SEARCH · Search NASA

Results for “flexure”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 415 records · Page 23

All sources NASA NTRS DOE OSTI

NASA NTRS1990

Nicalon/siliconoxycarbide ceramic composites

A series of silsesquioxane copolymers was synthesized by acid hydrolysis and condensation of trimethoxysilanes of the form RSi(OCH3)3, where R = methyl or phenyl. By varying pH, water/methoxy and methyl/phenyl ratios, the molecular structure, polymer rheology and ceramic composition can be controlled. The polymers form an amorphous siliconoxycarbide on pyrolysis. Composites of Nicalon/siliconoxycarbide were fractured in four-point flexure and in tension to evaluate the influence of matrix composition, final fabrication temperature and use of filler on composite mode of failure, modulus, strain capability and strength. Incorporation of filler was found to increase matrix compressive strength. Employment of processing temperatures of 1375 to 1400 C enhanced strain to failure and reduced the tendency toward brittle fracture. Mixed mode (compression/shear and tension/shear) failures were observed in flexural samples processed to the higher temperatures, giving rise to nonlinear stress-strain curves. Tensile samples pyrolyzed to 1400 C showed linear-elastic behavior and failed by fracture of fiber bundles. Matrix material was found to be adherent to the fiber surface after failure. These results demonstrate the need for tensile testing to establish composite behavior.

Hurwitz, Frances I.↗

NASA NTRS1990

The effect of thermal damage on the mechanical properties of polymer regrinds

Reprocessed polymers are subjected to high processing temperatures that result in the breakdown of molecular chains and changes in the molecular structures. These phenomena are reflected in the mechanical properties of materials. Practically every regrind is seen as a new material. These experiments deal with the molding, regrinding, and reprocessing of test specimens for the study of their mechanical properties. The comparative test data from each recycled material would give students an insight of the molecular structures and property degradation. Three important rheological and mechanical properties such as melt flow, impact strength, and flexural strength are to be determined. These properties play key roles in the selection of engineering materials. The material selected for demonstration was Makrolon 3000L, a polycarbonate thermoplastic from Bayer AG. The thermal degradation due to repeated processing is reflected in the decrease in molecular weight and breakdown of molecular chains causing increase in melt flow. The Izod-impact resistance and the flexural strength deteriorate gradually.

Kundu, Nikhil K.↗

NASA NTRS1990

The effects of structural flap-lag and pitch-lag coupling on soft inplane hingeless rotor stability in hover

A 1.62-m-diameter rotor model was tested in hover to examine the effects of structural flap-lag and pitch-lag coupling on isolated rotor blade lead-lag stability. Flap-lag coupling was introduced by inclining the principal axes of the blade structure up to 60 degrees. Pitch-lag coupling was obtained either alone or in combination with flap-lag coupling through the use of skewed flexural hinges. The principal results confirm the predictions of theory, and show that both structural flap-lag and pitch-lag coupling when used separately are beneficial to blade stability. Moreover, when the couplings are combined, the lead-lag damping is significantly greater than it would be if the individual contributions were superimposed. Pitch-flap coupling is shown to have only a minor effect on blade lead-lag damping. Differences between theory and experiment observed at zero blade pitch and flexure angles during the initial testing were determined in a second test to be caused by stand flexibility. Other differences between theory and experiment warrant further investigation.

Bousman, William G.↗

NASA NTRS1989

Ames Research Center cryogenic mirror testing program - A comparison of the cryogenic performance of metal and glass mirrors with different types of mounts

A summary of the cryogenic testing of glass and metal mirrors performed at NASA Ames Research Center (ARC) and two other places is presented. Recent improvements to the ARC Cryogenic Optics Test Facility are described. The purposes of the tests were to determine: (1) how glass mirrors would perform at cryogenic temperatures compared with metal mirrors and (2) how various mirror mounts would affect the cryogenic performance of mirrors. Details of a cryogenic test of a 50 cm 'double arch', fused-silica mirror with a three-point mount and with a radially-compliant, flexured mount are given. Within the accuracy of the measurements, it was determined that the flexured mount did not induce appreciable distortion in the double arch mirror. Results of the cryogenic tests of a number of glass mirrors and two beryllium mirrors are included. The cryogenic distortion of the glass mirrors was found to be less than that for the beryllium mirrors. Within the accuracy of the measurements, no hysteresis was found in the glass mirrors. It was possible to measure hysteresis in one of the beryllium mirrors.

Miller, Jacob H.↗

NASA NTRS1990

Variations in effective elastic thickness of the North American lithosphere

A technique for estimating flexural rigidity that is not limited to sedimentary basins is used here to map variations in the effective elastic thickness of the North American lithosphere. The effective elastic thickness ranges from a minimum of about 4 km in the Basin and Range Province to more than 100 km in the Precambrian core of the continent. This finding supports the idea that flexural rigidity has increased with time since the last thermal event.

Bechtel, Timothy D.↗

NASA NTRS1990

Experiments on active control of vibrational power flow using piezoceramic actuators and sensors

The active control of flexural power flow in both semiinfinite and finite elastic beams is experimentally investigated. The experimental results demonstrate that piezoceramic transducers when used in conjunction with an adaptive least mean squares controller, can effectively control flexural power flow in thin beam systems. The piezoceramic transducers offer distinct size and weight advantages over conventional transducers. The experiments also demonstrate the use of an axial scanning laser vibrometer to determine out of plane velocity and power flow.

Gibbs, Gary P.↗

NASA NTRS1990

Thermal effects on the mechanical properties of SiC fibre reinforced reaction-bonded silicon nitride matrix composites

The elevated temperature four-point flexural strength and the room temperature tensile and flexural strength properties after thermal shock were measured for ceramic composites consisting of 30 vol pct uniaxially aligned 142 micron diameter SiC fibers in a reaction bonded Si3N4 matrix. The elevated temperature strengths were measured after 15 min of exposure in air at temperatures to 1400 C. Thermal shock treatment was accomplished by heating the composite in air for 15 min at temperatures to 1200 C and then quenching in water at 25 C. The results indicate no significant loss in strength properties either at temperature or after thermal shock when compared with the strength data for composites in the as-fabricated condition.

Bhatt, R. T.↗

NASA NTRS1991

Relationship between voids and interlaminar shear strength of polymer matrix composites

The effect of voids on the interlaminar shear strength of a polyimide matrix composite system is described. The AS4 graphite/PMR-15 composite was chosen for study because this system can be readily processed by using the standard specified cure cycle to produce void-free composites and because preliminary work in this study had shown that the processing parameters of this resin matrix system can be altered to produce cured composites of varying void contents. Thirty-eight 12-ply unidirectional composite panels were fabricated for this study. A significant range of void contents (0 to 10 percent) was produced. The panels were mapped, ultrasonically inspected, and sectioned into interlaminar shear, flexure, and fiber content specimens. The density of each specimen was measured and interlaminar shear and flexure strength measurements were then made. The fiber content was measured last. The results of these tests were evaluated by using ultrasonic results, photomicrographs, statistical methods, theoretical relationships derived by other investigators, and comparison of the test data with the Integrated Composite Analyzer (ICAN) computer program developed at the Lewis Research Center for predicting composite ply properties. The testing is described in as much detail as possible in order to help others make realistic comparisons.

Bowles, Kenneth J.↗

NASA NTRS1991

Integration of mechanism and control for large-angle slew maneuvers of flexible structures

A rolling contact noncircular gear system is applied to assist a desired controller in the slewing of a flexible space structure. The varying gear ratio in cooperation with the controller results in lower feedback gains at the controller, as well as considerably reducing flexural vibrations of the space structure. The noncircular gears consist of a pair of convex noncircular cylinders with specially designed profiles that are synthesized in conjunction with the optimal controller gains for minimizing the flexural vibrations of flexible structure during a slew maneuver. Convexity of the cylindrical profiles for this noncircular gear device must be ensured to maintain rolling contact between the two cylinders. Simulations of slewing control tasks for two kinds of flexible space structures, such as a planar flexible beam and the planar articulated flexible beams, are presented.

Chew, Meng-Sang↗

NASA NTRS1991

Flexible fluoropolymer filled protective coatings

Metal oxide films such as SiO2 are known to provide an effective barrier to the transport of moisture as well as gaseous species through polymeric films. Such thin film coatings have a tendency to crack upon flexure of the polymeric substrate. Sputter co-deposition of SiO2 with 4 to 15 percent fluoropolymers was demonstrated to produce thin films with glass-like barrier properties that have significant increases in strain to failure over pure glass films which improves their tolerance to flexure on polymeric substrates. Deposition techniques capable of producing these films on polymeric substrates are suitable for durable food packaging and oxidation/corrosion protection applications.

Banks, Bruce A.↗

NASA NTRS1991

Soft hub for bearingless rotors

Soft hub concepts which allow the direct replacement of articulated rotor systems by bearingless types without any change in controllability or need for reinforcement to the drive shaft and/or transmission/fuselage attachments of the helicopter were studied. Two concepts were analyzed and confirmed for functional and structural feasibility against a design criteria and specifications established for this effort. Both systems are gimballed about a thrust carrying universal elastomeric bearing. One concept includes a set of composite flexures for drive torque transmittal from the shaft to the rotor, and another set (which is changeable) to impart hub tilting stiffness to the rotor system as required to meet the helicopter application. The second concept uses a composite bellows flexure to drive the rotor and to augment the hub stiffness provided by the elastomeric bearing. Each concept was assessed for weight, drag, ROM cost, and number of parts and compared with the production BO-105 hub.

Dixon, Peter G. C.↗

NASA NTRS1991

Toughening of PMR composites by semi-interpenetrating networks

Polymerization of monomer reactants (PMR-15) type polyimide and RP46 prepregs were drum wound using IM-7 fibers. Prepregging and processing conditions were optimized to yield good quality laminates with fiber volume fractions of 60 percent (+/- 2 percent). Samples were fabricated and tested to determine comprehensive engineering properties of both systems. These included 0 deg flexure, short beam shear, transverse flexure and tension, 0 deg tension and compression, intralaminar shear, short block compression, mode 1 and 2 fracture toughness, and compression after impact properties. Semi-2-IPN (interpenetrating polymer networks) toughened PMR-15 and RP46 laminates were also fabricated and tested for the same properties.

Tiwari, S. N.↗

NASA NTRS1991

Relationship between voids and interlaminar shear strength of polymer matrix composites

The effect of voids on the interlaminar shear strength of a polyimide matrix composite system is described. The AS4 graphite/PMR-15 composite was chosen for study because this system can be readily processed by using the standard specified cure cycle to produce void-free composites and because preliminary work in this study had shown that the processing parameters of this resin matrix system can be altered to produce cured composites of varying void contents. Thirty-eight 12-ply unidirectional composite panels were fabricated for this study. A significant range of void contents (0 to 10 percent) was produced. The panels were mapped, ultrasonically inspected, and sectioned into interlaminar shear, flexure, and fiber content specimens. The density of each specimen was measured and interlaminar shear and flexure strength measurements were then made. The fiber content was measured last. The results of these tests were evaluated by using ultrasonic results, photomicrographs, statistical methods, theoretical relationships derived by other investigations, and comparison of the test data with the Integrated Composite Analyzer (ICAN) computer program developed at the Lewis Research Center for predicting composite ply properties. The testing is described in as much detail as possible in order to help others make realistic comparisons.

Bowles, Kenneth J.↗

NASA NTRS1990

Design of a prototype secondary mirror assembly for the Space Infrared Telescope Facility

The design of a liquid helium temperature prototype secondary mirror assembly (PSMA) under development for the NASA Space Infrared Telescope Facility (SIRTF) program is described. The SIRTF assembly must operate below 4 K and provide the functions of highly precise two-axis periodic tilting ('chopping') in addition to the conventional functions of focus and collimation adjustment. The PSMA design employs a fused quartz mirror kinematically attached at its center to an aluminum cruciform. The mirror/cruciform assembly is driven in tilt about its combined center of mass using an aluminum flexure pivot of unique design and a four-actuator control system with feedback provided by pairs of differential position sensors. The voice coil actuators are mounted on a second flexure-pivoted mass to enhance servosystem stability and isolate the telescope from vibration-induced disturbances. The mirror/cruciform and reaction mass are attached to opposite sides of an aluminum mounting plate whose position relative to the outer housing is controlled by a six-degree-of-freedom focus and centering mechanism using pivoted actuation levers driven by lead screw/harmonic drive/stepper motor assemblies.

Stier, Mark T.↗

NASA NTRS1992

Design of a D-Strut and its application results in the JPL, MIT, and LaRC test beds

High damping at very low magnitudes of vibration is provided by an 'Arched Flexure' D-Strut. Five of these devices have been incorporated in JPL's CSI truss structure, and five more in MIT's interferometer test bed. Two were also temporarily installed in the LaRC CSI test structure. Damping factors in the 50 percent category are being provided at vibration motions as small as 50 nm. The 'Arched Flexure' used in the D-Strut maximizes the volumetric stiffness parameter relative to its axial stiffness. In turn, this enables high damping factors, even in a very rigid truss structure. This paper includes a presentation and discussion of both element and system test data taken at JPL, LaRC, and MIT. Also provided is a detailed description of the D-Strut, the dynamic model, and several graphs showing design parameter variations that will make it possible to evaluate the D-Strut for other applications.

Davis, L. P.↗

NASA NTRS1992

Evidence for retrograde lithospheric subduction on Venus

Annular moats and outer rises around large Venus coronas such as Artemis, Latona, and Eithinoha are similar in arcuate planform and topography to the trenches and outer rises of terrestrial subduction zones. On earth, trenches and outer rises are modeled as the flexural response of a thin elastic lithosphere to the bending moment of the subducted slab; this lithospheric flexure model also accounts for the trenches and outer rises outboard of the major coronas on Venus. Accordingly, it is proposed that retrograde lithospheric subduction may be occurring on the margins of the large Venus coronas while compensating back-arc extension is occurring in the expanding coronas interiors. Similar processes may be taking place at other deep arcuate trenches or chasmata on Venus such as those in the Dali-Diana chasmata area of aestern Aphrodite Terra.

Sandwell, David T.↗

NASA NTRS1993

Active Suppression Of Vibrations On Elastic Beams

Pairs of colocated piezoelectric transducers, independently controlled by multichannel adaptive controller, employed as actuators and sensors to achieve simultaneous attenuation of both extensional and flexural motion. Single pair used to provide simultaneous control of flexural and extensional waves, or two pairs used to control torsional motion also. Capability due to nature of piezoelectric transducers, when bonded to surfaces of structures and activated by oscillating voltages, generate corresponding oscillating distributions of stresses in structures. Phases and amplitudes of actuator voltages adjusted by controller to impede flow of vibrational energy simultaneously, in waves of various forms, beyond locations of actuators. Concept applies equally to harmonic or random response of structure and to multiple responses of structure to transverse bending, torsion, and compression within structural element. System has potential for many situations in which predominant vibration transmission path through framelike structure.

Silcox, Richard J.↗

NASA NTRS1992

Development and mechanical properties of construction materials from lunar simulant

Development of versatile engineering materials from locally available materials in space is an important step toward the establishment of outposts on the Moon and Mars. Development of the technologies for manufacture of structural and construction materials on the Moon, utilizing local lunar soil (regolith), without the use of water, is an important element for habitats and explorations in space. It is also vital that the mechanical behavior such as strength and tensile, flexural properties, fracture toughness, ductility, and deformation characteristics are defined toward establishment of the ranges of engineering applications of the materials developed. The objectives include two areas: (1) thermal 'liquefaction' of lunar simulant (at about 1100 C) with different additives (fibers, powders, etc.), and (2) development and use of a new triaxial test device in which lunar simulants are first compacted under cycles of loading, and then tested with different vacuums and initial confining or in situ stress. Details of the development of intermediate ceramic composites (ICC) and testing for their flexural and compression characteristics were described in various reports and papers. The subject of behavior of compacted simulant under vacuum was described in previous progress reports and publications; since the presently available device allows vacuum levels up to only 10(exp -4) torr, it is recommended that a vacuum pump that can allow higher levels of vacuum be utilized for further investigation.

Desai, Chandra S.↗