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NASA NTRS1988

Advanced probabilistic methods for quantifying the effects of various uncertainties in structural response

The effects of actual variations, also called uncertainties, in geometry and material properties on the structural response of a space shuttle main engine turbopump blade are evaluated. A normal distribution was assumed to represent the uncertainties statistically. Uncertainties were assumed to be totally random, partially correlated, and fully correlated. The magnitude of these uncertainties were represented in terms of mean and variance. Blade responses, recorded in terms of displacements, natural frequencies, and maximum stress, was evaluated and plotted in the form of probabilistic distributions under combined uncertainties. These distributions provide an estimate of the range of magnitudes of the response and probability of occurrence of a given response. Most importantly, these distributions provide the information needed to estimate quantitatively the risk in a structural design.

Nagpal, Vinod K.↗

NASA NTRS1988

Unsteady pressure and structural response measurements of an elastic supercritical wing

Results are presented which define unsteady flow conditions associated with high dynamic response experienced on a high aspect ratio elastic supercritical wing at transonic test conditions while being tested in the NASA Langley Transonic Dynamics Tunnel. The supercritical wing, designed for a cruise Mach number of 0.80, experienced the high dynamic response in the Mach number range from 0.90 to 0.94 with the maximum response occurring at a Mach number of approximately 0.92. At the maximum wing response condition the forcing function appears to be the oscillatory chordwise movement of strong shocks located on both the wing upper and lower surfaces in conjunction with the flow separating and reattaching in the trailing edge region.

Eckstrom, Clinton V.↗

NASA NTRS1988

Aeroelastic response of metallic and composite propfan models in yawed flow

An analytical investigation of aeroelastic response of metallic and composite propfan models in yawed flow was performed. The analytical model is based on the normal modes of a rotating blade and the three dimensional unsteady lifting surface aerodynamic theory including blade mistuning. The calculated blade stresses or strains are compared with published wind tunnel data on two metallic and three composite propfan wind tunnel models. The comparison shows a good agreement between theory and experiment. Additional parametric results indicate that blade response is very sensitive to the blade stiffness and also to blade frequency and mode shape mistuning. From these findings, it is concluded that both frequency and mode shape mistuning should be included in aeroelastic response analysis. Furthermore, both calculated and measured strains show that combined blade frequency and mode shape mistuning has beneficial effects on response due to yawed flow.

Kaza, Krishna Rao V.↗

NASA NTRS1987

Optokinetic and vestibulo-ocular reflex responses to an unpredictable stimulus

Horizontal plane optokinetic (OKR) and vestibuloocular reflex eye movements were obtained from normal subjects in response to pseudorandom rotational stimulation. Results showed large intersubject variability in OKR responses. Typical OKR responses had approximately constant gain over 0.02-1.5 Hz. Response phase was near zero below 0.1 Hz with increasing phase lags as frequency increased consistent with time delays of 180 ms. Results suggested that visual motion information could be significant in the control of eye movements up to 6.1 Hz. Pseudorandom optokinetic stimulation induced motion sickness symptoms in approximately 20 percent of the 213 subjects tested.

Peterka, Robert J.↗

NASA NTRS1988

Unsteady pressure and structural response measurements on an elastic supercritical wing

Results are presented which define unsteady flow conditions associated with high dynamic response experienced on a high aspect ratio elastic supercritical wing at transonic test conditions while being tested in the NASA Langley Transonic Dynamics Tunnel. The supercritical wing, designed for a cruise Mach number of 0.80, experienced the high dynamic response in the Mach number range from 0.90 to 0.94 with the maximum response occurring at a Mach number of approximately 0.92. At the maximum wing response condition the forcing function appears to be the oscillatory chordwise movement of strong shocks located on both the wing upper and lower surfaces in conjuction with the flow separating and reattaching in the trailing edge region.

Eckstrom, Clinton V.↗

NASA NTRS1987

Evanescent pressure gradient response in the upper ocean to subinertial wind stress forcing of finite wavelength

A schematic model is used to interpret field observations related to the mixed layer response to wind stress at subinertial frequencies. It is shown that subinertial density and pressure fluctuations can arise locally from the finite wavelength character of the wind stress forcing as a fundamental part of the upper ocean transient, wind-driven response on time scales of 2-10 pendulum days. Evanescent vertical motions arise which alter the density field of the pycnocline, and hence the pressure field over the entire upper ocean. It is thus found that in the real ocean driven by wind stress, a transient geostrophic response exists which can be as large or larger than the transient Eckman response.

White, Warren B.↗

NASA NTRS1989

Theory of the pulse response from a small antenna in a magnetized plasma

The electrostatic plasma response to a small pulsed antenna in a magnetic field is analyzed. The ringing of the plasma at three discrete frequencies--the upper-hybrid frequency and two resonance cone branch frequencies--is evidenced, and the amplitudes of these frequency responses is determined as a function of the characteristic plasma frequencies, the angle of observation with respect to the magnetic field, and the pulse length. Applications to plasma diagnostics are discussed. It is shown that the upper hybrid response and the response at either of the resonance cone branch frequencies is adequate information to determine the plasma density, and the magnetic field magnitude and angle.

Grabbe, Crockett L.↗

NASA NTRS1988

Aeroelastic response of metallic and composite propfan models in yawed flow

An analytical investigation of aeroelastic response of metallic and composite propfan models in yawed flow was performed. The analytical model is based on the normal modes of a rotating blade and the three-dimensional unsteady lifting surface aerodynamic theory including blade mistuning. The calculated blade stresses or strains are compared with published wind tunnel data on two metallic and three composite propfan wind tunnel models. The comparison shows a good agreement between theory and experiment. Additional parametric results indicate that blade response is very sensitive to the blade stiffness and also to blade frequency and mode shape mistuning. From these findings, it is concluded that both frequency and mode shape mistuning should be included in aeroelastic response analysis. Furthermore, both calculated and measured strains show that combined blade frequency and mode shape mistuning has beneficial effects on response due to yawed flow.

Kaza, Krishna Rao↗

NASA NTRS1989

Effects of mistuning and matrix structure on the topology of frequency response curves

The stability of a frequency response curve under mild perturbations of the system's matrix is investigated. Using recent developments in the theory of singularities of differentiable maps, it is shown that the stability of a response curve depends on the structure of the system's matrix. In particular, the frequency response curves of a cylic system are shown to be unstable. Consequently, slight parameter variations engendered by mistuning will induce a significant difference in the topology of the forced response curves, if the mistuning transformation crosses the bifurcation set.

Afolabi, Dare↗

NASA NTRS1989

Static and free-vibrational response of semi-circular graphite-epoxy frames with thin-walled open sections

Experiments were conducted to measure the three-dimensional static and free vibrational response of two graphite-epoxy, thin-walled, open section frames. The frames are semi-circular with a radius of three feet, and one specimen has an I cross section and the other has a channel cross section. The flexibility influence coefficients were measured in static tests for loads applied at midspan with the ends of the specimens clamped. Natural frequencies and modes were determined from vibrational tests for free and clamped end conditions. The experimental data is used to evaluate a new finite element which was developed specifically for the analysis of curved, thin-walled structures. The formulation of the element is based on a Vlasov-type, thin-walled, curved beam theory. The predictions from the finite element program generally correlated well with the experimental data for the symmetric I-specimen. Discrepancies in some of the data were found to be due to flexibility in the clamped end conditions. With respect to the data for the channel specimen, the correlation was less satisfactory. The finite element analysis predicted the out-of-plane response of the channel specimen reasonably well, but large discrepancies occurred between the predicted in-plane response and the experimental data. The analysis predicted a much more compliant in-plane response than was observed in the experiments.

Collins, J. Scott↗

NASA NTRS1990

Inelastic response of metal matrix composites under biaxial loading

Elements of the analytical/experimental program to characterize the response of silicon carbide titanium (SCS-6/Ti-15-3) composite tubes under biaxial loading are outlined. The analytical program comprises prediction of initial yielding and subsequent inelastic response of unidirectional and angle-ply silicon carbide titanium tubes using a combined micromechanics approach and laminate analysis. The micromechanics approach is based on the method of cells model and has the capability of generating the effective thermomechanical response of metal matrix composites in the linear and inelastic region in the presence of temperature and time-dependent properties of the individual constituents and imperfect bonding on the initial yield surfaces and inelastic response of (0) and (+ or - 45)sub s SCS-6/Ti-15-3 laminates loaded by different combinations of stresses. The generated analytical predictions will be compared with the experimental results. The experimental program comprises generation of initial yield surfaces, subsequent stress-strain curves and determination of failure loads of the SCS-6/Ti-15-3 tubes under selected loading conditions. The results of the analytical investigation are employed to define the actual loading paths for the experimental program. A brief overview of the experimental methodology is given. This includes the test capabilities of the Composite Mechanics Laboratory at the University of Virginia, the SCS-6/Ti-15-3 composite tubes secured from McDonnell Douglas Corporation, a text fixture specifically developed for combined axial-torsional loading, and the MTS combined axial-torsion loader that will be employed in the actual testing.

Mirzadeh, F.↗

NASA NTRS1989

Solar activity, the QBO, and tropospheric responses

The suggestion that galactic cosmic rays (GCR) as modulated by the solar wind are the carriers of the component of solar variability that affects weather and climate has been discussed in the literature for 30 years, and there is now a considerable body of evidence that supports it. Variations of GCR occur with the 11 year solar cycle, matching the time scale of recent results for atmospheric variations, as modulated by the quasibiennial oscillation of equatorial stratospheric winds (the QBO). Variations in GCR occur on the time scale of centuries with a well defined peak in the coldest decade of the little ice age. New evidence is presented on the meteorological responses to GCR variations on the time scale of a few days. These responses include changes in the vertical temperature profile in the troposphere and lower stratosphere in the two days following solar flare related high speed plasma streams and associated GCR decreases, and in decreases in Vorticity Area Index (VAI) following Forbush decreases of GCR. The occurrence of correlations of GCR and meteorological responses on all three time scales strengthens the hypothesis of GCR as carriers of solar variability to the lower atmosphere. Both short and long term tropospheric responses are understandable as changes in the intensity of cyclonic storms initiated by mechanisms involving cloud microphysical and cloud electrification processes, due to changes in local ion production from changes in GCR fluxes and other high energy particles in the MeV to low GeV range. The nature of these mechanisms remains undetermined. Possible stratospheric wind (particularly QBO) effects on the transport of HNO3 and other constituents incorporated in cluster ions and possible condensation and freezing nuclei are considered as relevant to the long term variations.

Tinsley, Brian A.↗

NASA NTRS1989

Scaling effects in the impact response of graphite-epoxy composite beams

In support of crashworthiness studies on composite airframes and substructure, an experimental and analytical study was conducted to characterize size effects in the large deflection response of scale model graphite-epoxy beams subjected to impact. Scale model beams of 1/2, 2/3, 3/4, 5/6, and full scale were constructed of four different laminate stacking sequences including unidirectional, angle ply, cross ply, and quasi-isotropic. The beam specimens were subjected to eccentric axial impact loads which were scaled to provide homologous beam responses. Comparisons of the load and strain time histories between the scale model beams and the prototype should verify the scale law and demonstrate the use of scale model testing for determining impact behavior of composite structures. The nonlinear structural analysis finite element program DYCAST (DYnamic Crash Analysis of STructures) was used to model the beam response. DYCAST analysis predictions of beam strain response are compared to experimental data and the results are presented.

Jackson, Karen E.↗

NASA NTRS1989

Carotid baroreflex response following 30 days exposure to simulated microgravity

The mechanism of the carotid-baroreflex response to weightlessness was investigated in human subjects exposed to simulated microgravity (30 days of 6-day head-down bed rest followed by 5 days of recovery). Baroreceptor-cardiac reflex responses were elicited by a complex sequence of pressure changes delivered to a neck chamber device. The shape of the sigmoid baroreceptor-cardiac response curve was examined for alterations and the occurrence of resetting, as well as for a possible association of the impaired baroreflex function with hypotension during the postexposure orthostatic stress. It was found that the exposure to head-down bed rest caused a significant shift on the R-R interval axis, which paralleled reductions and elevations in baseline HR such that the baseline R-R (operational point) remained in the same position on the response curve. This shift in the location of the reflex relation indicates a significant resetting of the carotid baroreceptors, which may represent an appropriate adaptation which contributes to the maintenance of a constant resting arterial blood pressure before, during, and after bed rest, observed in these study.

Convertino, V. A.↗

NASA NTRS1990

Effect of a central redistribution of fluid volume on response to lower-body negative pressure

Cardiovascular responses to lower-body negative pressure (LBNP) were studied following 1 hour of 6-deg head-down tilt to determine whether a redistribution of blood volume toward the central circulation modifies the subsequent response to orthostatic stress. Responses of 12 men, ages 30-39 years, were evaluated by electrocardiography, impedance cardiography, sphygmomanometry, and measurement of calf circumference. During the LBNP that followed head-down tilt, as compared with control LBNP (no preceding head-down tilt) subjects, had smaller stroke volume and cardiac output, greater total peripheral resistance, and less calf enlargement. These differences reflect differences in the variables immediately preceding LBNP. Magnitudes of the responses from pre-LBNP to each pressure stage of the LBNP procedure did not differ between protocols. Mean and diastolic arterial pressures were slightly elevated after LBNP-control, but they fell slightly during LBNP post-tilt.

Tomaselli, Clare M.↗

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 NTRS1990

Infrared detector responsivity measured simultaneously at multiple frequencies

The responsivity (the ratio of the output electrical signal to the incident radiation power) of an IR detector is measured at several frequencies simultaneously. A computer-controlled variable frequency chopper is used. Chopper modulation of a blackbody is square-wave-like, hence measurements can be taken not only at the fundamental frequency but also at the harmonics. The technique gives excellent agreement with the classical technique of measuring the fundamental response alone at a set of modulation frequencies. With a small enough aperture and a high enough chopper speed, agreement can be within five percent out to 31 times the fundamental frequency. The technique allows the rapid and automated acquisition of the frequency response of IR detectors, which may then be screened for responsivity, noise, and noise equivalent power.

Brasunas, John C.↗

NASA NTRS1989

Nonlinear response of composite tubes under combined thermomechanical loading

An analytical investigation was conducted to study the effect of material nonlinearities on the response of graphite/epoxy composite tubes subjected to combined axial and torsional loading. The endochronic theory of Valanis (1971) was employed to model the nonlinear response of the individual graphite/epoxy layers. Thermal loading was included in the analysis in order to incorporate the effect of residual stresses on subsequent mechanical response. The predictions of the analytical model are compared with the response of P75/934 graphite/epoxy tubes with a stacking sequence of (15/0/+/- 10/0/-15)s. Good qualitative correlation between analysis and experiment is obtained when microstructural details of the tube's cross-section, residual stresses and nonlinear effects are taken into account.

Derstine, M. S.↗