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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Influence of transverse shear on an axial crack in a cylindrical shell

An axial crack in a cylindrical shell is investigated by use of a 10th order shell theory, which accounts for transverse shear deformations as well as a special kind of orthotropy. The symmetric problem is formulated in terms of two coupled singular integral equations, which are solved numerically. The asymptotic membrane and bending stress fields ahead of the crack are found to be self similar. Stress intensity factors are given as a function of the shell parameter for various values of the ratio crack length to shell thickness. Considerable differences from 8th order shell theory results are found for the bending stresses, while the membrane stresses of the 8th order theory seems to be a lower limit reached for very thin shells.

Krenk, S.↗
NASA NTRS1978

Influence of transverse shear on an axial crack in a cylindrical shell

An axial crack in a cylindrical shell is investigated by use of a 10th order shell theory, which accounts for transverse shear deformations as well as a special kind of orthotropy. The symmetric problem is formulated in terms of two coupled singular integral equations, which are solved numerically. The asymptotic membrane and bending stress fields ahead of the crack are found to be self similar. Stress intensity factors are given as a function of the shell parameter for various values of the ratio shell radius to shell thickness. Considerable differences from 8th order shell theory results are found for the bending stresses, while the membrane stresses of the 8th order theory seems to be a lower limit reached for very thin shells.

Krenk, S.↗
NASA NTRS2010

Methods for In-Flight Wing Shape Predictions of Highly Flexible Unmanned Aerial Vehicles: Formulation of Ko Displacement Theory

The Ko displacement theory is formulated for a cantilever tubular wing spar under bending, torsion, and combined bending and torsion loading. The Ko displacement equations are expressed in terms of strains measured at multiple sensing stations equally spaced on the surface of the wing spar. The bending and distortion strain data can then be input to the displacement equations to calculate slopes, deflections, and cross-sectional twist angles of the wing spar at the strain-sensing stations for generating the deformed shapes of flexible aircraft wing spars. The displacement equations have been successfully validated for accuracy by finite-element analysis. The Ko displacement theory that has been formulated could also be applied to calculate the deformed shape of simple and tapered beams, plates, and tapered cantilever wing boxes. The Ko displacement theory and associated strain-sensing system (such as fiber optic sensors) form a powerful tool for in-flight deformation monitoring of flexible wings and tails, such as those often employed on unmanned aerial vehicles. Ultimately, the calculated displacement data can be visually displayed in real time to the ground-based pilot for monitoring the deformed shape of unmanned aerial vehicles during flight.

Ko, William L.↗
NASA NTRS1979

Minimization theory of induced drag subject to constraint conditions

Exact analytical solutions in terms of induced drag influence coefficients can be attained which define the spanwise loading with minimized induced drag, subject to specified constraint conditions, for any nonplanar wing shape or number of lift plus wing bending moment about a given wing span station. Example applications of the theory are made to a biplane, a wing in ground effect, a cruciform wing, a V-wing, a planar-wing winglet, and linked wingtips in formation flying. For minimal induced drag, the spanwise loading, relative to elliptic, is outboard for the biplane and is inboard for the wing in ground effect and for the planar-wing winglet. A spinoff of the triplane solution provides mathematically exact equations for downwash and sidewash about a planar vorticity sheet having an arbitrary loading distribution.

Deyoung, J.↗
NASA NTRS1994

An Accurate Quartic Force Field and Vibrational Frequencies for HNO and DNO

An accurate ab initio quartic force field for HNO has been determined using the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, CCSD(T), in conjunction with the correlation consistent polarized valence triple zeta (cc-pVTZ) basis set. Improved harmonic frequencies were determined with the cc-pVQZ basis set. Fundamental vibrational frequencies were determined using a second-order perturbation theory analysis and also using variational calculations. The N-0 stretch and bending fundamentals are determined well from both vibrational analyses. The H-N stretch, however, is shown to have an unusually large anharmonic correction, and is not well determined using second-order perturbation theory. The H-N fundamental is well determined from the variational calculations, demonstrating the quality of the ab initio quartic force field. The zero-point energy of HNO that should be used in isodesmic reactions is also discussed.

Dateo, Christopher E.↗
NASA NTRS1992

Analysis of delamination related fracture processes in composites

An anisotropic thin walled closed section beam theory was developed based on an asymptotical analysis of the shell energy functional. The displacement field is not assumed a priori and emerges as a result of the analysis. In addition to the classical out-of-plane torsional warping, two new contributions are identified namely, axial strain and bending warping. A comparison of the derived governing equations confirms the theory developed by Reissner and Tsai. Also, explicit closed form expressions for the beam stiffness coefficients, the stress and displacement fields are provided. The predictions of the present theory were validated by comparison with finite element simulation, other closed form analyses and test data.

Armanios, Erian A.↗
NASA NTRS1968

Notes on nonlinear shell theory.

Nonlinear membrane shell theory and small perturbations of pressurized membrane shells, noting bending, buckling and vibration of stressed shells

Budiansky, B.↗
NASA NTRS1973

Nonlinear problems of the theory of heterogeneous slightly curved shells

An account if given of the variational method of the solution of physically and geometrically nonlinear problems of the theory of heterogeneous slightly curved shells. Examined are the bending and supercritical behavior of plates and conical and spherical cupolas of variable thickness in a temperature field, taking into account the dependence of the elastic parameters on temperature. The bending, stability in general and load-bearing capacity of flexible isotropic elastic-plastic shells with different criteria of plasticity, taking into account compressibility and hardening. The effect of the plastic heterogeneity caused by heat treatment, surface work hardening and irradiation by fast neutron flux is investigated. Some problems of the dynamic behavior of flexible shells are solved. Calculations are performed in high approximations. Considerable attention is given to the construction of a machine algorithm and to the checking of the convergence of iterative processes.

Kantor, B. Y.↗
NASA NTRS1974

An experimental and theoretical study of low-aspect ration swept and oblique wings at Mach numbers between 0.6 and 1.4

Experimental forces and moments for two wing-body combinations having (a) a swept wing and (b) an oblique wing are compared. At all Mach numbers, the oblique wing (at its optimum sweep angle) had higher maximum lift-to-drag ratios than the fixed, swept wing. At high angles of attack, the direction of the pitching or rolling tendencies of the oblique wing was a function of the spanwise distribution of wing bend or washout for the two bends being investigated. At low angles of attack, linear theory gave satisfactory predictions of the subsonic lift-curve slope, the aerodynamic-center travel with Mach number, and the maximum lift-to-drag ratio.

Hopkins, E. J.↗
NASA NTRS1980

Stability of large horizontal-axis axisymmetric wind turbines

The stability of large horizontal axis, axi-symmetric, power producing wind turbines was examined. The analytical model used included the dynamic coupling of the rotor, tower and power generating system. The aerodynamic loading was derived from blade element theory. Each rotor blade was permitted tow principal elastic bending degrees of freedom, one degree of freedom in torsion and controlled pitch as a rigid body. The rotor hub was mounted in a rigid nacelle which may yaw freely or in a controlled manner. The tower can bend in two principal directions and may twist. Also, the rotor speed can vary and may induce perturbation reactions within the power generating equipment. Stability was determined by the eigenvalues of a set of linearized constant coefficient differential equations. All results presented are based on a 3 bladed, 300 ft. diameter, 2.5 megawatt wind turbine. Some of the parameters varied were; wind speed, rotor speed structural stiffness and damping, the effective stiffness and damping of the power generating system and the principal bending directions of the rotor blades. Unstable or weakly stable behavior can be caused by aerodynamic forces due to motion of the rotor blades and tower in the plane of rotation or by mechanical coupling between the rotor system and the tower.

Hirschbein, M. S.↗
NASA NTRS1985

Electrical, structural and chemical characterization of Si sheet material

Web Dendritic Si ribbons are deformed under a well-defined four point bending stress. Since the mechanical properties of Si are known to depend on the 0 content of the material, 0 measurements using Fourier Transform Infrared Spectroscopy were done to determine a proper control sample of non-defected single crystal Si. Four point bending revealed a unique two step bending behavior for the Web Si ribbons. An initial theory for this behavior involves the interaction of the dislocations generated by the deformation with the central twin planes of the ribbons. Measurements showed a uniformly high 0 content for the Web Si ribbons, approximately 10 to the 18th power atoms/cu.cm. The Web samples had a much broader absorption peak at 9 microns than is usually seen for well-annealed single crystal Si. This broadening is thought to be related to stress in the Web Si ribbons. Two samples containing a known amount of residual stress support this hypothesis. Also, a shoulder on the infrared absorption peak associated with interstitial 0 in Si appears in the transmission vs. wavenumber plots for some of the Web samples. This shoulder has been associated with 0-vacancy complexes or with 0 at dislocations. The O content and configuration do not seem to correlate with the growth configuration of the Web Si ribbon.

Ast, D. G.↗
NASA NTRS1988

Viscous induced drag

A fundamentally new approach to the aircraft minimum induced drag problem is presented. The method, a 'viscous lifting line', is based on the minimum entropy production principle and does not require the planar wake assumption. An approximate, closed form solution is obtained and compared with several classical results. In addition, the problem of optimizing in-plane wing sweep with constant wing root bending moment is considered. Like the classical lifting line theory, this theory predicts that induced drag is proportional to the square of the lift coefficient and inversely proportional to the wing aspect ratio. Unlike the classical theory, it predicts that in-plane wing sweep may significantly reduce induced drag, that induced drag is Reynolds number dependent, and that the optimum spanwise circulation distribution is non-elliptic.

Greene, George C.↗
NASA NTRS1990

The effect of steady aerodynamic loading on the flutter stability of turbomachinery blading

An aeroelastic analysis is presented which accounts for the effect of steady aerodynamic loading on the aeroelastic stability of a cascade of compressor blades. The aeroelastic model is a two degree of freedom model having bending and torsional displacements. A linearized unsteady potential flow theory is used to determine the unsteady aerodynamic response coefficients for the aeroelastic analysis. The steady aerodynamic loading was caused by the addition of airfoil thickness and camber and steady flow incidence. The importance of steady loading on the airfoil unsteady pressure distribution is demonstrated. Additionally, the effect of steady loading on the tuned flutter behavior and flutter boundaries indicates that neglecting either airfoil thickness, camber or incidence could result in nonconservative estimates of flutter behavior.

Smith, Todd E.↗
NASA NTRS1991

The effect of steady aerodynamic loading on the flutter stability of turbomachinery blading

An aeroelastic analysis is presented which accounts for the effect of steady aerodynamic loading on the aeroelastic stability of a cascade of compressor blades. The aeroelastic model is a two degree of freedom model having bending and torsional displacements. A linearized unsteady potential flow theory is used to determine the unsteady aerodynamic response coefficients for the aeroelastic analysis. The steady aerodynamic loading was caused by the addition of airfoil thickness and camber and steady flow incidence. The importance of steady loading on the airfoil unsteady pressure distribution is demonstrated. Additionally, the effect of steady loading on the tuned flutter behavior and flutter boundaries indicates that neglecting either airfoil thickness, camber or incidence could result in nonconservative estimates of flutter behavior.

Smith, Todd E.↗
NASA NTRS1992

Propagation Of Sound In Curved Ducts

Monograph presents concise, comprehensive summary of knowledge of propagation of acoustic waves in ducts and pipes including bends. Pulls together information from Lord Rayleigh's book Theory Of Sound, published in 1878, and from 33 papers scattered throughout various scientific journals published between 1945 and 1989. Monograph useful to scientists and engineers interested in such diverse topics as musical instruments, air-conditioning ducts, and jet engines. Material not available in current texts.

Rostafinski, Wojciech↗
NASA NTRS2001

Analysis of Graphite Reinforced Cementitious Composites

This paper describes analytical methods that can be used to determine the deflections and stresses in highly compliant graphite-reinforced cementitious composites. It is demonstrated that the standard transform section fails to provide accurate results when the elastic modulus ratio exceeds 20. So an alternate approach is formulated by using the rule of mixtures to determine a set of effective material properties for the composite. Tensile tests are conducted on composite samples to verify this approach; and, when the effective material properties are used to characterize the deflections of composite beams subject to pure bending, an excellent agreement is obtained. Laminated composite plate theory is also investigated as a means for analyzing even more complex composites, consisting of multiple graphite layers oriented in different directions. In this case, composite beams are analyzed by incorporating material properties established from tensile tests. Finite element modeling is used to verity the results and, considering the complexity of the samples, a very good agreement is obtained.

Vaughan, Robert E.↗
NASA NTRS2002

Analysis of Graphite-Reinforced Cementitious Composites

Strategically embedding graphite meshes in a compliant cementitious matrix produces a composite material with relatively high tension and compressive properties as compared to steel-reinforced structures fabricated from a standard concrete mix. Although these composite systems are somewhat similar, the methods used to analyze steel-reinforced composites often fail to characterize the behavior of their more advanced graphite-reinforced counterparts. This Technical Memorandum describes some of the analytical methods being developed to determine the deflections and stresses in graphite-reinforced cementitious composites. It is initially demonstrated that the standard transform section method fails to provide accurate results when the elastic moduli ratio exceeds 20. An alternate approach is formulated by using the rule of mixtures to determine a set of effective material properties for the composite. Tensile tests are conducted on composite samples to verify this approach. When the effective material properties are used to characterize the deflections of composite beams subjected to pure bending, an excellent agreement is obtained. Laminated composite plate theory is investigated as a means for analyzing even more complex composites, consisting of multiple graphite layers oriented in different directions. In this case, composite beams are analyzed using the laminated composite plate theory with material properties established from tensile tests. Then, finite element modeling is used to verify the results. Considering the complexity of the samples, a very good agreement is obtained.

Vaughan, R. E.↗