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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1971

Effects of loading sequence for notched specimens under high-low two-step fatigue loading

The effects of loading sequence on crack-initiation period were investigated for notched aluminum-alloy specimens under high-low two-step loading with special emphasis on local cyclic stresses and strains at the notch root. Local stress and strain were determined by a procedure based on an equation proposed by Neuber which relates elastoplastic stress and strain at a notch. Local stress and strain were also measured experimentally to verify the Neuber equation. The effects of initial high load on the crack-initiation periods were demonstrated with notched specimens and were simulated in unnotched specimens fatigue tested with local stress sequences. An analysis of the results indicated that sequence effects were not caused solely by local residual stresses, as is usually assumed; the existence of a damaging effect, resulting from the high local strain cycles, was demonstrated. The sequence effects observed with notched specimens were interpreted as the combined result of residual stresses and high local strain cycles.

Crews, J. H., Jr.↗
NASA NTRS1992

The rhodopsins: structure and function. Introduction

Nature makes use of the propensity of retinal for light-dependent double-bond isomerization in a number of systems and in a variety of ways. The common theme for light receptors based on this kind of chemistry is that (1) the retinal is bound in most cases to a small membrane protein via a protonated lysine-retinal Schiff base, (2) the absorption maximum in the visible is tuned to a suitable wavelength largely by electrostatic interaction with polar protein residues, and (3) the light-induced bond rotations and strains in the retinal set off reaction chains during which at least part of the excess free energy acquired is transferred to the protein and causes pK shifts of acidic residues and/or backbone conformational changes. The physiological consequence of the process initiated by absorption of light is either the activation of an information transfer chain (sensory and visual rhodopsins) or energy transduction which drives the electrogenic movement of ions across the membrane (ion-motive rhodopsins). Rhodopsins with these functions occur in bacteria and in higher organisms; from an evolutionary standpoint they are not related to one another. Nevertheless, all of these proteins are remarkably similar and form a distinct family.

NASA Discipline Number 52-30↗
NASA NTRS1988

Composite transport wing technology development

The design, fabrication, testing, and analysis of stiffened wing cover panels to assess damage tolerance criteria are discussed. The damage tolerance improvements were demonstrated in a test program using full-sized cover panel subcomponents. The panels utilized a hard skin concept with identical laminates of 44-percent 0-degree, 44-percent plus or minus 45-degree, and 12-percent 90-degree plies in the skins and stiffeners. The panel skins were impacted at midbay between the stiffeners, directly over the stiffener, and over the stiffener flange edge. The stiffener blades were impacted laterally. Impact energy levels of 100 ft-lb and 200 ft-lb were used. NASTRAN finite-element analyses were performed to simulate the nonvisible damage that was detected in the panels by nondestructive inspection. A closed-form solution for generalized loading was developed to evaluate the peel stresses in the bonded structure. Two-dimensional delamination growth analysis was developed using the principle of minimum potential energy in terms of closed-form solution for critical strain. An analysis was conducted to determine the residual compressive stress in the panels after impact damage, and the analytical predictions were verified by compression testing of the damaged panels.

Madan, Ram C.↗
NASA NTRS2002

Effects of Thermomechanical History on the Tensile Behavior of Nitinol Ribbon

Shape memory alloys (SMAs) have enormous potential for a wide variety of applications. A large body of work exists on the characterization of the microstructure and stress-strain behavior of these alloys, Nitinol (NiTi) in particular. However, many attributes of these materials are yet to be fully understood. Previous work at NASA Langley Research Center (LaRC) has included fabrication of hybrid composite specimens with embedded Nitinol actuators and modeling of their thermomechanical behavior. An intensive characterization effort has been undertaken to facilitate fundamental understanding of this alloy and to promote implementation of Nitinol in aerospace applications. Previous work revealed attributes of the Nitinol ribbon that were not easily rationalized with existing data in the literature. In particular, tensile behavior at ambient temperature showed significant dependence on the thermomechanical history prior to testing. The present work is focused on characterizing differences in the microstructure of Nitinol ribbons exposed to four different thermomechanical histories and correlation of the microstructure with tensile properties. Differential scanning calorimetry (DSC) and x-ray diffraction (XRD) analysis were employed to rationalize the microstructures present after exposure to various thermomechanical histories. Three of the Nitinol ribbon conditions were reversible upon heating (in the DSC) through the reverse transformation temperature (A(sub f) to transform the microstructure to austenite. However, the prior thermomechanical conditioning for the Nitinol ribbon that reflected the entire fabrication procedure (4% thermal cycle condition) was found to have an irreversible effect on the microstructure, as it remained unchanged after repeated complete thermal cycles. Tensile tests were conducted to determine the effect of prior thermomechancal conditioning on both the tensile behavior of the Nitinol ribbons and the stress state of the microstructure. The stress-strain behavior of the Nitinol actuators appears to be governed by the interplay between two major variables: namely, microstructural constituents such as the R-phase and the martensite; and the stress state of these constituents (whether twinned with low residual stresses, or detwinned with high residual stresses). The most significant difference in the stress-strain behavior of the four conditions, the critical stress required to achieve an initial stress plateau, was found to depend on both the amount and stress state (twinned or detwinned) of R-phase present in the initial microstructure. Thus, the effect of prior thermomechanical processing is critical to the resulting tensile behavior of the Nitinol actuator. For numerical modeling inputs one must take into account the entire fabrication process on the Nitinol actuator.

Lach, Cynthia L.↗
NASA NTRS1989

Piecewise-Linear Computation Of Creep

Theory of elastic and inelastic stresses and strains in isotropic materials blended with finite-element computer programs to calculate residual stresses due to creep in structures. Analogy drawn between thermal expansion and creep, enabling use of thermal-stress computational approach to calculate creep stresses and strains. Overall transient solution obtained by piecewise-linear iterations. Creep stresses in structure computed in iterative process in which cumulative creep strains treated as fictitious changes in coefficients of thermal expansion.

Jenkins, Jerald↗
NASA NTRSDate not supplied

Icing Formation Strains on Airfoil Skin

Aircraft icing has long presented a safety hazard to the aerospace industry. One problem inherent to aircraft icing is predicting the adhesive properties of the ice-aircraft interface; values commonly reported span multiple orders of magnitude, making shedding prediction and surface evaluation difficult. An effort has been undertaken at the NASA Glenn Research Center to understand the cause of the scatter in the data. Geometry dependencies, variation in testing procedures, and residual stresses in ice samples have previously been identified as significant contributors to the problem. Residual stresses were singled out for this study, which investigated the strains at the leading edge of a Common Research Model (CRM) wind tunnel model in the NASA Glenn Icing Research Tunnel. Sprays were conducted at a particle size median volumetric diameter (MVD) of 20 µm, with liquid water contents (LWC) spanning from 0.4 to 3.0 g/m3. Total temperatures were varied between 0 and -25 °C, and test section velocity between 51 and 118 m/s. The leading edge was instrumented with custom strain gages, resistance temperature detectors (RTDs), optical fibers, and a pressure mat. Results indicate Δ strains, which were the difference between recorded strains and those predicted from isotropic thermal expansion of aluminum, below -400 µε with consistent patterns of strain across a wide variety of tunnel conditions. Isotropic, linear elastic finite element analysis (FEA) was conducted by varying the temperature of simplified airfoil and ice geometries, producing similar strains to those observed in testing. This allowed induced stresses to be predicted. Predicted stresses were above the expected tensile strength for ice, and suggest a source for previously observed cracking in tunnel ice.

Ice Adhesion↗
NASA NTRS1990

InAs/Ga(1-x)In(x)Sb superlattices for infrared detector applications

The successful growth of InAs/Ga(1-x)In(x)Sb superlattices and their optical and structural characterization is discussed. Samples were grown by molecular beam epitaxy at fairly low substrate temperatures (less than 400 C). Structural quality was assessed by reflection high energy electron difrraction, transmission electron microscopy, and x ray diffraction. Excellent structures were achieved for growth on thick, strain relaxed GaSb buffer layers on GaAs substrates, despite a residual threading dislocation density of 10(exp 9)cm(exp -2) originating at the GaSb/GaAs interface. Despite a lattice mismatch of 1.7 percent, InAs/Ga(0.75)In(0.25)Sb superlattices are observed to be free of misfit dislocations at the thicknesses examined here, owing to the close lattice match between the superlattice and GaSb, which evenly distributes compressive and tensile stresses between the InAs and Ga(0.75)In(0.25)Sb layers. Photoluminescence and photoconductivity measurements indicate that the energy gaps of the strain-layer superlattices are smaller than those of InAs/GaSb superlattices with the same layer thicknesses, and are in agreement with the theoretical predictions of Smith and Mailhiot. Energy gaps of 80 to 250 meV (15 to 5 microns) have been measured for InAs/Ga(0.75)In(0.25)Sb superlattices with 45 to 25 A/25 A layer thickness. Results demonstrate that far-infrared cutoff wavelengths are compatible with the thin superlattice layers required for strong optical absorption in type-II superlattices.

Miles, Richard H.↗
NASA NTRS1987

Comparison of measured temperatures, thermal stresses and creep residues with predictions on a built-up titanium structure

Temperature, thermal stresses, and residual creep stresses were studied by comparing laboratory values measured on a built-up titanium structure with values calculated from finite-element models. Several such models were used to examine the relationship between computational thermal stresses and thermal stresses measured on a built-up structure. Element suitability, element density, and computational temperature discrepancies were studied to determine their impact on measured and calculated thermal stress. The optimum number of elements is established from a balance between element density and suitable safety margins, such that the answer is acceptably safe yet is economical from a computational viewpoint. It is noted that situations exist where relatively small excursions of calculated temperatures from measured values result in far more than proportional increases in thermal stress values. Measured residual stresses due to creep significantly exceeded the values computed by the piecewise linear elastic strain analogy approach. The most important element in the computation is the correct definition of the creep law. Computational methodology advances in predicting residual stresses due to creep require significantly more viscoelastic material characterization.

Jenkins, Jerald M.↗
NASA NTRS2014

Bi-Metallic Composite Structures With Designed Internal Residual Stress Field

Shape memory alloys (SMA) have a unique ability to recover small amounts of plastic strain through a temperature induced phase change. For these materials, mechanical displacement can be accomplished by heating the structure to induce a phase change, through which some of the plastic strain previously introduced to the structure can be reversed. This paper introduces a concept whereby an SMA phase is incorporated into a conventional alloy matrix in a co-continuous reticulated arrangement forming a bi-metallic composite structure. Through memory activation of the mechanically constrained SMA phase, a controlled residual stress field is developed in the interior of the structure. The presented experimental data show that the memory activation of the SMA composite component significantly changes the residual stress distribution in the overall structure. Designing the structural arrangement of the two phases to produce a controlled residual stress field could be used to create structures that have much improved durability and damage tolerance properties.

Brice, Craig A.↗
NASA NTRS1990

Survivability characteristics of composite compression structure

Test and evaluation was performed to determine the compression residual capability of graphite reinforced composite panels following perforation by high-velocity fragments representative of combat threats. Assessments were made of the size of the ballistic damage, the effect of applied compression load at impact, damage growth during cyclic loading and residual static strength. Several fiber/matrix systems were investigated including high-strain fibers, tough epoxies, and APC-2 thermoplastic. Additionally, several laminate configurations were evaluated including hard and soft laminates and the incorporation of buffer strips and stitching for improved damage resistance of tolerance. Both panels (12 x 20-inches) and full scale box-beam components were tested to assure scalability of results. The evaluation generally showed small differences in the responses of the material systems tested. The soft laminate configurations with concentrated reinforcement exhibited the highest residual strength. Ballistic damage did not grow or increase in severity as a result of cyclic loading, and the effects of applied load at impact were not significant under the conditions tested.

Avery, John G.↗
NASA NTRS1986

Analysis of crack closure under plane strain conditions

The phenomenon of plasticity-induced crack closure is associated with the development of residual material on the flanks of an advancing fatigue crack. While it is easy to see that this residual material can come from the side faces of a specimen under plane stress conditions, it is difficult to discover the origin of this extra volume of material on the crack flanks when it is assumed that plane deformations occur and plastic flow is incompressible. The purpose of this paper is to determine whether plasticity-induced fatigue crack closure occurs in an elastic-perfectly plastic body under plane strain conditions.

Fleck, N. A.↗
NASA NTRS1988

Analysis of crack closure under plane strain conditions

The phenomenon of plasticity-induced crack closure is associated with the development of residual material on the flanks of an advancing fatigue crack. While it is easy to see that this residual material can come from the side faces of a specimen under plane stress conditions, it is difficult to discover the origin of this extra volume of material on the crack flanks when it is assumed that plane deformations occur and plastic flow is incompressible. The purpose of this paper is to determine whether plasticity-induced fatigue crack closure occurs in an elastic-perfectly plastic body under plane strain conditions.

Fleck, Norman A.↗
NASA NTRS2024

Tensile Behavior and High-Cycle Fatigue Durability of a SiC/SiC Ceramic-Matrix Composite Without and With Simulated Cooling Holes

High-cycle fatigue (HCF) behavior of a melt-infiltrated, prepreg SiC/SiC composite was investigated using specimens without and with simulated cooling holes at 816 and 1,316 °C. The ceramic-matrix composite (CMC) material used for this study was fabricated by GE Aviation and had a ply configuration of [0/90/0] 3 . HCF tests were conducted with an R-ratio of 0.6 and a cyclic frequency of 30 Hz. Maximum tensile gross-section stresses used in the HCF tests ranged from 152 to 359 MPa. Runout in HCF was defined as 30 million cycles (278 h) at both temperatures. Monotonic tensile tests were conducted at room temperature (RT) on as-received CMC specimens and on HCF runout specimens without and with holes to determine residual tensile strengths. Digital image correlation (DIC) and acoustic emission (AE) techniques were used to monitor progression of damage in RT tensile tests on specimens without and with holes. Fractography was performed on a limited number of specimens without and with holes to determine morphology of fracture surfaces. Representative RT stress-strain behavior, elevated temperature HCF behavior at the two temperatures, and RT residual tensile strengths of HCF runout specimens are documented in this report. Select examples of DIC and AE data from RT tensile tests on specimens without and with holes and typical features observed on fracture surfaces of specimens tested under tensile and HCF conditions are also discussed.

Ceramic matrix composite↗
NASA NTRS1971

Investigation of flaw geometry and loading effects on plane strain fracture in metallic structures

The effects on fracture and flaw growth of weld-induced residual stresses, combined bending and tension stresses, and stress fields adjacent to circular holes in 2219-T87 aluminum and 5AI-2.5Sn(ELI) titanium alloys were evaluated. Static fracture tests were conducted in liquid nitrogen; fatigue tests were performed in room air, liquid nitrogen, and liquid hydrogen. Evaluation of results was based on linear elastic fracture mechanics concepts and was directed to improving existing methods of estimating minimum fracture strength and fatigue lives for pressurized structure in spacecraft and booster systems. Effects of specimen design in plane-strain fracture toughness testing were investigated. Four different specimen types were tested in room air, liquid nitrogen and liquid hydrogen environments using the aluminum and titanium alloys. Interferometry and holograph were used to measure crack-opening displacements in surface-flawed plexiglass test specimens. Comparisons were made between stress intensities calculated using displacement measurements, and approximate analytical solutions.

Hall, L. R.↗
NASA NTRS1983

Experimental creep data for a built-up aluminum/titanium structure subjected to heating and loading

Experimental creep, temperature, and strain data resulting from a laboratory experiment on a built-up aluminum/titanium structure are presented. The structure and the experiment are described in detail. A heating and loading experiment lasting approximately six hours is conducted on a test structure. Considerable creep strain resulted from compressive stresses in the heated skin. Large residual stresses were found after the experiment was completed. The residual stresses in the substructure frames were large enough to preclude further cycles of creep experiments with this built-up structure because of concern that the frame webs would buckle.

Jenkins, J. M.↗
NASA NTRS1983

Stress studies in EFG

Experimental work in support of stress studies in high speed silicon sheet growth has been emphasized in this quarter. Creep experiments utilizing four-point bending have been made in the temperature range from 1000 C to 1360 C in CZ silicon as well as on EFG ribbon. A method to measure residual stress over large areas using laser interferometry to map strain distributions under load is under development. A fiber optics sensor to measure ribbon temperature profiles has been constructed and is being tested in a ribbon growth furnace environment. Stress and temperature field modeling work has been directed toward improving various aspects of the finite element computing schemes. Difficulties in computing stress distributions with a very high creep intensity and with non-zero interface stress have been encountered and additional development of the numerical schemes to cope with these problems is required. Temperature field modeling has been extended to include the study of heat transfer effects in the die and meniscus regions.

Source record↗
NASA NTRS1998

Wide Panel Testing Technique for Evaluating Repair Weld Strengths

This paper describes a new tensile testing technique for evaluating the overall effect of a repair weld on the strength of a welded joint. Previously, repair weld strengths have been evaluated using one-inch width tensile specimens, but this technique does not capture all of the effects that result from a repair. The new technique involves testing of "wide panel" tensile specimens which contain the full length of a repair weld within a longer initial weld, allowing the specimen to capture the combined effects of residual stresses, local strength degradation, and load redistribution around a repair. The development of strains in the repair area of standard aluminum alloy specimens and new high-performance aluminum-lithium alloy specimens was observed and evaluated using photoelastic material. The results of this evaluation show an increased sensitivity to repair welding residual stresses in the aluminum-lithium alloy specimens.

Rogers, Patrick R.↗