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Williams, J. G.

Publications and source records attributed to Williams, J. G..

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Compression-Failure Mechanisms in Composite Laminates

Failure mechanisms observed using transparent fiberglass/epoxy birefringent materials. Technique based on use of transparent fiberglass/epoxy birefringent material. Transparency allows visual observation of location of initial laminate failure and of subsequent failure propagation; birefringence allows laminate stress distribution to be observed during test and also after test if permanent residual stresses occur. Nondestructive technique developed to observe failure as it develops and as propagates within laminate.

Shuart, M. J.↗
NASA NTRS1986

The effect of resin toughness and modulus on compressive failure modes of quasi-isotropic graphite/epoxy laminates

Compressive failure mechanisms in quasi-isotropic graphite/epoxy laminates were characterized for both unnotched and notched specimens and also following damage by impact. Two types of fibers (Thornel 300 and 700) and four resin systems (Narmco 5208, American Cyanamid BP907, and Union Carbide 4901/MDA and 4901/mPDA) were studied. For all material combinations, failure of unnotched specimens was initiated by kinking of fibers in the 0-degree plies. A major difference was observed, however, in the mode of failure propagation after the 0-degree ply failure. The strength of quasi-isotropic laminates in general increased with increasing resin tensile modulus. The laminates made with Thornel 700 fibers exhibited slightly lower compressive strengths than did the laminates made with Thornel 300 fibers. The notch sensitivity as measured by the hole strength was lowest for the BP907 resin and highest for the 5208 resin. For the materials studied, however, the type of fiber had no effect on the notch sensitivity.

Sohi, M. M.↗
NASA NTRS1986

Compression failure mechanisms in unidirectional composites

Compression failure mechanisms in unidirectional composites were examined. Possible failure modes of constituent materials are summarized and analytical models for fiber microbuckling are reviewed from a unified viewpoint. Due to deficiencies in available models, a failure model based on nonlinear properties and initial fiber curvature is proposed. The effect of constituent properties on composite compression behavior was experimentally investigated using two different graphite fibers and four different epoxy resins. The predominant microscopic scale failure mode was found to be shear crippling. In a soft resin, shear crippling was in the form of buckling of fibers on a microscopic scale. However, stiff resins failure was characterized by the formation of a kink band. For unidirectional laminates, compressive strength, and compressive modulus to a less extent, were found to increase with increasing magnitude of resin modulus. The change in compressive strength with resin modulus was predicted using the proposed nonlinear model.

Hahn, H. T.↗
NASA NTRS1986

Reference frame studies at JPL/CALTECH

Studies being undertaken in order to interconnect the three principal celestial coordinate systens are reviewed. These systems are the optical frame (FK4/FK5) based on positions of Galactic stars, the planetary/lunar ephemeris frame based on the major celestial bodies of the solar system, and the radio frame constructed from observations of quasars. The optical frame is being connected to the radio frame by VLBI observations of radio-emitting stars, and the radio frame is being tied to the ephemeris frame in several ways; for example, differential VLBI measurements between quasars and planet-orbiting spacecraft. Prospects for future interconnection studies are briefly discussed.

Dickey, J. O.↗
NASA NTRS1986

Multi-station lunar laser ranging - An analysis of data quality and earth rotation results

The lunar laser ranging (LLR) results obtained from the MERIT period (Sept. 1983 through Oct. 1984) as well as from the post-MERIT period (Nov. 1, 1984 through Aug. 12, 1985) are presented. The ranging targets on the moon include the Apollo 11, 14, and 15 reflectors and a reflector on Lunokhod 2; the LLR network includes McDonald, Texas; CERGA, France; Haleakala, Hawaii; Orroral, Australia; and Crimea, USSR, stations. Data acquired with these systems are reported, and the data quality is assessed, with particular emphasis on recent ranges. During the MERIT period, sixty-five earth rotation values (UTO) were derived from LLR data, with the best accuracy being 0.25 msec; during the post-MERIT period, 115 determinations of UTO were calculated with the best inherent accuracy of about 0.1 msec. The results are compared with those from other techniques.

Dickey, J. O.↗
NASA NTRS1986

The coordinate frame of the lunar laser ranging network

The geocentric coordinates for four instruments, which were derived using lunar laser ranging, are compared with the 84L02 coordinates determined from the Lageos satellite. The determination of the geocentric coordinates for the 2.7 m and McDonald Observatory laser ranging system telescopes at McDonald Observatory, the Haleakala site, and the CERGA site near Grasse, France is described. Consideration is given to the McDonald Observatory colocation and station motion due to continential drift. A rms difference of 18 cm is determined for the two sets of geocentric coordinates; however, removing a data anomaly reduces the rms difference to 13 cm.

Williams, J. G.↗
NASA NTRS1985

Earth orientation from lunar laser ranging and an error analysis of polar motion services

Lunar laser ranging (LLR) data are obtained on the basis of the timing of laser pulses travelling from observatories on earth to retroreflectors placed on the moon's surface during the Apollo program. The modeling and analysis of the LLR data can provide valuable insights into earth's dynamics. The feasibility to model accurately the lunar orbit over the full 13-year observation span makes it possible to conduct relatively long-term studies of variations in the earth's rotation. A description is provided of general analysis techniques, and the calculation of universal time (UT1) from LLR is discussed. Attention is also given to a summary of intercomparisons with different techniques, polar motion results and intercomparisons, and a polar motion error analysis.

Dickey, J. O.↗
NASA NTRS1985

A VLBI survey at 2.29 GHz

VLBI observations at 2.29 GHz with fringe spacings of about 3 milliarcsec have been performed on 1398 radio sources spread over the entire sky. 917 sources were detected, including 93 percent of the identified BL Lacertae objects, 86 percent of the quasars, and 36 percent of the galaxies. The resulting catalog of compact radio sources is useful for various astrophysical studies and in the formation of VLBI celestial reference frames.

Preston, R. A.↗
NASA NTRS1985

Failure mechanisms of laminates transversely loaded by bolt push-through

Stiffened composite panels proposed for fuselage and wing design utilize a variety of stiffener-to-skin attachment concepts including mechanical fasteners. The attachment concept is an important factor influencing the panel's strength and can govern its performance following local damage. Mechanical fasteners can be an effective method for preventing stiffener-skin separation. One potential failure mode for bolted panels occurs when the bolts pull through the stiffener attachment flange or skin. The resulting loss of support by the skin to the stiffener and by the stiffener to the skin can result in local buckling and subsequent panel collapse. The characteristic failure modes associated with bolt push-through failure are described and the results of a parametric study of the effects that different material systems, boundary conditions, and laminates have on the forces and displacements required to cause damage and bolt pushthrough failure are presented.

Waters, W. A., Jr.↗
NASA NTRS1985

A VLBI survey at 2.29 GHz

The Deep Space Network (DSN) is establishing a high-accuracy VLBI celestial reference frame. The results of a search for suitable radio sources to be used in constructing this frame are given. The VLBI observations using DSN baselines at 2.29 GHz with fringe spacings of about 3 milliarcseconds have been performed on 1398 radio sources spread over the entire sky. Of those, 917 sources were detected including 93% of the identifed BL Lacertae objects, 86% of Quasars and 36% of galaxies. The resulting catalog of compact radio sources is also useful for various astrophysical studies and in the formation of VLBI celestial reference frames.

Morabito, D. D.↗
NASA NTRS1985

Clarifications concerning the definition and determination of the celestial ephemeris pole

The precise meaning and consequences of the introduction beginning in 1984 of the 1980 IAU theory of nutation (Wahr, 1980; Seidelmann, 1982) based on the celestial ephemeris pole (CEP) rather than the instantaneous pole of rotation (IPR) of the previous theory (Woolard, 1953) are analyzed and discussed. The CEP implicitly accounts for the forced diurnal polar motion and thus implies a fixed change of 0.0087 arcsec in the mean celestial pole. It is pointed out that past determinations of the obliquity and the celestial pole which were assumed to be referred to the IPR were actually referred to the CEP. A number of clarified formulations of the new theory are proposed.

Capitaine, N.↗
NASA NTRS1985

Continuous-Filament Isogrid Composite Panel

Damaged panels do not fail catastrophically when fabricated this way. Isogrid stiffener formed in slotted silicon-rubber tool. At grid intersections, slots offset to prevent excessive buildup of composite material and widened to give room for overlapping filaments to spread when compressed. Following total failure of skin, isogrid stiffener still carries 350 kN/m (2,000 1b/in.) and remains intact.

Williams, J. G.↗
NASA NTRS1985

Small Solar System Bodies

The feasibility of using ranging data to a spacecraft lander on a synchronously rotating satellite to determine the satellite's orbit, the planet's orbit, the lander coordinates, and the principal term of forced physical librations was studied. Classifications of asteroids into their proper elements and families are also presented. Measurements and reduction of positions on comets and the minor planets with less common orbits was the focus for the astrometry program.

Williams, J. G.↗
NASA NTRS1984

Effect of Impact Damage and Open Holes on the Compression Strength of Tough Resin/high Strain Fiber Laminates

Tests to assess the performance of composite structures damaged by impact or containing local discontinuities such as cutouts are discussed. In particular, two plate specimen test configurations which were developed to define the serverity of compression strength reduction are described; one for impact damage and the other open hole specimens. The test technique for impact specimens involves damaging the plate at selected energies, measuring the size of damage by ultrasonic C-scan techniques and measuring the residual strength in a compression load test. Open-hole specimen compression tests are conducted for several different hole diameters and the failure strain and load and mode of failure recorded. The plate specimen used in these tests is designed with length, width, thickness and laminate stiffness to ensure that overall plate buckling is not responsible for initiating failure.

Williams, J. G.↗
NASA NTRS1984

Investigating compression failure mechanisms in composite laminates with a transient fiberglass-epoxy birefringent material

An experimental study is reported in which a nondestructive technique involving the use of a transparent fiberglass-epoxy composite birefringent material has been used to investigate compression failure mechanisms in graphite-epoxy laminates. It is shown that the birefringency and transparency of the fiberglass-epoxy material permits regions of high stress to be located and the mechanisms of local failure propagation to be identified within the laminate. The material may also be useful for studying stress fields and for identifying failure initiation and propagation mechanisms in a wide variety of composite-structure problems.

Shuart, M. J.↗
NASA NTRS1984

Comparison of toughened composite laminates using NASA standard damage tolerance tests

The proposed application of composite materials to transport wing and fuselage structures prompted the search for tougher materials having improved resistance to impact damage and delamination. Several resin/graphite fiber composite materials were subjected to standard damage tolerance tests and the results were compared to ascertain which materials have superior toughness. In addition, test results from various company and NASA laboratories were compared for repeatability.

Williams, J. G.↗
NASA NTRS1984

Compression failure mechanisms in unidirectional composites

Compression failure mechanisms in unidirectional composites were examined. Possible failure modes of constituent materials are summarized and analytical models for fiber microbuckling are reviewed from a unified viewpoint. Due to deficiencies in available models, a failure model based on nonlinear properties and initial fiber curvature is proposed. The effect of constituent properties on composite compression behavior was experimentally investigated using two different graphite fibers and four different epoxy resins. The predominant microscopic scale failure mode was found to be shear crippling. In a soft resin, shear crippling was in the form of buckling of fibers on a microscopic scale. However, stiff resins failure was characterized by the formation of a kink band. For unidirectional laminates, compressive strength, and compressive modulus to a less extent, were found to increase with increasing magnitude of resin modulus. The change in compressive strength with resin modulus was predicted using the proposed nonlinear model.

Hahn, H. T.↗