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At least 343 records · Page 19

All sources NASA NTRS DOE OSTI

DOE OSTI2022

A directional microcrack damage theory for brittle solids based on continuous hyperplasticity

We present a novel microcrack-damage theory for brittle solids under compression. Instead of using internal variables like zeroth, second or fourth rank damage tensors, the state of material damage is represented by an internal function that encapsulates the information of direction, density and size of microcracks. Just like other internal variables, the evolution of this state function must obey the second law of thermodynamics for arbitrary loading paths. This is done by casting the model in the framework of continuous hyperplasticity and enforcing a non-negative dissipation rate functional. The proposed framework offers predictions on the continuous evolution of microcrack density and the induced material anisotropy along with the macroscopic stress-strain curves. The use of continuous damage function grants the model significantly enhanced resolution in characterizing the direction-dependent response of cracked solids compared to classical models that are based on damage tensors. Two scenarios are considered in developing the theory, one assumes frictionless cracks and the other incorporates friction between crack surfaces. The results highlight that inelasticity, pressure dependence, and loading-unloading hysteresis exhibited by brittle solids are natural consequences of frictional microcracks. The proposed theory offers a generic and versatile framework to upscale micromechanical processes operating at individual crack scale to explain the macroscopic behavior of cracked solids.

Materials Science↗

NASA NTRS1976

The size frequency distribution and rate of production of microcraters

The paper contends that lunar crater size distributions vary to a degree that cannot be explained by variations in lunar surface orientation of the crater detectors or changes in micrometeoroid flux. In an attempt to investigate this size frequency distribution, a flux of micrometeoroid particles producing 0.1 micron diameter craters of approximately 300/sq cm/steradian/yr was obtained. No anisotropy was observed in the 0.1 micron flux between the ecliptic plane and the normal in the direction of lunar north.

Morrison, D. A.↗

NASA NTRS1995

Hot gas in superclusters and microwave background distortions

Variations in the electron pressure can produce significant distortions of the cosmic microwave background radiation (CMBR) spectrum through the Sunyaev-Zel'dovich effect. In this paper, we compute the amplitude of these distortions in various cosmological scenarios using the results of hydrodynamical simulations combined with analytic theory. For reionized cold dark matter (CDM) with h = 0.5 probed with the OVRO experiment (Theta approximately = 0.1 deg), we find that the amplitude of these fluctuations is (Delta T/T)(sub rms) = 1 x 10(exp -5) (Omega(sub i)/0.06) if the experiment looks at a random part of the sky and 6 x 10(exp -6)(Omega(sub i)/(0.06) if bright areas are avoided. Here Omega is the density of ionized baryons in units of the critical density, rho(sub c) = 2.78 x 10(exp 11)h(exp 2)/cu Mpc. Primary fluctuations would generate (Delta T/T)(sub rms) approximately = 2 x 10(exp -6) for this model. These distortions could be a dominant source of CMBR anisotropics on small and intermediate angular scales. It is important to note that these signals are generated relatively recently, when nonlinear effects are improtant, and do not require early reionization. We also examine the contribution to anisotropies from the Doppler effect, in both the linear and nonlinear regimes. In this case, the bulk of the signal is generated before a redshift of 10, provided that the intercluster medium is ionized back to the surface of last scattering. On the smallest scales thermal effects are bigger, while, on larger scales the Doppler-induced fluctuations are greater with the exact boundary determined by the scenario. However, over a wide range of scales, the two signals are equal to within a factor of a few.

Persi, Fred M.↗

NASA NTRS1974

Wave transmission characteristics and anisotropy of canine carotid arteries

A method was developed to generate and record three types of small amplitude waves (pressure, torsion and axial) in the exposed carotid artery of anesthetized dogs. The pressure waves were studied with the aid of miniature pressure transducers; electro-optical tracking units monitored the axial and circumferential surface displacements. Results from 6 dogs are presented in the form of the phase velocities and attenuation of three types of waves. The data demonstrate incompatibility with an isotropic elastic model for the mechanical behavior of the artery. The measured damping appears to be primarily due to the viscoelastic properties of the vessel wall material.

Moritz, W. E.↗

DOE OSTI2023

Room temperature colossal superparamagnetic order in aminoferrocene–graphene molecular magnets

Intensive studies are published for graphene-based molecular magnets due to their remarkable electric, thermal, and mechanical properties. However, to date, most of all produced molecular magnets are ligand based and subject to challenges regarding the stability of the ligand(s). The lack of long-range coupling limits high operating temperature and leads to a short-range magnetic order. Herein, we introduce an aminoferrocene-based graphene system with room temperature superparamagnetic behavior in the long-range magnetic order that exhibits colossal magnetocrystalline anisotropy of 8 × 10 5 and 3 × 10 7 J/m 3 in aminoferrocene and graphene-based aminoferrocene, respectively. These values are comparable to and even two orders of magnitude larger than pure iron metal. Aminoferrocene [C 10 H 11 FeN] + is synthesized by an electrophilic substitution reaction. It was then reacted with graphene oxide that was prepared by the modified Hammers method. The phase structure and functionalization of surface groups were characterized and confirmed by XRD, FT-IR, and Raman spectroscopy. To model the behavior of the aminoferrocene between two sheets of hydroxylated graphene, we have used density functional theory by placing the aminoferrocene molecule between two highly ordered hydroxylated sheets and allowing the structure to relax. Here, the strong bowing of the isolated graphene sheets suggests that the charge transfer and resulting magnetization could be strongly influenced by pressure effects. In contrast to strategies based on ligands surface attachment, our present work that uses interlayer intercalated aminoferrocene opens routes for future molecular magnets as well as the design of qubit arrays and quantum systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

NASA NTRS1984

Measurements of mantle wave velocities and inversion for lateral heterogeneity and anisotropy. II - Analysis by the single-station method

In the present investigation, the single-station method reported by Brune et al. (1960) is utilized for an analysis of long-period Love(G) and Rayleigh(R) waves recorded on digital seismic networks. The analysis was conducted to study the lateral heterogeneity of surface wave velocities. The data set is examined, and a description is presented of the single-station method. Attention is given to an error analysis for velocity measurements, the estimation of geographical distribution of surface wave velocities, the global distribution of surface wave velocities, and the correlation between the surface wave velocities and the heat flow on the geoid. The conducted measurements and inversions of surface wave velocities are used as a basis to derive certain conclusions. It is found that the application of the single-station method to long-period surface waves recorded on digital networks makes it possible to reach an accuracy level comparable to great circle velocity measurements.

Nakanishi, I.↗

DOE OSTI2021

An FEM-Based Peridynamic Model for Failure Analysis of Unidirectional Fiber-Reinforced Laminates

To predict the mixed damage modes of unidirectional fiber-reinforced polymer (FRP) laminates under dynamic loading, an FEM-based peridynamic model is introduced in this paper. Based on its geometric structure and material composition, a long fiber lamina is considered a transversely isotropic medium as a result of homogenization at the meso-scale. The laminated structure is modeled by stacking surface mesh layers with arbitrary fiber angles along the thickness direction. The peridynamic bonds between Gauss points connect the separated elements. These bonds are classified as inner-layer bonds and inter-layer bonds. To represent the anisotropy of a laminate, the micro-elastic modulus of the inner-layer and inter-layer bonds is calculated from the anisotropic engineering material constants separately. To capture complex failure behaviors of laminate structures, an empirical damage model is proposed for the tension/ compression breakage of peridynamic bonds. This damage model can control the in-plane and delamination failure process. Finally, benchmark tests are conducted to validate the elastic response of laminates under dynamic loading. In terms of damage analysis, the proposed model can capture the complex damage modes and resistive force of laminate structures.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Concentric core-shell tracks and spectroscopic properties of SrTiO 3 under intense electronic excitation

A deeper understanding of the intrinsic link between the irradiation-induced microstructures and the corresponding spectroscopic properties is becoming increasingly attractive for the research fields of materials science, physics, and information technology. In this work, the structural damage response, spectroscopic features, and associated physical mechanisms of SrTiO 3 single crystals under swift heavy ion irradiation were comparatively analyzed using a combination of experimental and theoretical approaches. Corresponding to 0.11–5.00 MeV/u ion irradiation with electronic energy loss ranging from 4.0 to 29.3 keV/nm, the inelastic thermal spike calculations combined with molecular dynamics simulations are compared with the experimental observations, revealing the track fine structures (individual spherical defects with a disordered region, and discontinuous and continuous tracks consisting of an amorphous core and a disordered outer shell) and demonstrating the dominant effects of the deposition energy and lattice temperature on track damage formation and evolution; thus, two essential thresholds for defect formation of ~0.60 eV/atom and amorphous region formation of ~1.81 eV/atom were identified to better describe the concentric core-shell track structure. The enhanced nanohillock formation in the surface region is attributed to the combined action of kinetic and potential energy depositions, and the fluence dependence in regulating the hillock dimensions is also presented. The measured refractive index profiles along different crystal axes further indicate the anisotropy of irradiation-induced lattice expansion. With increasing ion fluence and damage level, the intrinsic bandgap gradually decreased, and the concentration of additional radiative recombination centers (1.81 eV, 2.31 eV, and 2.42 eV) accordingly increased in the SrTiO 3 crystal, providing the possibility of regulating related defects to achieve tunable/selective photon emission for more critical technological applications.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2021

Copper-Tantalum Metal Matrix Composites Consolidated from Powder Blends by Severe Plastic Deformation

We investigated the effectiveness of severe plastic deformation by equal channel angular extrusion (ECAE) for consolidation of metal powders into metal matrix composites. Equal volumes of copper (Cu) and tantalum (Ta) powders were consolidated at ambient temperature via different ECAE routes. Composites processed by ECAE routes 4E and 4Bc were also processed at 300 °C. The resulting materials were characterized by scanning electron microscopy (SEM) and compression testing. Processing by route 4Bc at 300 °C resulted in the highest compressive strength, lowest anisotropy, and least strain rate sensitivity. We conclude that the superior properties achieved by this route arise from mechanical bonding due to interlocking Cu and Ta phases as well as enhanced metallurgical bonds from contact of pristine metal surfaces when the material is sheared along orthogonal planes.

36 MATERIALS SCIENCE↗

NASA NTRS1992

Assessing spatial and seasonal variations in grasslands with spectral reflectances from a helicopter platform

The helicopter system data acquisition technique has shown to be a viable means of gathering surface data with spectral detail adequate for intersite, intrasite, and temporal characterizations and for assessing temporal and spatial variability throughout the FIFE 1987 IFCs. The successful employment of nadir measurements for grassland assessments is notable given the reflectance anisotropy (Middleton, 1992). Though only five sites were repetitively observed, the conclusions reached from this particular sample of sites agree well with assessments from other data sources (Sellars et al., 1990 and Kittel et al., 1990).

Walthall, Charles L.↗

NASA NTRS1994

Velocity surface measurements for ZnO films over /001/-cut GaAs

A potential application for a piezoelectic film deposited on a GaAs substrate is the monolithic integration of surface acoustic wave (SAW) devices with GaAs electronics. Knowledge of the SAW properties of the filmed structure is critical for the optimum design of such devices. In this article, the measurements of the velocity surface, which directly affects the SAW diffraction, on the bare and metallized ZnO/SiO2 or Si3N4/GaAs /001/-cut samples are reported using two different techniques: (1) knife-edge laser probe, (2) line-focus-beam scanning acoustic microscope. Comparisons, such as measurement accuracy and tradeoffs, between the former (dry) and the latter (wet) method are given. It is found that near the group of zone axes (110) propagation direction the autocollimating SAW property of the bare GaAs changes into a noncollimating one for the layered structure, but a reversed phenomenon exists near the group of zone axes (100) direction. The passivation layer of SiO2 or Si3N4 (less than 0.2 micrometer thick) and the metallization layer change the relative velocity but do not significantly affect the velocity surface. On the other hand, the passivation layer reduces the propagation loss by 0.5-1.3 dB/microseconds at 240 MHz depending upon the ZnO film thickness. Our SAW propagation measurements agree well with theorectical calculations. We have also obtained the anisotropy factors for samples with ZnO films of 1.6, 2.8, and 4.0 micrometer thickness. Comparisons concerning the piezoelectric coupling and acoustic loss between dc triode and rf magnetron sputtered ZnO films are provided.

Kim, Yoonkee↗

NASA NTRS1986

Mechanical property anisotropy in superalloy El-929 directionally solidified by an exothermic technique

Directional solidification (DS) of the nickel-based superalloy El-929 was carried out by employing the exothermic technique for preparing several 150 mm long x 55 mm diameter rods. Specimens machined from blanks cut at 0, 45, 75 and 90 deg to the chill surface were tensile and stress-rupture tested at different temperatures. The air-melted DS alloy, when loaded parallel to the growth direction, shows considerable improvement in stress-rupture life and tensile ductility as compared with the vacuum induction melted, forged and heat-treated alloy. However, these property advantages rapidly degrade with the increasing deviation of the load axis from the growth direction.

Pradhan, D. C.↗

NASA NTRS2012

The Exact Solution for Linear Thermoelastic Axisymmetric Deformations of Generally Laminated Circular Cylindrical Shells

A detailed exact solution is presented for laminated-composite circular cylinders with general wall construction and that undergo axisymmetric deformations. The overall solution is formulated in a general, systematic way and is based on the solution of a single fourth-order, nonhomogeneous ordinary differential equation with constant coefficients in which the radial displacement is the dependent variable. Moreover, the effects of general anisotropy are included and positive-definiteness of the strain energy is used to define uniquely the form of the basis functions spanning the solution space of the ordinary differential equation. Loading conditions are considered that include axisymmetric edge loads, surface tractions, and temperature fields. Likewise, all possible axisymmetric boundary conditions are considered. Results are presented for five examples that demonstrate a wide range of behavior for specially orthotropic and fully anisotropic cylinders.

Nemeth, Michael P.↗

NASA NTRS1993

Assessment of damage in ceramics and ceramic matrix composites using ultrasonic techniques

This paper addresses the application of ultrasonic sensing to damage assessment in ceramics and ceramic matrix composites. It focuses on damage caused by thermal shock or oxidation at elevated temperatures, which often results in elastic anisotropy. This damaged-induced anisotropy is determined by measuring the velocities of ultrasonic waves in different propagation directions. Thermal shock damage is assessed in ceramic samples of reaction bonded silicon nitride (RBSN). Thermal shock treatment from different temperatures up to 1000 C is applied to produce the microcracks. Results indicate that most microcracks produced by thermal shock are located near sample surfaces. Ultrasonic measurements using the surface wave method are found to correlate well with measurements of degradation of mechanical properties obtained independently by other authors using destructive methods. Oxidation damage is assessed in silicon carbide fiber/reaction bonded silicon nitride matrix (SCS-6/RBSN) composites. The oxidation is done by exposing the samples in a flowing oxygen environment at elevated temperatures, up to 1400 C, for 100 hr. The Youngs' modulus in the fiber direction as obtained from ultrasonic measurements decreases significantly at 600 C but retains its original value at temperatures above 1200 C. This agrees well with the results of destructive tests by other authors. On the other hand, the transverse moduli obtained from ultrasonic measurements decrease continually until 1200 C. Measurements on the shear stiffnesses show behavior similar to the transverse moduli. The results of this work show that the damage-induced anisotropy in both ceramics and ceramic matrix composites can be determined successfully by ultrasonic methods. This suggests the possibility of assessing damage severity using ultrasonic techniques.

Chu, Y. C.↗

DOE OSTI2025

Monte Carlo Simulations of Polarized Radiative Transfer in Neutron Star Atmospheres

Soft X-ray emission from neutron stars affords powerful diagnostic tools for uncovering their surface and interior properties, as well as their geometric configurations. In the atmospheres of neutron stars, the presence of magnetic fields alters the photon-electron scattering cross sections, resulting in nontrivial angular dependence of intensity and polarization of the emergent signals. This paper presents recent developments of our Monte Carlo simulation, MAGTHOMSCATT, which tracks the complex electric field vector for each photon during its transport. Our analysis encompasses the anisotropy and polarization characteristics of X-ray emission for field strengths ranging from nonmagnetic to extremely magnetized regimes that are germane to magnetars. In the very low field domain, we reproduced the numerical solution to the radiative transfer equation for nonmagnetic Thomson scattering, and provided analytical fits for the angular dependence of the intensity and the polarization degree. These fits can be useful for studies of millisecond pulsars and magnetic white dwarfs. By implementing a refined injection protocol, we show that, in the magnetar regime, the simulated intensity and polarization pulse profiles of emission from extended surface regions becomes invariant with respect to the ratio of photon (ω) and electron cyclotron (ω B ) frequencies once ω/ω B ≲ 0.01. This circumvents the need for simulations pertinent to really high magnetic field strengths, which are inherently slower. Our approach will be employed elsewhere to model observational data to constrain neutron star geometric parameters and properties of emitting hot spots on their surfaces.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1974

Improving impact resistance of ceramic materials by energy absorbing surface layers

Energy absorbing surface layers were used to improve the impact resistance of silicon nitride and silicon carbide ceramics. Low elastic modulus materials were used. In some cases, the low elastic modulus was achieved using materials that form localized microcracks as a result of thermal expansion anisotropy, thermal expansion differences between phases, or phase transformations. In other cases, semi-vitreous or vitreous materials were used. Substantial improvements in impact resistance were observed at room and elevated temperatures.

Kirchner, H. P.↗

DOE OSTI2024

In-plane anisotropic magnetoresistance in detwinned BaFe 2-x Ni x As 2 (x = 0, 0.6)

Understanding the magnetoresistance (MR) of a magnetic material forms the basis for uncovering the orbital mechanisms and charge-spin interactions in the system. Although the parent state of iron-based high-temperature superconductors, including BaFe 2 As 2 , exhibits unusual electron transport properties resulting from spin and charge correlations, there is still valuable insight to be gained by understanding the in-plane MR effect due to twin domains in the orthorhombic antiferromagnetic ordered state. Here, we study the in-plane magnetoresistance anisotropy in detwinned BaFe 2 As 2 and compare the results to the nonmagnetic Ni-doped sample. We find that in the antiferromagnetically ordered state, BaFe 2 As 2 exhibits anisotropic MR that becomes large at low temperatures and high fields. Both transverse and longitudinal MRs are highly anisotropic and dependent on the field and current orientations. Furthermore, these results cannot be fully explained by calculations considering only the anisotropic Fermi surface. Instead, the spin orientation of the ordered moment also affects the MR effect, suggesting the presence of a large charge-spin interaction in BaFe 2 As 2 that is not present in the Ni-doped material.

36 MATERIALS SCIENCE↗

NASA NTRS2012

Comparison of PARASOL Observations with Polarized Reflectances Simulated Using Different Ice Habit Mixtures

Insufficient knowledge of the habit distribution and the degree of surface roughness of ice crystals within ice clouds is a source of uncertainty in the forward light scattering and radiative transfer simulations required in downstream applications involving these clouds. The widely used MODerate Resolution Imaging Spectroradiometer (MODIS) Collection 5 ice microphysical model assumes a mixture of various ice crystal shapes with smooth-facets except aggregates of columns for which a moderately rough condition is assumed. When compared with PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar) polarized reflection data, simulations of polarized reflectance using smooth particles show a poor fit to the measurements, whereas very rough-faceted particles provide an improved fit to the polarized reflectance. In this study a new microphysical model based on a mixture of 9 different ice crystal habits with severely roughened facets is developed. Simulated polarized reflectance using the new ice habit distribution is calculated using a vector adding-doubling radiative transfer model, and the simulations closely agree with the polarized reflectance observed by PARASOL. The new general habit mixture is also tested using a spherical albedo differences analysis, and surface roughening is found to improve the consistency of multi-angular observations. It is suggested that an ice model incorporating an ensemble of different habits with severely roughened surfaces would potentially be an adequate choice for global ice cloud retrievals.

Cole, Benjamin H.↗