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At least 811 records · Page 45

All sources NASA NTRS DOE OSTI

NASA NTRS2015

Effects of Unpredictable Variable Prenatal Stress (UVPS) on Bdnf DNA Methylation and Telomere Length in the Adult Rat Brain

In utero exposure to stress can shape neurobiological and behavioral outcomes in offspring, producing vulnerability to psychopathology later in life. Animal models of prenatal stress likewise have demonstrated long-‐term alterations in brain function and behavioral deficits in offspring. For example, using a rodent model of unpredictable variable prenatal stress (UVPS), in which dams are exposed to unpredictable, variable stress across pregnancy, we have found increased body weight and anxiety-‐like behavior in adult male, but not female, offspring. DNA methylation (addition of methyl groups to cytosines which normally represses gene transcription) and changes in telomere length (TTAGGG repeats on the ends of chromosomes) are two molecular modifications that result from stress and could be responsible for the long-‐term effects of UVPS. Here, we measured methylation of brain-‐derived neurotrophic factor (bdnf), a gene important in development and plasticity, and telomere length in the brains of adult offspring from the UVPS model. Results indicate that prenatally stressed adult males have greater methylation in the medial prefrontal cortex (mPFC) compared to non-‐stressed controls, while females have greater methylation in the ventral hippocampus compared to controls. Further, prenatally stressed males had shorter telomeres than controls in the mPFC. These findings demonstrate the ability of UVPS to produce epigenetic alterations and changes in telomere length across behaviorally-‐relevant brain regions, which may have linkages to the phenotypic outcomes.

epigenetic developmental programming↗

DOE OSTI2020

Deformation behavior of nanoscale Al–Al 2 Cu eutectics studied by in situ micropillar compression

We report deformation behavior of nanoscale laser processed Al–Al 2 Cu eutectics at room temperature is characterized through in situ micro-pillar compression testing in a scanning transmission microscope. Interlamellar spacing of Al–Al 2 Cu eutectics varies from hundreds of nanometers to 20 nm. Three different sizes of micro-pillars are fabricated in order to study the deformation behaviors of single colony and multiple colonies, corresponding to the single crystal and polycrystal respectively. For single colonies, lamellar orientations parallel, normal or inclined to the loading direction were tested. The main findings are: 1) the plasticity mechanisms strongly depend on loading orientation: buckling and kinking in the parallel-loaded eutectics, planar sliding along Al–Al 2 Cu lamellar interfaces in the incline-loaded eutectics and localized shearing in the normal-loaded eutectics. 2) the incline-loaded eutectics exhibits the lowest compression flow strength, and the normal-loaded eutectics has the highest compression flow strength. 3) with decreasing inter-lamellar spacing, the strength increases and plasticity is uniformly distributed, as opposed to shear localization. Highest compressive plasticity is observed in polycrystalline eutectics with an ensemble of lamellar orientations with ~20 nm average spacing: 17.9% at flow stress of 1.63 GPa, and degenerate, bimodal morphology: 11.1% at flow stress of 1.36 GPa.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Magnetotransport of Sm 2 Ir 2 O 7 across the pressure-induced quantum-critical phase boundary

Rare-earth pyrochlore iridates host two interlocking magnetic sublattices of corner-sharing tetrahedra and can harbour a unique combination of frustrated moments, exotic excitations and highly correlated electrons. They are also the first systems predicted to display both topological Weyl semimetal and axion insulator phases. We have measured the transport and magnetotransport properties of single-crystal Sm 2 Ir 2 O 7 up to and beyond the pressure-induced quantum critical point for all-in-all-out (AIAO) Ir order at p c = 63 kbar previously identified by resonant X-ray scattering and close to which Weyl semimetallic behavior has been previously predicted. Our findings overturn the accepted expectation that the suppression of AIAO order should lead to metallic conduction persisting down to zero temperature. Instead, the resistivity-minimum temperature, which tracks the decrease in the AIAO ordering temperature for pressures up to 30 kbar, begins to increase under further application of pressure, pointing to the presence of a second as-yet unidentified mechanism leading to non-metallic behavior. The magnetotransport does track the suppression of Ir magnetism, however, with a strong hysteresis observed only within the AIAO phase boundary, similar to that found for Ho 2 Ir 2 O 7 and attributed to plastic deformation of Ir domains. Around pc we find the emergence of a new type of electronic phase, characterized by a negative magnetoresistance with small hysteresis at the lowest temperatures, and hysteresis-free positive magnetoresistance above approximately 5K. The temperature dependence of our low-temperature transport data are found to be best described by a model consistent with a Weyl semimetal across the entire pressure range.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Magnetotransport of Sm 2 Ir 2 O 7 across the pressure-induced quantum-critical phase boundary

Rare-earth pyrochlore iridates host two interlocking magnetic sublattices of corner-sharing tetrahedra and can harbour a unique combination of frustrated moments, exotic excitations and highly correlated electrons. They are also the first systems predicted to display both topological Weyl semimetal and axion insulator phases. We have measured the transport and magnetotransport properties of single-crystal Sm 2 Ir 2 O 7 up to and beyond the pressure-induced quantum critical point for all-in-all-out (AIAO) Ir order at $p_c$ = 63 kbar previously identified by resonant X-ray scattering and close to which Weyl semimetallic behavior has been previously predicted. Our findings overturn the accepted expectation that the suppression of AIAO order should lead to metallic conduction persisting down to zero temperature. Instead, the resistivity-minimum temperature, which tracks the decrease in the AIAO ordering temperature for pressures up to 30 kbar, begins to increase under further application of pressure, pointing to the presence of a second as-yet unidentified mechanism leading to non-metallic behavior. The magnetotransport does track the suppression of Ir magnetism, however, with a strong hysteresis observed only within the AIAO phase boundary, similar to that found for Ho 2 Ir 2 O 7 and attributed to plastic deformation of Ir domains. Around $p_c$ we find the emergence of a new type of electronic phase, characterized by a negative magnetoresistance with small hysteresis at the lowest temperatures, and hysteresis-free positive magnetoresistance above approximately 5 K. The temperature dependence of our low-temperature transport data are found to be best described by a model consistent with a Weyl semimetal across the entire pressure range.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Shear strain gradient in Cu/Nb nanolaminates: Strain accommodation and chemical mixing

Disentangling the intragranular and interfacial plasticity contribution to the overall strain accommodation is crucial to understanding the microstructural evolution and mass transport upon deformation in materials with the nanoscale feature size. In this study, we devise a new approach to tackle the issue by introducing shear strain gradients into Cu/Nb nanolaminates of different layer thicknesses with the shear perpendicular to the laminate interfaces. The measurement of the strain gradient and the resultant lattice disorientation enables a quantitative understanding of the intragranular and interfacial plasticity contribution. We found that intragranular slip entirely governs the deformation in the 300 nm-layer laminate and, unexpectedly, contributes ~80% of the total plasticity in the 30 nm-layer laminate. The high intragranular plasticity in the thin laminate is attributed to the large width of confined slip planes and their remnant potential for storing dislocations. In addition, substantial forced chemical mixing is observed in the top region of the 30 nm-layer laminate where the effective layer thickness is reduced below 8 nm. The transition of deformation mechanism from confined layer slip to dislocation transmission is largely responsible for the initiation of substantial mixing. Our method and findings shed light on the deformation mechanism and deformation-induced mass transport behavior in nanostructured materials.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Heterogeneous microstructural evolution during hydrodynamic penetration of a high-velocity copper microparticle impacting copper

Microparticle hydrodynamic penetration (HDP) may be associated with the erosion regime in cold spray processing and other high-velocity impact events. Here, in an experimental approach where we can individually launch particles and study the impact sites, we explore copper microparticles impacted on copper substrates at velocities above 900 m/s where HDP begins. We lift cross-sectional lamellae from the impact sites with a focused-ion beam for further microstructural characterization using electron backscatter diffraction and scanning transmission electron microscopy. Due to the gradients of strain, strain rate, and temperature associated with HDP, heterogeneous microstructures result. The structural evolution processes observed include deformation twinning and multiple dislocation-mediated grain recrystallization mechanisms—geometric dynamic recrystallization (gDRX), discontinuous DRX (dDRX), and meta DRX (mDRX). The higher strains at the interface lead to the most significant structural changes and complex mechanisms. In contrast, there is a gradient to more conventional dislocation plasticity away from the interface (on either the particle or substrate side). Here, these microstructural observations are consistent with the deformation map for copper and extend the observations of impact-induced recrystallization across new regimes of behavior.

Cold spray process↗

DOE OSTI2022

Characterization of Tri-lab β-Tin (Sn)

This report documents details of the microstructure and mechanical properties of -tin (Sn), that is used in the Tri-lab (Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratory (LLNL), Sandia National Laboratories (SNL)) collaboration project on Multi-phase Tin Strength. We report microstructural features detailing the crystallographic texture and grain morphology of as-received -tin from electron back scatter diffraction (EBSD). Temperature and strain rate dependent mechanical behavior was investigated by multiple compression tests at temperatures of 200K to 400K and strain rates of 0.0001 /s to 100 /s. Tri-lab tin showed significant temperature and strain rate dependent strength with no significant plastic anisotropy. A sample to sample material variation was observed from duplicate compression tests and texture measurements. Compression data was used to calibrate model parameters for temperature and rate dependent strength models, Johnson-Cook (JC), Zerilli-Armstrong (ZA) and Preston-Tonks-Wallace (PTW) strength models.

36 MATERIALS SCIENCE↗

NASA NTRS1986

Damping characteristics of damaged fiber composite components

Defects in fiber composite components produce changes with respect to the vibrational characteristics of the material. These changes can be recognized in the form of a frequency shift or an alteration of the damping process. The present investigation is concerned with questions regarding the possibility of a utilization of the changes in suitable defect-detecting inspection procedures. A description is given of a method for measuring the damping characteristics of a specimen. This method provides a spectrum of the damping coefficients of the sample as a basis for a comprehensive evaluation of the damping behavior. The correlation between defects and change in the damping characteristics is demonstrated with the aid of results obtained in measurements involving specimens of carbon-fiber composites and a component consisting of glass-fiber-reinforced plastics.

Eberle, K.↗

NASA NTRS1988

Relationship between fatigue life in the creep-fatigue region and stress-strain response

On the basis of mechanical tests and metallographic studies, strainrange partitioned lives were predicted by introducing stress-strain materials parameters into the Universal Slopes Equation. This was the result of correlating fatigue damage mechanisms and deformation mechanisms operating at elevated temperatures on the basis of observed mechanical and microstructural behavior. Correlation between high temperature fatigue and stress strain properties for nickel base superalloys and stainless steel substantiated the method. Parameters which must be evaluated for PP- and CC- life are the maximum stress achievable under entirely plastic and creep conditions respectively and corresponding inelastic strains, and the elastic modulus. For plasticity/creep interaction conditions (PC and CP) two more pairs of stress strain parameters must be ascertained.

Berkovits, A.↗

NASA NTRS1988

Relationship between fatigue life in the creep-fatigue region and stress-strain response

On the basis of mechanical tests and metallographic studies, strainrange partitioned lives were predicted by introducing stress-strain materials parameters into the Universal Slopes Equation. This was the result of correlating fatigue damage mechanisms and deformation mechanisms operating at elevated temperatures on the basis of observed mechanical and microstructural behavior. Correlation between high temperature fatigue and stress strain properties for nickel base superalloys and stainless steel substantiated the method. Parameters which must be evaluated for PP- and CC- life are the maximum stress achievable under entirely plastic and creep conditions respectively and corresponding inelastic strains, and the two more pairs of stress strain parameters must be ascertained.

Berkovits, A.↗

NASA NTRS2017

Grain Size-Dependent Crystal Plasticity Constitutive Model for Polycrystal Materials

A new method to introduce grain size-dependence in classical crystal plasticity constitutive model is developed by considering the resistance to dislocation motion in the grain boundary influence region as equivalent to that of a work hardening. A general framework for the size-dependent constitutive model is derived by implementing this method on a core and mantle model. The work hardening, equivalent to grain boundary effect, is realized by introducing a fictitious, pre-existing, plastic strain grain boundary influence region (mantle) following the principles of classical crystal plasticity. This fictitious plastic strain, in effect, increases the yield strength and decreases the initial hardening coefficient of the grain. With the thickness of grain boundary influence region and the distribution of introduced plastic strain remaining the same, the grain boundary effect increased as the grain size becomes smaller. A simplified model that considers the grain boundary effect on a grain average sense is also developed under this general framework. Implementation of this general framework to the specific cases of crystal plasticity constitutive models is demonstrated by considering the case of power law flow rule and hyperbolic-secant hardening rule. Finally, the grain size-dependent constitutive model is validated by comparing the predicted stress-strain behavior of polycrystal samples with different average grain sizes under uniaxial loading with the experimental results.

Grain Size↗

NASA NTRS1992

A micromechanics-based strength prediction methodology for notched metal matrix composites

An analytical micromechanics based strength prediction methodology was developed to predict failure of notched metal matrix composites. The stress-strain behavior and notched strength of two metal matrix composites, boron/aluminum (B/Al) and silicon-carbide/titanium (SCS-6/Ti-15-3), were predicted. The prediction methodology combines analytical techniques ranging from a three dimensional finite element analysis of a notched specimen to a micromechanical model of a single fiber. In the B/Al laminates, a fiber failure criteria based on the axial and shear stress in the fiber accurately predicted laminate failure for a variety of layups and notch-length to specimen-width ratios with both circular holes and sharp notches when matrix plasticity was included in the analysis. For the SCS-6/Ti-15-3 laminates, a fiber failure based on the axial stress in the fiber correlated well with experimental results for static and post fatigue residual strengths when fiber matrix debonding and matrix cracking were included in the analysis. The micromechanics based strength prediction methodology offers a direct approach to strength prediction by modeling behavior and damage on a constituent level, thus, explicitly including matrix nonlinearity, fiber matrix debonding, and matrix cracking.

Bigelow, C. A.↗

NASA NTRS1993

A micromechanics-based strength prediction methodology for notched metal-matrix composites

An analytical micromechanics-based strength prediction methodology was developed to predict failure of notched metal matrix composites. The stress-strain behavior and notched strength of two metal matrix composites, boron/aluminum (B/Al) and silicon-carbide/titanium (SCS-6/Ti-15-3), were predicted. The prediction methodology combines analytical techniques ranging from a three-dimensional finite element analysis of a notched specimen to a micromechanical model of a single fiber. In the B/Al laminates, a fiber failure criteria based on the axial and shear stress in the fiber accurately predicted laminate failure for a variety of layups and notch-length to specimen-width ratios with both circular holes and sharp notches when matrix plasticity was included in the analysis. For the SCS-6/Ti-15-3 laminates, a fiber failure based on the axial stress in the fiber correlated well with experimental results for static and postfatigue residual strengths when fiber matrix debonding and matrix cracking were included in the analysis. The micromechanics-based strength prediction methodology offers a direct approach to strength prediction by modeling behavior and damage on a constituent level, thus, explicitly including matrix nonlinearity, fiber matrix debonding, and matrix cracking.

Bigelow, C. A.↗

DOE OSTI2020

Modeling the role of local crystallographic correlations in microstructures of Ti-6Al-4V using a correlated structure visco-plastic self-consistent polycrystal plasticity formulation

This paper presents a multi-level crystal plasticity-based simulation framework for modeling mechanical response and microstructure evolution of Ti-6Al-4V with α-lath/lamellar microstructures. Here, the model is a correlated structure visco-plastic self-consistent (CS-VPSC) formulation linking three scales: a single crystals micro-scale, a lath/lamellar colony meso-scale, and a lath/lamellar aggregate macro-scale. A selected hardening law for the evolution of critical resolved shear stress per slip system used in CS-VPSC is phenomenological. However, it adjusts the resistances of basal and prismatic slip systems based on the geometry of slip transfer between adjacent lamellae. Consistent with experimental evidences, the resolved shear stress on the pyramidal slip planes is dependent not only on the stress in the direction of slip but also on the two orthogonal shear stress components and the three normal stress components (non-Schmid effects). Electron backscatter diffraction (EBSD) data in conjunction with a procedure relying on α→β phase transformation is used to construct paired variants of α-laths/lamellae satisfying their local crystallographic correlations. The procedure fits volume fractions of individual laths/lamellae with the experimental EBSD data and selects a distribution of habit planes between adjacent variants with respect to the loading direction. The simulation framework is applied to interpret the deformation behavior in tension and compression along two sample directions of Ti-6Al-4V fabricated via laser powder bed fusion. Moreover, the model is used to simulate texture evolution during rolling of the material to large strains. It is demonstrated that the model is capable of predicting plastic anisotropy/asymmetry and the concomitant texture evolution. While the model reveals a significant effect of habit plane inclination with respect to the loading direction on yield stress, the comparison of the data and model predictions shows that a random distribution of habit planes fits the flow response. It is further inferred that the tension-compression asymmetry arises from the non-Schmid effects.

36 MATERIALS SCIENCE↗

NASA NTRS1975

Anelastic deformation of boron fibers

The flexural deformation behavior of vapor-deposited boron fibers is investigated in the temperature range from 100 to 1100 K by stress-relaxation and internal friction methods. Strong thermally-activated anelasticity is uniquely observed, and no evidence of plasticity is found up to surface strains of 0.006. The parameters governing the relaxation processes within the anelastic spectra of untreated and annealed fibers are determined and correlated with the results of X-ray structural studies to develop preliminary models for the possible causes of boron's anelasticity. The large relaxation strengths of the dominant Ia processes together with their relaxation times and energies are shown to suggest a sliding mechanism between certain basic structural subunits common to both the beta-rhombohedral and vapor-deposited boron structures.

Dicarlo, J. A.↗

NASA NTRS1978

The role of thermal shock in cyclic oxidation

The effect of thermal shock on the spalling of oxides from the surfaces of several commercial alloys was determined. The average cooling rate was varied from approximately 240 C per second to less than 1.0 C per second during cyclic oxidation tests in air. The tests consisted of one hundred cycles of one hour at the maximum temperature (1100 or 1200 C). The alloys were HOS-875, TD-Ni, TD-NiCrAl, IN-601, IN-702, and B-1900 plus Hf. All of these alloys exhibited partial spalling within the oxide rather than total oxide loss down to bare metal. Thermal shock resulted in deformation of the metal which in turn resulted, in most cases, in changing the oxide failure mode from compressive to tensile. Tensile failures were characterized by cracking of the oxide and little loss, while compressive failures were characterized by explosive loss of platelets of oxide. This behavior was confirmed by examination of mechanically stressed oxide scales. The thermally shocked oxides spalled less than the slow cooled samples with the exception of TD-NiCrAl. This material failed in a brittle manner rather than by plastic deformation.

Lowell, C. E.↗

NASA NTRS1984

Plastic flow of plasma sprayed ceramics

The plastic flow of plasma-sprayed ZrO2-8Y2O3 ceramic has been measured at temperatures up to 1250 C and compared to the plastic flow of pressed and sintered ZrO2-8Y2O3. Plasma spraying of binary oxide ceramics is found to result in a metastable state which is inelastic at high temperature but can also be stabilized or devitrified through heat treating so as to decrease plastic properties. The mechanical properties of the as-plasma sprayed and devitrified ceramic sheet material was measured. An improved algorithm that incorporates the inherently nonlinear thermomechanical field equations was used to determine the influence of inelastic material behavior on the thermomechanical response of ceramic coated seal components. Significant creep was found during the thermal shock and steady heating periods with insufficient time during thermal quench to reverse the process, thereby inducing significant residual stresses into the components.

Padovan, J.↗

NASA NTRS2014

Customizing Countermeasure Prescriptions using Predictive Measures of Sensorimotor Adaptability

Astronauts experience sensorimotor disturbances during the initial exposure to microgravity and during the readapation phase following a return to a gravitational environment. These alterations may lead to disruption in the ability to perform mission critical functional tasks during and after these gravitational transitions. Astronauts show significant inter-subject variation in adaptive capability following gravitational transitions. The ability to predict the manner and degree to which each individual astronaut will be affected would improve the effectiveness of a countermeasure comprised of a training program designed to enhance sensorimotor adaptability. Due to this inherent individual variability we need to develop predictive measures of sensorimotor adaptability that will allow us to predict, before actual space flight, which crewmember will experience challenges in adaptive capacity. Thus, obtaining this information will allow us to design and implement better sensorimotor adaptability training countermeasures that will be customized for each crewmember's unique adaptive capabilities. Therefore the goals of this project are to: 1) develop a set of predictive measures capable of identifying individual differences in sensorimotor adaptability, and 2) use this information to design sensorimotor adaptability training countermeasures that are customized for each crewmember's individual sensorimotor adaptive characteristics. To achieve these goals we are currently pursuing the following specific aims: Aim 1: Determine whether behavioral metrics of individual sensory bias predict sensorimotor adaptability. For this aim, subjects perform tests that delineate individual sensory biases in tests of visual, vestibular, and proprioceptive function. Aim 2: Determine if individual capability for strategic and plastic-adaptive responses predicts sensorimotor adaptability. For this aim, each subject's strategic and plastic-adaptive motor learning abilities are assessed using a test of locomotor function designed specifically to delineate both mechanisms. Aim 3: Develop predictors of sensorimotor adaptability using brain structural and functional metrics. We will measure individual differences in regional brain volumes (structural MRI), white matter integrity (diffusion tensor imaging, or DTI), functional network integrity (resting state functional connectivity MRI), and sensorimotor adaptation task-related functional brain activation (functional MRI). We decided to complete the data collection for Specific Aims 1, 2 and 3 simultaneously on the same subjects to increase data capture. By having the same subjects perform all three specific aims we can enhance our ability to detect how a wider range of factors can predict adaptability in a specific individual. This provides a much richer database and potentially a better understanding of the predictive power of the selected factors. In this presentation I will discuss preliminary data obtained to date.

Bloomberg, J. J.↗