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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 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Advanced collapsible tank for liquid containment

Tanks for bulk liquid containment will be required to support advanced planetary exploration programs. Potential applications include storage of potable, process, and waste water, and fuels and process chemicals. The launch mass and volume penalties inherent in rigid tanks suggest that collapsible tanks may be more efficient. Collapsible tanks are made of lightweight flexible material and can be folded compactly for storage and transport. Although collapsible tanks for terrestrial use are widely available, a new design was developed that has significantly less mass and bulk than existing models. Modelled after the shape of a sessible drop, this design features a dual membrane with a nearly uniform stress distribution and a low surface-to-volume ratio. It can be adapted to store a variety of liquids in nearly any environment with constant acceleration field. Three models of 10L, 50L, and 378L capacity have been constructed and tested. The 378L (100 gallon) model weighed less than 10 percent of a commercially available collapsible tank of equivalent capacity, and required less than 20 percent of the storage space when folded for transport.

Flanagan, David T.↗
NASA NTRS1993

Numerical analysis of the Iosipescu specimen for composite materials

A finite element analysis of the Iosipescu shear tests for unidirectional and cross-ply composites is presented. It is shown that an iterative analysis procedure must be used to model the fixture-specimen kinematics. The correction factors which are needed to compensate for the nonuniformity of stress distribution in calculating shear modulus are shown to be dependent on the material orthotropic ratio and the finite element loading models. Test section strain distributions representative of typical graphite-epoxy specimens are also presented.

Ho, H.↗
NASA NTRS1993

Effects of initial conditions on the development of curved wakes

An experimental investigation has been conducted on the three-dimensional structure of curved plane wakes developing from tripped and untripped initial boundary layers. The effects of mild stream-wise curvature on a wake generated at the trailing edge of a slowly tapering splitter plate were investigated at a Reynolds number of about 30,000. With the initial boundary layers turbulent, spatially-stationary streamwise vorticity was not observed. The curvature affected the wake growth and defect-decay rates, but in different ways for each of the two initial conditions. The effects of curvature were also apparent in the Reynolds stress results, especially in the primary shear stress distributions, which showed that the levels on the unstable side were increased significantly compared to those for a straight wake, while those on the stable side were decreased, with the effect stronger in the initially laminar wake.

Weygandt, James H.↗
NASA NTRS1992

Liquid hydrogen slosh waves excited by constant reverse gravity acceleration of geyser initiation

The requirement to settle or to position liquid fuel over the outlet end of the spacecraft propellant tank before main engine restart poses a microgravity fluid behavior problem. Resettlement or reorientation of liquid propellant can be accomplished by providing the optimal acceleration to the spacecraft such that the propellant is reoriented over the tank outlet. In this study slosh wave excitation induced by the resettling flowfield during the course of liquid reorientation with the initiation of geyser for liquid-filled levels of 30, 50, 65, 70, and 80 percent have been studied. Characteristics of slosh waves with various frequencies excited are discussed. Slosh wave excitations will affect the fluid stress distribution exerted on the container wall and shift the fluid mass distribution inside the container, which imposes the time-dependent variations in the moment of inertia of the container. This information is important for the spacecraft control during the course of liquid reorientation.

Hung, R. J.↗
NASA NTRS1993

A displacement gradient BEM for accurate stress computation near boundaries in 2-D anisotropic problems

A displacement gradient method for 2D anisotropic elasticity problems is presented, which effectively minimizes the boundary layer effect through a two-step procedure. First, the boundary integral equations are solved for the unknown boundary displacements and tractions. Second, a direct integral equation for displacement gradients is developed in terms of boundary tractions. Three methods based on different evaluation procedures and locations for determining the displacement gradients are proposed. In the first method the displacement gradients are averaged at nodes common to adjacent elements. The second method stores the gradients element-wise. In the third method, the gradients are evaluated at the nodes of discontinuous elements. The three methods are applied to near-isotropic plates with circular and elliptic cutouts. It is concluded that all three methods can yield accurate stress distributions.

Sistla, R.↗
NASA NTRS1993

A mixed formulation for interlaminar stresses in dropped-ply laminates

A structural model is developed for the linear elastic response of structures consisting of multiple layers of varying thickness such as laminated composites containing internal ply drop-offs. The assumption of generalized plane deformation is used to reduce the solution domain to two dimensions while still allowing some out-of-plane deformation. The Hellinger-Reissner variational principle is applied to a layerwise assumed stress distribution with the resulting governing equations solved using finite differences.

Harrison, Peter N.↗
NASA NTRS1993

An investigation of shock wave turbulent boundary layer interaction with bleed through slanted slots

Flow-field characteristics are simulated numerically in an oblique shock wave/turbulent boundary layer interactions with six different bleed slot configurations. The strong conservation-law form of the 2D compressible Navier-Stokes equations and the k-epsilon equations are solved throughout the interaction region and inside the bleed slot. The computed results are presented for a normal and 20-deg slanted bleed slots at three different locations, upstream, across and downstream of the impingement point of an oblique shock of sufficient strength to cause boundary layer separation without bleed. The detailed flow characteristics in the interaction zone and inside the bleed slot are compared for the different bleed slot configurations. The resulting surface pressure and shear stress distributions as well as the boundary layer characteristics downstream of the interaction region are also presented for the mix bleed configurations at different bleed mass flows up to choking.

Hamed, A.↗
NASA NTRS1954

Stress analysis of circular semimonocoque cylinders with cutouts by a perturbation load technique

A method is presented for analyzing the stresses about a cutout in a circular cylinder of semimonocoque construction. The method involves the use of so-called perturbation solutions which are superposed on the stress distribution that would exist in the structure with no cutout in such a way as to give the effects of a cutout. The method can be used for any loading case for which the structure without the cutout can be analyzed and is sufficiently versatile to account for stringer and shear reinforcement about the cutout. (author)

Mccomb, Harvey G , Jr↗
NASA NTRS1948

Determination of stresses in gas-turbine disks subjected to plastic flow and creep

A finite-difference method previously presented for computing elastic stresses in rotating disks is extended to include the computation of the disk stresses when plastic flow and creep are considered. A finite-difference method is employed to eliminate numerical integration and to permit nontechnical personnel to make the calculations with a minimum of engineering supervision. Illustrative examples are included to facilitate explanation of the procedure by carrying out the computations on a typical gas-turbine disk through a complete running cycle. The results of the numerical examples presented indicate that plastic flow markedly alters the elastic-stress distribution.

Millenson, M B↗
NASA NTRS1949

Recommendations for numerical solution of reinforced-panel and fuselage-ring problems

Procedures are recommended for solving the equations of equilibrium of reinforced panels and isolated fuselage rings as represented by the external loads and the operations table established according to Southwell's method. From the solution of these equations the stress distribution can be easily determined. The method of systematic relaxations, the matrix-calculus method, and several other methods applicable in special cases are discussed. Definite recommendations are made for obtaining the solution of reinforced-panel problems which are generally designated as shear lag problems. The procedures recommended are demonstrated in the analysis of a number of panels. In the case of fuselage rings it is not possible to make definite recommendations for the solution of the equilibrium equations for all rings and loadings. However, suggestions based on the latest experience are made and demonstrated on several rings.

Hoff, N J↗
NASA NTRS1953

Some problems on the theory of creep

The term creep of metals is applied to the phenomenon in which, at temperatures beyond a certain limit, the metal subjected to a load slowly undergoes deformation with time. For the case of steel, the creep phenomenon must be taken into account at temperatures above 400 degrees C. Very slow deformations for a prolonged period are cumulative and lead either to inadmissible changes in the dimensions of a structural part or to its failure. The theory of creep constitutes part of the mechanics of dense media and the mechanical formulation of the problem may be given as the following: a body is subjected to the action of a given system of forces, or initial displacements are prescribed on its surface. It is required to find the stress distribution in the body and the changes or its deformations with time. For the solution of the problem of creep as thus formulated, a mechanical theory of creep is required. Such theory must necessarily bear an external, overall character based, not on the investigation of microprocesses, but on the results of mechanical tests.

METALS - CREEP↗
NASA NTRS1994

Elongation Transducer For Tensile Tests

Extensometer transducer measures elongation of tensile-test specimen with negligible distortion of test results. Used in stress-versus-strain tests of small specimens of composite materials. Clamping stress distributed more evenly. Specimen clamped gently between jaw and facing surface of housing. Friction force of load points on conical tips onto specimen depends on compression of spring, adjusted by turning cover on housing. Limp, light nylon-insulated electrical leads impose minimal extraneous loads on measuring elements.

Roberts, Paul W.↗
NASA NTRS1993

Thermoelastic response of metal matrix composites with large-diameter fibers subjected to thermal gradients

A new micromechanical theory is presented for the response of heterogeneous metal matrix composites subjected to thermal gradients. In contrast to existing micromechanical theories that utilize classical homogenization schemes in the course of calculating microscopic and macroscopic field quantities, in the present approach the actual microstructural details are explicitly coupled with the macrostructure of the composite. Examples are offered that illustrate limitations of the classical homogenization approach in predicting the response of thin-walled metal matrix composites with large-diameter fibers when subjected to thermal gradients. These examples include composites with a finite number of fibers in the thickness direction that may be uniformly or nonuniformly spaced, thus admitting so-called functionally gradient composites. The results illustrate that the classical approach of decoupling micromechanical and macromechanical analyses in the presence of a finite number of large-diameter fibers, finite dimensions of the composite, and temperature gradient may produce excessively conservative estimates for macroscopic field quantities, while both underestimating and overestimating the local fluctuations of the microscopic quantities in different regions of the composite. Also demonstrated is the usefulness of the present approach in generating favorable stress distributions in the presence of thermal gradients by appropriately tailoring the internal microstructure details of the composite.

Aboudi, Jacob↗
NASA NTRS1993

Loading, electromyograph, and motion during exercise

A bicycle ergometer system has been developed to determine forces acting in specific muscles and muscle groups for both cycling and isometric exercise. The bicycle has been instrumented with encoders, accelerometers, and load cells. A harnessing system has been developed to keep subjects in place during isometric exercise. EMG data will also be collected with electrodes attached to various muscles on the subject's leg. Data has been collected for static loading and will be collected for cycling in both an earth-based laboratory and on the KC-135. Once the data is analyzed, the forces will be entered into finite element models of bones of the lower extremities. A finite element model of the tibia-fibula has been generated from the experimental subject's MRI data. The linear elastic isoparametric brick elements representing the bones are connected by linear elastic isoparametric shell elements placed at the locations of ligaments. Models will be generated for the calcaneus and the femur. Material properties for the various tissues will be taken from the literature. The experimentally determined muscle forces will be applied to the models to determine the stress distribution which is created in the bones.

Todd, Beth A.↗
NASA NTRS1994

Analysis of new composite architectures

Global/local method in conjunction with a special macro finite element is used to gain computational efficiency in the simulation of textile composites behavior. Results are included to demonstrate the effectiveness of the method. Also, 2D finite elements are used to study boundary effects in plain weave composite specimens subjected to extension, shear, and flexure loads. Effective extension, shear, and flexural moduli were found to be quite sensitive to specimen size. For extension and flexure loads stress distributions were affected by a free surface, but the free surface boundary effect did not appear to propagate very far into the interior. For shear load, the boundary effect appeared to propagate much further into the interior. The report is in the form of two technical articles, the first describes the global/local method while the second describes the use of the 2D finite elements.

Whitcomb, John D.↗
NASA NTRS1994

Manufacturing and testing of a magnetically suspended composite flywheel energy storage system

This paper presents the work performed to develop a multiring composite material flywheel and improvements of a magnetically suspended energy storage system. The flywheel is constructed of filament would graphite/epoxy and is interference assembled for better stress distribution to obtain higher speeds. The stationary stack in the center of the disk supports the flywheel with two magnetic bearings and provides power transfer to the flywheel with a motor/generator. The system operates under a 10(exp -4) torr environment and has been demonstrated to 20,000 rpm with a total stored energy of 15.9 Wh. When this flywheel cycles between its design speeds (45,000 to 90,000 rpm), it will deliver 242 Wh and have a usable specific energy density of 42.6 Wh/kg.

Wells, Stephen↗
NASA NTRS1994

Size effect in composite materials and structures: Basic concepts and design considerations

Composite materials display strength characteristics that are similar to those of brittle ceramics, whose strengths are known to decrease with increasing volume for a uniform state of stress (size effect) and also are dependent on stress distribution. These similarities raise the question of whether there is also a size effect in composite materials and structures. There is significant, but inconclusive experimental evidence for the existence of a size effect in composites. Macroscopic and micromechanical statistical models have been developed which predict a size effect and are in general agreement with experimental data. The existence of a significant size effect in composites would be of great importance. For example, it would mean that use of standard test coupons to establish design allowables for large structures could be very nonconservative. Further, it would be necessary to analyze the strength of large composite structures using statistical methods, as is done for ceramics.

Zweben, Carl↗