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At least 523 records · Page 29

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NASA NTRS1985

Short-wavelength buckling and shear failures for compression-loaded composite laminates

The short-wavelength buckling (or the microbuckling) and the interlaminar and inplane shear failures of multi-directional composite laminates loaded in uniaxial compression are investigated. A laminate model is presented that idealizes each lamina. The fibers in the lamina are modeled as a plate, and the matrix in the lamina is modeled as an elastic foundation. The out-of-plane w displacement for each plate is expressed as a trigonometric series in the half-wavelength of the mode shape for laminate short-wavelength buckling. Nonlinear strain-displacement relations are used. The model is applied to symmetric laminates having linear material behavior. The laminates are loaded in uniform end shortening and are simply supported. A linear analysis is used to determine the laminate stress, strain, and mode shape when short-wavelength buckling occurs. The equations for the laminate compressive stress at short-wavelength buckling are dominated by matrix contributions.

Shuart, M. J.↗

NASA NTRS2011

Vertical Drop Testing and Simulation of Anthropomorphic Test Devices

A series of 14 vertical impact tests were conducted using Hybrid III 50th Percentile and Hybrid II 50th Percentile Anthropomorphic Test Devices (ATDs) at NASA Langley Research Center. The purpose of conducting these tests was threefold: to compare and contrast the impact responses of Hybrid II and Hybrid III ATDs under two different loading conditions, to compare the impact responses of the Hybrid III configured with a nominal curved lumbar spine to that of a Hybrid III configured with a straight lumbar spine, and to generate data for comparison with predicted responses from two commercially available ATD finite element models. The two loading conditions examined were a high magnitude, short duration acceleration pulse, and a low magnitude, long duration acceleration pulse, each created by using different paper honeycomb blocks as pulse shape generators in the drop tower. The test results show that the Hybrid III results differ from the Hybrid II results more for the high magnitude, short duration pulse case. The comparison of the lumbar loads for each ATD configuration show drastic differences in the loads seen in the spine. The analytical results show major differences between the responses of the two finite element models. A detailed discussion of possible sources of the discrepancies between the two analytical models is also provided.

Polanco, Michael A.↗

NASA NTRS2018

Adaptive Load Control of Flexible Aircraft Wings Using Fiber Optic Sensing

Over the past century aircraft wing design has transformed from the morphing wing used on the Wright Flyer to rigid wings with little to no shape-tailoring abilities. Modern day wings are designed to fly at a single trim condition and optimized to have a maximum aerodynamic efficiency at only this condition. Shape morphing wings on the other hand have the potential to undergo geometric changes allowing them to adapt to their mission profiles. Several flight demonstrations have been conducted over the decades using morphing-wing technologies. Active wing-twist control was demonstrated on the X-53 Active Aeroelastic Wing (AAW) research project by utilizing multiple leading- and trailing-edge control surfaces. Passive morphing technology has been demonstrated on the Rockwell RPRV-870 Highly Maneuverable Aircraft Technology (HiMAT) aircraft. In the current study, the wings of a small unmanned aerial system (sUAS) were modified to have segmented control surfaces (SCS). The modifications include segmenting the original wing control surfaces (one flap and one aileron per wing) into 44 individual sections, each section having its own independent servo control motor. The wings were also instrumented with a network of over 1800 fiber-optic strain sensors (on four sensing fibers distributed over the top and bottom surfaces of the wing) monitoring the strain response of the wing to aerodynamic loading. The SCS positions were manipulated in real time to modify the spanwise lift distribution of the wings on the sUAS. The change in the structural response of the wings caused by load redistribution was quantified by measuring the bending strains on the upper and lower wing surfaces using an on-board compact fiber-optic strain sensing (cFOSS) system. A feedback controller was developed to control the SCS positions using strain-based shape estimations from the Displacement Transfer Function (DTF). Post-processing of the strain data allowed for the transverse displacement distributions and load distributions to be compared for the conventional and segmented control surface cases using displacements and loads algorithms developed by Richards and Ko at AFRC (refs. 5-10). While the current study focused on the shifting of the spanwise aerodynamic loads as quantified by displacements, future applications for loads or displacement control might include active gust alleviation and flutter suppression.

Pena, Francisco↗

NASA NTRS2000

On Flowfield Periodicity in the NASA Transonic Flutter Cascade: Experimental Study - Part 1

An extensive study to improve flow uniformity and periodicity in the NASA Transonic Flutter Cascade is presented here. The results are reported in two independent parts dealing with the experimental approach and the analytical approach. The first part, the Experimental Study, focuses first on the data sets acquired in this facility in the past and explains several discrepancies, particularly the questions of actual flow incidence and cascade back pressure levels. Next, available means for control and modifications of the cascade flowfield, boundary layer bleed and tailboard settings are presented in detail. This is followed by experimental data sets acquired in modified test facility configurations that were based on analytical predictions of the cascade flowfield. Finally, several important conclusions about improving the cascade flowfield uniformity and blade load periodicity are summarized. The important conclusions are: (1) boundary layer bleed does not improve the cascade flow periodicity; (2) tunnel wall contours must be carefully matched to the expected shape of cascade streamlines; (3) actual flow incidence for each cascade configuration rather must be measured instead of relying on the tunnel geometry; and (4) the current cascade configuration exhibits a very high blade load uniformity over six blades from blade #2 to blade #7, and the facility is now ready for unsteady pressure data acquisition.

Lepicovsky, J.↗

NASA NTRS1991

Simulation of iced wing aerodynamics

The sectional and total aerodynamic load characteristics of moderate aspect ratio wings with and without simulated glaze leading edge ice were studied both computationally, using a three dimensional, compressible Navier-Stokes solver, and experimentally. The wing has an untwisted, untapered planform shape with NACA 0012 airfoil section. The wing has an unswept and swept configuration with aspect ratios of 4.06 and 5.0. Comparisons of computed surface pressures and sectional loads with experimental data for identical configurations are given. The abrupt decrease in stall angle of attack for the wing, as a result of the leading edge ice formation, was demonstrated numerically and experimentally.

Potapczuk, M. G.↗

NASA NTRS1991

Simulation of iced wing aerodynamics

Potapczuk, M. G.↗

NASA NTRS1965

A Tension Shell Structure for Application to Entry Vehicles

The shape of a shell of revolution designed t o have only tensile stresses under axisymmetric aerodynamic loadings has been derived on the basis of linear membrane theory. values of circumferential tension is also given. The results of this paper indicate that the design of an entry vehicle based on the tension shell concept leads to desirable aerodynamic and structural characteristics, namely, high drag and low weight.

Anderson, Melvin S.↗

NASA NTRS1978

Reproducibility of structural strength and stiffness for graphite-epoxy aircraft spoilers

Structural strength reproducibility of graphite epoxy composite spoilers for the Boeing 737 aircraft was evaluated by statically loading fifteen spoilers to failure at conditions simulating aerodynamic loads. Spoiler strength and stiffness data were statistically modeled using a two parameter Weibull distribution function. Shape parameter values calculated for the composite spoiler strength and stiffness were within the range of corresponding shape parameter values calculated for material property data of composite laminates. This agreement showed that reproducibility of full scale component structural properties was within the reproducibility range of data from material property tests.

Howell, W. E.↗

NASA NTRS1979

Theoretical study of aerodynamic characteristics of wings having vortex flow

The aerodynamic characteristics of slender wings having separation induced vortex flows are investigated by employing three different computer codes--free vortex sheet, quasi vortex lattice, and suction analogy methods. Their capabilities and limitations are examined, and modifications are discussed. Flat wings of different configurations: arrow, delta, and diamond shapes, as well as cambered delta wings, are studied. The effect of notch ratio on the load distributions and the longitudinal characteristics of a family of arrow and diamond wings is explored. The sectional lift coefficients and the accumulated span loadings are determined for an arrow wing and are seen to be unusual in comparison with the attached flow results. The theoretically predicted results are compared with the existing experimental values.

Reddy, C. S.↗

NASA NTRS1981

Toroidal cell and battery

A toroidal storage battery designed to handle relatively high amp-hour loads is described. The cell includes a wound core disposed within a pair of toroidal channel shaped electrodes spaced apart by nylon insulator. The shape of the case electrodes of this toroidal cell allows a first planar doughnut shaped surface and the inner cylindrical case wall to be used as a first electrode and a second planar doughnut shaped surface and the outer cylindrical case wall to be used as a second electrode. Connectors may be used to stack two or more toroidal cells together by connecting substantially the entire surface area of the first electrode of a first cell to substantially the entire surface area of the second electrode of a second cell. The central cavity of each toroidal cell may be used as a conduit for pumping a fluid through the toroidal cell to thereby cool the cell.

Nagle, W. J.↗

NASA NTRS1987

Buckling Analysis of Rectangular Plates With Holes

BUCKO is computer program developed to predict buckling of rectangular compression-loaded orthotropic plate with centrally located cutout. Plate assumed balanced, symmetric laminate of uniform thickness. Cutout shape elliptical, circular, rectangular, or square. Package includes sample data demonstrating essence of program and ease of use. Written in FORTRAN V.

Nemeth, M. P.↗

NASA NTRS1987

DYCAST: A finite element program for the crash analysis of structures

DYCAST is a nonlinear structural dynamic finite element computer code developed for crash simulation. The element library contains stringers, beams, membrane skin triangles, plate bending triangles and spring elements. Changing stiffnesses in the structure are accounted for by plasticity and very large deflections. Material nonlinearities are accommodated by one of three options: elastic-perfectly plastic, elastic-linear hardening plastic, or elastic-nonlinear hardening plastic of the Ramberg-Osgood type. Geometric nonlinearities are handled in an updated Lagrangian formulation by reforming the structure into its deformed shape after small time increments while accumulating deformations, strains, and forces. The nonlinearities due to combined loadings are maintained, and stiffness variation due to structural failures are computed. Numerical time integrators available are fixed-step central difference, modified Adams, Newmark-beta, and Wilson-theta. The last three have a variable time step capability, which is controlled internally by a solution convergence error measure. Other features include: multiple time-load history tables to subject the structure to time dependent loading; gravity loading; initial pitch, roll, yaw, and translation of the structural model with respect to the global system; a bandwidth optimizer as a pre-processor; and deformed plots and graphics as post-processors.

Pifko, A. B.↗

NASA NTRS1992

Finite-element-analysis model and preliminary ground testing of controls-structures interaction evolutionary model reflector

Results of two different nonlinear finite element analyses and preliminary test results for the final design of the Controls-Structures Interaction Evolutionary Model are presented. Load-deflection data bases are generalized from analysis and testing of the 16-foot diameter, dish shaped reflector. Natural frequencies and mode shapes are obtained from vibrational analysis. Experimental and analytical results show similar trends; however, future test hardware modifications and finite element model refinement would be necessary to obtain better correlation. The two nonlinear analysis procedures are both adequate techniques for the analysis of prestressed structures with complex geometries.

Reaves, Mercedes C.↗

NASA NTRS1994

Crystalline structure and symmetry dependence of acoustic nonlinearity parameters

A quantitative measure of elastic wave nonlinearity in crystals is provided by the acoustic nonlinearity parameters. The nonlinearity parameters are defined for arbitrary propagation modes for solids of arbitrary crystalline symmetry and are determined along the pure mode propagation directions for 33 crystals of cubic symmetry from data reported in the literature. The magnitudes of the nonlinearity parameters are found to exhibit a strong dependence on the crystalline structure and symmetries associated with the modal direction in the solid. Calculations based on the Born-Mayer potential for crystals having a dominant repulsive contribution to the elastic constants from the interatomic pair potential suggest that the origin of the structure dependence is associated with the shape rather than the strength of the potential. Considerations based on variations in crystal symmetry during loading along pure mode propagation directions of face-centered-cubic solids provide a qualitative explanation for the dependence of the acoustic nonlinearity parameters on modal direction.

Cantrell, John H.↗

NASA NTRS1994

Development of the Toggle Deployment Mechanism

The Toggle Deployment Mechanism (TDM) is a two fault tolerant, single point, low shock pyro/mechanical releasing device. Many forms of releasing are single fault tolerant and involve breaking of primary structure. Other releasing mechanisms, that do not break primary structure, are only pyrotechnically redundant and not mechanically redundant. The TDM contains 3 independent pyro actuators, and only one of the 3 is required for release. The 2 separating members in the TDM are held together by a toggle that is a cylindrical stem with a larger diameter spherical shape on the top and flares out in a conical shape at the bottom. The spherical end of the toggle sits in a socket with the top assembly and the bottom is held down by 3 pins or hooks equally spaced around the conical shaped end. Each of the TDM's 3 independent actuators shares a third of the separating load and does not require as much pyrotechnic energy as many single fault tolerant actuators. Other single separating actuators, i.e., separating nuts or pin pullers, have the pyrotechnic energy releasing the entire preload holding the separating members together. Two types of TDM's ,described in this paper, release the toggle with pin pullers, and the third TDM releases the toggle with hooks. Each design has different advantages and disadvantages. This paper describes the TDM's construction and testing up to the summer of 1993.

Brown, Christopher W.↗

NASA NTRS2001

Thermomechanical Response of Shape Memory Alloy Hybrid Composites

This study examines the use of embedded shape memory alloy (SMA) actuators for adaptive control of the thermomechanical response of composite structures. A nonlinear thermomechanical model is presented for analyzing shape memory alloy hybrid composite (SMAHC) structures exposed to steady-state thermal and dynamic mechanical loads. Also presented are (1) fabrication procedures for SMAHC specimens, (2) characterization of the constituent materials for model quantification, (3) development of the test apparatus for conducting static and dynamic experiments on specimens with and without SMA, (4) discussion of the experimental results, and (5) validation of the analytical and numerical tools developed in the study. Excellent agreement is achieved between the predicted and measured SAMHC responses including thermal buckling, thermal post-buckling and dynamic response due to inertial loading. The validated model and thermomechanical analysis tools are used to demonstrate a variety of static and dynamic response behaviors including control of static (thermal buckling and post-buckling) and dynamic responses (vibration, sonic fatigue, and acoustic transmission). and SMAHC design considerations for these applications. SMAHCs are shown to have significant advantages over conventional response abatement approaches for vibration, sonic fatigue, and noise control.

Turner, Travis L.↗

NASA NTRS1994

Finite Element Analysis of the Random Response Suppression of Composite Panels at Elevated Temperatures using Shape Memory Alloy Fibers

A feasibility study on the use of shape memory alloys (SMA) for suppression of the random response of composite panels due to acoustic loads at elevated temperatures is presented. The constitutive relations for a composite lamina with embedded SMA fibers are developed. The finite element governing equations and the solution procedures for a composite plate subjected to combined acoustic and thermal loads are presented. Solutions include: 1) Critical buckling temperature; 2) Flat panel random response; 3) Thermal postbuckling deflection; 4) Random response of a thermally buckled panel. The preliminary results demonstrate that the SMA fibers can completely eliminate the thermal postbuckling deflection and significantly reduce the random response at elevated temperatures.

Turner, Travis L.↗

NASA NTRS2004

Optical Diagnostic System for Solar Sails: Phase 1 Final Report

NASA's In-Space Propulsion program recently selected AEC-ABLE Engineering and L'Garde, Inc. to develop scale-model solar sail hardware and demonstrate its functionality on the ground. Both are square sail designs with lightweight diagonal booms (<100 g/m) and ultra-thin membranes (<10 g/sq m). To support this technology, the authors are developing an integrated diagnostics instrumentation package for monitoring solar sail structures such as these in a near-term flight experiment. We refer to this activity as the "Optical Diagnostic System (ODS) for Solar Sails" project. The approach uses lightweight optics and photogrammetric techniques to measure solar sail membrane and boom shape and dynamics, thermography to map temperature, and non-optical sensors including MEMS accelerometers and load cells. The diagnostics package must measure key structural characteristics including deployment dynamics, sail support tension, boom and sail deflection, boom and sail natural frequencies, sail temperature, and sail integrity. This report summarizes work in the initial 6-month Phase I period (conceptual design phase) and complements the final presentation given in Huntsville, AL on January 14, 2004.

Pappa, Richard S.↗