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At least 757 records · Page 42

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NASA NTRS1987

Surface flaw reliability analysis of ceramic components with the SCARE finite element postprocessor program

The SCARE (Structural Ceramics Analysis and Reliability Evaluation) computer program on statistical fast fracture reliability analysis with quadratic elements for volume distributed imperfections is enhanced to include the use of linear finite elements and the capability of designing against concurrent surface flaw induced ceramic component failure. The SCARE code is presently coupled as a postprocessor to the MSC/NASTRAN general purpose, finite element analysis program. The improved version now includes the Weibull and Batdorf statistical failure theories for both surface and volume flaw based reliability analysis. The program uses the two-parameter Weibull fracture strength cumulative failure probability distribution model with the principle of independent action for poly-axial stress states, and Batdorf's shear-sensitive as well as shear-insensitive statistical theories. The shear-sensitive surface crack configurations include the Griffith crack and Griffith notch geometries, using the total critical coplanar strain energy release rate criterion to predict mixed-mode fracture. Weibull material parameters based on both surface and volume flaw induced fracture can also be calculated from modulus of rupture bar tests, using the least squares method with known specimen geometry and grouped fracture data. The statistical fast fracture theories for surface flaw induced failure, along with selected input and output formats and options, are summarized. An example problem to demonstrate various features of the program is included.

Gyekenyesi, John P.↗

NASA NTRS1987

Interlaminar shear fracture toughness and fatigue thresholds for composite materials

Static and cyclic end notched flexure tests were conducted on a graphite epoxy, a glass epoxy, and graphite thermoplastic to determine their interlaminar shear fracture toughness and fatigue thresholds for delamination in terms of limiting values of the mode II strain energy release rate, G-II, for delamination growth. The influence of precracking and data reduction schemes are discussed. Finite element analysis indicated that the beam theory calculation for G-II with the transverse shear contribution included was reasonably accurate over the entire range of crack lengths. Cyclic loading significantly reduced the critical G-II for delamination. A threshold value of the maximum cyclic G-II below which no delamination occurred after one million cycles was identified for each material. Also, residual static toughness tests were conducted on glass epoxy specimens that had undergone one million cycles without delamination. A linear mixed-mode delamination criteria was used to characterize the static toughness of several composite materials; however, a total G threshold criterion appears to characterize the fatigue delamination durability of composite materials with a wide range of static toughness.

Obrien, T. Kevin↗

NASA NTRS1986

Dual-pilot tone calibration technique

Pilot-based calibration techniques are used to reduce the effects of multipath fading in mobile satellite receivers. One of the more recent of these techniques, namely the tone calibration technique (TCT), suggests transmitting double sideband modulation with the pilot tone located at the center of its spectrum where the amplitude and phase characteristics of the channel are most stable. To 'make room' for the pilot in the presence of the Doppler shift, the equivalent low-pass data sidebands must be shaped so as to have zero response in the neighborhood of dc. Other techniques such as transparent tone-in-band (TTIB) similarly 'notch out' a hole in the center of the data spectrum for location of the pilot. An alternate possibility which is at the same time much more bandwidth efficient than TCT is a dual-pilot tone calibration technique (DPTCT) that symmetrically locates a pair of pilots outside the data spectrum near the band edges of the channel. The operation and performance of DPTCT are analyzed, and its effectiveness is compared to that of the single tone TCT technique.

Simon, Marvin K.↗

NASA NTRS1987

Surface flaw reliability analysis of ceramic components with the SCARE finite element postprocessor program

Gyekenyesi, J. P.↗

NASA NTRS1987

Current status of the Iosipescu shear test method

During the past several years, considerable study of the Iosipescu shear test method has been conducted by a number of investigators, including the present authors. Several different test fixture designs have been developed, and variations of the specimen size and notch configuration have been considered. Materials with properties ranging from isotropic to highly orthotropic have been tested. The response of such materials has also been predicted using finite element techniques, and attempts have been made to correlate these predictions with available experimental data. This recent work is summarized here, with particular emphasis being given to the correlations and contradictions resulting from the published works of these various investigators.

Adams, Donald F.↗

NASA NTRS1986

On fracture of fibrous composites

The problem of an existing through-the-thickness center notch in a unidirectional fibrous composite is investigated using a homogeneous anisotropic elasticity analysis in conjunction with various crack growth criteria. Tensor polynomial, strain energy density, and normal stress ratio criteria are applied as crack growth direction criteria and compared to experimental observations. For the cases and criteria studied, only the normal stress ratio criterion exhibits an ability to correctly predict crack growth direction. The normal stress ratio criterion is subsequently applied to the same cases as a failure criterion. Comparison with experiment indicates that these predictions exhibit reasonable correlation with observed critical stresses. Finally, the macroscopic approach to the problem used in this study is compared to a microscopic shear lag analysis of experimental results in the literature. The comparison indicates that both approaches yield essentially identical predictions. The comparisons made with other criteria and approaches indicate that application of the normal stress ratio within a macroscopic level analysis has great potential as an approach to modeling composite crack growth.

Beuth, J. L., Jr.↗

NASA NTRS1988

Computational Methods For Composite Structures

Selected methods of computation for simulation of mechanical behavior of fiber/matrix composite materials described in report. For each method, report describes significance of behavior to be simulated, procedure for simulation, and representative results. Following applications discussed: effects of progressive degradation of interply layers on responses of composite structures, dynamic responses of notched and unnotched specimens, interlaminar fracture toughness, progressive fracture, thermal distortions of sandwich composite structure, and metal-matrix composite structures for use at high temperatures. Methods demonstrate effectiveness of computational simulation as applied to complex composite structures in general and aerospace-propulsion structural components in particular.

Chamis, Christos C.↗

NASA NTRS1985

Constitutive modeling for isotropic materials

The objective is to develop a unified constitutive model for finite element structural analysis of turbine engine hot-section components. This effort constitutes a different approach for non-linear finite-element computer codes which have heretofore been based on classical inelastic methods. The unified constitutive theory to be developed will avoid the simplifying assumptions of classical theory and should more accurately represent the behavior of superalloy materials under cyclic loading conditions and high temperature environments. During the first two years of the program, extensive experimental correlations were made with two representative unified models. The experiments were both uniaxial and biaxial at temperatures up to 1093 C (2000 F). In addition, the unified models were adopted to the MARC finite element code and used for stress analysis of notched bar and turbine blade geometries.

Lindholm, Ulric S.↗

NASA NTRS1987

The use of airborne imaging spectrometer data to determine experimentally induced variation in coniferous canopy chemistry

It was experimentally determined whether induced differences in forest canopy chemical composition can be detected using data from the Airborne Imaging Spectrometer (AIS). Treatments were applied to an even-aged forest of Douglas fir trees. Work to date has stressed wet chemical analysis of foilage samples and correction of AIS data. Plot treatments were successful in providing a range of foliar N2 concentrations. Much time was spent investigating and correcting problems with the raw AIS data. Initial problems with groups of drop out lines in the AIS data were traced to the tape recorder and the tape drive. Custom adjustment of the tape drive led to recovery of most missing lines. Remaining individual drop out lines were replaced using average of adjacent lines. Application of a notch filter to the Fourier transform of the image in each band satisfactorily removed vertical striping. The aspect ratio was corrected by resampling the image in the line direction using nearest neighbor interpolation.

Swanberg, Nancy A.↗

NASA NTRS1987

Toward detecting California shrubland canopy chemistry with AIS data

Airborne Imaging Spectrometer (AIS)-2 data of coastal sage scrub vegetation were examined for fine spectral features that might be used to predict concentrations of certain canopy chemical constituents. A Fourier notch filter was applied to the AIS data and the TREE and ROCK mode spectra were ratioed to a flat field. Portions of the resulting spectra resemble spectra for plant cellulose and starch in that both show reduced reflectance at 2100 and 2270 nm. The latter are regions of absorption of energy by organic bonds found in starch and cellulose. Whether the relationship is sufficient to predict the concentration of these chemicals from AIS spectra will require testing of the predictive ability of these wavebands with large field sample sizes.

Price, Curtis V.↗

NASA NTRS1988

Preliminary In-Flight Boundary Layer Transition Measurements on a 45 Deg Swept Wing at Mach Numbers Between 0.9 and 1.8

A preliminary flight experiment was flown to generate a full-scale supersonic data base to aid the assessment of computational codes, to improve instrumentation for measuring boundary layer transition at supersonic speeds, and to provide preliminary information for the definition of follow-on programs. The experiment was conducted using an F-15 aircraft modified with a small cleanup test section on the right wing. Results are presented for Mach (M) numbers from 0.9 to 1.8 at altitudes from 25,000 to 55,000 ft. At M greater than or = 1.2, transition occurred near or at the leading edge for the clean configuration. The furthest aft that transition was measured was 20 percent chord at M = 0.9 and M = 0.97. No change in transition location was observed after the addition of a notch-bump on the leading edge of the inboard side of the test section which was intended to minimize attachment line transition problems. Some flow visualization was attempted during the flight experiment with both subliming chemicals and liquid crystals. However, difficulties arose from the limited time the test aircraft was able to hold test conditions and the difficulty of positioning the photo chase aircraft during supersonic test points. Therefore, no supersonic transition results were obtained.

Johnson, J. Blair↗

NASA NTRS1988

Unified constitutive models for high-temperature structural applications

Unified constitutive models are characterized by the use of a single inelastic strain rate term for treating all aspects of inelastic deformation, including plasticity, creep, and stress relaxation under monotonic or cyclic loading. The structure of this class of constitutive theory pertinent for high temperature structural applications is first outlined and discussed. The effectiveness of the unified approach for representing high temperature deformation of Ni-base alloys is then evaluated by extensive comparison of experimental data and predictions of the Bodner-Partom and the Walker models. The use of the unified approach for hot section structural component analyses is demonstrated by applying the Walker model in finite element analyses of a benchmark notch problem and a turbine blade problem.

Lindholm, U. S.↗

NASA NTRS1988

Characterization of Mode 1 and Mode 2 delamination growth and thresholds in graphite/peek composites

Composite materials often fail by delamination. The onset and growth of delamination in AS4/PEEK, a tough thermoplastic matrix composite, was characterized for mode 1 and mode 2 loadings, using the Double Cantilever Beam (DCB) and the End Notched Flexure (ENF) test specimens. Delamination growth per fatigue cycle, da/dN, was related to strain energy release rate, G, by means of a power law. However, the exponents of these power laws were too large for them to be adequately used as a life prediction tool. A small error in the estimated applied loads could lead to large errors in the delamination growth rates. Hence strain energy release rate thresholds, G sub th, below which no delamination would occur were also measured. Mode 1 and 2 threshold G values for no delamination growth were found by monitoring the number of cycles to delamination onset in the DCB and ENF specimens. The maximum applied G for which no delamination growth had occurred until at least 1,000,000 cycles was considered the threshold strain energy release rate. Comments are given on how testing effects, facial interference or delamination front damage, may invalidate the experimental determination of the constants in the expression.

Martin, Roderick H.↗

NASA NTRS1988

Fatique crack growth behavior of a single crystal alloy as observed through an in situ fatigue loading stage

An in situ fatigue loading stage inside a scanning electron microscope (SEM) was used to determine the fatigue crack growth behavior of a PWA 1480 single-crystal nickel-based superalloy. The loading stage permits real-time viewing of the fatigue damage processes at high magnification. The PWA 1480 single-crystal, single-edge notch specimens were tested with the load axis parallel to the (100) orientation. Two distinct fatigue failure mechanisms were identified. The crack growth rate differed substantially when the failure occurred on a single slip system in comparison to multislip system failure. Two processes by which crack branching is produced were identified and are discussed. Also discussed are the observed crack closure mechanisms.

Telesman, Jack↗

NASA NTRS1986

Fracture mechanics

The application of fracture mechanics to the design of ceramic structures will require the precise measurement of crack growth and fracture resistance of these materials over their entire range of anticipated service temperatures and standardized test methods for making such measurements. The development of a standard test for measuring the plane strain fracture toughness is sought. Stress intensity factor coefficients were determined for three varieties of chevron-notch specimens, and fracture toughness measurements were made on silicon nitrides, silicon carbides, and aluminum oxides to assess the performance of each specimen variety. It was determined that silicon nitride and silicon carbides have flat crack growth resistance curves, but aluminum oxide does not. Additionally, batch-to-batch differences were noticed for the aluminum oxide. Experiments are continuing to explain the rising crack growth resistance and batch-to-batch variations for the aluminum oxide.

Shannon, John L., Jr.↗

NASA NTRS1988

Properties of two composite materials made of toughened epoxy resin and high-strain graphite fiber

Results are presented from an experimental evaluation of IM7/8551-7 and IM6/18081, two new toughened epoxy resin, high strain graphite fiber composite materials. Data include ply-level strengths and moduli, notched tension and compression strengths and compression-after-impact assessments. The measured properties are compared with those of other graphite-epoxy materials.

Dow, Marvin B.↗

NASA NTRS1988

Thermo-viscoelastic analysis of composite materials, volume 1

Advanced composite materials, especially graphite/epoxy, are being applied to aircraft structures in order to improve performance and save weight. An important consideration in composite design is the residual strength of a structure containing holes, delaminations, or interlaminar damage when subjected to compressive loads. Recent studies have revealed the importance of viscoelastic effects in polymer-based composites. The viscoelastic effect is particularly significant at elevated temperature/moisture conditions since the matrix material is strongly affected by the environment. The solution of viscoelastic problems in composites was limited to special cases which can be solved by classical lamination theory. A finite element procedure is presented for calculating time-dependent stresses and strains in composite structures with general configurations and complicated boundary conditions. Using this procedure the in-plane and interlaminar stress distributions and histories in notched and unnotched composites were obtained for mechanical and thermal loads. Both two-dimensional and three-dimensional viscoelastic problems are analyzed. The effects of layup orientation and load spectrum on creep response and stress relaxation were also studied.

Lin, K. Y.↗

NASA NTRS1988

Constitutive modeling for isotropic materials

The third and fourth years of a 4-year research program, part of the NASA HOST Program, are described. The program goals were: (1) to develop and validate unified constitutive models for isotropic materials, and (2) to demonstrate their usefulness for structural analysis of hot section components of gas turbine engines. The unified models selected for development and evaluation were those of Bodner-Partom and of Walker. The unified approach for elastic-viscoplastic constitutive equations is a viable method for representing and predicting material response characteristics in the range where strain rate and temperature dependent inelastic deformations are experienced. This conclusion is reached by extensive comparison of model calculations against the experimental results of a test program of two high temperature Ni-base alloys, B1900+Hf and Mar-M247, over a wide temperature range for a variety of deformation and thermal histories including uniaxial, multiaxial, and thermomechanical loading paths. The applicability of the Bodner-Partom and the Walker models for structural applications has been demonstrated by implementing these models into the MARC finite element code and by performing a number of analyses including thermomechanical histories on components of hot sections of gas turbine engines and benchmark notch tensile specimens. The results of the 4-year program have been published in four annual reports. The results of the base program are summarized in this report. The tasks covered include: (1) development of material test procedures, (2) thermal history effects, and (3) verification of the constitutive model for an alternative material.

Chan, K. S.↗