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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,783 records · Page 99
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

Nonlinear crack analysis with finite elements

The application of finite element techniques to the analytic representation of the nonlinear behavior of arbitrary two-dimensional bodies containing cracks is discussed. Specific methods are proposed using which it should be possible to obtain information concerning: the description of the maximum, minimum, and residual near-tip stress and strain fields; the effects of crack closure on the near-tip behavior of stress and strain fields during cyclic loading into the plastic range; the stress-strain and displacement field behavior associated with a nonstationary crack; and the effects of large rotation near the crack tip.

Armen, H., Jr.↗
NASA NTRS1978

Environmental effects on composites for aircraft

A number of ongoing, long-term environmental effects programs for composite materials are evaluated. The flight service experience was evaluated for 142 composite aircraft components after more than 5 years and 1 million successful component flight hours. Ground-based outdoor exposures of composite material coupons after 3 years of exposure at 5 sites have reached equilibrium levels of moisture pickup which are predictable. Solar ultraviolet-induced material loss is discussed for these same exposures. No significant degradation was observed in residual strength for either stressed or unstressed specimens, or for exposures to aviation fuels and fluids.

Pride, R. A.↗
NASA NTRS1978

Interim results of long-term environmental exposures of advanced composites for aircraft applications

Interim results from a number of ongoing, long-term environmental effects programs for composite materials are reported. The flight service experience is evaluated for 142 composite aircraft components after more than five years and one million successful component flight hours. Ground-based outdoor exposures of composite material coupons after 3 years of exposure at five sites have reached equilibrium levels of moisture pickup which are predictable. Solar ultraviolet-induced material loss is discussed for these same exposures. No significant degradation has been observed in residual strength for either stressed or unstressed specimens, or for exposures to aviation fuels and fluids.

Pride, R. A.↗
NASA NTRS1981

Characterization of metal-polymer boundaries using Stoneley and Leaky waves

Optical and acoustooptical techniques for bonded surface inspection and interface wave detection are discussed. Interface wave propagation models based on viscous, elastic, and viscoelastic mechanical properties are reviewed. Gross boundary anomalies such as large defects and localized disbond regions were measured using conventional high frequency pulse echo and pitch catch ultratechniques. Bond durabilty is estimated by comparing detected wave attenuation versus frequency as a function of prebond surface preparation with the experimental limit obtained for perfect welded contact. Sensitivity of interface acoustic field periodicity to both residual and dc applied stress at the boundary surface is observed as well as hysteresis due to viscoelastic polymeric behavior as the applied stress is cycled.

Claus, R. O.↗
NASA NTRS1984

The slab geometry laser. I - Theory

Slab geometry solid-state lasers offer significant performance improvements over conventional rod-geometry lasers. A detailed theoretical description of the thermal, stress, and beam-propagation characteristics of a slab laser is presented. The analysis includes consideration of the effects of the zig-zag optical path, which eliminates thermal and stress focusing and reduces residual birefringence.

Eggleston, J. M.↗
NASA NTRS1991

Local delamination in laminates with angle ply matrix cracks. Part 2: Delamination fracture analysis and fatigue characterization

Constant amplitude tension-tension fatigue tests were conducted on AS4/3501-6 graphite/epoxy (02/ theta sub 2/ -(theta sub 2))sub s laminates, where theta was 15, 20, 25, or 30 degrees. Fatigue tests were conducted at a frequency of 5 Hz and an R-ratio of 0.1. Dye penetrant enhanced x-radiography was used to document the onset of matrix cracking in the central -(theta) degree plies, and the subsequent onset of local delaminations in the theta/ -(theta) interface at the intersection of the matrix cracks and the free edge, as a function of the number of fatigue cycles. Two strain energy release rate solutions for local delamination from matrix cracks were derived: one for a local delamination growing from an angle ply matrix crack with a uniform delamination growing from an angle ply matrix crack with a triangular shaped delamination area that extended only partially into the laminate width from the free edge. Plots of G(max) vs. N were generated to assess the accuracy of these G solutions. The influence of residual thermal and moisture stresses on G were also quantified. However, a detailed analysis of the G components and a mixed-mode fatigue failure criterion for this material may be needed to predict the fatigue behavior of these laminates.

Obrien, T. Kevin↗
NASA NTRS1990

Residual strength of repaired graphite/epoxy laminates after 5 years of outdoor exposure

The NASA Langley Research Center has sponsored research to develop generic repair techniques and processes for advanced graphite/epoxy (Gr/Ep) composites applicable to secondary structures for commercial transport aircraft. The long-term durability of such repairs is being addressed in a 10-year outdoor exposure program at the Langley Research Center. Details of the program and results of residual strength tests after 5 years of outdoor exposure are presented. Four repair methods are being evaluated. These include: (1) externally bolted aluminum-plus adhesive; (2) precured, bonded external Gr/Ep; (3) cure-in-place external Gr/Ep; and (4) cure-in-place flush Gr/Ep. Repaired specimens as well as undamaged and damaged unrepaired controls are being exposed outdoors for 1, 3, 5, 7, and 10 years. The residual tensile strength of stressed, unstressed, and fatigue specimens from each group is reported and compared with the tensile strength of baseline specimens which received no outdoor exposure. Identification of the commercial products and companies is used to describe adequately the test materials. The identification of these commercial products does not constitute endorsement, expressed or implied, of such products by the National Aeronautics and Space Administration.

Deaton, Jerry W.↗
NASA NTRS2003

Probabilistic Evaluation of Blade Impact Damage

The response to high velocity impact of a composite blade is probabilistically evaluated. The evaluation is focused on quantifying probabilistically the effects of uncertainties (scatter) in the variables that describe the impact, the blade make-up (geometry and material), the blade response (displacements, strains, stresses, frequencies), the blade residual strength after impact, and the blade damage tolerance. The results of probabilistic evaluations results are in terms of probability cumulative distribution functions and probabilistic sensitivities. Results show that the blade has relatively low damage tolerance at 0.999 probability of structural failure and substantial at 0.01 probability.

Chamis, C. C.↗
DOE OSTI2020

In situ measurement and modelling of the growth and length scale of twins in α -uranium

Twinning is an important deformation mechanism in crystalline materials. Twins nucleate, then grow, forming a discrete pattern of deformation bands with a certain thickness. The mechanism for twin growth is not completely understood. We present a simple phase field model which captures the physics of twin growth and reproduces the details observed during in situ electron backscatter diffraction experiments on α-uranium. The interaction between residual dislocations at twin interfaces and mobile dislocations in untwinned regions increases the stress needed for twin growth. This is described by a nonlocal term in the proposed constitutive equations: the nucleation stress of a twin increases proportionally to the twin phase field in a neighborhood. Competition between slip and twinning favors the nucleation of a new twin rather than the growth of a preexisting twin. The phase field model is coupled with a crystal plasticity finite element solver that includes the plastic deformation due to twinning. Simulations are able to reproduce the number of twins and their thicknesses as a function of the strain observed during in situ electron backscatter diffraction experiments. The stress-strain curve is also reproduced. This comparison allows the values of the nucleation stress and the interaction strength between residual and mobile dislocations to be found. It also gives an estimation of the density of residual dislocations that is consistent with the observed twin thickness. Finally, this model can be applied to understand microstructural effects in materials using twinning as a strengthening mechanism.

36 MATERIALS SCIENCE↗
NASA NTRS1981

Elevated temperature mechanical properties and residual tensile properties of two cast superalloys and several nickel-base oxide dispersion strengthened alloys

The elevated temperature tensile, stress-rupture and creep properties and residual tensile properties after creep straining have been determined for two cast superalloys and several wrought Ni-16Cr-4Al-yttria oxide dispersion strengthened (ODS) alloys. The creep behavior of the ODS alloys is similar to that of previously studied ODS nickel alloys. In general, the longitudinal direction is stronger than the long transverse direction, and creep is at least partially due to a diffusional creep mechanism as dispersoid-free zones were observed after creep-rupture testing. The tensile properties of the nickel-base superalloy B-1900 and cobalt-base superalloy MAR-M509 are not degraded by prior elevated temperature creep straining (at least up to 1 pct) between 1144 and 1366 K. On the other hand, the room temperature tensile properties of ODS nickel-base alloys can be reduced by prior creep strains of 0.5 pct or less between 1144 and 1477 K, with the long transverse direction being more susceptible to degradation than the longitudinal direction.

Whittenberger, J. D.↗
DOE OSTI2023

Residual strain orientation in rolled titanium determined with synchrotron X-ray Laue microdiffraction

Previously, synchrotron X-ray Laue microdiffraction has been used to measure the magnitudes of residual strain in materials. Recently the method was advanced to determine the orientation of the strain ellipsoid and applied to naturally deformed quartzites; however, the deformation history of these quartzites is ambiguous due to their natural origin. Here, in this study, synchrotron X-ray Laue microdiffraction (µXRD) is used to measure the residual strain for the first time in a sample with known stress history, rolled titanium. A deviatoric strain tensor is calculated from each Laue diffraction image collected with two µXRD scans of a rolled titanium sheet in different sample orientations. The principal strain axes are calculated using an eigen decomposition of the deviatoric strain tensors. The results show that the principal axis of compression is aligned with the normal direction of the titanium sheet, and the principal axis of extension is aligned with the rolling direction. Pole figures are used to represent the 3D distribution of residual strain axes.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Staphylococcus aureus CymR Is a New Thiol-based Oxidation-sensing Regulator of Stress Resistance and Oxidative Response

As a human pathogen, Staphylococcus aureus must cope with oxidative stress generated by the human immune system. Here, we report that CymR utilizes its sole Cys-25 to sense oxidative stress. Oxidation followed by thiolation of this cysteine residue leads to dissociation of CymR from its cognate promoter DNA. In contrast, the DNA binding of the CymRC25S mutant was insensitive to oxidation and thiolation, suggesting that CymR senses oxidative stress through oxidation of its sole cysteine to form a mixed disulfide with low molecular weight thiols. The determined crystal structures of the reduced and oxidized forms of CymR revealed that Cys-25 is oxidized to Cys-25-SOH in the presence of H{sub 2}O{sub 2}. Deletion of cymR reduced the resistance of S. aureus to oxidative stresses, and the resistance was restored by expressing a C25S mutant copy of cymR. In a C25S substitution mutant, the expression of two genes, tcyP and mccB, was constitutively repressed and did not respond to hydrogen peroxide stress, whereas the expression of the genes were highly induced under oxidative stress in a wild-type strain, indicating the critical role of Cys-25 in redox signaling in vivo. Thus, CymR is another master regulator that senses oxidative stress and connects stress responses to virulence regulation in S. aureus.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2019

Relationship of non-invasive quantification of myocardial blood flow to arrhythmic events in patients with implantable cardiac defibrillators

Background: Ischemia contributes to arrhythmogenesis though its role is incompletely understood. Abnormal myocardial perfusion measured by PET imaging may predict ventricular arrhythmias (VAs) in a high-risk population. Methods: Patients with implantable cardiac defibrillators who had undergone rubidium-82 cardiac PET imaging were identified. Patients were stratified by median MBF and MFR values for analysis. The Cox proportional hazards model was used to assess the impact of myocardial perfusion on survival free of VT independent of critical covariates. Results: A total of 159 patients (124 (78%) males, median age 65.9 years, IQR [56.76-72.63]) were followed for 1.43 years IQR [0.83-2.21]. VA occurred in 29 patients (23.7%). After adjustment for ejection fraction, age, and sex, impaired stress MBF was associated with an increased risk of VA (adjusted HR per ml/min/g 1.52, 95% CI (1.01-2.31), P = 0.04). Summed rest and stress scores were not predictive of VA. Among patients with severe LV dysfunction, stress MBF remained an independent predictor of VA (adjusted HR per 1 ml/min/g HR 1.69, 95% CI (1.03-11.36), P = 0.03), while residual EF, summed rest, and summed stress scores were not (P > 0.05). Conclusions: Impaired stress myocardial blood flow was associated with less survival free of ventricular arrhythmias.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2021

Extracellular vesicles promote epithelial-to-mesenchymal transition of lens epithelial cells under oxidative stress

Posterior capsule opacification (PCO), resulting from residual lens epithelial cell (LEC) epithelial–mesenchymal transition (EMT), abnormal proliferation, and migration, is the most common complication of cataract surgery. A recent study determined that extracellular vesicles (EVs) and reactive oxygen species (ROS) regulate the EMT process during cutaneous wound healing and tumour metastasis. However, their underlying mechanism in PCO is unclear. In this study, we examined the secreted EVs from a scratch model in vitro. We found that the production of ROS was increased after mechanical injury, especially at the wound edge, and there was an increased viability of LECs, which can be blocked by diphenyleneiodonium, an NADPH oxidase inhibitor. Cell viability and migration were increased upon treatment with 1 μM H{sub 2}O{sub 2}, but significantly reduced when the concentration of H{sub 2}O{sub 2} increased to 100 μM. Transwell assay showed that both post-surgery LECs and LECs treated with 1 μM H{sub 2}O{sub 2} significantly induced the migration of normal LECs by EV secretion. Extraction and quantification of EVs derived from injured and H{sub 2}O{sub 2}-treated LECs showed a similar increase in production. Co-incubation of EVs from both injured and H{sub 2}O{sub 2}-treated LECs with normal LECs and organ-cultured mouse lenses activated EMT, which was attenuated by a ROS inhibitor. These results suggest that EVs participate in ROS-induced lens EMT, making EVs a potential target for treating PCO.

60 APPLIED LIFE SCIENCES↗
NASA NTRS2004

Stressed Oxidation Life Prediction for C/SiC Composites

The residual strength and life of C/SiC is dominated by carbon interface and fiber oxidation if seal coat and matrix cracks are open to allow oxygen ingress. Crack opening is determined by the combination of thermal, mechanical and thermal expansion mismatch induced stresses. When cracks are open, life can be predicted by simple oxidation based models with reaction controlled kinetics at low temperature, and by gas phase diffusion controlled kinetics at high temperatures. Key life governing variables in these models include temperature, stress, initial strength, oxygen partial pressure, and total pressure. These models are described in this paper.

Levine, Stanley R.↗
DOE OSTI2020

Experimental investigation on the effects of fabric architectures on mechanical and damage behaviors of carbon/epoxy woven composites

The mechanical behaviors and damage evolutions of carbon/epoxy woven fabric composites with three different geometries, i.e., one plain weave and two twill weave patterns with different areal densities, are studied under tensile loading. The effects of weave patterns on mechanical properties are investigated by monotonic and cyclic tension tests. Remarkable variations in stress–strain curve, Poisson’s ratio, residual strain and strain map exist in the three composites. Crimp ratio is found to be a critical factor to govern the mechanical properties. With smaller crimp ratio, a quasi-linear stress–strain curve with higher elastic modulus and strength is observed. The stress–strain curves of composites with higher crimp ratio contain transition stages with significant tangent modulus degradation. Elastic modulus, strength and damage initiation are all correlated with the crimp ratio linearly regardless of the fabric pattern. Dramatic nonlinear evolution in Poisson’s ratio occurs in the composite with higher crimp ratio. Cyclic tension results indicate that the residual strain is a more appropriate damage indicator than the unloading elastic modulus. Microstructure examination shows that damage developments are essentially related to the fabric geometry, and result in various mechanical behaviors. This work provides important insights into the geometry-deformation mechanism-mechanical property relationship of the woven composites.

36 MATERIALS SCIENCE↗
NASA NTRS1979

The effect of load sequence on the statistical fatigue of composites

A theoretical model to predict the effect of loading sequence on the statistical distributions of the fatigue life and the residual strength under n-stress levels of cyclic loading is derived on the basis of a three-parameter model. In particular, the dual stress fatigue cumulative damage is studied, showing that the Miner's sum (the cumulative damage sum at fatigue failure) is a statistical variable. It is proved theoretically that the Miner's sum is always greater than or equal to unity for the high-low load sequence, while it is always smaller than or equal to unity for the low-high load sequence, with the deviation from unity increasing as the difference between the high and the low stress levels increases. An experimental test program using graphite/epoxy angle-ply laminates, carried out to generate statistically meaningful data for verifying the proposed model, is discussed.

Yang, J. N.↗
NASA NTRS1982

The mechanics of fatigue in composite laminates

A status report is presented for the development of the mechanics of fatigue in composite materials. A concensus is noted on the need to construct mechanics treatments on the basis of precisely defined damage states which are both characteristic of the material and independent of as many extensive variables, such as load history, as possible. This approach is in keeping with the use of a single crack as the characteristic damage state in homogeneous materials, which has provided a well defined and precise boundary value problem that can be used in the description and prediction of damaged material response. The utility of stress redistribution concepts in determining residual strength, stiffness, and life in composite laminates is also recognized.

Reifsnider, K. L.↗