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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Soft X-ray/extreme ultraviolet images of the solar atmosphere with normal incidence multilayer optics

High-resolution images of the sun in the soft-X-ray/XUV range have been obtained with normal-incidence multilayer coated optics (MCOs) during a sounding-rocket flight on October 23, 1987 from White Sands. MCOs were used in two Cassegrain telescopes, as off-axis primary mirrors, and as tertiary mirrors operating with a Wolter-Schwarzschild (WS) grazing-incidence mirror. The inherent energy-selective property of MCOs allowed distinct groups of emission lines to be isolated in the solar corona and the transition region. Images were recorded at 173 and 256 A with the Cassegrain telescopes, at 256 A with a Herschelian telescope, and at 44, 173, and 256 A with the WS hybrid telescope. In addition, soft-X-ray images in the 8-18-A bandpass were obtained at the prime focus of the WS optics. The images show many features of the solar corona and transition region, including magnetically confined loops of hot solar plasma, coronal plumes, polar coronal holes, supergranulation, and features associated with overlying cool prominences.

Lindblom, Joakim F.↗
NASA NTRS1989

A detailed investigation of the micromechanisms of compressive failure in open hole composite laminates

Micromechanisms responsible for compressive failure in open-hole composite laminates were investigated. Specimens containing circular center holes were loaded in compression to failure at a slow rate, and the surface damage development during loading was monitored. Several coupon compression tests were interrupted prior to catastrophic failure to allow for the assessment of fiber shear crippling and/or fiber microbuckling by NDE techniques. From these sectioning studies, the extent and mode of failure were determined, and a three-dimensional schematic of the shear crippling zone was developed. Results show that the majority of the damage in compression-loaded open-hole composite laminates is shear crippled or microbuckled 0-deg fibers; in many cases, however, the shear crippling damage within the 0-deg plies produced shear strains and damage in the adjacent off-axis plies.

Guynn, E. Gail↗
NASA NTRS1990

Two-Wavelength Optical-Path-Difference Mapping

Proposed technique for measuring shapes and alignments of reflectors based on use of two-wavelength absolute-distance interferometer to generate optical-path-difference maps of reflecting or refracting surfaces. Facilitates such tasks as determining manufacturing and alignment errors of off-axis segment of large-aperture paraboloidal telescope mirror, or aligning all segments of such mirror. Suitable for use where reflecting surfaces highly aspherical, initial misalignments large, and/or surface errors exceed optical wavelengths.

Manhart, Paul K.↗
NASA NTRS1989

Mechanics of damping for fiber composite laminates including hygro-thermal effects

An integrated mechanics theory was developed for the modeling of composite damping from the micromechanics to the laminate level. Simplified, design oriented equations based on hysteretic damping are presented for on-axis plies, off-axis plies, and laminates including the effect of temperature, moisture, and interply hysteretic damping. The temperature rise within vibrating composite laminates resulting from strain energy dissipation is also modeled, and their coupled hygro-thermo-mechanical response is predicted. The method correlates well with reported damping measurements. Application examples illustrate the effect of various ply, laminate, and hygro-thermal parameters on the overall damping performance of composite laminates.

Saravanos, D. A.↗
NASA NTRS1989

Orthotropic elasto-plastic behavior of AS4/APC-2 thermoplastic composite in compression

Uniaxial compression tests were performed on off-axis coupon specimens of unidirectional AS4/APC-2 thermoplastic composite at various temperatures. The elasto-plastic and strength properties of AS4/APC-2 composite were characterized with respect to temperature variation by using a one-parameter orthotropic plasticity model and a one-parameter failure criterion. Experimental results show that the orthotropic plastic behavior can be characterized quite well using the plasticity model, and the matrix-dominant compressive strengths can be predicted very accurately by the one-parameter failure criterion.

Sun, C. T.↗
NASA NTRS1988

Coherent phasing of segmented mirrors

The technical issues associated with a coherently phased segmented mirror can be divided into two types. The first involves the issues of manufacturing the surface quality of the mirror segments themselves (coherent phasing of individual segments). The second involves assembly issues of initializing in 1 G and retaining in 0 G an aggregate segmented mirror (coherent phasing between individual segments). Using a rectangular coordinate system at the vertex of a mirror segment, the rigid body motions are the six translational and rotational degrees of freedom. A first cut primary mirror wavefront error budget is presented. For a minimum operational wavelength of 30 micrometers, the derived values from the budget are: segment surface quality (0.45 micrometers RMS), radius mismatch (50 PPM), segment piston error (1.3 micrometers, segment tip/tilt error (0.6 microrad). Either active or passive segmented mirrors can be addressed, but if the surface quality of the off-axis segment and the radius matching requirements can be passively met, then only segment alignment need be sensed and controlled during operation in orbit. For the active mirror case, dimensional stability of the mirror material during operation is a key factor in establishing the degree of active figure control required.

Jones, Peter A.↗
NASA NTRS1990

Feasibility study for the advanced one-dimensional high temperature optical strain measurement system, phase 3

The Instrumentation and Control Technology Division is developing optical strain measurement systems for applications using high temperature wire and fiber specimens. This feasibility study has determined that stable optical signals can be obtained from specimens at temperatures beyond 2,400 C. A system using an area array sensor is proposed to alleviate off-axis decorrelation arising from rigid body motions. A digital signal processor (DSP) is recommended to perform speckle correlations at a rate near the data acquisition rate. Design parameters are discussed, and fundamental limits on the speckle shift strain measurement technique are defined.

Lant, Christian T.↗
NASA NTRS1990

A constitutive model for AS4/PEEK thermoplastic composites under cyclic loading

Based on the basic and essential features of the elastic-plastic response of the AS4/PEEK thermoplastic composite subjected to off-axis cyclic loadings, a simple rate-independent constitutive model is proposed to describe the orthotropic material behavior for cyclic loadings. A one-parameter memory surface is introduced to distinguish the virgin deformation and the subsequent deformation process and to characterize the loading range effect. Cyclic softening is characterized by the change of generalized plastic modulus. By the vanishing yield surface assumption, a yield criterion is not needed and it is not necessary to consider loading and unloading separately. The model is compared with experimental results and good agreement is obtained.

Rui, Yuting↗
NASA NTRS1989

A 58 x 62 pixel Si:Ga array camera for 5 - 14 micron astronomical imaging

A new infrared array camera system has been successfully applied to high background 5 - 14 micron astronomical imaging photometry observations, using a hybrid 58 x 62 pixel Si:Ga array detector. The off-axis reflective optical design incorporating a parabolic camera mirror, circular variable filter wheel, and cold aperture stop produces diffraction-limited images with negligible spatial distortion and minimum thermal background loading. The camera electronic system architecture is divided into three subsystems: (1) high-speed analog front end, including 2-channel preamp module, array address timing generator, bias power suppies, (2) two 16 bit, 3 microsec per conversion A/D converters interfaced to an arithmetic array processor, and (3) an LSI 11/73 camera control and data analysis computer. The background-limited observational noise performance of the camera at the NASA/IRTF telescope is NEFD (1 sigma) = 0.05 Jy/pixel min exp 1/2.

Gezari, D. Y.↗
NASA NTRS1988

Monochromatic X-ray and XUV imaging with multilayer optics

The development of techniques for the fabrication of multilayer coated mirrors which can function as energy selective X-ray and XUV mirrors at normal incidence has greatly expanded the options available to the astronomical spectroscopist. A rocket spectroheliograph which utilizes multilayer optics in three configurations has been developed. These are: (1) as Cassegrain telescopes for imaging at 256 A (He II) and 173 A (Fe IX, X); (2) as tertiary mirrors used with a conventional Wolter I telescope for imaging at 44 A (Si XI, XII), 173 A, and 256 A; and (3) as off-axis spherical mirrors for imaging at 44 A and 256 A. The paper reports on laboratory images and performance measurements obtained with these optical systems, and briefly on Solar Observations.

Walker, A. B. C., Jr.↗
NASA NTRS1990

Nonlinear response of unidirectional boron/aluminum

Experimental results obtained for unidirectional boron/aluminum subjected to combined loading using off-axis tension, compression and Iosipescu shear specimens are correlated with a nonlinear micromechanics model. It is illustrated that the nonlinear response in the principal material directions is markedly influenced by the different loading modes and different ratios of the applied stress components. The observed nonlinear response under pure and combined loading is discussed in terms of initial yielding, subsequent hardening, stress-interaction effects and unloading-reloading characteristics. The micromechanics model is based on the concept of a repeating unit cell representative of the composite-at-large and employs the unified theory of Bodner and Partom to model the inelastic response of the matrix. It is shown that the employed micromechanics model is sufficiently general to predict the observed nonlinear response of unidirectional boron/aluminum with good accuracy.

Pindera, M.-J.↗
NASA NTRS1989

Shear testing of fiber reinforced metal matrix composites

This paper outlines the elements of a combined experimental/analytical methodology for accurate shear characterization of unidirectional composites in the linear and nonlinear range, with particular attention devoted to metal matrix composites. It is illustrated that consistent results can be obtained for a large class of composites from two commonly employed shear test methods currently in use by composites researchers when the influence of various factors that affect the determination of the actual shear response is properly analyzed. These factors include the effects of material anisotropy, specimen geometry, manner of load introduction, and test fixture design on the stress and deformation fields in the test section of off-axis and Iosipescu specimens. Common errors associated with the measurement of deformation fields and calculation of stress fields are discussed and quantified. Particular problems in the determination of the shear response of unidirectional boron/aluminum using the Iosipescu test are illustrated and discussed.

Pindera, Marek-Jerzy↗
NASA NTRS1990

Status of structural analysis of 30 cm diameter ion optics

Three structural finite element programs are compared with theory, experimental data, and each other to evaluate their usefulness for modeling the thermomechanical deflection of ion engine electrodes. Two programs, NASTRAN and MARC, used a Cray XMP and the third, Algor, used an IBM compatible personal computer. The shape of the applied temperature gradient greatly affects off-axis displacement, implying that an accurate temperature distribution is required to analyze new designs. The use of bulk material constants to model the perforated electrodes was investigated. The stress and displacement predictions are shown to be sensitive to the temperature gradient and the Young's modulus, and insensitive to number of nodes, above some minimum value, and the Poisson ratio used. The models are shown to be useful tools for evaluating designs. Experimental measurement of temperatures and displacements was identified as the most critical area for further work.

Macrae, Gregory S.↗
NASA NTRS1990

Experimental observations and finite element analysis of the initiation of fiber microbuckling in notched composite laminates

A better understanding of the factors that affect the semi-circular edge-notched compressive strength is developed, and the associated failure mode(s) of thermoplastic composite laminates with multidirectional stacking sequences are identified. The primary variables in this investigation are the resin nonlinear shear constitutive behavior, stacking sequence (orientation of plies adjacent to the 0 degree plies), resin-rich regions between the 0 degree plies and the off-axis supporting plies, fiber/matrix interfacial bond strength, and initial fiber waviness. Two thermoplastic composite material systems are used in this investigation. The materials are the commercial APC-2 (AS4/PEEK) and a poor interface experimental material, AU4U/PEEK, designed for this investigation. Notched compression specimens are studied at 21, 77, and 132 C. Geometric and material nonlinear two-dimensional finite element analysis is used to model the initiation of fiber microbuckling of both the ideal straight fiber and the more realistic initially wavy fiber. The effects of free surface, fiber constitutive properties, matrix constitutive behavior, initial fiber curvature, and fiber/matrix interfacial bond strength on fiber microbuckling initiation strain levels are considered.

Guynn, E. Gail↗
NASA NTRS1990

Temperature-dependent tensile and shear response of P100/6061 graphite-aluminum

This paper summarizes the results of an investigation conducted to characterize tensile and shear response of P100/6061 graphite-aluminum composite in the temperature range -101 to 260 C. The off-axis tension test and Iosipescu shear test methods were employed to meet this objective. The experimental results indicate that both the fiber-dominated and matrix-dominated elastic properties exhibit relatively small changes in the considered temperature range. The effect of temperature on yielding and strength, however, is significant under longitudinal and shear loading. Residual stresses play a significant role in controlling the variation of the yield stress with temperature. The observed low transverse strength of the composite is explained by interfacial defects between adjacent precursor wires induced during fabrication.

Fujita, Takahiro↗
NASA NTRS1989

Composite fracture using the normal stress ratio theory

Experiments were performed on center-notched specimens of unidirectional AS4/3501-6 graphite-epoxy subjected to varied far-field loading conditions. Specimens tested include on-axis and off-axis tensile coupons and Iosipescu shear specimens, allowing the study of normal, shear, and combined far-field loading conditions. From the tests, the direction of initial crack extension and the far-field stresses at crack initiation and fracture are obtained. The normal stress ratio theory is subsequently applied within a macroscopic-level homogeneous anisotropic stress solution to predict the direction of crack extension, critical stresses, and other crack growth behavior for the experiments. Comparison between experimental results and theoretical predictions indicates that application of the normal stress ratio theory within a macroscopic-level stress analysis has significant potential as an approach to modeling composite crack growth.

Beuth, J. L., Jr.↗
NASA NTRS1990

Polarization properties of non-symmetric retroreflectors

The on- and off-axis polarizing properties of asymmetric retroreflectors are studied in detail for various angles of incidence and for various incident linear polarization states. An analytic model is developed by applying Fresnel law to the incident and reflecting radiation on each facet of the retroreflector. It is shown that the polarization state of retroreflected radiation is a sensitive function of incident angle, incident polarization rate, and retroreflector material. These characteristics may be applicable to the determination of the relative angular position between the retroreflector and the analyzer.

An, C.-H.↗
NASA NTRS1986

Large Deformation Dynamic Bending of Composite Beams

Studies were conducted on the large deformation response of composite beams subjected to a dynamic axial load. The beams were loaded with a moderate eccentricity to promote bending. The study was primarily experimental but some finite element results were obtained. Both the deformation and the failure of the beams were of interest. The static response of the beams was also studied to determine potential differences between the static and dynamic failure. Twelve different laminate types were tested. The beams tested were 23 in. by 2 in. and generally 30 plies thick. The beams were loaded dynamically with a gravity-driven impactor traveling at 19.6 ft/sec and quasi-static tests were conducted on identical beams in a displacement controlled manner. For laminates of practical interest, the failure modes under static and dynamic loadings were identical. Failure in most of the laminate types occurred in a single event involving 40% to 50% of the plies. However, failure in laminates with 300 or 150 off-axis plies occurred in several events. All laminates exhibited bimodular elastic properties. The compressive flexural moduli in some laminates was measured to be 1/2 the tensile flexural modulus. No simple relationship could be found among the measured ultimate failure strains of the different laminate types. Using empirically determined flexural properties, a finite element analysis was reasonably accurate in predicting the static and dynamic deformation response.

Derian, E. J.↗