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At least 1,711 records · Page 95

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NASA NTRS1978

Radiation damage of germanium detectors

Energetic particles can produce interstitial-vacancy pairs in a crystal by knocking the atoms from their normal positions. Detectors are unique among semiconductor devices in depending on very low concentrations of electrically active impurities, and also on efficient transport of holes and electrons over relatively large distances. Because the dense regions of damage produced by energetic particles may result in donors and/or acceptors, and also provide trapping sites for holes and electrons, detectors are very sensitive to radiation damage. In addition to these effects occurring within the detector, radiation may also change the characteristics of the exposed surfaces causing unpredictable effects on the detector leakage current. Radiation-induced surface degradation has rarely, if ever, been observed for germanium detectors. The possibility of minimizing hole trapping in charge collection by the use of a high-purity germanium coaxial detector configured with the p (+) contact on the coaxial periphery is discussed.

Pehl, R. H.↗

NASA NTRS1978

Annealing of GaAs solar cells damaged by electron irradiation

Measurements of thermal annealing of GaAlAs/GaAs solar cells damaged by 1 MeV electron irradiation are reported, and the magnitude of the short-circuit current recovery is discussed. The damaged cells are annealed in a vacuum at 200 C. A cell irradiated at 10 to the 13th power electrons per sq cm recovers all its lost short-circuit current after 15 hours of annealing. Possible application of the annealing process to solar cells in space is also considered.

Walker, G. H.↗

NASA NTRS1978

Recovery of shallow junction GaAs solar cells damaged by electron irradiation

Solar cells operated in space are subject to degradation from electron and proton radiation damage. It has been found that for deep junction p-GaAlAs/p-GaAs solar cells some of the electron radiation damage is removed by annealing the cells at 200 C. The reported investigation shows that shallow junction p-GaAlAs/p-GaAs/n-GaAs heteroface solar cells irradiated with 1 MeV electrons show a more complete recovery of short-circuit current than do the deep junction cells. The heteroface p-GaAlAs/p-GaAs/n-GaAs solar cells studied were fabricated using the etch-back epitaxy process.

Walker, G. H.↗

NASA NTRS1979

Recent development in the design, testing and impact-damage tolerance of stiffened composite panels

Structural technology of laminated filamentary-composite stiffened-panel structures under combined inplane and lateral loadings is discussed. Attention is focused on: (1) methods for analyzing the behavior of these structures under load and for determining appropriate structural proportions for weight-efficient configurations; and (2) effects of impact damage and geometric imperfections on structural performance. Recent improvements in buckling analysis involving combined inplane compression and shear loadings and transverse shear deformations are presented. A computer code is described for proportioning or sizing laminate layers and cross-sectional dimensions, and the code is used to develop structural efficiency data for a variety of configurations, loading conditions, and constraint conditions. Experimental data on buckling of panels under inplane compression is presented. Mechanisms of impact damage initiation and propagation are described.

Williams, J. G.↗

NASA NTRS1979

Experimental and analytical study of fatigue damage in notched graphite/epoxy laminates

Both tension and compression fatigue behaviors were investigated in four notched graphite/epoxy laminates. After fatigue loading, specimens were examined for damage type and location using visual inspection, light microscopy, scanning electron microscopy, ultrasonic C-scans, and X-radiography. Delamination and ply cracking were found to be the dominant types of fatigue damage. In general, ply cracks did not propagate into adjacent plies of differing fiber orientation. To help understand the varied fatigue observations, the interlaminar stress distribution was calculated with finite element analysis for the regions around the hole and along the straight free edge. Comparison of observed delamination locations with the calculated stresses indicated that both interlaminar shear and peel stresses must be considered when predicting delamination. The effects of the fatigue cycling on residual strength and stiffness were measured for some specimens of each laminate type. Fatigue loading generally caused only small stiffness losses. In all cases, residual strengths were greater than or equal to the virgin strengths.

Whitcomb, J. D.↗

NASA NTRS1979

Radiation damage in high-voltage silicon solar cells

Three types of open circuit high voltage solar cells were tested to determine their performance after exposure to 1 MeV electron irradiations. The cells with a relatively deep n-type emitter were more susceptible to radiation damage than other high open circuit high voltage cells. The use of diffused or ion implanted junctions leads to open circuit high voltage cell designs that are less susceptible to radiation damage. These latter two types of cells show degradations that are typical of the 0.1 ohm-cm material from which they are fabricated. Furthermore, exposure to ionizing radiation causes oxide degradation and decreased cell performance in cells that depend on a charged oxide to achieve significant cell properties. Hence, the combination of a charged oxide and a relatively deep n-type emitter is not recommended for incorporation into a cell designed for use in the particulate radiation environment of space.

Weinberg, I.↗

NASA NTRS1981

Wrench for smooth or damaged fasteners

Smooth-surfaced or damaged fasteners that cannot be gripped by conventional wrench can be unscrewed by special wrench. It can be used in tight spaces and will not damage adjacent structures. Wrench consists of central handle and 2 independent jaws with serrated teeth. Teeth are placed on fastener to be removed, and handle is rotated until fastener is gripped with positive locking action. Rotation of wrench handle removes fastener.

Carrillo, R.↗

NASA NTRS1980

Space solar cells: High efficiency and radiation damage

The progress and status of efforts to increase the end-of-life efficiency of solar cells for space use is assessed. High efficiency silicon solar cells, silicon solar cell radiation damage, GaAs solar cell performance and radiation damage and 30 percent devices are discussed.

Brandhorst, H., Jr.↗

NASA NTRS1980

A NASTRAN investigation of simulated projectile damage effects on a UH-1B tail boom model

A NASTRAN model of a UH-1B tail boom that had been designed for another project was used to investigate the effect on structural integrity of simulated projectile damage. Elements representing skin, and sections of stringers, longerons and bulkheads were systematically deleted to represent projectile damage. The structure was loaded in a manner to represent the flight loads that would be imposed on the tail boom at a 130 knot cruise. The deflection of four points on the rear of the tail boom relative to the position of these points for the unloaded, undamaged condition of the tail boom was used as a measure of the loss of structural rigidity. The same procedure was then used with the material properties of the aluminum alloys replaced with the material properties of T300/5208 high strength graphite/epoxy fibrous composite material, (0, + or - 45, 90)s for the skin and (0, + or - 45)s for the longerons, stringers, and bulk heads.

Futterer, A. T.↗

NASA NTRS1979

Effect of impact damage and holes on the compressive strength of a graphite/epoxy laminate

An experimental investigation has been conducted to determine the effect of low-velocity impact damage and unloaded circular holes on the compressive strength of a 48-ply orthotropic graphite/epoxy flat laminate. Specimens were impacted by a 1.27-cm-diameter aluminum sphere with speeds from 52 to 101 m/x to simulate momenta typical of low-velocity impact hazards that can occur in commercial aircraft service. It is shown that low-velocity impact damage can significantly degrade the static compressive strength of the laminate. Specimens that fail at axial strains above 0.008 in the undamaged condition can fail at strains as low as 0.0031 when impacted at 100 m/s. Circular holes also reduce the static compressive strength of the laminate. The failure strain decreases as the hole diameter increases.

Starnes, J. H., Jr.↗

NASA NTRS1980

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

Carbon and graphite fibers are known to be electrically conductive. The rapidly accelerating use of carbon fibers as the reinforcement in filamentary composite materials brought up the possibility of accidental release of carbon fibers from the burning of crashed commercial airliners with carbon composite parts. Such release could conceivably cause widespread damage to electrical and electronic equipment. The experimental and analytical results of a comprehensive investigation of the various elements necessary to assess the extent of such potential damage in terms of annual expected costs and maximum losses at low probabilities of occurrence are presented. A review of NASA materials research program to provide alternate or modified composite materials to overcome any electrical hazards from the use of carbon composites in aircraft structures is described.

Bell, V. L.↗

NASA NTRS1980

Annealing radiation damaged silicon solar cells with a copper halide laser

The use of a multiply pulsed copper halide laser to significantly anneal out the damage to silicon solar cells caused by a simulated space radiation environment is investigated. Preliminary experiments demonstrate that the amount of damage can be decreased by 41% as measured by the maximum power generated.

Pivirotto, T. J.↗

NASA NTRS1980

Simultaneous radiation damage and annealing of GaAs solar cells

In this report, concepts and equations are developed to describe the simultaneous radiation damage and annealing of solar cells. The annealing characteristics of three types of defects are employed to study three examples of continuous irradiation and annealing. One example shows that the effective damage could be reduced by a factor of fifty. The difficulty of a laboratory experiment properly simulating the effects in space are discussed. Potential advantages of continuous annealing such as reduction of stable defect formation and omission of radiation cover glasses are presented.

Heinbockel, J. H.↗

NASA NTRS1980

Proton damage annealing kinetics in silicon solar cells

Proton damage annealing as a method for prolonging the life of solar power systems in space is discussed. Variables are minimized and fundamental characteristics of proton damage annealing are considered. The usefulness of annealing for prolonging space missions is evaluated. A preliminary determination of optimum annealing conditions is made, and base data provided for more detailed research programs.

Horne, W. E.↗

NASA NTRS1982

Improved damage tolerance of titanium by adhesive lamination

Basic damage tolerance properties of Ti-6A1-4V titanium plate can be improved by laminating thin sheets of titanium with adhesives. Compact tension and center cracked tension specimens made from thick plate, thin sheet, and laminated plate (six plies of thin sheet) were tested. The fracture toughness of the laminated plate was 39 percent higher than the monolithic plate. The laminated plate's through the thickness crack growth rate was about 20 percent less than that of the monolithic plate. The damage tolerance life of the surface cracked laminate was 6 to over 15 times the life of a monolithic specimen. A simple method of predicting crack growth in a crack ply of a laminate is presented.

Johnson, W. S.↗

NASA NTRS1982

Damage tolerant design using collapse techniques

A new approach to the design of structures for improved global damage tolerance is presented. In its undamaged condition the structure is designed subject to strength, displacement and buckling constraints. In the damaged condition the only constraint is that the structure will not collapse. The collapse load calculation is formulated as a maximization problem and solved by an interior extended penalty function. The design for minimum weight subject to constraints on the undamaged structure and a specified level of the collapse load is a minimization problem which is also solved by a penalty function formulation. Thus the overall problem is of a nested or multilevel optimization. Examples are presented to demonstrate the difference between the present and more traditional approaches.

Haftka, R. T.↗

NASA NTRS1982

Three dimensional finite element analysis of damage accumulation in composite laminate

A three dimensional finite-element computer program has been developed to analyze layered fiber-reinforced composite laminate. This program is capable of: (1) calculating the detailed stress distribution, (2) identifying the damage zone and mode of failure, (3) analyzing the damage accumulation, and (4) determining the ultimate strength of the composite laminate.

Lee, J. D.↗

NASA NTRS1981

A thermochemical model of radiation damage and annealing applied to GaAs solar cells

Calculations of the equilibrium conditions for continuous radiation damage and thermal annealing are reported. The calculations are based on a thermochemical model developed to analyze the incorporation of point imperfections in GaAs, and modified by introducing the radiation to produce native lattice defects rather than high-temperature and arsenic atmospheric pressure. The concentration of a set of defects, including vacancies, divacancies, and impurity vacancy complexes, are calculated as a function of temperature. Minority carrier lifetimes, short circuit current, and efficiency are deduced for a range of equilibrium temperatures. The results indicate that GaAs solar cells could have a mission life which is not greatly limited by radiation damage.

Conway, E. J.↗