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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
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

Towards a damage tolerance philosophy for composite materials and structures

A damage-threshold/fail-safe approach is proposed to ensure that composite structures are both sufficiently durable for economy of operation, as well as adequately fail-safe or damage tolerant for flight safety. Matrix cracks are assumed to exist throughout the off-axis plies. Delamination onset is predicted using a strain energy release rate characterization. Delamination growth is accounted for in one of three ways: either analytically, using delamination growth laws in conjunction with strain energy release rate analyses incorporating delamination resistance curves; experimentally, using measured stiffness loss; or conservatively, assuming delamination onset corresponds to catastrophic delamination growth. Fail-safety is assessed by accounting for the accumulation of delaminations through the thickness. A tension fatigue life prediction for composite laminates is presented as a case study to illustrate how this approach may be implemented. Suggestions are made for applying the damage-threshold/fail-safe approach to compression fatigue, tension/compression fatigue, and compression strength following low velocity impact.

O'Brien, T. Kevin↗
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

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 measurements of temperatures and displacements was identified as the most critical area.

Macrae, Gregory S.↗
NASA NTRS1990

Analyses of composite fuselage structure under various loading conditions

With the characteristics of high strength/weight and stiffness/weight ratios in advanced composite materials, composite plate structures have been successfully applied to secondary load-carrying structural components in the aerospace industry for the past two decades. Recently, filament wound composite shells are being considered for design of primary fuselage structures. To implement the design the structural response under various loading conditions needs to be predicted accurately. Therefore, it is essential to establish a rigorous analytical solution in the area of composite laminated shells. A closed form solution is presented that predicts the response of a composite shell subjected to internal pressure, axial tension, bending, and torsion. The material of the shell is assumed to be general cylindrically anisotropic. Based on the theory of cylindrical anisotropic elasticity coupled partial differential governing equations are developed using Lehknitskii's stress function approach. The general expressions for the stresses and displacements in the composite cylinders under these loading conditions is discussed. Three examples (45) off-axis unidirectional, (45/45) unsymmetric, and (45/45)(sub s) symmetric angle-ply fiber-reinforced laminated shells are shown to illustrate the effect of radius-to-thickness ratios, coupling, and stacking sequence.

Yuan, Fuh-Gwo↗
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

Design and analysis of multilayer x ray/XUV microscope

The design and analysis of a large number of normal incidence multilayer x ray microscopes based on the spherical mirror Schwarzschild configuration is examined. Design equations for the spherical mirror Schwarzschild microscopes are summarized and used to evaluate mirror parameters for microscopes with magnifications ranging from 2 to 50x. Ray tracing and diffraction analyses are carried out for many microscope configurations to determine image resolution as a function of system parameters. The results are summarized in three publication included herein. A preliminary study of advanced reflecting microscope configurations, where aspherics are used in place of the spherical microscope mirror elements, has indicated that the aspherical elements will improve off-axis image resolution and increase the effective field of view.

Shealy, David L.↗
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 NTRS1990

Mechanical characterization of unnotched SCS6/Ti-15-3 metal matrix composites at room temperature

The Ti-15-3 metal matrix composites containing silicon-carbide (SCS6) fibers, in five different lay-ups, have been tested at room temperature to determine static strengths and mechanical properties. Experimental data and predicted values of the laminate properties and strengths showed good correlation. The off-axis laminate plies (that is, 90 and 45 deg) suffered fiber/matrix interface failures at stress levels as low as 20 ksi, thus significantly affecting the mechanical properties of the laminate. Edge replicas were used to verify the fiber/matrix separations. Microscopic examinations determined that the fiber/matrix failures were occurring in the titanium/silicon reaction layer. Fatigue tests were performed on unnotched specimens to determine the number of cycles to failure versus cyclic stress level. It was found that the stress in the 0 deg fiber could be used to correlate the fatigue life of different laminates containing 0 deg plies.

Johnson, W. S.↗
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 NTRS1991

Analysis of laminated composite shells under internal pressure

A closed form solution is presented that predicts the response of filament wound composite shells subjected to internal pressure. The material of the shell is assumed to be general cylindrically anisotropic. Based on the theory of cylindrical anisotropic elasticity coupled differential equations are developed using Lekhnitskii's stress function approach. Two composite systems, graphite/epoxy and glass/epoxy, are selected to demonstrate the influence of degree of material anisotropy and fiber orientations on the axial and induced twisting deformation. Detailed stress distributions of (45) off-axis unidirectional and (45/-45)s symmetric angle-ply fiber-reinforced laminated shells are shown to illustrate the effect of radius-to-thickness ratio and stacking sequence.

Yuan, F. G.↗
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 NTRS1991

Micro-optic lens for data storage

A new type of microlens for data storage applications that has improved off-axis performance is described. The lens consists of a micro Fresnel pattern on a curved substrate. The radius of the substrate is equal to the focal length of the lens. If the pattern and substrate are thin, the combination satisfies the Abbe sine condition. Therefore, the lens is free of coma. We analyze a 0.5 numerical aperture, 0.50 mm focal length lens in detail. A 0.16 numerical aperture lens was fabricated holographically, and results are presented.

Milster, T. D.↗
NASA NTRS1991

A comparison of three popular test methods for determining the shear modulus of composite materials

Three popular shear tests (the 10 degree off-axis, the plus or minus 45 degree tensile, and the Iosipescu specimen tested in the modified Wyoming fixture) for shear modulus measurement are evaluated for a graphite-epoxy composite material system. A comparison of the shear stress-strain response for each test method is made using conventional strain gage instrumentation and moire interferometry. The uniformity and purity of the strain fields in the test sections of the specimens are discussed, and the shear responses obtained from each test technique are presented and compared. For accurate measurement of shear modulus, the 90 degree Iosipescu specimen is recommended.

Ho, Henjen↗
NASA NTRS1991

Characterization of elastic-viscoplastic properties of an AS4/PEEK thermoplastic composite

The elastic-viscoplastic properties of an AS4/PEEK (APC-2) thermoplastic composite were characterized at 24 C (75 F) and 121 C (250 F) by using a one-parameter viscoplasticity model. To determine the strain-rate effects, uniaxial tension tests were performed on unidirectional off-axis coupon specimens with different monotonic strain rates. A modified Bodner and Partom's model was also used to describe the viscoplasticity of the thermoplastic composite. The experimental results showed that viscoplastic behavior can be characterized quite well using the one-parameter overstress viscoplasticity model.

Yoon, K. J.↗
NASA NTRS1990

Straylight comparison of SIRTF designs with different aperture stop locations

The APART/PADE program is used to examine whether the SIRTF aperture stop can be moved back to the primary mirror without jeopardizing the requirement for natural background-limited observations. The straylight performance of the baseline design, which has the aperture stop at the secondary mirror, is compared to that of a revised design with the stop at the primary mirror. Both designs include reimaging optics and a LYOT stop, as well as a chopping secondary mirror. The straylight due to off-axis sources and telescope thermal emission is investigated at 50 and 550 microns. It is found that in SIRTF, where every instrument will include reimaging optics and a LYOT stop, the aperture stop may be moved to the primary mirror with modest degradation of the straylight performance.

Dinger, Ann S.↗
NASA NTRS1991

A comparison of three shear test methods for composite materials

A graphite-epoxy composite material system is used to evaluate the performance of three popular shear tests - the 10-deg off-axis, the +/-45-deg tension, and the Iosipescu specimen tested in the modified Wyoming fixture. A comparison of the shear stress-strain response for each test method is made using a conventional strain gage instrument and moire interferometry. The uniformity and purity of the strain fields in the test sections of the specimens are discussed, and the shear responses obtained from each test are presented and compared. It is shown that the shear stress-strain responses obtained by instrumenting only one face of the 90-deg Iosipescu specimen could give erroneous results. The +/-45-deg tensile and 0-deg Iosipescu specimens were not affected by front-to-back face shear strain variations. Correction factors could be applied to bring all responses together, within the limits of the material uniformity, which was itself documented in the moire fringe patterns.

Ho, H.↗
NASA NTRS1991

Aerodyne Research mobile infrared methane monitor

An improved real-time methane monitor based on infrared absorption of the 3.39 micron line of a HeNe laser is described. Real time in situ measurement of methane has important applications in stratospheric and tropospheric chemistry, especially when high accuracy measurements can be made rapidly, providing fine spatial-scale information. The methane instrument provides 5 ppb resolution in a 1 sec averaging time. A key feature in this instrument is the use of magnetic (Zeeman) broadening to achieve continuous tunability with constant output power over a range of 0.017/cm. The instruments optical absorption path length is 47 m through sampled air held at 50 torr in a multipass cell of the Herriott (off-axis resonator) type. A microprocessor controls laser frequency and amplitude and collects data with minimal operator attention. The instrument recently has been used to measure methane emissions from a variety of natural and artificial terrestrial sources.

Mcmanus, J. B.↗