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At least 451 records · Page 25

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NASA NTRS1974

Some effects of applied stress on early stages of cavitation damage

The phenomenon of cavitation flow damage is discussed. The initial phases of damage and the effect of external stresses on the extent to which damage is incurred are analyzed. Three experimental facilities were used to procure the data required: (1) a water loop with venturi, (2) a mercury loop with venturi, and (3) a vibratory facility (stationary specimen, nonflow system). A description of each system is provided for the clarification of test conditions. Photographs of typical cavitation damage instances are included.

Kemppainen, D. J.↗

NASA NTRS1974

Satellite detection of vegetative damage and alteration caused by pollutants emitted by a zinc smelter

The author has identified the following significant results. Field observations and data collected by low flying aircraft were used to verify the accuracy of maps produced from the satellite data. Although areas of vegetation as small as six acres can accurately be detected, a white pine stand that was severely damaged by sulfur dioxide could not be differentiated from a healthy white pine stand because spectral differences were not large enough. When winter data were used to eliminate interference from herbaceous and deciduous vegetation, the damage was still undetectable. The analysis was able to produce a character map that accurately delineated areas of vegetative alteration due to high zinc levels accumulating in the soil. The map depicted a distinct gradient of less damage and alteration as the distance from the smelter increased. Although the satellite data will probably not be useful for detecting small acreages of damaged vegetation, it is concluded that the data may be very useful as an inventory tool to detect and delineate large vegetative areas possessing differing spectral signatures.

Mcmurtry, G. J.↗

NASA NTRS1975

Modeling damaged wings: Element selection and constraint specification

The NASTRAN analytical program was used for structural design, and no problems were anticipated in applying this program to a damaged structure as long as the deformations were small and the strains remained within the elastic range. In this context, NASTRAN was used to test three-dimensional analytical models of a damaged aircraft wing under static loads. A comparison was made of calculated and experimentally measured strains on primary structural components of an RF-84F wing. This comparison brought out two sensitive areas in modeling semimonocoque structures. The calculated strains were strongly affected by the type of elements used adjacent to the damaged region and by the choice of multipoint constraints sets on the damaged boundary.

Stronge, W. J.↗

NASA NTRS1975

Electron and proton damage coefficients in low-resistivity silicon

The electron and proton damage coefficients for low resistivity p-type boron-doped silicon were determined from minority-carrier lifetime measurements on bulk material and diffusion length measurements on solar cells. The bulk samples were irradiated with electrons at three energy levels (0.5, 1.5, and 2.5 MeV) using a Dynamitron. Lifetime measurements were made with a steady-state photoconductivity apparatus, and comparison measurements of diffusion length were obtained using the steady-state surface photovoltage method (Goodman, 1961). The diffusion-length damage coefficients increased with decreasing resistivity for boron-doped silicon; this dependence can be qualitatively accounted for using a two-level Hall-Shockley-Read model. The damage coefficients for solar cells were larger than for their bulk-material counterparts. The damage coefficient was apparently independent of the dislocation density in the 0.1 ohm-cm bulk samples and solar cells investigated.

Srour, J. R.↗

NASA NTRS1977

Image dissector photocathode solar damage test program

Image dissector sensors of the same type which will be used in the NASA shuttle star tracker were used in a series of tests directed towards obtaining solar radiation/time damage criteria. Data were evaluated to determine the predicted level of operability of the star tracker if tube damage became a reality. During the test series a technique for reducing the solar damage effect was conceived and verified. The damage concepts are outlined and the test methods and data obtained which were used for verification of the technique's feasibility are presented. The ability to operate an image dissector sensor with the solar image focussed on the photocathode by a fast optical lens under certain conditions is feasible and the elimination of a mechanical protection device is possible.

Smith, R. A.↗

NASA NTRS1977

Fatigue damage of notched boron/epoxy laminates under constant-amplitude loading

Fatigue damage in (0, plus or minus 45) and (0, plus or minus 45, 90) boron/epoxy laminates was studied with X-ray radiography and scanning electron microscopy. In addition, limited tests for residual strength and stiffness were performed. Initially, fatigue damage in both (0, plus or minus 45) and (0, plus or minus 45, 90) laminates occurred as intralaminar cracks around the edge of the hole. Then, whenever further damage developed, intralaminar cracks in the plus or minus 45-deg plies began to propagate from the edge of the hole. Finally, in both types of laminates, primarily plus or minus 45-deg fibers broke (prior to two-piece failure) where intralaminar cracks in the plus or minus 45-deg plies had occurred. In the (0, plus or minus 45) laminates, the 45-deg plies developed intralaminar and transthickness cracks along lines parallel to the loading axis and tangent to the hole in the test specimen. This damage, which was most pronounced under compressive loads, had little effect on either strength or stiffness. In contrast, in the (0, plus or minus 45, 90) laminates, the plus or minus 45-deg plies developed intralaminar cracks transverse to the loading axis.

Roderick, G. L.↗

NASA NTRS1978

Using reflectance and photography to detect ozone damage to cantaloupe plants

Laboratory and field reflectance measurements showed that ozone-damaged cantaloupe (Cucumis melo L.) leaves had lower water contents and higher reflectance than nondamaged leaves. Cantaloupe plants with lightly, severely, and very severely ozone-damaged leaves were distinguishable from nondamaged plants by reflectance measurements in the 1.35-2.5-micron near-IR water absorption band. Ozone-damaged leaf areas were detected photographically 16 hours before the damage was visible.

Gausman, H. W.↗

NASA NTRS1979

Dynamic response of damaged angleplied fiber composites

The effects of low level damage induced by monotonic load, cyclic load and/or residual stresses on the vibration frequencies and damping factors of fiber composite angleplied laminates were investigated. Two different composite systems were studied - low modulus fiber and ultra high modulus fiber composites. The results obtained show that the frequencies and damping factors of angleplied laminates made from low modulus fiber composites are sensitive to low level damage while those made from ultra high modulus composites are not. Vibration tests may not be sufficiently sensitive to assess concentrated local damage in angleplied laminates. Dynamic response determined from low-velocity impact coupled with the Fast Fourier Transform and packaged in a minicomputer can be a convenient procedure for assessing low-level damage.

Chamis, C. C.↗

NASA NTRS1980

Effects of moisture, residual thermal curing stresses and mechanical load on the damage development in quasi-isotropic laminates

Classical laminate theory and a finite element model were used to predict stress states prior to the first formation of damage in laminates fabricated from T/300/5208. Crack patterns characteristic of the laminate in a wet or dry condition were also predicted using a shear lag model. Development of edge damage was recorded and observed during the test by transferring an image of the damage from the edge surface on to a thin acetate sheet such that the damage imprinted could be immediately viewed on a microfiche card reader. Moisture was shown to significantly alter the interior and edge dry stress states due to swelling and a reduction of elastic properties and to reduce the transverse strength in 90 deg plies. A model was developed in order to predict changes in first ply failure laminate loads due to differences in stacking sequence together with a wet or dry environmental condition.

Kriz, R. D.↗

NASA NTRS1980

The potential for damage from the accidental release of conductive carbon fibers from aircraft composites

The paper considers the potential for damage from the accidental release of conductive carbon fibers from aircraft composites. The electrical conductivity of carbon and graphite fibers has led to electrical equipment damage from the inadvertent release of virgin fibers into the atmosphere; an accidental release of carbon fibers from filamentary composites from the burning of crashed commercial airliners could damage electrical and electronic equipment. The experimental and analytical results by NASA of the methods of assessing the extent of potential damage in terms of costs is presented; the NASA materials research program to provide alternate or modified composites to overcome electrical hazards of carbon composites in aircraft structures is described.

Bell, V. L.↗

NASA NTRS1979

Dynamic response of damaged angleplied fiber composites

An investigation was conducted to determine the effects of low level damage induced by monotonic load, cyclic load and/or residual stresses on the vibration frequencies and damping factors of fiber composite angleplied laminates. Two different composite systems were studied - low modulus fiber and ultra high modulus fiber composites. The results obtained showed that the frequencies and damping factors of angleplied laminates made from low modulus fiber composites are sensitive to low level damage while those made from ultra high modulus composites are not. Also, vibration tests may not be sufficiently sensitive to assess concentrated local damage in angleplied laminates. And furthermore, dynamic response determined from low-velocity impact coupled with the Fast Fourier Transform and packaged in a minicomputer can be a convenient procedure for assessing low-level damage in fiber composite angleplied laminates.

Chamis, C. C.↗

NASA NTRS1981

Development of an engineering analysis of progressive damage in composites during low velocity impact

A computerized, analytical methodology was developed to study damage accumulation during low velocity lateral impact of layered composite plates. The impact event was modeled as perfectly plastic with complete momentum transfer to the plate structure. A transient dynamic finite element approach was selected to predict the displacement time response of the plate structure. Composite ply and interlaminar stresses were computed at selected time intervals and subsequently evaluated to predict layer and interlaminar damage. The effects of damage on elemental stiffness were then incorporated back into the analysis for subsequent time steps. Damage predicted included fiber failure, matrix ply failure and interlaminar delamination.

Humphreys, E. A.↗

NASA NTRS1982

Effect of lubricant environment on saw damage in silicon wafers

The chemomechanical effect of lubricant environments on the inner diameter (ID) sawing induced surface damage in Si wafers was tested for four different lubricants: water, dielectric oil, and two commercial cutting solutions. The effects of applying different potential on Si crystals during the sawing were also tested. It is indicated that the number and depth of surface damage are sensitive to the chemical nature of the saw lubricant. It is determined that the lubricants that are good catalysts for breaking Si bonds can dampen the out of plane blade vibration more effectively and produce less surface damage. Correlations between the applied potential and the depth of damage in the dielectric oil and one of the commercial cutting solutions and possible mechanisms involved are discussed.

Kuan, T. S.↗

NASA NTRS1982

Pre- and post-annealing of mechanical damage in silicon wafers

Basic properties of mechanical damage in silicon consisting of cracks and abrasion were studied using transmission electron microscopy. The crystallographic structure of mechanical damage was determined before and after high temperature annealing. The main findings include that stresses in silicon around crack tips are not plastically relieved at room temperature and that abrasion at room temperature introduces shear loops into the silicon. It was also found that cracks of micron size can be annealed out, specifically, if cleavage occurs on 111 planes. The healing products of such cracks are 60 deg and 90 deg dislocations. Submicron cracks transform into stacking faults during annealing. Likewise high concentrations of shear loops due abrasion were found to anneal into stacking faults. A one to one correlation between surface areas containing small cracks and stacking faults was made. Measurements of damage removal on silicon surfaces through chemical-mechanical etching techniques are presented. It is shown that silicon dioxide repolishes damaged silicon surfaces most effectively.

Schwottke, G. H.↗

NASA NTRS1980

Behavior of damaged graphite/epoxy laminates under compression loading

The influence of three different resin systems on the damage tolerance of graphite/polymer laminates was evaluated. Testing consisted of both static compression and cyclic compression evaluation of 10.2 by 15.2 by 0.5 cm (4 by 6 by 0.2 in) laminates with circular holes, simulated delaminations, and low velocity impact. Damage growth under steadily increasing compression and cyclic compression loading was monitored. Damage size and impact-induced failures for the three materials were compared. Of the three material systems evaluated, the one most tolerant to impact damage exhibited the least delamination within the cross section due to impact, the highest transverse tension strain to failure, and the largest crack opening force, as determined from double cantilever beam tests.

Byers, B. A.↗

NASA NTRS1981

The effect of lamination-induced stresses on fatigue damage development at internal flaws

The effects of stresses induced by the lamination of off-axis plies to O-deg lamina on the development of damage during the fatigue loading of the O-deg plies are discussed. The transverse normal stresses in the plane of the laminae and the laminate created by the laminating constraints when an axial force is applied to the laminate are calculated in terms of a differential Poisson ratio between the ply in question in the unconstrained and constrained states, and significant differences in the constraint environments of an unnotched specimen joined to plies of 45 and 90 deg inclination are noted which correspond to an increase in longitudinal splitting in the 90 deg case and a marked decrease in longitudinal splitting in the 45 deg case. If a notch is present, shear and crack-opening damage is found to be very effectively suppressed in 45-deg laminates, and less so in the 90-deg case. It is pointed out that whereas the 45-deg laminate represents the least damage situation, it does not have the greatest notched strength. It is concluded that an understanding and prediction of damage development in laminates requires knowledge of the stress fields caused by the lamination constraints.

Reifsnider, K. L.↗

NASA NTRS1981

Enhanced annealing of GaAs solar cell radiation damage

Solar cells are degraded by radiation damage in space. Investigations have been conducted concerning possibilities for annealing this radiation damage in GaAs solar cells, taking into account the conditions favoring such annealing. It has been found that continuous annealing as well as the combination of injection annealing with thermal annealing can lead to recovery from radiation damage under particularly favorable conditions in GaAs solar cells. The damage caused by both electrons and protons in GaAs solar cells can be substantially reduced by annealing at temperatures as low as 150 C, under appropriate conditions. This possibility makes the GaAs solar cells especially attractive for long space missions, or for missions in severe radiation environments. Attention is given to results concerning periodic thermal annealing, continuous annealing, and injection annealing combined with thermal annealing.

Loo, R.↗

NASA NTRS1981

Damage and coefficients and thermal annealing of irradiated silicon and GaAs solar cells

Electron and proton damage coefficients have been measured for several types of silicon cells of recent manufacture using 1 MeV electrons and protons with energies of 8.3, 17.5 and 40 MeV. LPE (AlGa)As-GaAs cells were also irradiated and pseudodamage coefficients derived under the assumption that the irradiation changes the diffusion length in only one of the n- or p-type layers. After irradiation the cells were annealed isochronally up to 450 C. The damage coefficient and annealing data for silicon cells are in substantial agreement with previous work. The GaAs cells have pseudo-damage coefficients which are dependent on fluence, and have about the same energy dependence as Si damage coefficients.

Anspaugh, B. E.↗