inhibiting their use to explore novel materials for transition fatigue resistance. Reduced-order, microstructure-sensitive models are needed to accelerate the search for next-generation, fatigue-resistant materials. In a recent study, Paulson et al. (2018) extended the materials knowledge system (MKS) framework for rank-ordering the HCF resistance of polycrystalline microstructures. The efficacy of this approach lies in the reduced-dimensional representation of microstructures through 2-point spatial correlations and principal component analysis (PCA), in addition to the characterization of the HCF response with a small set of performance measures. In this work, these same protocols are critically evaluated for their applicability to rank-order the transition fatigue resistance of the same class of polycrystalline microstructures subjected to increased strain amplitudes. Success in this endeavor requires the formation of homogenization linkages that account for the significantly higher levels of local inelastic deformation and stress redistribution in transition fatigue as compared to HCF. Finally, a set of 12 α-titanium microstructures generated using the open access DREAM.3D software (Groeber and Jackson, 2014) are employed for this evaluation.

2-point correlations↗
DOE OSTI2018

Anisotropic finite hyper-elastoplasticity of geomaterials with Drucker-Prager/Cap type constitutive model formulation

The formulation of large strain anisotropic hyper-elastoplasticity of geomaterials is examined. Attention is given to the role of structure tensors (also called fabric tensors), especially in context of the Eshelby-Mandel stress and large inelastic volume changes attributable to porosity. Both (hyper-)elastic and inelastic orthotropic symmetry, reducing to the particular case of transverse isotropy, are considered. Specific material assumptions and constitutive choices are identified for the development of a novel Anisotropic Drucker-Prager/Cap (ADPC) model formulated within the intermediate configuration consistent with multiplicative split of the deformation gradient. The model is calibrated to existing experimental measurements, including high pressure large strain triaxial compression of lithographic (Solnhofen) limestone and triaxial compression measurements on Tournemire shale assessing elastoplastic anisotropy. Manifest implications of constitutive theory are investigated, including consequences of recognizing (or not) the Eshelby-Mandel stress as energy conjugate to the plastic velocity gradient and including (or not) contribution from the skew-symmetric parts of the Mandel stress to the plastic anisotropy. Numerical simple shear experiments and large deformation simulated indentation experiments are provided in order to investigate model predictions and demonstrate the overall robustness in finite element modeling.

42 ENGINEERING↗
DOE OSTI2023

Deformation, dislocation evolution and the non-Schmid effect in body-centered-cubic single- and polycrystal tantalum

A physically-informed continuum crystal plasticity model is presented to elucidate deformation mechanisms, dislocation evolution and the non-Schmid effect in body-centered-cubic (bcc) tantalum widely used as a key structural material for mechanical and thermal extremes. We show the unified structural modeling framework informed by mesoscopic dislocation dynamics simulations is capable of capturing salient features of the large inelastic behavior of tantalum at quasi-static (10 -3 s -1 ) to extreme strain rates (5000 s -1 ) and at low (77 K) to high temperatures (873 K) at both single- and polycrystal levels. Here we also present predictive capabilities of the model for microstructural evolution in the material. To this end, we investigate the effects of dislocation interactions on slip activities, instability and the non-Schmid behavior at the single crystal level. Furthermore, ex situ measurements on crystallographic texture evolution and dislocation density growth are carried out for polycrystal tantalum specimens at increasing strains. Numerical simulation results also support that the modeling framework is capable of capturing the main features of the polycrystal behavior over a wide range of strains, strain rates and temperatures. The theoretical, experimental and numerical results at both single- and polycrystal levels provide critical insight into the underlying physical pictures for micro- and macroscopic responses and their relations in this important class of refractory bcc materials undergoing large inelastic deformations.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Revealing the unusual rate-dependent mechanical behaviors of nematic liquid crystal elastomers

Liquid crystal elastomers (LCEs) exhibit unique mechanical properties of soft elasticity and enhanced energy dissipation with rate dependency. They are potentially transformative materials for applications in mechanical impact mitigation and vibration isolation. However, previous studies have primarily focused on the mechanics of LCEs under equilibrium and quasistatic loading conditions. Critical knowledge gaps exist in understanding their rate-dependent behaviors, which are a complex mixture of traditional network viscoelasticity and the soft elastic behaviors with changes in the mesogen orientation and order parameter. Together, these inelastic mechanisms lead to unusual rate-dependent energy absorption responses of LCEs. In this work, we developed a viscoelastic constitutive theory for monodomain nematic LCEs to investigate how multiple underlying sources of inelasticity manifest in the rate-dependent and dissipative behaviors of monodomain LCEs. Here, the theoretical modeling framework combines the neo-classical network theory with evolution rules for the mesogen orientation and order parameter with conventional viscoelasticity. The model is calibrated with uniaxial tension and compression data spanning six decades of strain rates. The established 3D constitutive model enables general loading predictions taking the initial mesogen orientation and order parameter as inputs. Additionally, parametric studies were performed to further understand the rate dependence of monodomain LCEs in relation to their energy absorption characteristics. Based on the parametric studies, particularly loading scenarios are identified as conditions where LCEs outperform conventional elastomers regarding energy absorption.

42 ENGINEERING↗
DOE OSTI2016

The cross-aisle seismic performance of storage rack base connections

Steel storage racks used in retail stores and warehouses are seismically designed as moment resisting frames in the down-aisle direction, and braced frames in the cross-aisle direction. While their down-aisle response is relatively well understood, there is little understanding of their cross-aisle response, especially as it pertains to the desired mode of inelastic deformation and associated design methods. In this paper, results are presented from six full scale tests on braced frames representing storage racks in the cross-aisle direction. These tests investigate the base plate thickness and dimensions, and the upright (column) cross section. The experiments indicate that inelastic deformation in the base plate provides stable hysteretic response with significant ductility and energy dissipation. Ductile tearing is also observed in welds connecting the base plate to the upright. However, it does not appear to negatively influence the hysteretic response. The tests are complemented by Finite Element (FE) simulations of the base connections. These simulations provide insights into internal force distributions within the connections. Based on these insights, analytical equations are proposed for characterizing the backbone curve of the hysteretic response, for use in displacement based design methods. It is determined that the current approach for characterizing design forces in the anchors is unconservative, since it does not incorporate the effects of strain hardening or the membrane action as the base plate undergoes large deformations. A new approach which incorporates these phenomena is presented, and determined to be significantly more accurate. Finally, limitations of the study are outlined and directions for future work are identified.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nd ordering, cluster formation, and the origin of negative magnetization in NdMn 0.8 Fe 0.2 O 3+δ

The orthorhombic pseudo-perovskite series NdMn 1-x Fe x O 3+δ has negative magnetization, i.e. magnetization directed opposite to an applied field under certain conditions, for x=0.2 and x=0.25. We have explored the microscopic origin of this phenomenon for x=0.2 by using elastic and inelastic scattering techniques including neutron backscattering, X-ray magnetic circular dichroism at the Mn K and Nd L 2 edges, neutron powder diffraction, and powder inelastic neutron scattering. We find that Nd 3+ ions possess zero field ordered moments of 1.6 μ B per ion at 1.5 K oriented parallel to the Mn moments. Due to the neutron diffraction, there is a freezing of magnetic clusters before the long-range order sets in. The Nd crystal field levels shift with temperature due to a molecular field that is 1.1 meV at 1.5 K. These results are consistent with a magnetic domain/cluster model for the negative magnetization in NdMn 0.8 Fe 0.2 O 3+δ .

36 MATERIALS SCIENCE↗
DOE OSTI2017

Dynamic restoration of severely predeformed, ultrafine-grained pure Cu at 373 K observed in situ

Highlights: • Discontinuous grain coarsening during tension at 373 K generated new grains exceeding significantly grain size of UFG Cu. • The coarsening was preferentially located at shear bands. • There are indications that the grain coarsening occurs by recrystallization. - Abstract: Microstructure changes of ultrafine-grained pure copper prepared by equal-channel angular pressing were investigated during in situ tensile testing inside a scanning electron microscope at constant total strain rate of 10{sup −4} s{sup −1} at 373 K (~ 0.28 T{sub m}). After fast work hardening the flow stress saturated at an inelastic strain of 0.06. Significant local coarsening of grains was observed to start at about 2/3 of the maximum flow stress. However, these localized events did not significantly change the mean grain size up to an inelastic strain of 0.09. Surface observations showed that the coarsening was preferentially located at shear bands.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Enabling event-by-event precision in γ-ray cascades for neutron-induced reactions

Neutron-induced γ-ray spectra provide key inputs for modern active interrogation applications. A precise modeling of the nuclear reaction and subsequent emission of γ rays is challenging and often impossible due to limitations on evaluated data file formats and nuclear transport simulation codes. We present a framework that addresses these challenges by combining experimental data and reaction-model calculation outputs into an extended candidate version of the Generalized Nuclear Data Structure (GNDS) file, the successor format for the legacy Evaluated Nuclear Data File (ENDF-6). This proposed GNDS hierarchical format contains all the necessary ingredients for inline γ-ray cascade reproduction with event-by-event precision, including continuum–continuum and continuum–discrete transitions following neutron-capture and inelastic neutron scattering reactions. Cascade-event generation based on our approach demonstrates improved energy conservation on an event-by-event basis and permits the use of γ-γ coincidences in applications. This work offers, for the first time, a method to generate neutron-capture and inelastic neutron-scattering γ-ray cascades where energy conservation, correlations, and experimental primaries are fully accounted for.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Opportunities for Undergraduate Research in Nuclear Physics

University of Dallas (UD) physics majors are offered a variety of undergraduate research opportunities in nuclear physics through an established program at the University of Kentucky Accelerator Laboratory (UKAL). The 7-MV Model CN Van de Graaff accelerator and the neutron production and detection facilities located there are used by UD students to investigate how neutrons scatter from materials that are important in nuclear energy production and for our basic understanding of how neutrons interact with matter. Recent student projects include modeling of the laboratory using the neutron transport code MCNP to investigate the effectiveness of laboratory shielding, testing the long-term gain stability of C 6 D 6 liquid scintillation detectors, and deducing neutron elastic and inelastic scattering cross sections for 12 C. Finally, results of these student projects are presented that indicate the pit below the scattering area reduces background by as much as 30%; the detectors show no significant gain instabilities; and new insights into existing 12 C neutron inelastic scattering cross-section discrepancies near a neutron energy of 6.0 MeV are obtained.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Future directions for spectroscopy at the Spallation Neutron Source

Inelastic neutron scattering (spectroscopy) is in high demand to further our understanding of energy and condensed matter. Furthermore, this article highlights some of the developments, which may be anticipated for the coming decade and beyond, of the inelastic instrument suite at the Spallation Neutron Source in Oak Ridge.

47 OTHER INSTRUMENTATION↗
DOE OSTI2017

New measurement of the lifetime of the 2 + 1 state of 12 Be

The lifetime of the $2_1^+$ state at 2.1 MeV in $^{12}$Be has been measured using inelastic scattering of a $^{12}$Be beam at 43 MeV/nucleon with a gold target. Through the Doppler shift attenuation method, the mean-life of the $2_1^+$ state has been determined as 2.5±0.7(stat)±0.3 (syst) ps, which gives a B(E2; $2_1^+$ → $0_{g.s.}^+$)) value of 4.9±1.3±0.5 in Weisskopf units. The result shows a large quadrupole strength in the ground state transition, providing further evidence on the disappearance of the N=8 magic number. The B(E2; $2_1^+$ → $0_{g.s.}^+$) value together with the deformation length measured by proton inelastic scattering yields a neutron quadrupole matrix element two times larger than those for $^{14}$C and ${16}$O.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

Miniband Transport in a Two-Dimensional Electron Gas with a Strong Periodic Unidirectional Potential Modulation

In this paper, we study the Bloch oscillations of a two-dimensional electron gas with a strong periodic potential-modulation and miniband transport along the field at low temperatures, assuming a free motion in the transverse direction. The dependence of the current on the field, the electron density, and the temperature is investigated by using a relaxation-time approximation for inelastic scattering. Moreover, for a fixed total scattering rate, the field dependence of the current is sensitive to the ratio of the elastic and inelastic scattering rates in contrast with the recent result of a multiband but otherwise similar model with a weak potential modulation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Application of multi-angle scattering maps to stepped surfaces

This study examines channeling, multiple scattering, and neutralization/re-ionization of ions scattered along the stepped Al(332) plane. Our experimental approach involves probing the surface with 1–2 keV He + and Ne + beams, and then systematically mapping the scattered ion fluxes over a large solid angle. This provides comprehensive ion channeling information over all directions, rather than along a few low-index azimuths, as is common practice in ion scattering spectroscopy. We first probe the surface with 2 keV He + at near-normal incidence, and then map the backscattered particle flux (both ions and neutrals) via time of flight (TOF) spectrometry. The features contained in these maps can be correlated with axial and inter-planar channeling effects, and are reproduced well via binary collision simulations. Sensitivity to the stepped surface topography is heightened considerably for oblique ion incidence in the forward-scattering direction. In this geometry, we used 2 keV Ne + to probe the surface and mapped the corresponding scattered fluxes of both single and multiply-charged ions. In both cases, the scattering intensity depends strongly on the precise trajectory taken along the surface, and is particularly sensitive to how extensively the incident ions interact with the step edges. We interpret the information contained in these maps by considering several mechanisms for charge transfer and double ion production. The formation of Ne ++ appears to be correlated with a previously observed inelastic mechanism that occurs when the collision apsis, R min , is less than 0.65 Å. In conclusion, this contributes to an energy loss of 48 ± 8 eV for Ne + undergoing single scattering; the R min threshold for this inelastic step coincides with the emergence of a distinct Ne ++ peak. Using the information gained from the maps, we propose methods for extending this approach to chemisorbed layers.

47 OTHER INSTRUMENTATION↗