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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 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Time-temperature behavior of a unidirectional graphite/epoxy composite

A testing program to determine the time-temperature response of unidirectional T300/934 graphite/epoxy materials is presented. The short-term creep test results of tension specimens with the load at various angles to the fiber direction and at various temperatures are reported, showing that the material is elastic at all temperatures when the fiber is in the load direction. However, when the load is transverse to the fibers, the viscoelastic response varies from small amounts at room temperature to large amounts at temperatures above the 180 C transition temperature. The time-temperature superposition principle or the method of reduced variables were used to determine compliance master curves for each fiber angle, and a viscoelastic analog to the elastic orthotropic transformation equation was used incrementally to predict the master curves for the tensile compliance of the off-axis specimen.

Yeow, Y. T.↗
NASA NTRS1980

Accelerated characterization of graphite/epoxy composites

A method to predict the long term compliance of unidirectional off-axis laminates from short term laboratory tests is presented. The method uses an orthotropic transformation equation and the time-stress-temperature superposition principle. Short term tests are used to construct master curves for two off-axis unidirectional laminates with fiber angles of 10 and 90 degrees. Analytical predictions of long term compliance for 30 and 60 degrees laminates are made. Comparisons with experimental data are also given.

Griffith, W. I.↗
NASA NTRS1981

Creep and creep rupture of laminated graphite/epoxy composites

An incremental numerical procedure based on lamination theory is developed to predict creep and creep rupture of general laminates. Existing unidirectional creep compliance and delayed failure data is used to develop analytical models for lamina response. The compliance model is based on a procedure proposed by Findley which incorporates the power law for creep into a nonlinear constitutive relationship. The matrix octahedral shear stress is assumed to control the stress interaction effect. A modified superposition principle is used to account for the varying stress level effect on the creep strain. The lamina failure model is based on a modification of the Tsai-Hill theory which includes the time dependent creep rupture strength. A linear cumulative damage law is used to monitor the remaining lifetime in each ply.

Dillard, D. A.↗
NASA NTRS1980

Creep rupture behavior of unidirectional advanced composites

A 'material modeling' methodology for predicting the creep rupture behavior of unidirectional advanced composites is proposed. In this approach the parameters (obtained from short-term tests) required to make the predictions are the three principal creep compliance master curves and their corresponding quasi-static strengths tested at room temperature (22 C). Using these parameters in conjunction with a failure criterion, creep rupture envelopes can be generated for any combination of in-plane loading conditions and ambient temperature. The analysis was validated experimentally for one composite system, the T300/934 graphite-epoxy system. This was done by performing short-term creep tests (to generate the principal creep compliance master curves with the time-temperature superposition principle) and relatively long-term creep rupture tensile tests of off-axis specimens at 180 C. Good to reasonable agreement between experimental and analytical results is observed.

Yeow, Y. T.↗
NASA NTRS1980

Accelerated characterization of graphite/epoxy composites

A method to predict the long-term compliance of unidirectional off-axis laminates from short-term laboratory tests is presented. The method uses an orthotropic transformation equation and the time-stress-temperature superposition principle. Short-term tests are used to construct master curves for two off-axis unidirectional laminates with fiber angles of 10 deg and 90 deg. In addition, analytical predictions of long-term compliance for 30 deg and 60 deg laminates are made. Comparisons with experimental data are also given.

Griffith, W. I.↗
NASA NTRS1981

The viscoelastic response of a graphite/epoxy laminate

An accelerated characterization procedure has been developed for time-dependent compliances and strengths of resin-matrix composites. The procedure, which is based on a time-temperature-stress superposition principle, is applied to a graphite/epoxy laminate T300/934. Creep rupture analytical models are discussed, and model predictions are compared with experimental data. Master curves for the creep compliances are given.

Brinson, H. F.↗
NASA NTRS1982

The prediction of long term viscoelastic properties of fiber reinforced plastics

A method for the experimental and analytical accelerated characterization of long term viscoelastic properties is presented. The time-temperature-stress superposition principle (TTSSP) is shown to serve as the basis for determining long term compliance data from short term creep test results. Nonlinear viscoelastic theories are discussed to provide the framework for the mathematically modeling of such a process. A time dependent Tsai-Hill-Zhurkov failure theory is used to determine long term failure properties from short term data. Compliance and failure data are incorporated in an incremental lamination theory to make long term laminate predictions. Comparisons are made between theory and experiment.

Brinson, H. F.↗
NASA NTRS1983

Spectral characterization of the LANDSAT thematic mapper sensors

Data collected on the spectral characteristics of the LANDSAT-4 and LANDSAT-4 backup thematic mapper instruments, the protoflight (TM/PF) and flight (TM/F) models, respectively, are presented and analyzed. Tests were conducted on the instruments and their components to determine compliance with two sets of spectral specifications: band-by-band spectral coverage and channel-by-channel within-band spectral matching. Spectral coverage specifications were placed on: (1) band edges--points at 50% of peak response, (2) band edge slopes--steepness of rise and fall-off of response, (3) spectral flatness--evenness of response between edges, and (4) spurious system response--ratio of out-of-band response to in-band response. Compliance with the spectral coverage specifications was determined by analysis of spectral measurements on the individual components contributing to the overall spectral response: filters, detectors, and optical surfaces.

Markham, B. L.↗
NASA NTRS1984

Spectral Characterization of the LANDSAT Thematic Mapper Sensors

Data collected on the spectral characteristics of the LANDSAT-4 and LANDSAT-4 backup thematic mapper instruments, the protoflight (TM/PF) and flight (TM/F) models, respectively, are presented and analyzed. Tests were conducted on the instruments and their components to determine compliance with two sets of spectral specifications: band-by-band spectral coverage and channel-by-channel within-band spctral matching. Spectral coverage specifications were placed on: (1) band edges - points at 50% of peak response, (2) band edge slopes - steepness of rise and fall-off of response, (3) spectral flatness - evenness of response between edges, and (4) spurious system response - ratio of out-of-band response to in-band response. Compliance with the spectral coverage specifications was determined by analysis of spectral measurements on the individual components contributing to the overall spectral response: filters, detectors, and optical surfaces.

Markham, B. L.↗
NASA NTRS1983

Environmental effects and viscoelastic behavior of laminated graphite/epoxy composites

Primarily because of the polymeric matrix, graphite/epoxy composites are viscoelastic materials which exhibit creep and delayed failures. Guided by the time-temperature superposition principle, the authors are developing accelerated characterization techniques to predict the long term compliance behavior and creep rupture strength of composite materials based on short term tests at elevated temperatures. The effect of a post-cure cycle on the compliance and creep rupture strength of composite materials is discussed. The Zhurkov type failure law is applied to experimental creep rupture data of a typical laminate and is shown to correlate the data. A beneficial mechanical strengthening effect is described which significantly affects the failure strengths of specimens which have been under creep loading for a period of time. This aging effect is reversible if the specimen is allowed to recover before being loaded to failure. Other environmental aspects of composite materials are also reviewed.

Dillard, D. A.↗
NASA NTRS1983

A numerical procedure for predicting creep and delayed failures in laminated composites

A numerical procedure is described for predicting the viscoelastic response of general laminates. A nonlinear compliance model is used to predict the creep response of the individual laminae. A biaxial delayed failure model predicts ply failure. The numerical procedure, based on lamination theory, increases by increments through time to predict creep compliance and delayed failures in laminates. Numerical stability problems and experimental verification are discussed. Although the program has been quite successful in predicting creep of general laminates, the assumptions associated with lamination theory have resulted in erroneous bounds on the predicted material response. Delayed failure predictions have been conservative. Several improvements are suggested to increase the accuracy of the procedure.

Dillard, D. A.↗
NASA NTRS1984

The structures and the role of an international agency for the control of satellites

Legal questions involved in the liability of a proposed agency which would control internationally owned satellites for monitoring worldwide compliance with arms control agreements are discussed. Difficulties in acquiring the signed consent of all the relevant nations, and guaranteeing satisfactory compliance with the terms of such an agreement are noted. Additional problems to be solved comprise the construction of the ground based facilities and the satellites, the funding for the venture, and the reconciliation of the functions of the proposed agency with the sovereignty of individual states. The agency would gather, treat, and format data for signatories of arms control agreements and provide technical assistance in crisis conditions. It is concluded that the existence and functioning of the agency would reduce the amount of classified information and would consequently reduce the level of international tensions.

Dupuy, R. J.↗
NASA NTRS1986

Evaluation of a high-torque backlash-free roller actuator

The results are presented of a test program that evaluated the stiffness, accuracy, torque ripple, frictional losses, and torque holding capability of a 16:1 ratio, 430 N-m (320 ft-lb) planetary roller drive for a potential space vehicle actuator application. The drive's planet roller supporting structure and bearings were found to be the largest contributors to overall drive compliance, accounting for more than half of the total. In comparison, the traction roller contacts themselves contributed only 9 percent of the drive's compliance based on an experimentally verified stiffness model. The drive exhibited no backlash although 8 arc sec of hysteresis deflection were recorded due to microcreep within the contact under torque load. Because of these load-dependent displacements, some form of feedback control would be required for arc second positioning applications. Torque ripple tests showed the drive to be extremely smooth, actually providing some damping of input torsional oscillations. The drive also demonstrated the ability to hold static torque with drifts of 7 arc sec or less over a 24 hr period at 35 percent of full load.

Steinetz, Bruce M.↗
NASA NTRS1987

Expansion of epicyclic gear dynamic analysis program

The multiple mesh/single stage dynamics program is a gear tooth analysis program which determines detailed geometry, dynamic loads, stresses, and surface damage factors. The program can analyze a variety of both epicyclic and single mesh systems with spur or helical gear teeth including internal, external, and buttress tooth forms. The modifications refine the options for the flexible carrier and flexible ring gear rim and adds three options: a floating Sun gear option; a natural frequency option; and a finite element compliance formulation for helical gear teeth. The option for a floating Sun incorporates two additional degrees of freedom at the Sun center. The natural frequency option evaluates the frequencies of planetary, star, or differential systems as well as the effect of additional springs at the Sun center and those due to a flexible carrier and/or ring gear rim. The helical tooth pair finite element calculated compliance is obtained from an automated element breakup of the helical teeth and then is used with the basic gear dynamic solution and stress postprocessing routines. The flexible carrier or ring gear rim option for planetary and star spur gear systems allows the output torque per carrier and ring gear rim segment to vary based on the dynamic response of the entire system, while the total output torque remains constant.

Boyd, Linda Smith↗
NASA NTRS1986

Finite element analysis of the end notched flexure specimen for measuring Mode II fracture toughness

The paper presents a finite element analysis of the end-notched flexure (ENF) test specimen for Mode II interlaminar fracture testing of composite materials. Virtual crack closure and compliance techniques employed to calculate strain energy release rates from linear elastic two-dimensional analysis indicate that the ENF specimen is a pure Mode II fracture test within the constraints of small deflection theory. Furthermore, the ENF fracture specimen is shown to be relatively insensitive to process-induced cracks, offset from the laminate midplane. Frictional effects are investigated by including the contact problem in the finite element model. A parametric study investigating the influence of delamination length, span, thickness, and material properties assessed the accuracy of beam theory expressions for compliance and strain energy release rate, GII. Finite element results indicate that data reduction schemes based upon beam theory underestimate GII by approximately 20-40 percent for typical unidirectional graphite fiber composite test specimen geometries. Consequently, an improved data reduction scheme is proposed.

Gillespie, J. W., Jr.↗
NASA NTRS1987

Spin resistance evaluation of a light airplane

A brief history of stall/spin technology for light general aviation airplanes and proposed criteria to describe desirable characteristics of a spin-resistant airplane are presented. Flight tests of a representative light airplane to evaluate compliance with and usefulness of the criteria are presented. The baseline airplane configuration would not meet the spin resistance criteria. Tests of the airplane with a wing leading edge modification to enhance its spin resistance showed compliance with the proposed criteria.

Stough, H. Paul, III↗
NASA NTRS1987

Evaluation of airplane spin resistance using proposed criteria for light general aviation airplanes

Proposed criteria to describe desirable characteristics of a spin-resistant airplane are reviewed. Flight tests of two representative light airplanes to evaluate compliance with the criteria are also presented. The baseline airplane configurations would not meet the spin resistance criteria. Tests of the airplanes with a wing leading edge modification to enhance spin resistance showed compliance with the proposed criteria. The tests demonstrated the usefulness of the criteria when the criteria are applied in the manner described.

Stough, H. Paul, III↗
NASA NTRS1988

Evaluation of a high-torque backlash-free roller actuator

The results are presented of a test program that evaluated the stiffness, accuracy, torque ripple, frictional losses, and torque holding capability of a 16:1 ratio, 430 N-m (320 ft-lb) planetary roller drive for a potential space vehicle actuator application. The drive's planet roller supporting structure and bearings were found to be the largest contributors to overall drive compliance, accounting for more than half the total. In comparison, the traction roller contacts themselves contributed only 9 percent of the drive's compliance based on an experimentally verified stiffnesss model. Torque ripple tests showed the drive to be extremely smooth, actually providing some damping of input torsional oscillations. The drive also demonstrated the ability to hold static torque with drifts of 7 arc sec or less over a 24-hour period at 35 percent of full load.

Steinetz, Bruce M.↗