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At least 127 records · Page 7

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NASA NTRS1979

The effects of thermal cycling on the thermal deformation of graphite-polyimide

The effects of repetitive thermal cycling on the thermal deformation of graphite-fiber-reinforced polyimide laminates are investigated. Unsymmetric and unidirectional laminate specimens were cycled in an oven between room temperature and 180 or 315 C five times, and temperature and deformation were recorded as a function of time in order to measure thermal deformation. The results of the lower temperature cycling reveal a hysteresis-like effect in the temperature-thermal deformation relation for a single cycle, while no differences are detected from one cycle to the next. The response of an eight-layer curved specimen is found to agree well with predictions based on a piecewise-linear lamination theory, however, four-layer laminate response could not be explained. The results of the high-temperature tests also indicate a dependence of the strain on whether the specimen was being heated or cooled, however quantitative results are considered unreliable, due to strain gage difficulties.

Hyer, M. W.↗

NASA NTRS1979

Structure of deformed silicon and implications for low-cost solar cells

The paper reports on an investigation of the microstructure and minority carrier lifetime of silicon in uniaxially compressed silicon samples, the objective of which was to determine if it is feasible to produce silicon solar cells from sheet formed by high temperature deformation. It is reported that recrystallization was found to be incomplete in both fine and large grained materials, and that the major mode of recrystallization appears to be migration of existing boundaries into the deformed regions. Also, minority carrier diffusion length was found to be drastically reduced after deformation, perhaps due to contamination or cooling rate, and recovered only slightly with annealing. It is concluded that these results suggest that high temperature deformation of silicon for direct production of sheet for high efficiency solar cells is not practical. It is noted that potential may exist for its use as a coarse grained substrate.

Mardesich, N.↗

NASA NTRS1981

Some effects of thermal-cycle-induced deformation in rocket thrust chambers

The deformation process observed in the hot gas side wall of rocket combustion chambers was investigaged for three different liner materials. Five thrust chambers were cycled to failure by using hydrogen and oxygen as propellants at a chamber pressure of 4.14 MN/cu m. The deformation was observed nondestructively at midlife points and destructively after failure occurred. The cyclic life results are presented with an accompanying discussion about the problems of life prediction associated with the types of failures encountered in the present work. Data indicating the deformation of the thrust chamber liner as cycles are accumulated are presented for each of the test thrust chambers. From these deformation data and observation of the failure sites it is evident that modeling the failure process as classic low cycle thermal fatigue is inadequate as a life prediction method.

Hannum, N. P.↗

NASA NTRS1982

Elastic deformation effects on aerodynamic characteristics for a high-aspect-ratio supercritical-wing model

The results of an investigation of the deformations of a high-aspect-ratio, force/pressure, supercritical-wing model during wind tunnel tests and the effects these deformations have on the wing aerodynamics are presented. A finite element model of the wing was developed, and then, for conditions corresponding to wind tunnel test points, experimental aerodynamic loads and theoretical aerodynamic loads were applied to the finite element model. Comparisons were made between the results of these load conditions for changes in structural deflections and for changes in aerodynamic characteristics. The results show that the deformations are quite small and that the pressure data are not significantly affected by model deformation.

Watson, J. J.↗

NASA NTRS1982

On finite deformation elasto-plasticity

Lee (1969) has proposed a theory based on the decomposition of the total deformation gradient to an elastic and plastic part, and from it has concluded that the additive decomposition of the strain rates holds only approximately. Lubarda and Lee (1981) have declared that Lee's 'exact finite-deformation kinematics shows the almost universal assumption that the total velocity strain or rate of deformation is the sum of elastic and plastic rates to be in error'. Hence questions are raised regarding the validity of essentially all finite deformation elasto-plasticity theories. The present investigation is concerned with these questions. It is shown that the additive decomposition of the strain rates follows from all common finite elasto-plasticity concepts. Lee's theory is examined, and it is shown that this theory also leads to an additive strain rate decomposition, and therefore, his conclusion stems from misinterpretation. It is found that the elastic and the plastic strain rates considered by Lee do not correspond to the same configuration. They are, therefore, not compatible measures.

Nemat-Nasser, S.↗

NASA NTRS1982

Fatigue and creep-fatigue deformation of several nickel-base superalloys at 650 C

Transmission electron microscopy has been used to study the bulk deformation characteristics of seven nickel-base superalloys tested in fatigue and creep-fatigue at 650 C. The alloys were Waspalloy, HIP Astroloy, H plus F Astroloy, H plus F Rene 95, IN 100, MERL 76, and NASA IIB-7. The amount of bulk deformation observed in all the alloys was low. In tests with inelastic strain amplitudes less than about 0.003, only some grains exhibited yielding and the majority of those had the 110 line near the tensile axis. Deformation occurred on octahedral systems for all of the alloys except MERL 76 which also showed abundant primary cube slip. Creep-fatigue cycling occasionally produced extended faults between partial dislocations, but otherwise deformation was much the same as for fatigue cycling.

Miner, R. V.↗

NASA NTRS1982

Postseismic deformation due to subcrustal viscoelastic relaxation following dip-slip earthquakes

The deformation of the Earth following a dip-slip earthquake is calculated using a three layer rheological model and finite element techniques. The three layers are an elastic upper lithosphere, a standard linear solid lower lithosphere, and a Maxwell viscoelastic asthenosphere-a model previously analyzed in the strike-clip case (Cohen, 1981, 1982). Attention is focused on the magnitude of the postseismic subsidence and the width of the subsidence zone that can develop due to the viscoelastic response to coseismic reverse slip. Detailed analysis for a fault extending from the surface to 15 km with a 45 deg dip reveals that postseismic subsidence is sensitive to the depth to the asthenosphere but is only weakly dependent on lower lithosphere depth. The greatest subsidence occurs when the elastic lithosphere is about 30 km thick and the asthenosphere lies just below this layer (asthenosphere depth = 2 times the fault depth). The extremum in the subsidence pattern occurs at about 5 km from the surface trace of the fault and lies over the slip plane. In a typical case after a time t = 30 tau (tau = Maxwell time) following the earthquake the subsidence at this point is 60% of the coseismic uplift. Unlike the horizontal deformation following a strike slip earthquake, significant vertical deformation due to asthenosphere flow persists for many times tau and the magnitude of the vertical deformation is not necessarily enhanced by having a partially relaxing lower lithosphere.

Cohen, S. C.↗

NASA NTRS1983

Reports on crustal movements and deformations

This Catalog of Reports on Crustal Movements and Deformation is a structured bibliography of scientific papers on the movements of the Earth crust. The catalog summarizes by various subjects papers containing data on the movement of the Earth's surface due to tectonic processes. In preparing the catalog we have included studies of tectonic plate motions, spreading and convergence, microplate rotation, regional crustal deformation strain accumulation and deformations associated with the earthquake cycle, and fault motion. We have also included several papers dealing with models of tectonic plate motion and with crustal stress. Papers which discuss tectonic and geologic history but which do not present rates of movements or deformations and papers which are primarily theoretical analyses have been excluded from the catalog. An index of authors cross-referenced to their publications also appears in the catalog. The catalog covers articles appearing in reviewed technical journals during the years 1970-1981. Although there are citations from about twenty journals most of the items come from the following publications: Journal of Geophysical Research, Tectonophysics, Geological Society of America Bulletin of the Seismological Society of America, Nature, Science, Geophysical Journal of the Royal Astronomical Society, Earth and Planetary Science Letters, and Geology.

Cohen, S. C.↗

NASA NTRS1983

Estimating crustal deformations from a combination of baseline measurements and geophysical models

Three possible estimation algorithms are presented for the monitoring of crustal deformations from a combination of repeated baseline measurements and prior deformation information. Attention is given to best linear minimum bias estimation, best linear estimation, and Bayesian estimation. The application of the deformation estimators to the maintenance of a conventional terrestrial reference system is considered. It is shown that if no prior deformation model is available, the free adjustment is the preferred algorithm. In the presence of prior information, however, it is better to use a somewhat incorrect model than to ignore it altogether. The best estimation model of the ones tested is the best linear estimation.

Bock, Y.↗

NASA NTRS1984

Postseismic deformation due to subcrustal viscoelastic relaxation following dip-slip earthquakes

The deformation of the earth following a dip-slip earthquake is calculated using a three layer rheological model and finite element techniques. The three layers are an elastic upper lithosphere, a standard linear solid lower lithosphere, and a Maxwell viscoelastic asthenosphere - a model previously analyzed in the strike-clip case (Cohen, 1981, 1982). Attention is focused on the magnitude of the postseismic subsidence and the width of the subsidence zone that can develop due to the viscoelastic response to coseismic reverse slip. Detailed analysis for a fault extending from the surface to 15 km with a 45 deg dip reveals that postseismic subsidence is sensitive to the depth to the asthenosphere but is only weakly dependent on lower lithosphere depth. The greatest subsidence occurs when the elastic lithosphere is about 30 km thick and the asthenosphere lies just below this layer (asthenosphere depth = 2 times the fault depth). The extremum in the subsidence pattern occurs at about 5 km from the surface trace of the fault and lies over the slip plane. In a typical case after a time t = 30 tau (tau = Maxwell time) following the earthquake, the subsidence at this point is 60 percent of the coseismic uplift. Unlike the horizontal deformation following a strike slip earthquake, significant vertical deformation due to asthenosphere flow persists for many times tau and the magnitude of the vertical deformation is not necessarily enhanced by having a partially relaxing lower lithosphere. Previously announced in STAR as N83-13683

Cohen, S. C.↗

NASA NTRS1984

Hygrothermal effects on stresses and deformations in a bonded fiber-reinforced plastic/aluminum system

The hygrothermal effect on time dependent mechanical behavior of a model of nonsymmetrical bonded fiber-reinforced plastic (FRP)/aluminum system was investigated analytically. It was found that a linear viscoelastic closed form solution is sufficient for the prediction model deformations for short periods of time and small strains, while a numerical finite-element solution is preferable for determining adhesive stresses and deformational behavior of the model at any time. The thermoelastic numerical solution shows a certain decrease with time of adhesive stresses, which does not seem to affect significantly the global deformations of the model. On the other hand, it was found that the presence of moisture within the polymeric adhesive layer causes significant decrease in adhesive stresses, which affects deformational changes with time, far beyond the level predicted by the static solution.

Yaniv, G.↗

NASA NTRS1984

Determination of near-surface plastic deformation in sliding contacts

It is pointed out that substantial plastic deformation occurs on and near the contact surfaces, when two solid bodies slide against each other without lubrication. It has been found that this deformation plays an important role in the tribological behavior of the sliding contact. The present investigation has the objective to develop an analytical model to predict the near-surface plastic deformation resulting from a single pass of one metallic surface over another. A finite element viscoplasticity program was written relating velocities to forces in a two-dimensional domain. The program was employed in the study of plastic deformation during a single pass of a hardened tool steel slider over a copper rub specimen. It was found that essentially the only material set in motion by the slider was directly under the contact zone. The agreement between values obtained in the analysis and experimental data is reasonably good.

Kennedy, F. E., Jr.↗

NASA NTRS1986

Transpression as the main deformational event in an Archaean greenstone belt, northeastern Minnesota

Deformed and metamorphosed sedimentary and volcanic rocks of the Vermilion district constitute an Archean greenstone belt trending east-west between higher grade rocks of the Vermilion Granitic Complex to the north and the Giants Range batholith to the south. Metamorphic grade is low throughout, being lowest in the center of the belt (chlorite zone of the greenschist facies). All the measured strain, a cleavage or schistosity, and a mineral lineation in this belt are attributed to the main phase of deformation D sub 2 that followed an earlier nappe-forming event D sub 1, which left little evidence of penetrative fabric. Previous work assumed that the D sub 2 deformation resulted from north-south compression across the district. It is now believed that a significant component of this deformation resulted from dextral shear across the whole region. Thus the Vermilion fault, a late-state largely strike-slip structure that bounds the Vermilion district to the north, may simply be the latest, most brittle expression of a shear regime that was much more widespread in space and time. Features that are indicative of shear include ductile shear zones with sigmoidal foliation patterns, highly schistose zones with the development of shear bands, feldspar clasts or pyrite cubes with asymmetric pressure shadows, and the fact that the asymmetry of the F sub 2 folds is predominantly Z for at least 15 km south of the Vermilion fault.

Hudleston, P. J.↗

NASA NTRS1986

Plate motions and deformations from geologic and geodetic data

Research effort on behalf of the Crustal Dynamics Project focused on the development of methodologies suitable for the analysis of space-geodetic data sets for the estimation of crustal motions, in conjunction with results derived from land-based geodetic data, neo-tectonic studies, and other geophysical data. These methodologies were used to provide estimates of both global plate motions and intraplate deformation in the western U.S. Results from the satellite ranging experiment for the rate of change of the baseline length between San Diego and Quincy, California indicated that relative motion between the North American and Pacific plates over the course of the observing period during 1972 to 1982 were consistent with estimates calculated from geologic data averaged over the past few million years. This result, when combined with other kinematic constraints on western U.S. deformation derived from land-based geodesy, neo-tectonic studies, and other geophysical data, places limits on the possible extension of the Basin and Range province, and implies significant deformation is occurring west of the San Andreas fault. A new methodology was developed to analyze vector-position space-geodetic data to provide estimates of relative vector motions of the observing sites. The algorithm is suitable for the reduction of large, inhomogeneous data sets, and takes into account the full position covariances, errors due to poorly resolved Earth orientation parameters and vertical positions, and reduces baises due to inhomogeneous sampling of the data. This methodology was applied to the problem of estimating the rate-scaling parameter of a global plate tectonic model using satellite laser ranging observations over a five-year interval. The results indicate that the mean rate of global plate motions for that interval are consistent with those averaged over several million years, and are not consistent with quiescent or greatly accelerated plate motions. This methodology was also used to provide constraints on deformation in the western U.S. using very long baseline interferometry observations over a two-year period.

Jordan, T. H.↗

NASA NTRS1986

Viscoelastic deformation near active plate boundaries

Model deformations near the active plate boundaries of Western North America using space-based geodetic measurements as constraints are discussed. The first six months of this project were spent gaining familarity with space-based measurements, accessing the Crustal Dynamics Data Information Computer, and building time independent deformation models. The initial goal was to see how well the simplest elastic models can reproduce very long base interferometry (VLBI) baseline data. From the Crustal Dynamics Data Information Service, a total of 18 VLBI baselines are available which have been surveyed on four or more occasions. These data were fed into weighted and unweighted inversions to obtain baseline closure rates. Four of the better quality lines are illustrated. The deformation model assumes that the observed baseline rates result from a combination of rigid plate tectonic motions plus a component resulting from elastic strain build up due to a failure of the plate boundary to slip at the full plate tectonic rate. The elastic deformation resulting from the locked plate boundary is meant to portray interseismic strain accumulation. During and shortly after a large interplate earthquake, these strains are largely released, and points near the fault which were previously retarded suddenly catch up to the positions predicted by rigid plate models. Researchers judge the quality of fit by the sum squares of weighted residuals, termed total variance. The observed baseline closures have a total variance of 99 (cm/y)squared. When the RM2 velocities are assumed to model the data, the total variance increases to 154 (cm/y)squared.

Ward, S. N.↗

NASA NTRS1985

Earthquake deformation cycle in the Andean Back Arc, western Argentina

It is pointed out that the 1977 Caucete earthquake was an east-west compressive event which occurred at a depth of about 17 km in the Andean back arc approximately 450 km east of the Chile trench. The present study is primarily concerned with the mechanics of fault behavior prior to and after the 1977 earthquake, as evidenced by preseismic and postseismic vertical deformation. Simple fault models for each phase of deformation are presented. The models of preseismic slip, coseismic slip, and postseismic slip consist of uniform dislocations in an elastic half-space following the method of Chinnery (1961). The basically elastic models presented demonstrate that deformations in the vicinity of the 1977 Caucete, Argentina, earthquake are consistent with activity on some portion of the coseismic fault. This is evidence that these deformations are directly related to the processes of strain accumulation and release associated with the event.

Reilinger, R.↗

NASA NTRS1986

Extension of continental lithosphere - A model for two scales of basin and range deformation

The development of a model for deformation in an extending continental lithosphere that is stratified in density and strength is described. The lithosphere model demonstrates that the necking instabilities at two wavelengths originate due to a strong upper crust, a mantle layer, and a weak lower crust. It is observed that the dominant wavelengths of necking are controlled by layer thickness and the strength of the layers control the amplitude of the instabilities. The model is applied to the Basin and Range Province of the western U.S. where deformations in ranges and tile domains are detected. The relation between the Bouguer gravity anomaly and the deformations is studied. The data reveal that the horizontal scale of short wavelength necking correlates with the spacings of individual basins and ranges, and the longer wavelength corresponds to the width of tilt domains. The control of the Basin and Range deformation by two scales of extensional instability is proposed.

Zuber, M. T.↗

NASA NTRS1986

Strain accumulation and surface deformation along the San Andreas, California

Stressing and rupture of a locked zone adjacent to a creeping fault segment was studied with special reference to strength heterogeneity depthwise and along-strike. The resulting precursory temporal and spatial variations of surface strain rate profiles were compared to geodetic measurements on the San Andreas fault in central California. Crustal deformation in great California earthquake cycles was also studied with special reference to the temporal decay of strain rate observed since the 1957 and 1906 great earthquakes, and comtemporary surface strain rate and velocity profiles at several locations along the San Andreas. The effect of viscoelastic response in the deep aseismic shear zone on the surface deformation behavior was examined. Work was begun on a fundamental reformulation of the crustal deformation problem focusing on the crustal deformation process affected by deep aseismic slip as the slip zone progresses toward an instability and as deep seismic slip continues postseismically, the 3-D nature of the problem due to geometry and material heterogeneity, and the time-dependent source coming from the lithosphere/astenospheric coupling process.

Li, Victor C.↗