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At least 703 records · Page 39

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NASA NTRS1984

A simple application of the Bailey-Orowan creep model to Fe-39.8 at. pct Al and gamma/gamma prime - alpha

The results of a study to determine the recovery rates and work-hardening coefficients for creep from constant cross-head speed compressive tests are presented. Stressing and straining rates are computed from measured time-load curves obtained from compression testing between 1200 and 1400 K of several B2 crystal structure Fe-39.8 Al intermetallic materials and the directionally solidified eutectic alloy gamma/gamma prime alpha. These quantities are then fitted to the universal form of the Bailey-Orowan equation for creep. The recovery rates were found to be functions of nominal strain rate, stress, and temperature, while the hardening coefficients were dependent only on temperature. While the work-hardening coefficient for gamma/gamma prime - alpha was about 0.05 of the elastic modulus, the work-hardening coefficients for Fe-39.8 at. pct Al were less than 0.002 of the modulus.

Whittenberger, J. D.↗

NASA NTRS1982

Strain rate characterization of unidirectional graphite/epoxy composite

For an investigation of problems of composites at high strain rates with dominant wave propagation effects, such as dynamically loaded composite jet engine blades, the longitudinal, transverse, and in-plane shear properties of unidirectional SP 288/AS graphite/epoxy were measured at various strain rates ranging from quasi-static to over 500/s. The test specimens were rings 10.16 cm in diameter, 2.54 cm long, 6 to 8 plies thick, and with fibers at 0, 90, and 10 deg off-axis. Quasi-static testing was conducted in a fixture which applies hydraulic pressure to the ring specimens, and dynamic testing was conducted by applying an internal pressure pulse through a liquid in a special fixture. Dynamic stress-strain curves are presented. For the 0-deg specimens, the modulus increases with strain up to 20 percent of the static value. The modulus and strength increase sharply in the 90-deg rings with strain rate reaching values two to three times the corresponding static rate.

Daniel, I. M.↗

NASA NTRS1984

Processing of sintered alpha SiC

Processing methods of sintered alpha SiC for engine applications are developed in a cost effective manner, using a submicron sized powder blended with sintering aids (boron and carbon). The processes for forming a green powder compact, such as dry pressing, cold isostatic pressing and green machining, slip casting, aqueous extrusion, plastic extrusion, and injection molding, are described. Dry pressing is the simplest route to component fabrication, and is carried out at approximately 10,000 psi pressure, while in the cold isostatic method the pressure could go as high as 20,000 psi. Surfactants are added to control settling rates and casting characteristics in the slip casting. The aqueous extrusion process is accomplished by a hydraulic ram forcing the aqueous mixture through a die. The plastic forming processes of extrusion and injection molding offer the potential of greater diversity in shape capacity. The physical properties of sintered alpha SiC (hardness, Young's modulus, shear modulus, and thermal diffusivity) are extensively tested. Corrosion resistance test results of silicon carbide are included.

Storm, R. S.↗

NASA NTRS1984

Effects of Constituent Properties on Compression Failure Mechanisms

Compression failure mechanisms were investigated through the analysis of matrix-embedded fiber bundles. The use of fiber bundle specimens can provide much needed information on compression failure mechanisms because failure of the bundle is well contained and can be monitored during testing. The method can clearly distinguish between buckling-induced failure and shear-induced failure. The results indicate that WY and T300 graphite fibers and E-glass fiber fail in buckling while the high-modulus P75S graphite fiber fails in shear. Buckling-induced failure becomes more evident with low-modulus fiber in softer epoxy.

Hahn, H. T.↗

NASA NTRS1985

Crack problems for bonded nonhomogeneous materials under antiplane shear loading

The singular nature of the crack tip stress field in a nonhomogeneous medium having a shear modulus with a discontinuous derivative was investigated. The problem is considered for the simplest possible loading and geometry, namely the antiplane shear loading of two bonded half spaces in which the crack is perpendicular to the interface. It is shown that the square-root singularity of the crack tip stress field is unaffected by the discontinuity in the derivative of the shear modulus. The problem is solved for a finite crack and extensive results are given for the stress intensity factors.

Erdogan, F.↗

NASA NTRS1985

Solid Rocket Motor Case (SRMC) dynamic sensitivity to changes in composite properties

Traditional composite lamination theory was used to compute the constitutive matrices using a potential range of lamina properties. Sensitivity of vibrational frequencies was examined for variations in constituent materials and geometric properties. The material component interactions examined were the lamina longitudinal and transverse Young's modulus, the lamina Poisson's ratio, and the shear modulus. Fiber and resin properties were related to the frequency response through lamina properties. Geometric properties examined are void ratio and helical wind angle. Capabilities and applications of the SPAR and NASTRAN computer codes are examined for laminated cylindrical shells.

Tolbert, R. N.↗

NASA NTRS1985

An Elastic Analysis of a Plated Bone to Determine Fracture Gap Motion

An elastic analysis to determine fracture gap motions occurring in the osteotomized and plated canine femur was performed using the finite element program NASTRAN. The femur was idealized as a hollow right cylinder, and transverse anisotropy was assumed for the elastic properties of the bone. A 3-D 360 degree model consisting of 224 isoparametric quadrilateral hexahedral and 11 beam elements was created. A range of plate stiffnesses was tested by varying the modulus of elasticity of the plate from 207 GPa to 1 GPA. Moments were applied in the plane of the plate, about the axis of the plate, and in the plane of the screws. Results showed that, for plates of typical geometry and elastic modulus under 10 GPa, the contribution to fracture gap motion occurring due to deformation in the bone was negligible compared to that contribution from deformation in the plate.

Cooke, F. W.↗

NASA NTRS1984

Interply layer degradation effects on composite structural response

Recent research activities at NASA Lewis Research Center to computationally evaluate the effects of interply layer progressive weakening (degradation) on the structural response of a composite beam are summarized. The structural responses of interest include: (1) bending, (2) buckling, (3) free vibrations, (4) periodic excitation, and (5) impact. Finite element analysis was used for the computational evaluations. The interply layer degradation effects on the various structural responses were determined and assessed as a function of the interply layer modulus varying from 1 million psi down to 1000 psi and even lower for some limiting cases. The results obtained show that the interply layer degradation has generally negligible effectss on composite structural response and, therefore, structural integrity, unless the interply layer modulus degrades to about 10,000 psi or less.

Chamis, C. C.↗

NASA NTRS1985

A redetermination of the luminosity, distance, and reddening of Beta Lyrae

The most likely distance to the Beta Lyrae system is redetermined to lie at 370 pc, corresponding to a distance modulus of 7.8 mag, while the correct modulus value probably lies in the 7.7-8.0 mag range, corresponding to a distance range between 345 and 400 pc. On the basis of Wilson's (1974) model, the phase-averaged absolute visual magnitude of the primary component is -4.1 mag; this star is therefore a very luminous giant, classifiable as B8.5 or B9, with a luminosity class of II-Ib. The present data are based on optical and IUE scans of the brightest visual companion to Beta Lyrae, HD 174664, together with an analysis of the H line profiles in its spectrum.

Dobias, J. J.↗

NASA NTRS1985

Characterization of solid particle erosion resistance of ductile metals based on their properties

This paper presents experimental results pertaining to spherical glass bead and angular crushed glass particle impingement. A concept of energy absorption to explain the failure of material is proposed and is correlated with the erosion characteristics of several pure metals. Analyses of extensive erosion data indicate that the properties - surface energy, specific melting energy, strain energy, melting point, bulk modulus, hardness, atomic volume - and the product of the parameters - linear coefficient of thermal expansion x bulk modulus x temperature rise required for melting, and ultimate resilience x hardness - exhibit the best correlations. The properties of surface energy and atomic volume are suggested for the first time for correlation purposes and are found to correlate well with erosion rates at different angles of impingement. It further appears that both energy and thermal properties contribute to the total erosion.

Rao, P. V.↗

NASA NTRS1985

Investigation of composite materials property requirements for sonic fatigue research

Experimental techniques for determining the extensional and bending stiffness characteristics for symmetric laminates are presented. Vibrational test techniques for determining the dynamic modulus and material damping are also discussed. Partial extensional stiffness results intially indicate that the laminate theory used for predicting stiffness is accurate. It is clearly shown that the laminate theory can only be as accurate as the physical characteristics describing the lamina, which may vary significantly. It is recommended that all of the stiffness characteristics in both extension and bending be experimentally determined to fully verify the laminate theory. Dynamic modulus should be experimentally evaluated to determine if static data adequately predicts dynamic behavior. Material damping should also be ascertained because laminate damping is an order of magnitude greater than found in common metals and can significantly effect the displacement response of composite panels.

Patrick, H. V. L.↗

NASA NTRS1985

Fracture surface analysis in composite and titanium bonding

To understand the mechanical properties of fiber-reinforced composite materials, it is necessary to understand the mechanical properties of the matrix materials and of the reinforcing fibers. Another factor that can affect the mechanical properties of a composite material is the interaction between the fiber and the matrix. In general, composites with strong fiber matrix bonding will give higher modulus, lower toughness composites. Composites with weak bonding will have a lower modulus and more ductility. The situation becomes a bit more complex when all possibilities are examined. To be considered are the following: the properties of the surface layer on the fiber, the interactive forces between polymer and matrix, the surface roughness and porosity of the fiber, and the morphology of the matrix polymer at the fiber surface. In practice, the surface of the fibers is treated to enhance the mechanical properties of a composite. These treatments include anodization, acid etching, high temperature oxidation, and plasma oxidation, to name a few. The goal is to be able to predict the surface properties of carbon fibers treated in various ways, and then to relate surface properties to fiber matrix bonding.

Devilbiss, T. A.↗

NASA NTRS1985

Composite materials for space structures

Composite materials have been extensively used for spacecraft structures because of their unique combination of high specific strength and stiffness, low weight, good dimensional stability, and high specific damping capacity. High-modulus graphite reinforced epoxies have been the principal composite material system used. For early applications composites were selected primarily to reduce weight. However, the development of high precision structures such as The Space Telescope required high modulus composites with a coefficient of thermal expansion close to zero to meet design requirements. The use of advanced composites for space structures is reviewed in this paper. Barriers likely to limit further applications of composites are discussed and highlights of current research to improve composites are presented. New developments Jn composites technology which could impact future spacecraft systems are also reviewed.

Composites↗

NASA NTRS1986

Preliminary design methods for fiber reinforced composite structures employing a personal computer

The objective of this project was to develop a user-friendly interactive computer program to be used as an analytical tool by structural designers. Its intent was to do preliminary, approximate stress analysis to help select or verify sizing choices for composite structural members. The approach to the project was to provide a subroutine which uses classical lamination theory to predict an effective elastic modulus for a laminate of arbitrary material and ply orientation. This effective elastic modulus can then be used in a family of other subroutines which employ the familiar basic structural analysis methods for isotropic materials. This method is simple and convenient to use but only approximate, as is appropriate for a preliminary design tool which will be subsequently verified by more sophisticated analysis. Additional subroutines have been provided to calculate laminate coefficient of thermal expansion and to calculate ply-by-ply strains within a laminate.

Eastlake, C. N.↗

NASA NTRS1986

Temperature effects on the universal equation of state of solids

Recently it has been argued based on theoretical calculations and experimental data that there is a universal form for the equation of state of solids. This observation was restricted to the range of temperatures and pressures such that there are no phase transitions. The use of this universal relation to estimate pressure-volume relations (i.e., isotherms) required three input parameters at each fixed temperature. It is shown that for many solids the input data needed to predict high temperature thermodynamical properties can be dramatically reduced. In particular, only four numbers are needed: (1) the zero pressure (P=0) isothermal bulk modulus; (2)it P=0 pressure derivative; (3) the P=0 volume; and (4) the P=0 thermal expansion; all evaluated at a single (reference) temperature. Explicit predictions are made for the high temperature isotherms, the thermal expansion as a function of temperature, and the temperature variation of the isothermal bulk modulus and its pressure derivative. These predictions are tested using experimental data for three representative solids: gold, sodium chloride, and xenon. Good agreement between theory and experiment is found.

Vinet, P.↗

NASA NTRS1986

Leaf spring made of fiber-reinforced resin

A leaf spring made of a matrix reinforced by at least two types of reinforcing fibers with different Young's modulus is described in this Japanese patent. At least two layers of reinforcing fibers are formed by partially arranging the reinforcing fibers toward the direction of the thickness of the leaf spring. A mixture of different types of reinforced fibers is used at the area of boundary between the two layers of reinforced fibers. The ratio of blending of each type of reinforced fiber is frequently changed to eliminate the parts where discontinuous stress may be applied to the leaf spring. The objective of this invention is to prevent the rapid change in Young's modulus at the boundary area between each layer of reinforced fibers in the leaf spring.

Hori, J.↗

NASA NTRS1985

Densification and properties of alpha-silicon carbide

The sintering behavior of alpha-SiC with B and C sintering aids and without sintering aids was investigated experimentally. The sintering was performed for 30-60 min in argon flow at 2150 and 2200 C. The density of the sintered bars was determined by weighing and measuring the bars, four-point flexure tests were conducted on machined bars, and the Weibull modulus was determined by regression analysis. It is found that type 1 alpha-SiC powder, which does not densify, can be sintered to a high final density (in excess of 96 percent of the theoretical value) using B and C sintering aids. Flexure test results and Weibull modulus data suggest that the strength of the alpha-SiC ceramic is greatly affected by processing flaws and also by inclusions and machining flaws.

Dutta, S.↗

NASA NTRS1985

The crack problem for bonded nonhomogeneous materials under antiplane shear loading

Erdogan, F.↗