Showing papers by "Wright-Patterson Air Force Base published in 1975"

The deformation and fracture of TiAl at elevated temperatures

Abstract: The tensile properties of the intermetallic compound TiAl have been determined at several temperatures in the range 25 to 1000°C. Additional variables studied were the influence of strain rate and the effect of exposure to oxidizing conditions prior to testing. The modes of deformation under the various testing conditions were studied in the electron microscope, the modes of fracture were studied in the scanning electron microscope, and these data were correlated with the mechanical properties. The results indicate that the ductilebrittle transition behavior of TiAl at about 700°C is controlled by the trailinga/6 [112] partial dislocation components of thea [011] superdislocations overcoming their pinning barriers. It was also shown that prior exposure to oxidizing conditions does not markedly influence the mechanical properties of TiAl.

An energy release rate criterion for mixed mode fracture

Abstract: The initial energy release rate for a branch crack propagating at an arbitrary angle from an existing crack tip is obtained in a simple fashion and in closed form by using a continuity assumption. It is then postulated that the branch crack propagates in the direction which causes the energy release rate to be a maximum and that initiation occurs when the value of this release rate reaches a critical value. It is shown that these postulates yield results identical to the maximum stress theory, since the direction in which the maximum circumferential stress occurs is also the direction causing the maximum energy release rate.

Approximate Stresses in an Orthotropic Plate Containing a Circular Hole

Abstract: An approximate solution in the form of a polynomial is presented for the normal stress distribution adjacent to a circular hole in an infinite orthotropic plate. Comparison of the approximate solut...

Comparison of Shock-Induced Two- and Three- Dimensional Incipient Turbulent Seperation

Abstract: 2 Garnet, H. and Armen, H., Evaluation of Numerical Time Integration Methods as Applied to Elastic-Plastic Dynamic Problems Involving Wave Propagation, Rept. RE-475, March 1974, Research Dept., Grumman Aircraft Corp., Bethpage, N.Y. 3 Stoodley, G. R. and Ball, D. J., Mathematical Background of Two Numerical Integration Techniques for Ordinary Differential Equations, Memo. RM-192, Oct. 1961, Research Dept, Grumman Aircraft Corp., Bethpage, N.Y. 4 Mantus, M., Lerner, E., and Elkins, W., "Landing Dynamics of the Lunar Excursion Module (Method of Analysis)," Rept. LED-5206A, Revised April 10, 1967, Grumman Aircraft Corp., Bethpage, N.Y. 5 Lerner, E. and Mantus, M., "Dynamics of Unsymmetric Landing," Rept. ADR 02-10-10-62-1, Jan. 1963, Grumman Aircraft Corp, Bethpage, N.Y. 6 Donnell, L. H., "Longitudinal Wave Transmission and Impact," Transactions of the ASME, Vol, 52, 1930, pp. 153-167. 7 DeJuhasz, K. J., "Graphical Analysis of Impact of Bars Stressed Above the Elastic Range," Journal of the Franklin Institute, Vol. 248, No. 2, Aug. 1949, pp. 113-142. 8 Davids, N. and Kumar, S., "A Contour Method for One Dimensional Pulse Propagation in Elastic-Plastic Materials," Proceedings of the Third U.S. National Congress of Applied Mechanics, Brown University, Providence, R.I., 1958, pp. 502-512. 9 Garnet, H. and Armen, H., One Dimensional Elastic-Plastic Wave Propagation and Boundary Reflections, Memo, Grumman Research Department, Bethpage, N.Y. (in preparation).

Magnetocrystalline anisotropy of Sm Co 5 and its interpretation on a crystal-field model

Abstract: Results of magnetization measurements on carefully prepared single crystals of Sm${\mathrm{Co}}_{5}$ are presented over the temperature range 4-970 K. The contribution of the rare-earth sublattice magnetocrystalline anisotropy was evaluated using the data on Y${\mathrm{Co}}_{5}$ as "a blank." These results are interpreted on the basis of a singleion model. The following Hamiltonians were employed to obtain eigenvalues: $\mathcal{H}(\mathrm{parallel}\mathrm{to} c \mathrm{axis})=\ensuremath{\lambda}\stackrel{\ensuremath{\rightarrow}}{\mathrm{L}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}+{\mathcal{H}}_{\mathrm{CF}}+2{\ensuremath{\mu}}_{B}{S}_{z}{H}_{\mathrm{ex}}$, $\mathcal{H}(\mathrm{perpendicular}\mathrm{to} c \mathrm{axis})=\ensuremath{\lambda}\stackrel{\ensuremath{\rightarrow}}{\mathrm{L}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}+{\mathcal{H}}_{\mathrm{CF}}+2{\ensuremath{\mu}}_{B}{S}_{x}{H}_{\mathrm{ex}}$. ${H}_{\mathrm{ex}}$ was regarded as arising mainly from the cobalt sublattice. Contributions arising from the multiplets $^{6}H_{\frac{7}{2}}$ and $^{6}H_{\frac{9}{2}}$ to the ground multiplet $^{6}H_{\frac{5}{2}}$ were included. The crystal field was evaluated in terms of Racah's tensor-operator technique employing $3\ensuremath{-}j$ and $6\ensuremath{-}j$ symbols. With the two sets of eigenvalues obtained from the above expressions, the anisotropy energy to rotate the magnetization direction from the axis to the plane was calculated and compared with experimental results on single crystals of Sm${\mathrm{Co}}_{5}$. It is shown that the sign of the crystal-field parameter predicts the correct easy direction of magnetization and that the free-energy values calculated from 4 to 970 K are in reasonable agreement with experiment.

Stress intensity factors for some through-cracked fastener holes

Abstract: A stress intensity factor solution is developed for a large plate containing radial hole cracks loaded with arbitrary crack face pressure. When the pressure is defined as the unflawed hoop stress surrounding a mechanical fastener, stress intensity factor calibrations are readily computed by the linear superposition principle. Results obtained in this manner agree well with previous solutions determined for open holes loaded in remote tension. The potential usefulness of the present analysis is further demonstrated with application to specific fastener configurations, including interference fit fasteners, pin-loaded plates, and cold-worked holes.

Active Flutter Suppression—A Flight Test Demonstration

Abstract: The first flight test demonstration of active flutter suppression has been successfully completed. The Control Configured Vehicles (CCV) B-52 test airplane was twice flown 10 knots faster than its flutter speed relying solely on an automatic control system for adequate damping. The design, safety considerations, mechanization, ground testing, and flight testing of the flutter mode control system are reported. Comparisons between flight test and theoretical results are presented. The system was tested at heavy and light airplane weights and tested for compatibility with simultaneous ride control, maneuver load control, fatigue reduction, and augmented stability.

Numerical Solution for Supersonic Turbulent Flow over a Compression Ramp

Abstract: A modified eddy viscosity model is incorporated into the compressible Navier-Stokes equations. The modification attempts to reproduce the response of turbulence to a severe pressure gradient in the flowfield. This relaxation phenomenon is described by an exponential decay of the unperturbed eddy viscosity coefficient downstream of the perturbation in terms of a prescribed length scale. The system of equations is solved by MacCormack's time-splitting explicit numerical scheme for a series of compression corner configurations. Computations are performed for ramp angles varying from 15 to 25° at a Mach number of 2.96 and a Reynolds number of 10 7. Calculations utilizing the modified eddy viscosity for the interacting turbulent flow compare very well with experimental measurements, particularly in the prediction of the upstream pressure propagation and location of the separation and the reattachment points. Good agreement is also attained between the measured and calculated density profiles in the viscous-inviscid interaction region.

Residual strength characterization of laminated composites subjected to impact loading

Abstract: : An analogy between damage inflicted by a single point hard particle impact and damage inflicted by inserting a flaw of known dimensions in a static tensile coupon is discussed. The results suggest that residual strength can be predicted as a function of kinetic energy of impact by executing two experiments, a static tensile test on an unflawed specimen and a static tensile test on a coupon previously subjected to a single point impact. The model appears to be accurate for impact velocities which are less than the penetration velocity. For velocities above complete penetration, the residual strength is identical to the static strength of a coupon with a hole having the same diameter as the impacting particle. Comparison of various materials indicates that the impact strength of composite materials is strongly influenced by the strain energy to failure of the reinforcement. (Author)

Instability of plastic flow in the directions of pure shear: I. Theory

Abstract: The localization of plastic flow (plastic instability) is a prelude to fracture initiation. Prior treatments are based mainly on the phenomenon of necking (load maximum) which has for its basis fluctuations in cross-sectional area. A second basic mode of localization has been confirmed experimentally. This is the localization of flow within a plastic zone along characteristic directions (direction of pure shear or zero extensional strain). The localization is effected by the activation of tangential velocity discontinuities along the characteristics. A necessary condition is that the material simulate the ideal plastic state. The conditions for this second mode of localization are formulated. A general condition for instability involving both modes is derived. In situations in which both modes are possible, necking is always expected to precede the localization along characteristics.

High‐Temperature Creep of Y2O3‐Stabilized ZrO2

Abstract: The compression creep behavior of Y2O3-stabilized ZrO2 (YSZ) was studied at temperatures to 2000 ° C. The function of Y2O3 content and grain size was tested in specimens with various impurity concentrations and porosity distributions. For relatively fine-grained specimens, creep rates increased with the 1.5 power of the applied stress at low stresses and with the third power at high stresses. The results for coarse-grained specimens can, in general, be fit by the cube dependence. The 1.5 power can be reduced to a linear dependence by correcting for an apparent threshold stress, which decreases with increasing temperature. Creep activation energies for YSZ are 128 ± 10 kcal/mol, independent of Y2O3 content, impurity level, grain size, and porosity distribution. In addition, over a broad range of temperatures and stresses the absolute values of the steady-state creep rates are influenced only by grain size and O2 partial pressure.

Vibratory behavior of twisted cantilevered plates

Abstract: A combined numerical-experimental study of the effects of varying tip twist and increasing centrifugal loading on the resonant characteristics of cantilevered plates is presented. The finite element computer program, NASTRAN, is used to compute the natural frequencies, mode shapes, and normalized shear stress distribution for each mode of vibration for cantilevered plates having, a), varying degrees of tip twist and, b) increasing centrifugal loading. For the case of zero centrifugal load, the resulting mode shapes are compared to those obtained experimentally using holographic iiiterferometry. The agreement between the two is found to be quite good. For increasing centrifugal loading, it is found that the nodal lines of the flexural modes of vibration shift toward the plate root. Increased centrifugal loading is also found to strongly effect the character of some of the plate-like vibration modes of the cantilevered plate.

Ground-state wave function for shallow-donor electrons in silicon. I. Isotropic electron-nuclear-double-resonance hyperfine interactions

Abstract: In a new theoretical investigation of electrons bound to the shallow-donor impurities (P, As, Sb) in silicon we have calculated the Fermi-contact hyperfine-interaction constants for the ${\mathrm{Si}}^{29}$ lattice nuclei surrounding the impurity nucleus. We have used a model potential which represents the impurity potential, a wave-vector-dependent dielectric function which represents the screening of the impurity potential by the silicon lattice, and pseudopotential Bloch functions for the calculation of the wave-function density of the $1s({A}_{1})$ ground state at the nuclear sites. The restrictions of the effective-mass theory to a single band and to conduction-band-minima Bloch functions have been removed because we represent the wave function in terms of a Bloch-function expansion throughout the Brillouin zone for several bands. With the use of the Fermi-contact constants calculated from this wave function, we have been able to make definite matchings of ${\mathrm{Si}}^{29}$ lattice sites and the electron-nuclear-double-resonance (ENDOR) shells measured by Hale and Mieher. With these matchings we are able to explain the experimentally observed lack of inversion symmetry of the electronic wave function and to explain most of the donor dependence of the experimental ENDOR data. Tutorial type discussions of the comparison of this calculation with effective-mass calculations are presented. These results contain several features that are due to the complex values of the wave-function expansion coefficients and the complex values of the Bloch functions; these features cannot be obtained from any real-valued effective-mass Hamiltonian.

Influence of contaminants on the CO 2 electric-discharge laser

Abstract: In CO 2 -N 2 -He electric-discharge lasers, small concentrations of impurities may be present in the original gases, or new species may be generated by the discharge. In order to determine the effects of such impurities, controlled amounts of possible contaminants were introduced into the amplifier of a CO 2 probe-amplifier system. Small-signal gain was reduced in all cases when CO, O 2 , NO, N 2 O, or NO 2 molecules were added. It was found that less than 0.1 percent of NO 2 or N 2 O markedly reduces the laser gain and alters the discharge impedance. A discharge model which describes a three-component positive-column plasma (electrons, positive ions, and negative ions) yields results consistent with the observations. The influence of attachment and detachment rates on molecular gas discharge characteristics and stability are studied.

Correlated Hartree-Fock energy bands for diamond

Abstract: The screened-exchange-plus---Coulomb-hole method has been used to add correlation to Hartree-Fock energy bands These correlation corrections, or energy shifts, are state dependent due to the screened-exchange term The Coulomb-hole term is constant throughout the zone in our diagonal approximation for the screening (dielectric) function The energy shifts raise the occupied bands and lower the conduction bands with a resulting decrease in energy differences The calculation has been done for diamond using linear combination of atomic orbitals Hartree-Fock bands and the Penn-model dielectric function The energy shifts, which tend to flatten the bands, were computed at general points in the first zone Values of 56 and 76 eV were obtained for the indirect and direct band gaps respectively, both of which are in close agreement with experiment The diagonal part of the random-phase approximation was also used in the calculation and it was found to produce about two-thirds of the correlation obtained with the Penn model

Strength and Elastic Modulus of a Randomly-Distributed Short Fiber Composite:

Abstract: Experimental results for tests of strength and elastic modulus of a randomly-distributed short fiber composite are reported in this paper. Three different tests were employed: tension, 3-point flexure, and 4-point flexure. A statistical approach based on Weibull distribution was applied to account for the difference between tensile and flexural test results. Mate rial variation, as exhibited in the experimental scatter for elastic modulus, and its effect on strength were shown not to be negligible. Test results also showed wider experimental scatter than observed with continuous fiber composites. Consequently considerable penalty had to be paid by reducing the design strength in order to assure the same level of confidence and reliability.

Decomposition in Al-Zn alloys: Part I. Isothermal decomposition in an Al-22 at. pct Zn-0.1 at. pct Mg alloy

Abstract: A small-angle X-ray scattering (SAS) study has been made on solution treated and isothermally annealed specimens of an Al-22 at. pct Zn-0.1 at. pct Mg alloy. The changes in peak position and integrated area of the SAS spectra with time and temperature indicate that de composition is nearly complete immediately after quenching, in agreement with the earlier interpretation that Gerold and Merz placed on the results of Rundman and Hilliard in the binary Al-22 at. pct Zn alloy. Furthermore, structural changes occurring during annealing are consistent with a coarsening or maturation of the fluctuations in the solution. The domi nant wavelength varies ast 1/3 over a large time span and the temperature dependence of the coarsening process yields an activation energy of 94.2 kJ/mole. The effect of Mg is to re tard the formation of the equilibrium phases while having a small effect on the growth of the composition fluctuations during the coarsening process.

Work hardening of ordered Cu3Au

Abstract: Single crystals of fully-ordered Cu3Au of [001] orientation were subjected to tensile deformation and studied by stress-strain measurements, slip-line analysis, and transmission electron microscopy. The dislocation microstructure, which was studied as a function of plastic strain, consisted of jogged superdislocations and superdislocation dipoles at small strains. Dense tangles of superdislocation dipoles and multipole configurations were observed at large strains. The formation of antiphase boundary tubes is a rare occurrence and does not make a significant contribution to work hardening in ordered Cu3Au. Instead, the formation of non-aligned sessile jogs in superdislocations of screw character, the dragging stress of such jogs and the formation of superdislocation edge dipoles play an important role in the work hardening of multiple-slip oriented Cu3Au single crystals.

Screened-exchange plus Coulomb-hole correlated Hartree-Fock energy bands for LiF

Abstract: Green's-function techniques have been used to derive screened-exchange plus Coulomb-hole correlation corrections to Hartree-Fock energy bands. These correlation corrections or energy shifts consist of a statically screened exchange term which is state dependent, and a Coulomb-hole term which is constant in our diagonal approximation for the screening (dielectric) function. These energy shifts raise the occupied bands and lower the conduction bands with a resulting decrease in energy differences. The calculation was done for LiF at general points in the first zone using linear-combination-of-atomic-orbitals Hartree-Fock energy bands and the Penn-model dielectric function. A change in.the band gap at $\ensuremath{\Gamma}$ of 5.0 eV was obtained compared with the experimental value of 9.3 eV. The calculation was also done using the random-phase-approximation (RPA) dielectric function and it was found that the diagonal part of the RPA gives less than half of the correlation obtained with the Penn model.

Instability of plastic flow in the directions of pure shear: II. Experimental

Abstract: Two aspects of the phenomenon of plastic instability in direction of pure shear are examined, namely that the condition dσ = 0 (maximum in true flow stress) is necessary for localization of flow along characteristics as defined in continuum plasticity, and that fracture is initiated and propagates along characteristics. Two types of sheet specimens were employed, the standard-type flat sheet specimens, and specimens simulating both plane stress and plane strain. Grids were placed on gage sections and photographs were taken successively in the plastic range to enable strains to be calculated and instabilities to be observed and recorded. The principal variable in the flat specimen test was theW/T ratio (width to thickness). In the plane* strain specimens, both the gage length (constantW/T) and the strength level of the material (quenched and tempered AISI 4340 steel) were varied. A maximum in true flow stress is found consistently at the onset of instabilities. Fracture propagated consistently along the instability band-matrix interface. Variations in specimen geometry produces significant changes in stress state, directions of characteristics, and ductility. For a given specimen geometry, plane strain is more closely approached the higher the strength level of the material. In mixed mode fracture paths slant fracture is associated with the more embrittling stress state.

Atmospheric Distortions in a Retroreflected Laser Signal

Abstract: This paper presents and analyzes a model for the instantaneous received field that has been transmitted from a laser and then retroreflected from a corner cube, all in the presence of atmospheric turbulence and attenuation. The characteristics of this field are examined as a function of distance from the axis of the transmitted laser beam, and the merits of using shared optics (coaxial transmitter/receiver) from geometric and atmospheric considerations are examined. in particular, it is shown that angle-of-arrival (AOA) fluctuations induced by turbulence would be self-corrected in a retrosystem for a shared-optics configuration only if the transceiver and retroreflector are in each other’s effective near fields or are separated by less than a self-correction range as defined in the text. The results are shown to be in agreement with recent experimental observations.