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At least 145 records · Page 8

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NASA NTRS1977

Improved stress-intensity factors for semi-elliptical surface cracks in finite-thickness plates

Stress-intensity factors for shallow and deep semi-elliptical surface cracks in plates subjected to tension are presented. To verify the accuracy of the three-dimensional finite-element models employed, convergence was studied by varying the number of degrees of freedom in the models from 1500 to 6900. The 6900 degrees of freedom used here were more than twice the number used in previously reported solutions.

Raju, I. S.↗

NASA NTRS1976

Crack growth in Ti-8Al-1Mo-1V with real-time and accelerated flight-by-flight loading

Crack growth in Ti-8Al-1Mo-1V was measured and calculated for real-time and accelerated simulations of supersonic airplane loading and heating. Crack-growth rates calculated on the assumption that an entire flight could be represented by a single cycle predicted the experimental rates poorly. Calculated crack-growth rates were slower than the experimental rates for all tests with flight-by-flight loading. For room-temperature accelerated tests, the calculated rates agreed well with the experimental rates; but the calculations became progressively less accurate for progressively more complex test conditions (tests that included elevated temperature). Calculations of crack growth using the crack-closure concept can probably be improved through study of crack-opening stresses using finite-element models that account for variable-amplitude loading, residual stresses, and temperature effects. The calculations of crack growth could also be improved through detailed studies of material properties and interactions among stress, temperature, and time as appropriate for the real-time operating conditions of a supersonic transport airplane.

Imig, L. A.↗

NASA NTRS1977

Stability analysis of a reinforced carbon carbon shell

This paper presents the development of a stability analysis for the nose cap of the NASA Space Shuttle Orbiter. Stability is evaluated by the differential stiffness analysis of the NASTRAN finite-element computer code, addressing those nonstandard characteristics in the nose cap such as nonuniform curvature, asymmetrical and nonuniform loads, support fixity, and various combinations of membrane and bending stresses. A full-sized nose cap, thinner than production, was statically tested and stability analyzed. The failing load level correlated to within 30%. The region and mode of buckling that occurred during test was accurately predicted by analysis. The criterion for predicting instability is based on the behavior of the nonlinear deflections. The deflections are nonlinear elastic in that the stresses are well within the elastic range of the material, but the geometry-load relationship produces nonlinear deflections. The load-deflection relationship is well defined by differential stiffness analysis up to the zero-slope portion of the curve, the point of neutral stability or where the shell 'snaps through' just prior to general instability.

Agan, W. E.↗

NASA NTRS1977

Simplified and refined structural modeling for economical flutter analysis and design

A coordinated use of two finite-element models of different levels of refinement is presented to reduce the computer cost of the repetitive flutter analysis commonly encountered in structural resizing to meet flutter requirements. One model, termed a refined model (RM), represents a high degree of detail needed for strength-sizing and flutter analysis of an airframe. The other model, called a simplified model (SM), has a relatively much smaller number of elements and degrees-of-freedom. A systematic method of deriving an SM from a given RM is described. The method consists of judgmental and numerical operations to make the stiffness and mass of the SM elements equivalent to the corresponding substructures of RM. The structural data are automatically transferred between the two models. The bulk of analysis is performed on the SM with periodical verifications carried out by analysis of the RM. In a numerical example of a supersonic cruise aircraft with an arrow wing, this approach permitted substantial savings in computer costs and acceleration of the job turn-around.

Ricketts, R. H.↗

NASA NTRS1976

Cyclic structural analyses of air-cooled gas turbine blades and vanes

The creep-fatigue behavior of a fully impingement-cooled blade for four cyclic cases was analyzed by using the Elas 55, finite-element, nonlinear structural computer program. Expected cyclic lives were calculated by using the method of Strainrange Partitioning for reversed inelastic strains and time fractions for ratcheted tensile creep strains. Strainrange Partitioning was also applied to previous results from a one-dimensional cyclic analysis of a film-impingement-cooled vane. The analyses indicated that Strainrange Partitioning is more applicable to a constrained airfoil such as the film-impingement-cooled vane than to the relatively unconstrained fully impingement-cooled airfoil.

Kaufman, A.↗

NASA NTRS1977

10-deg off-axis test for shear properties in fiber composites

A combined theoretical and experimental investigation was conducted to assess the suitability of the 10-deg off-axis tensile test specimen for the intralaminar shear characterization of unidirectional composites. Composite mechanics, a combined-stress failure criterion, and a finite-element analysis were used to determine theoretically the stress-strain variation across the specimen width and the relative stress and strain magnitudes at the 10-deg plane. Strain gages were employed to measure the strain variation across the specimen width at specimen midlength and near the end tabs. Specimens from Mod-I/epoxy, T-300/epoxy, and S-glass/epoxy were used in the experimental program. It was found that the 10-deg off-axis tensile test specimen is suitable for intralaminar shear characterization, and it is recommended that it should be considered as a possible standard test specimen for such a characterization.

Chamis, C. C.↗

NASA NTRS1977

Compliance calibration of specimens used in the R-curve practice

The compliance calibrations for the compact (CS) and crack-line-wedge-loaded (CLWL) specimens have been determined by experimental measurements and by boundary-collocation analysis. The CS and CLWL specimen configurations were modeled more accurately than those used in previous analytical investigations. Polynomial expressions for the compliance at various stations along the crack line for CS and CLWL specimens are presented. The compliance calibrations for the center-crack tension (CCT) specimen have been determined theoretically by boundary-collocation and finite-element analysis. The calculated compliance values for the CCT specimen are compared with values obtained from the Irwin-Westergaard expression and from a modification to the Irwin-Westergaard expression proposed by Eftis and Liebowitz. The Eftis-Liebowitz expression was found to be in good agreement (plus or minus 2 percent) with both analyses for crack aspect ratios up to 0.8 and for gage half-span to specimen width ratios up to 0.5.

Mccabe, D. E.↗

NASA NTRS1978

Stress analysis under component relative interference fit

Finite-element computer program enables analysis of distortions and stresses occurring in components having relative interference. Program restricts itself to simple elements and axisymmetric loading situations. External inertial and thermal loads may be applied in addition to forces arising from interference conditions.

Taylor, C. M.↗

NASA NTRS1978

Structural performance analysis and redesign

Program performs stress buckling and vibrational analysis of large, linear, finite-element systems in excess of 50,000 degrees of freedom. Cost, execution time, and storage requirements are kept reasonable through use of sparse matrix solution techniques, and other computational and data management procedures designed for problems of very large size.

Whetstone, W. D.↗

NASA NTRS1977

Stress analysis of pressure vessel assembly

A radome assembly capable of withstanding large hydrostatic pressures was designed with a sapphire hemisperical dome and a plastic housing. These two parts were connected with a stainless steel flange ring. A finite-element stress analysis of this assembly was accomplished and an experimental load test was performed to verify the predicted stresses.

Eppink, R. T.↗

NASA NTRS1978

Mechanical behavior and fracture characteristics of off-axis fiber composites. 2: Theory and comparisons

The mechanical behavior and stresses inducing fracture modes of unidirectional high-modulus graphite-fiber/epoxy composites subjected to off-axis tensile loads were investigated theoretically. The investigation included the use of composite mechanics, combined-stress failure criteria, and finite-element stress analysis. The results are compared with experimental data and led to the formulation of criteria and convenient plotting procedures for identifying, characterizing, and quantifying these fracture modes.

Chamis, C. C.↗

NASA NTRS1978

Prospects for computational aerodynamics

The integral representations approach, for the solution of the Navier-Stokes equations is discussed as well as experience in its development and in applying available finite-difference and finite-element techniques to the treatment of three-dimensional problems, and the computation of turbulent flow. The magnitude of efforts required to develop turbulence models and three-dimensional algorithms indicates that the computational fluid dynamics research must have a broad base. Broader access to modern computing facilities that are in existence within NASA should be promoted for active researchers not directly affiliated with that agency.

Wu, J. C.↗

NASA NTRS1978

Comparison of two computer programs by predicting turbulent mixing of helium in a ducted supersonic airstream

Two parabolic flow computer programs, SHIP (a finite-difference program) and COMOC (a finite-element program), are used for predicting three-dimensional turbulent reacting flow fields in supersonic combustors. The theoretical foundation of the two computer programs are described, and then the programs are applied to a three-dimensional turbulent mixing experiment. The cold (nonreacting) flow experiment was performed to study the mixing of helium jets with a supersonic airstream in a rectangular duct. Surveys of the flow field at an upstream were used as the initial data by programs; surveys at a downstream station provided comparison to assess program accuracy. Both computer programs predicted the experimental results and data trends reasonably well. However, the comparison between the computations from the two programs indicated that SHIP was more accurate in computation and more efficient in both computer storage and computing time than COMOC.

Pan, Y. S.↗

NASA NTRS1978

Evaluation of a hybrid, anisotropic, multilayered, quadrilateral finite element

A multilayered finite element with bending-extensional coupling is evaluated for: (1) buckling of general laminated plates; (2) thermal stresses of laminated plates cured at elevated temperatures; (3) displacements of a bimetallic beam; and (4) displacement and stresses of a single-cell box beam with warped cover panels. Also, displacements and stresses for flat and spherical orthotropic and anisotropic segments are compared with results from higher order plate and shell finite-element analyses.

Robinson, J. C.↗

NASA NTRS1977

Three-dimensional modelling in magnetotelluric and magnetic variational sounding

The Galerkin finite-element method is used to obtain approximate solutions for the three-dimensional induction problem. A rectangular conductive prism is considered as an example, and solutions are obtained for linear and circularly polarized incident plane-wave fields. Magnetotelluric tensor impedances and magnetic transfer functions are computed. Polar diagrams of the tensor impedances and magnetic transfer functions along with their amplitude contour maps are presented. The dimensionality parameter, skew, is contoured at the surface of the earth. It is shown that the relative amplitudes and shapes of the additional and principal impedance polar diagrams can be used to determine the dimensionality of geoelectrical structures. Stations with skew values greater than 0.2 are significantly influenced by the three-dimensionality of the geoelectric structure. The amplitudes of the magnetic transfer function and the orientations of its polar diagrams exhibit large anomalies in the vicinity of the intersection of the lateral contacts.

Reddy, I. K.↗

NASA NTRS1978

Cost-effective use of minicomputers to solve structural problems

Minicomputers are receiving increased use throughout the aerospace industry. Until recently, their use focused primarily on process control and numerically controlled tooling applications, while their exposure to and the opportunity for structural calculations has been limited. With the increased availability of this computer hardware, the question arises as to the feasibility and practicality of carrying out comprehensive structural analysis on a minicomputer. This paper presents results on the potential for using minicomputers for structural analysis by (1) selecting a comprehensive, finite-element structural analysis system in use on large mainframe computers; (2) implementing the system on a minicomputer; and (3) comparing the performance of the minicomputers with that of a large mainframe computer for the solution to a wide range of finite element structural analysis problems.

Storaasli, O. O.↗

NASA NTRS1978

NASA technical advances in aircraft occupant safety

A NASA program to improve aircraft safety is discussed in terms of three areas of concentration: unexpected turbulence encounters, fire, and crash impact. To provide warning of clear air turbulence (CAT) so that the pilot can take evasive action, a laser Doppler system is described, which functions by measuring backscatter frequency radiation occurring in aerosols ahead of the aircraft. The system was found able to detect CAT, but at shorter than optimal ranges (10 km as opposed to 32 km). Fire safety has focused on both the early detection of fires through improved sensing methods, and on the development of fire-retardant materials, i.e., intumescent char-forming protective coatings. Crashworthiness is discussed in terms of the development of a survivable crash envelope and improved seat and restraint systems. To evaluate an aircraft for crashworthiness, finite-element computer programs are currently being developed which analyze both aircraft structural configurations and the intrinsic strength of aircraft materials.

Enders, J. H.↗

NASA NTRS1978

Time-domain Green's Function Method for three-dimensional nonlinear subsonic flows

The Green's Function Method for linearized 3D unsteady potential flow (embedded in the computer code SOUSSA P) is extended to include the time-domain analysis as well as the nonlinear term retained in the transonic small disturbance equation. The differential-delay equations in time, as obtained by applying the Green's Function Method (in a generalized sense) and the finite-element technique to the transonic equation, are solved directly in the time domain. Comparisons are made with both linearized frequency-domain calculations and existing nonlinear results.

Tseng, K.↗