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At least 433 records · Page 24

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NASA NTRS1984

Aerothermal performance and damage tolerance of a Rene 41 metallic standoff thermal protection system at Mach 6.7

A flight-weight, metallic thermal protection system (TPS) model applicable to Earth-entry and hypersonic-cruise vehicles was subjected to multiple cycles of both radiant and aerothermal heating in order to evaluate its aerothermal performance, structural integrity, and damage tolerance. The TPS was designed for a maximum operating temperature of 2060 R and featured a shingled, corrugation-stiffened corrugated-skin heat shield of Rene 41, a nickel-base alloy. The model was subjected to 10 radiant heating tests and to 3 radiant preheat/aerothermal tests. Under radiant-heating conditions with a maximum surface temperature of 2050 R, the TPS performed as designed and limited the primary structure away from the support ribs to temperatures below 780 R. During the first attempt at aerothermal exposure, a failure in the panel-holder test fixture severely damaged the model. However, two radiant preheat/aerothermal tests were made with the damaged model to test its damage tolerance. During these tests, the damaged area did not enlarge; however, the rapidly increasing structural temperature measuring during these tests indicates that had the damaged area been exposed to aerodynamic heating for the entire trajectory, an aluminum burn-through would have occurred.

Avery, D. E.↗

NASA NTRS1985

Field and airborne spectral characterization of suspected damage in red spruce (picea rubens) from Vermont

The utilization of remote sensing to monitor forest damage due to acid deposition is investigated. Spectral and water measurements and aircraft radiance data of red spruce and balsam fir, collected in Camels Hump Mountain and Ripton, Vermont between August 13-20, 1984, are analyzed to evaluate the damage levels of the trees. Variations in reflectance features and canopy moisture content are studied. It is observed that damage correlates with elevation (greater damage at higher elevations); xylem water column tension is greater at higher damage sites; and a 'blue shift' is indicated in the spectral data at high damage sites.

Rock, B. N.↗

NASA NTRS1986

Use of TMS/TM data for mapping of forest decline damage in the northeastern United States

Remote sensing systems were used to monitor forest decline damage suspected of being due to air pollution. Field activities and aircraft overflights were centered on montane spruce/fir forest sites. Using aircraft data acquired with the Thematic Mapper Simulator (TMS) and LANDSAT Thematic Mapper (TM) during the growing season, extensive areas of forest decline damage were accurately mapped. Seven levels of decline damage are discrininated and mapped and the levels of discriminated damage agree well (rsq-0.94) with visual assessment conducted on the ground. New areas of high damage were discovered. A band ratio (TM5/TM4) is most useful in discriminating and quantifying the various levels of forest decline damage.

Rock, B. N.↗

NASA NTRS1986

Correlation of Ultrasonic Polar Backscatter with the Deply Technique for Assessment of Impact Damage in Composite Laminates

Ultrasonic polar backscatter, a quantitative NDE technique introduced by O'Donnell and Miller (1981), was used to characterize impact damages in a 16-ply graphite-epoxy laminate. The results were correlated with the results obtained by the destructive deply technique of Freeman (1984). The size, shape, and orientation of damage correlated well between the polar backscatter technique and the deply technique. There was good quantitative correlation between the areas of damage indicated by the two techniques, suggesting that the polar backscatter technique is sensitive to specific orientations of damage. The polar backscatter technique provides a good qualitative image of the size and shape of the largest zone of damage in each of the principal orientations; a quantitative estimate of the extent of these largest damage zones can then be obtained.

Blodgett, E. D.↗

NASA NTRS1988

Surface crack analysis applied to impact damage in a thick graphite-epoxy composite

The residual tensile strength of a thick graphite/epoxy composite with impact damage was predicted using surface crack analysis. The damage was localized to a region directly beneath the impact site and extended only part way through the laminate. The damaged region contained broken fibers, and the locus of breaks in each layer resembled a crack perpendicular to the direction of the fibers. In some cases, the impacts broke fibers without making a visible crater. The impact damage was represented as a semi-elliptical surface crack with length and depth equal to that of the impact damage. The maximum length and depth of the damage were predicted with a stress analysis and a maximum shear stress criterion. The predictions and measurements of strength were in good agreement.

Poe, C. C., Jr.↗

NASA NTRS1988

The relationship between observed fatigue damage and life estimation models

Observations of the surface of laboratory specimens subjected to axial and torsional fatigue loadings has resulted in the identification of three damage fatigue phenomena: crack nucleation, shear crack growth, and tensile crack growth. Material, microstructure, state of stress/strain, and loading amplitude all influence which of the three types of fatigue damage occurs during a dominant fatigue life fraction. Fatigue damage maps are employed to summarize the experimental observations. Appropriate bulk stress/strain damage parameters are suggested to model fatigue damage for the dominant fatigue life fraction. Extension of the damage map concept to more complex loadings is presented.

Kurath, Peter↗

NASA NTRS1988

Extension and validation of a method for locating damaged members in large space trusses

The damage location approach employs the control system capabilities for the structure to test the structure and measure the dynamic response. The measurements are then used in a system identification algorithm to produce a model of the damaged structure. The model is compared to one for the undamaged structure to find regions of reduced stiffness which indicate the location of damage. Kabe's 3,4 stiffness matrix adjustment method was the central identification algorithm. The strength of his method is that, with minimal data, it preserves the representation of the physical connectivity of the structure in the resulting model of the damaged truss. However, extensive storage and computational effort were required as a result. Extension of the damage location method to overcome these problems is the first part of the current work. The central system identification algorithm is replaced with the MSMT method of stiffness matrix adjustment which was previously derived by generalizing an optimal-update secant method form quasi-Newton approaches for nonlinear optimization. Validation of the extended damage location method is the second goal.

Smith, Suzanne Weaver↗

NASA NTRS1989

STS-27R OV-104 Orbiter TPS damage review team, volume 1

Following the return to earth on December 2, 1988, of Orbiter OV-104, Atlantis, it was observed that there was substantial Thermal Protection System (TPS) tile damage present on the lower right fuselage and wing. Damage sites were more numerous than on previous flights and conversely, there was almost no damage present on Atlantis' left side. A review team investigated the cause beginning with a detailed inspection of the Atlantis TPS damage, and a review of related inspection reports to establish an indepth anomaly definition. An exhaustive data review followed. A fault tree and several failure scenarios were developed. Finally, the failure scenarios were categorized as either not possible, possible but not probable, or probable. This and other information gained during the review formed the basis for the team's findings and recommendations. The team concluded that the most probable cause of the severe STS-27R Orbiter tile damage is that the ablative insulating material covering the RH SRB Nose Cap dislodged and struck the Orbiter tile near 85 seconds into flight and possibly that debris from other sources, including repaired insulation and missing joint cork, caused minor tile damage. Findings are presented, and recommendations that are believed pertinent to minimizing the potential for inflight debris are described.

Thomas, John W.↗

NASA NTRS1989

Laser damage threshold of diamond films

The possibility that diamond films may inhibit laser-induced damage to optical components in laser systems films was investigated by measuring laser damage thresholds of free-standing diamond film windows, diamond films deposited on silicon substrates, and bare silicon substrate. Polycrystalline diamond films were deposited using a dc plasma-enhanced CVD process. It was found that free-standing diamond films had the highest laser damage threshold at 1064 nm. For a diamond film of 630 nm, the damage threshold was found to be 7 J/sq cm, as compared to a damage threshold of 4.5 J/sq cm for bare silicon, and a low value of 1.5 J/sq cm for the film/substrate combination. The damage mechanism is considered to involve melting or dielectric breakdown induced by laser radiation. The low value of the film/substrate combination is attributed to film stress and conditions of film deposition.

Albin, Sacharia↗

NASA NTRS1989

Radiation damage in scientific charge-coupled devices

Radiation damage is reported on CCDs (charge-coupled devices) that have been primarily exposed to 1.25-MeV gamma rays (Co-60 source). Two important classes of radiation damage are discussed, namely, bulk and ionization effects. Bulk damage or displacement damage is a process in which silicon atoms are displaced from their normal lattice positions by high-energy photons or particles. Single atomic displacements or cluster defect damage is produced, depending on the energy and type of radiation experienced by the detector. Bulk damage creates trapping sites within the CCD's signal channel which in turn degrades charge-transfer efficiency.

Janesick, James↗

NASA NTRS1989

Electron-beam induced damage in thin insulating films on compound semiconductors

Phosphorus rich plasma enhanced chemical vapor deposition (PECVD) of silicon nitride and silicon dioxide films on n-type indium phosphide (InP) substrates were exposed to electron-beam irradiation in the 5 to 40 keV range for the purpose of characterizing the damage induced in the dielectric. The electron-beam exposure was on the range of 10(exp -7) to 10(exp -3) C/sq cm. The damage to the devices was characterized by capacitance-voltage (C-V) measurements of the metal insulator semiconductor (MIS) capacitors. These results were compared to results obtained for radiation damage of thermal silicon dioxide on silicon (Si) MOS capacitors with similar exposures. The radiation induced damage in the PECVD silicon nitride films on InP was successfully annealed out in an hydrogen/nitrogen (H2/N2) ambient at 400 C for 15 min. The PECVD silicon dioxide films on InP had the least radiation damage, while the thermal silicon dioxide films on Si had the most radiation damage.

Pantic, Dragan M.↗

NASA NTRS1990

Electron beam induced damage in PECVD Si3N4 and SiO2 films on InP

Phosphorus rich plasma enhanced chemical vapor deposition (PECVD) of silicon nitride and silicon dioxide films on n-type indium phosphide (InP) substrates were exposed to electron beam irradiation in the 5 to 40 keV range for the purpose of characterizing the damage induced in the dielectic. The electron beam exposure was on the range of 10(exp -7) to 10(exp -3) C/sq cm. The damage to the devices was characterized by capacitance-voltage (C-V) measurements of the metal insulator semiconductor (MIS) capacitors. These results were compared to results obtained for radiation damage of thermal silicon dioxide on silicon (Si) MOS capacitors with similar exposures. The radiation induced damage in the PECVD silicon nitride films on InP was successfully annealed out in an hydrogen/nitrogen (H2/N2) ambient at 400 C for 15 min. The PECVD silicon dioxide films on InP had the least radiation damage, while the thermal silicon dioxide films on Si had the most radiation damage.

Pantic, Dragan M.↗

NASA NTRS1990

Summary of a study to determine low-velocity impact damage and residual tension strength for a thick graphite/epoxy motor case

Impacters of various shapes and masses were dropped from various heights onto 36 mm (1.4 in.) thick graphite/epoxy cylinders, which represented filament wound cases (FWC) for the booster motors of the Space Shuttle. Insert solid propellant was cast into some of the cylinders. The cylinders were impacted numerous times around the circumference and then cut into 51 mm (2.0 in.) wide tension specimens, each containing an impact site. Four indenters were used: a sharp corner, two hemispheres, and a bolt-like rod. The diameters of the hemispheres were 12.7 mm mm (0.5 in.) and 25.4 mm (1.0 in.), and the diameter of the rod was 6.3 mm (0.25 in.). Impacts with the rod were simulated by pressing the rod against the face of specimens. For the hemispheres, the damage initiated beneath the surface at a critical contact pressure and was not visible on the surface until an even larger pressure was exceeded. The damage consisted of matrix cracking and broken fiber. The rod an corner made visible surface damage in all tests. For the hemispheres, the tension strength was reduced considerably before the damage was visible on the surface, 30 percent for the 25.4 mm (1.0 in.) diameter hemisphere and 10 percent for the 12.7 mm (0.5 in.) diameter hemisphere. Analytical methods were used to predict the damage and residual tension strength. A factor of safety to account for nonvisible damage was determined.

Poe, C. C., Jr.↗

NASA NTRS1990

Surface crack analysis applied to impact damage in a thick graphite/epoxy composite

Poe, Clarence C., Jr.↗

NASA NTRS1991

Damage development under compression-compression fatigue loading in a stitched uniwoven graphite/epoxy composite material

Damage initiation and growth under compression-compression fatigue loading were investigated for a stitched uniweave material system with an underlying AS4/3501-6 quasi-isotropic layup. Performance of unnotched specimens having stitch rows at either 0 degree or 90 degrees to the loading direction was compared. Special attention was given to the effects of stitching related manufacturing defects. Damage evaluation techniques included edge replication, stiffness monitoring, x-ray radiography, residual compressive strength, and laminate sectioning. It was found that the manufacturing defect of inclined stitches had the greatest adverse effect on material performance. Zero degree and 90 degree specimen performances were generally the same. While the stitches were the source of damage initiation, they also slowed damage propagation both along the length and across the width and affected through-the-thickness damage growth. A pinched layer zone formed by the stitches particularly affected damage initiation and growth. The compressive failure mode was transverse shear for all specimens, both in static compression and fatigue cycling effects.

Vandermey, Nancy E.↗

NASA NTRS1990

Impact damage in composite laminates

A low-velocity impact-damage model for quasi-symmetric graphite-fiber composite plates is presented. The distribution of damage in each layer of the plate was calculated by employing Di Sciuva's composite laminate theory together with Hashin's failure criterion for fiber-reinforced composites. The dynamic deformation of the target plate was represented by the lower vibrational modes of the plate. The principle of virtual work was applied in the formulation of the problem. In the analysis, the material was regarded as 'damaged' when its designed strength was reduced by the failure of its constituents. The constituent failures consisted of matrix crackings, fiber breakages, and delamination between layers. According to damage modes, the moduli of material in the damaged zone were reduced according to the failure criteria. The interaction between layers and its role in damage propagation were also studied.

Wang, Chung-Yue↗

NASA NTRS1990

A model for predicting damage induced fatigue life of laminated composite structural components

This paper presents a model for predicting the life of laminated composite structural components subjected to fatigue induced microstructural damage. The model uses the concept of continuum damage mechanics, wherein the effects of microcracks are incorporated into a damage dependent lamination theory instead of treating each crack as an internal boundary. Internal variables are formulated to account for the effects of both matrix cracks and internal delaminations. Evolution laws for determining the damage variables as functions of ply stresses are proposed, and comparisons of predicted damage evolution are made to experiment. In addition, predicted stiffness losses, as well as ply stresses are shown as functions of damage state for a variety of stacking sequences.

Allen, David H.↗

NASA NTRS1990

Summary of a study to determine low-velocity impact damage and residual tension strength for a thick graphite/epoxy motor case

Impacters of various shapes and masses were dropped from various heights onto 36 mm (1.4 in.) thick graphite/epoxy cylinders, which represented filament wound cases (FWC) for the booster motors of the Space Shuttle. Insert solid propellant was cast into some of the cylinders. The cylinders were impacted numerous times around the circumference and then cut into 51 mm (2.0 in.) wide tension specimens, each containing an impact site. Four indenters were used: a sharp corner, two hemispheres, and a bolt-like rod. The diameters of the hemispheres were 12.7 mm (0.5 in.) and 25.4 mm (1.0 in.), and the diameter of the rod was 6.3 mm (0.25 in.). Impacts with the rod were simulated by pressing the rod against the face of specimens. For the hemispheres, the damage initiated beneath the surface at a critical contact pressure and was not visible on the surface until an even larger pressure was exceeded. The damage consisted of matrix cracking and broken fiber. The rod and corner made visible surface damage in all tests. For the hemispheres, the tension strength was reduced considerably before the damage was visible on the surface, 30 percent for the 25.4 mm (1.0 in.) diameter hemisphere and 10 percent for the 12.7 mm (0.5 in.) diameter hemisphere. Analytical methods were used to predict the damage and residual tension strength. A factor of safety to account for nonvisible damage was determined.

Poe, C. C., Jr.↗