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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Biopolymer Modified Cementitious Systems with Radically Superior Strength and Durability

This project was a pioneering study of the effect of nanochitin additives on setting time, rheological, and mechanical properties of cementitious materials. Chitin is the second most abundant biopolymer in nature after cellulose and is readily available from byproducts of the growing seafood industry. In this study, chitin powder from shrimp shells was processed into chitin nanocrystals (ChNCs) and chitin nanofibers (ChNFs). Ch-Cs delayed initial and final set times with their high ionic surface charges likely by electrostatic repulsion. The viscosity of fresh cement paste increased with Ch-NFs, while Ch-NCs did not significantly affect rheological properties. The distinct effects of the two nanochitin forms are attributed to the mobility of the different morphologies of the rod-like ChNCs vs. the entangled and restrained network of the ChNFs. Nanochitin forms showed encouraging results by significantly increasing flexural strength by ~40% at the optimum concentration of 0.05 wt% and significant increases in fracture energy. These bio-additives were found promising and future studies should continue to evaluate their use in concrete.

36 MATERIALS SCIENCE↗
NASA NTRS2004

Formulations for Stronger Solid Oxide Fuel-Cell Electrolytes

Tests have shown that modification of chemical compositions can increase the strengths and fracture toughnesses of solid oxide fuel-cell (SOFC) electrolytes. Heretofore, these solid electrolytes have been made of yttria-stabilized zirconia, which is highly conductive for oxygen ions at high temperatures, as needed for operation of fuel cells. Unfortunately yttria-stabilized zirconia has a high coefficient of thermal expansion, low resistance to thermal shock, low fracture toughness, and low mechanical strength. The lack of strength and toughness are especially problematic for fabrication of thin SOFC electrolyte membranes needed for contemplated aeronautical, automotive, and stationary power-generation applications. The modifications of chemical composition that lead to increased strength and fracture toughness consist in addition of alumina to the basic yttria-stabilized zirconia formulations. Techniques for processing of yttria-stabilized zirconia/alumina composites containing as much as 30 mole percent of alumina have been developed. The composite panels fabricated by these techniques have been found to be dense and free of cracks. The only material phases detected in these composites has been cubic zirconia and a alumina: this finding signifies that no undesired chemical reactions between the constituents occurred during processing at elevated temperatures. The flexural strengths and fracture toughnesses of the various zirconia-alumina composites were measured in air at room temperature as well as at a temperature of 1,000 C (a typical SOFC operating temperature). The measurements showed that both flexural strength and fracture toughness increased with increasing alumina content at both temperatures. In addition, the modulus of elasticity and the thermal conductivity were found to increase and the density to decrease with increasing alumina content. The oxygen-ion conductivity at 1,000 C was found to be unchanged by the addition of alumina.

Bansal, Narottam P.↗
NASA NTRS1992

What can we learn about impact mechanics from large craters on Venus?

More than 50 unequivocal peak-ring craters and multiringed impact basins have been identified on Venus from Earth-based Arecibo, Venera 15/16, and Magellan radar images. These ringed craters are relatively pristine, and so serve as an important new dataset that will further understanding of the structural and rheological properties of the venusian surface and of impact mechanics in general. They are also the most direct analogues for craters formed on the Earth in Phanerozoic time. Finite-element simulations of basin collapse and ring formation were undertaken in collaboration with V. J. Hillgren (University of Arizona). These calculations used an axisymmetric version of the viscoelastic finite element code TECTON, modeled structures on the scale of Klenova or Meitner, and demonstrated two major points. First, viscous flow and ring formation are possible on the timescale of crater collapse for the sizes of multiringed basins seen on Venus and heat flows appropriate to the plant. Second, an elastic lithosphere overlying a Newtonian viscous asthenosphere results mainly in uplift beneath the crater. Inward asthenospheric flow mainly occurs at deeper levels. Lithospheric response is dominantly vertical and flexural. Tensional stress maxima occur and ring formation by normal faulting is predicted in some cases, but these predicted rings occur too far out to explain observed ring spacings on Venus (or on the Moon). Overall, these estimates and models suggest that multiringed basin formation is indeed possible at the scales observed on Venus. Furthermore, due to the strong inverse dependence of solid-state viscosity on stress, the absence of Cordilleran-style ring faulting in craters smaller than Meitner or Klenova makes sense. The apparent increase in viscosity of shock-fluidized rock with crater diameter, greater interior temperatures accessed by larger, deeper craters, and decreased non-Newtonian viscosity associated with larger craters may conspire to make the transition with diameter from peak-ring crater to Orientale-type multiringed basin rather abrupt.

Mckinnon, William B.↗
DOE OSTI2022

Fibrous anisotropy and mineral gradients within the radula stylus of chiton: Controlled stiffness and damage tolerance in a flexible biological composite

Over hundreds of millions of years, organisms have evolved architected structures via precise control over hierarchically assembled components, including the integration of dissimilar materials. One such example is found in the radula system of chitons, intertidal mollusks that feed on algae growing on the rock. Their radula consists of multiple rows of ultrahard teeth, each integrated with a foldable belt-like substrate via a stiff, yet flexible stylus, which is essential for efficient rasping during the feeding process. Here, we investigate the nano and micro-scale components and architectures as well as regional mechanical properties of the stylus, and their subsequent role during the rasping of Cryptochiton stelleri. Three important factors were determined to contribute to the regio-specific stiffness of the stylus: the presence of mineral components, highly oriented chitinous fibers, and a chemically cross-linked protein matrix. All these factors are varied throughout the stylus. There is a high mineral content on the trailing edge close to the tooth and a cross-linked matrix on the leading edge, both with orientational specific oriented chitin fibers that provide force transduction to the tooth. Conversely, there is a significant lack of mineral or cross-linked matrix in the proximal end as well as a low degree of fiber orientation, resulting in a flexible region that can accommodate torsion and flexure during rasping. Understanding the graded composite structure of the stylus and applying this unique design to various engineering fields such as soft robotics, biotechnology, and the medical industry, can inspire the production of high-performance materials.

Lee, Jung-Eun↗
NASA NTRS2003

An Innovative Manufacturing of CCC Ion Thruster Grids by North Carolina A&T's RTM Carbon/Carbon Process

Electric ion thrusters are the preferred engines for deep space missions, because of very high specific impulse. The ion engine consists of screen and accelerator grids containing thousands of concentric very small holes. The xenon gas accelerates between the two grids, thus developing the impulse force. The dominant life-limiting mechanism in the state-of-the-art molybdenum thrusters is the xenon ion sputter erosion of the accelerator grid. Carbon/carbon composites (CCC) have shown to be have less than 1/7 the erosion rates than the molybdenum, thus for interplanetary missions CCC engines are inevitable. Early effort to develop CCC composite thrusters had a limited success because of limitations of the drilling technology and the damage caused by drilling. The proposed is an in-situ manufacturing of holes while the CCC is made. Special low CTE molds will be used along with the NC A&T s patented resin transfer molding (RTM) technology to manufacture the CCC grids. First, a manufacture process for 10-cm diameter thruster grids will be developed and verified. Quality of holes, density, CTE, tension, flexure, transverse fatigue and sputter yield properties will be measured. After establishing the acceptable quality and properties, the process will be scaled to manufacture 30-cm diameter grids. The properties of the two grid sizes are compared with each other.

Haag, Thomas W.↗
NASA NTRS2002

Strength and Fracture Toughness of Solid Oxide Fuel Cell Electrolyte Material Improved

Solid oxide fuel cells (SOFC) are being developed for various applications in the automobile, power-generation, and aeronautics industries. Recently, the NASA Glenn Research Center has been exploring the possibility of using SOFC's for aeropropulsion under its Zero Carbon Dioxide Emission Technology (ZCET) Program. 10-mol% yttriastabilized zirconia (10YSZ) is a very good anionic conductor at high temperatures and is, therefore, used as an oxygen solid electrolyte in SOFC. However, it has a high thermal expansion coefficient, low thermal shock resistance, low fracture toughness, and poor mechanical strength. For aeronautic applications, the thin ceramic electrolyte membrane of the SOFC needs to be strong and tough. Therefore, we have been investigating the possibility of enhancing the strength and fracture toughness of the 10YSZ electrolyte without degrading its electrical conductivity to an appreciable extent. We recently demonstrated that the addition of alumina to zirconia electrolyte increases its strength as well as its fracture toughness. Zirconia-alumina composites containing 0 to 30 mol% of alumina were fabricated by hot pressing. The hot pressing procedure was developed and various hot pressing parameters were optimized, resulting in dense, crackfree panels of composite materials. Cubic zirconia and a-alumina were the only phases detected, indicating that there was no chemical reaction between the constituents during hot pressing at elevated temperatures. Flexure strength sf and fracture toughness K(sub IC) of the various zirconia-alumina composites were measured at room temperature as well as at 1000 C in air. Both properties showed systematic improvement with increased alumina addition at room temperature and at 1000 C. Use of these modified electrolytes with improved strength and fracture toughness should prolong the life and enhance the performance of SOFC in aeronautics and other applications.

Bansal, Narottam P.↗
NASA NTRSDate not supplied

Development of a Composite Material Aerodynamic Demise Model for the Object Reentry Survival Analysis Tool (ORSAT)

The reentry demise of fiber-reinforced polymer (FRP) composites is an increasing concern for modern spacecraft at end of life. Unlike traditional materials such as metals, shredding of the material by aerodynamic forces appears to be a major component of the reentry demise mechanism. This paper will describe a new mechanical, strength-based, material demise model for the Object Reentry Survival Analysis Tool (ORSAT) Version 7.1. The model is based on laboratory in-situ and residual strength tests of several FRP materials performed during the NASA Orbital Debris Program Office’s Phase II Composite Material Demise test campaign. To test the residual strength of partially charred FRP materials, the authors tested 107 rectangular shaped samples of different thicknesses that were previously exposed to high-enthalpy flow at the University of Texas at Austin’s Inductively Coupled Plasma Torch facility. Tests were performed under normal atmospheric conditions at NASA Johnson Space Center’s Experimental Impact Laboratory using a Chatillon TCD1000 tensile test machine configured with a three-point bending jig. Mean fracture load, residual strength, delamination, and flexural modulus were measured for each test. An engineering model of aeromechanical demise for each material was developed by correlating the measured residual strength of the samples with the duration and magnitude of the applied heat flux and the mass loss and char progression. This model has been implemented in ORSAT 7.1 for the built-in, charring carbon fiber/epoxy and charring glass fiber/epoxy material models. The new model was verified by calculating the demise of hundreds of FRP composite fragments of various shapes and sizes and checking the calculations for consistency, with specific focus on material characterization in one-dimension. Further tests are planned in hypersonic flow facilities to validate the assumptions used in the model.

Priscilla A. Mendoza↗
NASA NTRSDate not supplied

Development of a Composite Material Aerodynamic Demise Model for the Object Reentry Survival Analysis Tool (ORSAT)

The reentry demise of fiber-reinforced polymer (FRP) composites is an increasing concern for modern spacecraft at end of life. Unlike traditional materials such as metals, shredding of the material by aerodynamic forces appears to be a major component of the reentry demise mechanism. This paper will describe a new mechanical, strength-based, material demise model for the Object Reentry Survival Analysis Tool (ORSAT) Version 7.1. The model is based on laboratory in-situ and residual strength tests of several FRP materials performed during the NASA Orbital Debris Program Office’s Phase II Composite Material Demise test campaign. To test the residual strength of partially charred FRP materials, the authors tested 107 rectangular shaped samples of different thicknesses that were previously exposed to high-enthalpy flow at the University of Texas at Austin’s Inductively Coupled Plasma Torch facility. Tests were performed under normal atmospheric conditions at NASA Johnson Space Center’s Experimental Impact Laboratory using a Chatillon TCD1000 tensile test machine configured with a three-point bending jig. Mean fracture load, residual strength, delamination, and flexural modulus were measured for each test. An engineering model of aeromechanical demise for each material was developed by correlating the measured residual strength of the samples with the duration and magnitude of the applied heat flux and the mass loss and char progression. This model has been implemented in ORSAT 7.1 for the built-in, charring carbon fiber/epoxy and charring glass fiber/epoxy material models. The new model was verified by calculating the demise of hundreds of FRP composite fragments of various shapes and sizes and checking the calculations for consistency, with specific focus on material characterization in one-dimension. Further tests are planned in hypersonic flow facilities to validate the assumptions used in the model.

Priscilla A. Mendoza↗
DOE OSTI2021

Modifying the pore size distribution in Fe-rich inorganic polymer mortars: An effective shrinkage mitigation strategy

Highlights: • Inorganic polymer mortar has mainly macropores and a drying shrinkage of 5.1 mm/m. • 2-methyl-2,4-pentanediol acts as air entrainer in inorganic polymer mortars. • Optimal dosage 2 wt% 2-methyl-2,4-pentanediol reduced drying shrinkage to 1.8 mm/m. • Heat curing inorganic polymer mortars reduced drying shrinkage more than 50%. • Blast Furnace slag addition increased drying shrinkage and reduced strength. Inorganic polymer (IP) binder is formed upon alkali-activation of Fe-rich (41 wt%) metallurgical slag leading to materials with mechanical properties comparable to ordinary Portland cement binders. Crack formation is reported in IPs, which can be related to volumetric stability, which is to date not thoroughly understood in IPs. This study determined the autogenous and drying shrinkage of IP mortars. Shrinkage mitigation strategies, such as the addition of 2-methyl-2,4-pentanediol (2MPL), blast furnace slag (GGBFS), and applying heat curing, were used separately or in combination. IP mortars exhibited autogenous expansion and high drying shrinkage. The addition of 2MPL led to a porosity increase, as air was entrained, decreasing the drying shrinkage with 64%. Additional heat curing had no effect on samples with 2MPL. Introducing GGBFS resulted in smaller pores, increasing drying shrinkage. Shrinkage in IP mortars is driven by its pore size distribution and higher shrinkage resulted in lower flexural strength.

36 MATERIALS SCIENCE↗
DOE OSTI2024

UHT-CAMANCHE: Ultra-High Temperature Ceramic Additively Manufactured Compact Heat Exchangers

The conceptual basis for this project is the convergence of advanced ultra-high temperature ceramic materials and additive manufacturing technologies to produce compact ceramic heat exchangers with complex internal flow path geometries. Task areas were broadly divided into materials and manufacturing development, heat exchanger design, component testing, and techno-economic analysis. Technical challenges included the design and commissioning of new test facilities, improving feature resolution and deposition rate of ceramic additive manufacturing techniques, establishing process-structure-property relationships in additively manufactured ultra-high temperature ceramics, and assessing high temperature materials compatibility in CO 2 environments. The primary candidate material evaluated in this work is a composite comprising zirconium diboride (ZrB2) with 30 vol. % silicon carbide (SiC) which was selected based on its desirable combination of high temperature mechanical properties, high thermal and electrical conductivities, and oxidation resistance. High solids loaded ZrB2-SiC pastes suitable for extrusion-based additive manufacturing were developed for the first time as part of this work. Materials compatibility studies indicate this material oxidizes in CO 2 to form a protective borosilicate scale which transforms to pure silica above 1000°C. Parts made by additive manufacturing displayed enlarged grain sizes produced by pressureless sintering as compared to hot-press sintering. Increases in microstructural coarseness have outsized effect on oxidation performance up to 1400°C due to incomplete oxidation of coarse large diameter SiC particles resulting in lower amounts of silica that apparently inhibit protective scale formation. Additive manufacturing as a forming technique did not appear to significantly affect thermal conductivity, hardness, or elastic modulus, though flexural strength was reduced by half or more as compared to traditionally hot-pressed materials. This effect was attributed to the presence of strength-limiting flaws (ca. 40 microns in size) originating from extrudate inhomogeneities that could potentially be eliminated with further process improvements. Attempts to attain economies of scale for production of multi-kilowatt scale heat exchangers by ceramic additive manufacturing proved difficult. Lack of automation and a modest extrusion rate while retaining fine feature resolution made the overall process labor intensive and limited experimental throughput. A number of full-scale components were taken through post-process heat treatments including drying, binder burnout, and sintering; however, none survived without significant flaws or cracks. Therefore, no operational data from a newly installed heat exchanger test loop were able to be obtained during the performance period. Continued research and development is recommended to improve economic feasibility of ceramic additive manufacturing by standardizing the use of advanced sensors, artificial intelligence, and automation tools to reduce associated labor costs and accelerate production rates. The materials and manufacturing techniques demonstrated in this work are likely to find applications in defense and energy applications.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
NASA NTRS1985

The 371 deg C mechanical properties of graphite/polyimide composites

A series of condensation polyimides based on pyromellitic dianhydride is synthesized and evaluated for potential application at 371 C. Several three-and four-ring benzenoid diamine systems containing oxygen bridging groups are investigated. Thermomechanical analysis of neat resin specimens indicate that the polyimide prepared from the dimethyl ester of pyrometallitic acid (PMDE) and 2,2-bis4-(4'-aminophenoxy) phenyl]-1,1,1,3,3,3- hexafluoropropane (BDAF) is the only resin system which has a glass transition temperature (Tg) above 371 C. The Tg of the PMDE/BDAF polyimide is found to be 390 C after a postcure air at 371 C for 24 hr. Unidirectional composites are fabricated from the PMDE/BDAF system and unsized Celion 6000 graphite fibers. Final cure temperatures in the range of 371 to 427 C with an applied pressure of 10.34 to 13.78 MPa are investigated. The void content of the composites ranges from 4.6 to 8.6 percent. Composites cured at 399 C under a pressure of 10.34 MPa and postcured in air at 371 C for 24 hr exhibit the highest 371 C interlaminar shear strength (ILSS, 40.7 MPa) and flexural strength (758 MPa). The thermo-oxidative stability of the composites is determined by subjecting specimens to isothermal exposure at 371 C in air at atmospheric pressure, as well as a pressure of 0.52 MPa. Specimens exposed at atmospheric pressure exhibit a weight loss of 12 percent after 200 hr of exposure and 88 percent retention of its original 371 C ILSS. In contrast, the specimens exposed at 0.52 MPa pressure exhibit a comparable weight loss after only 72 hr, and a 71 percent retention of its original 371 C ILSS.

Delvigs, P.↗
DOE OSTI2022

Enhanced polarization and abnormal flexural deformation in bent freestanding perovskite oxides

Abstract Recent realizations of ultrathin freestanding perovskite oxides offer a unique platform to probe novel properties in two-dimensional oxides. Here, we observe a giant flexoelectric response in freestanding BiFeO 3 and SrTiO 3 in their bent state arising from strain gradients up to 3.5 × 10 7 m −1 , suggesting a promising approach for realizing ultra-large polarizations. Additionally, a substantial change in membrane thickness is discovered in bent freestanding BiFeO 3 , which implies an unusual bending-expansion/shrinkage effect in the ferroelectric membrane that has never been seen before in crystalline materials. Our theoretical model reveals that this unprecedented flexural deformation within the membrane is attributable to a flexoelectricity–piezoelectricity interplay. The finding unveils intriguing nanoscale electromechanical properties and provides guidance for their practical applications in flexible nanoelectromechanical systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1987

Flexurally-resisted uplift of the Tharsis Province, Mars

The tectonic style of Mars is dominated by vertical motion, perhaps more than any of the terrestrial planets. The imprint of this tectonic activity has left a surface widely faulted even though younger volcanism has masked the expression of tectonism in many places. Geological activity associated with the Tharsis and, to a lesser extent, Elysium provinces is responsible for a significant portion of this faulting, while the origins of the remaining features are enigmatic in many cases. The origin and evolution of the Tharsis and Elysium provinces, in terms of their great elevation, volcanic activity, and tectonic style, has sparked intense debate over the last fifteen years. Central to these discussions are the relative roles of structural uplift and volcanic construction in the creation of immense topographic relief. For example, it is argued that the presence of very old and cratered terrain high on the Tharsis rise, in the vicinity of Claritas Fossae, points to structural uplift of an ancient crust. Others have pointed out, however, that there is no reason that this terrain could not be of volcanic origin and thus part of the constructional mechanism.

Phillips, R. J.↗
NASA NTRS2018

New Runtime Record Set for Free-Piston Stirling Machine

This article describes the recent progress GRC has made in the arena of long-life heat engines for power conversion. NASA has been interested in Stirling-cycle machines for power conversion since the 1970s. At that time, it was recognized that such a device could be designed to achieve long-life continuous operation with high reliability, by leveraging the free-piston configuration in favor of a kinematic engine. The free-piston variant eliminates the wear mechanisms associated with rolling-element bearings, piston rings, and connecting rods, typically found in kinematic Stirling engine designs. By restricting the moving components to linear motion only, there is no need to convert linear piston movement to rotating motion. With this, the moving components can be suspended within their cylinders by non-contacting bearings and seals can be effected with close clearances, also non-contacting. The use of a linear alternator attached directly to the piston enables direct conversion of heat, to mechanical, then to electrical energy, all in one hermetically sealed vessel, The electrical output only requires simple feedthroughs, and the complexity of sealed a rotating crankshaft is eliminated. In the free-piston variant, the displacer-to-piston phase is not implemented by a connection to a common rotating shaft. Instead, the pressure wave itself is used to drive the displacer. The displacer is designed to have a higher natural frequency of motion than the piston, which achieves a leading phase angle required for power production. The free-piston Stirling behaves like a spring-mass-damper system, with a forcing function created by the alternate heater and cooling of the working gas, and damping provided by the current flowing through the linear alternator. Two types of non-contacting bearings are available to designers: flexure and gas bearings. Flexure bearings consist of a spiral-cut disc of metal that has high radial stiffness but a deliberately engineering stiffness in the axial direction, to permit piston and displacer resonant motion. Gas bearings utilize a pressure source and precisely machined flow channels to provide a radial stiffness around the moving component. The pressure source can be supplied by the engine's pressure wave itself.

Oriti, Salvatore M.↗
NASA NTRS1991

A major advance in powder metallurgy

Ultramet has developed a process which promises to significantly increase the mechanical properties of powder metallurgy (PM) parts. Current PM technology uses mixed powders of various constituents prior to compaction. The homogeneity and flaw distribution in PM parts depends on the uniformity of mixing and the maintenance of uniformity during compaction. Conventional PM fabrication processes typically result in non-uniform distribution of the matrix, flaw generation due to particle-particle contact when one of the constituents is a brittle material, and grain growth caused by high temperature, long duration compaction processes. Additionally, a significant amount of matrix material is usually necessary to fill voids and create 100 percent dense parts. In Ultramet's process, each individual particle is coated with the matrix material, and compaction is performed by solid state processing. In this program, Ultramet coated 12-micron tungsten particles with approximately 5 wt percent nickel/iron. After compaction, flexure strengths were measured 50 percent higher than those achieved in conventional liquid phase sintered parts (10 wt percent Ni/Fe). Further results and other material combinations are discussed.

Williams, Brian E.↗
NASA NTRS1999

Fabrication Of Carbon-Boron Reinforced Dry Polymer Matrix Composite Tape

Future generation aerospace vehicles will require specialized hybrid material forms for component structure fabrication. For this reason, high temperature composite prepregs in both dry and wet forms are being developed at NASA Langley Research Center (LaRC). In an attempt to improve compressive properties of carbon fiber reinforced composites, a hybrid carbon-boron tape was developed and used to fabricate composite laminates which were subsequently cut into flexural and compression specimens and tested. The hybrid material, given the designation HYCARB, was fabricated by modifying a previously developed process for the manufacture of dry polymer matrix composite (PMC) tape at LaRC. In this work, boron fibers were processed with IM7/LaRC(TradeMark)IAX poly(amide acid) solution-coated prepreg to form a dry hybrid tape for Automated Tow Placement (ATP). Boron fibers were encapsulated between two (2) layers of reduced volatile, low fiber areal weight poly(amide acid) solution-coated prepreg. The hybrid prepreg was then fully imidized and consolidated into a dry tape suitable for ATP. The fabrication of a hybrid boron material form for tow placement aids in the reduction of the overall manufacturing cost of boron reinforced composites, while realizing the improved compression strengths. Composite specimens were press-molded from the hybrid material and exhibited excellent mechanical properties.

Belvin, Harry L.↗
NASA NTRS1996

Stress and Reliability Analysis of a Metal-Ceramic Dental Crown

Interaction of mechanical and thermal stresses with the flaws and microcracks within the ceramic region of metal-ceramic dental crowns can result in catastrophic or delayed failure of these restorations. The objective of this study was to determine the combined influence of induced functional stresses and pre-existing flaws and microcracks on the time-dependent probability of failure of a metal-ceramic molar crown. A three-dimensional finite element model of a porcelain fused-to-metal (PFM) molar crown was developed using the ANSYS finite element program. The crown consisted of a body porcelain, opaque porcelain, and a metal substrate. The model had a 300 Newton load applied perpendicular to one cusp, a load of 30ON applied at 30 degrees from the perpendicular load case, directed toward the center, and a 600 Newton vertical load. Ceramic specimens were subjected to a biaxial flexure test and the load-to-failure of each specimen was measured. The results of the finite element stress analysis and the flexure tests were incorporated in the NASA developed CARES/LIFE program to determine the Weibull and fatigue parameters and time-dependent fracture reliability of the PFM crown. CARES/LIFE calculates the time-dependent reliability of monolithic ceramic components subjected to thermomechanical and/Or proof test loading. This program is an extension of the CARES (Ceramics Analysis and Reliability Evaluation of Structures) computer program.

Anusavice, Kenneth J↗
NASA NTRS1985

Possible role of crustal flexure in the initial detachment of extensional allochthons

The existence of low-angle normal faults indicates that the ratio of shear stress (TAU) to normal stress (sigma sub N) needed to cause slip on faults is substantially less than would be predicted based on experimental data. Because the tensional strength of rock at a large scale is exceedingly low, the upper plate of a low-angle normal fault cannot be pulled down the fault ramp, but must be driven down it by its own weight. The active or recently active Sevier Desert detachment fault in western Utah dips regionally at 12 deg. The radio of shear stress to normal stress due to the weight of the upper plate on a 12 deg dipping fault surface is 0.2. In contrast, laboratory experiments indicate that slip on fracture surfaces occurs with almost all rock types when (tau/sigma) reaches values of 0.6 to 0.85, corresponding to normal-fault dips of 30 deg to 40 deg. Seismological data indicate that low deviatoric stresses are associated with movement on faults of other geometrics and are not unique to low-angle normal faults. It thus appears that approximately planar fault zones with surface areas of hundreds to thousands of square kilometers have different mechanical properties than would be predicted based on laboratory studies of fractured rock. Modeling of stresses indicates that deviatoric stresses greater than 5kb exist and are sustainable in oceanic lithosphere, and that failure occurs when tau/sigma sub N approaches 0.6 to 0.85.

Spencer, J. E.↗