Probabilistic constitutive relationships for cyclic material strength models
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Boyce, L.↗
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Simulation studies for surfaces and materials strength
The progress report for the period May 1, 1992 to October 31, 1992 is presented. During this period investigations were carried out in three areas. Binding energies and high energy binding sites were calculated for carbon atoms deposited on a (2 x 1) dimerized Si(100) surface. Structure- and energy-related properties of defects formed on relaxed diamond surfaces were investigated. Tersoff-type model functions for Si were employed to investigate the energetics of the following processes: (1) neutral monovacancy and divacancy formation and migration energies; (2) neutral bond-centered, site sharing, tetrahedral, and hexagonal self-interstitial formation and migration energies; and (3) the variation of these energies with distance from a bulk site of Frenkel defect formation and from (100) and (111) surfaces. Calculations are included in the appendices.
Halicioglu, Timur↗
Simulation studies for surfaces and materials strength
During this period investigations were carried out in three areas. Optimum energies and structures were estimated for single step ledges formed in the (111) surface of diamond. Binding energies of carbon atoms adsorbed on a (2x1) reconstructed Si(100) surface were calculated as a function of sub-monolayer coverages. Calculations were conducted to analyze the strain dependence of the binding energy of a carbon atom deposited on a (2x1) dimerized Si(100) surface. Results are discussed.
Halicioglu, Timur↗
Simulation studies for surfaces and materials strength
Optimum energies and structures were estimated for single step ledges formed on the diamond (111) surface. Binding energies of carbon atoms adsorbed on a (2x1) reconstructed Si(100) surface were calculated as a function of sub-monolayer coverages. Calculations were conducted to analyze the strain dependence of the binding energy of a carbon adatom deposited on a (2x1) dimerized Si(100) surface. Details and results of these studies are given as appendices.
Halicioglu, Timur↗
Particle Hydrodynamics with Material Strength for Multi-Layer Orbital Debris Shield Design
Three dimensional simulation of oblique hypervelocity impact on orbital debris shielding places extreme demands on computer resources. Research to date has shown that particle models provide the most accurate and efficient means for computer simulation of shield design problems. In order to employ a particle based modeling approach to the wall plate impact portion of the shield design problem, it is essential that particle codes be augmented to represent strength effects. This report describes augmentation of a Lagrangian particle hydrodynamics code developed by the principal investigator, to include strength effects, allowing for the entire shield impact problem to be represented using a single computer code.
Fahrenthold, Eric P.↗
Developing a Material Strength Design Value Based on Compression after Impact Damage for the Ares I Composite Interstage
The derivation of design values for compression after impact strength for two types of honeycomb sandwich structures are presented. The sandwich structures in this study had an aluminum core and composite laminate facesheets of either 16-ply quasi or 18-ply directional lay-ups. The results show that a simple power law curve fit to the data can be used to create A- and B-basis residual strength curves.
Nettles, A. T.↗
A New Database of Giant Impacts over a Wide Range of Masses and with Material Strength: A First Analysis of Outcomes
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Alexandre Emsenhuber↗
Testing methods and techniques: Strength of materials and components. A compilation
The methods, techniques, and devices used in testing the mechanical properties of various materials are presented. Although metals and metal alloys are featured prominently, some of the items describe tests on a variety of other materials, from concrete to plastics. Many of the tests described are modifications of standard testing procedures, intended either to adapt them to different materials and conditions, or to make them more rapid and accurate. In either case, the approaches presented can result in considerable cost savings and improved quality control. The compilation is presented in two sections. The first deals specifically with material strength testing; the second treats the special category of fracture and fatigue testing.
Source record↗
Strength of laminated composite materials.
Strength of structural composites of orthotropic materials with arbitrary thickness and orientation including lamination and anisotropy effects
STRUCTURAL RIGIDITY↗
Achievable space elevators for space transportation and starship acceleration
Space elevator concepts for low-cost space launches are reviewed. Previous concepts suffered from requirements for ultra-high-strength materials, dynamically unstable systems, or from danger of collision with space debris. The use of magnetic grain streams solves these problems. Magnetic grain streams can support short space elevators for lifting payloads cheaply into Earth orbit, overcoming the material strength problem in building space elevators. Alternatively, the stream could support an international spaceport circling the Earth daily tens of miles above the equator, accessible to advanced aircraft. Mars could be equipped with a similar grain stream, using material from its moons Phobos and Deimos. Grain-stream arcs about the sun could be used for fast launches to the outer planets and for accelerating starships to near lightspeed for interstellar reconnaisance. Grain streams are essentially impervious to collisions, and could reduce the cost of space transportation by an order of magnitude.
Pearson, Jerome↗
Probabilistic analysis for fatigue strength degradation of materials
This report presents the results of the first year of a research program conducted for NASA-LeRC by the University of Texas at San Antonio. The research included development of methodology that provides a probabilistic treatment of lifetime prediction of structural components of aerospace propulsion systems subjected to fatigue. Material strength degradation models, based on primitive variables, include both a fatigue strength reduction model and a fatigue crack growth model. Linear elastic fracture mechanics is utilized in the latter model. Probabilistic analysis is based on simulation, and both maximum entropy and maximum penalized likelihood methods are used for the generation of probability density functions. The resulting constitutive relationships are included in several computer programs, RANDOM2, RANDOM3, and RANDOM4. These programs determine the random lifetime of an engine component, in mechanical load cycles, to reach a critical fatigue strength or crack size. The material considered was a cast nickel base superalloy, one typical of those used in the Space Shuttle Main Engine.
Royce, Lola↗
Toward Ultralight High Strength Structural Materials via Collapsed Carbon Nanotube Bonding
The growing commercial availability of carbon nanotube (CNT) macro-assemblies such as sheet and yarn is making their use in structural composite components increasingly feasible. However, the mechanical properties of these materials continue to trail those of state-of-the-art carbon fiber composites due to relatively weak inter-tube load transfer. Forming covalent links between adjacent CNTs promises to mitigate this problem, but it has proven difficult in practice to introduce them chemically within densified and aligned CNT materials due to their low permeability. To avoid this limitation, this work explores the combination of pulsed electrical current, temperature, and pressure to introduce inter-CNT bonds. Reactive molecular dynamics simulations identify the most probable locations, configurations, and conditions for inter-nanotube bonds to form. This process is shown to introduce covalent linkages within the CNT material that manifest as improved macroscale mechanical properties. The magnitude of this effect increases with increasing levels of prealignment of the CNT material, promising a new synthesis pathway to ultralight structural materials with specific strengths and stiffnesses exceeding 1 and 100 GPa/(g/cu.cm), respectively.
Jensen, Benjamin D.↗
Determination of Parachute Joint Factors using Seam and Joint Testing
This paper details the methodology for determining the joint factor for all parachute components. This method has been successfully implemented on the Capsule Parachute Assembly System (CPAS) for the NASA Orion crew module for use in determining the margin of safety for each component under peak loads. Also discussed are concepts behind the joint factor and what drives the loss of material strength at joints. The joint factor is defined as a "loss in joint strength...relative to the basic material strength" that occurs when "textiles are connected to each other or to metals." During the CPAS engineering development phase, a conservative joint factor of 0.80 was assumed for each parachute component. In order to refine this factor and eliminate excess conservatism, a seam and joint testing program was implemented as part of the structural validation. This method split each of the parachute structural joints into discrete tensile tests designed to duplicate the loading of each joint. Breaking strength data collected from destructive pull testing was then used to calculate the joint factor in the form of an efficiency. Joint efficiency is the percentage of the base material strength that remains after degradation due to sewing or interaction with other components; it is used interchangeably with joint factor in this paper. Parachute materials vary in type-mainly cord, tape, webbing, and cloth -which require different test fixtures and joint sample construction methods. This paper defines guidelines for designing and testing samples based on materials and test goals. Using the test methodology and analysis approach detailed in this paper, the minimum joint factor for each parachute component can be formulated. The joint factors can then be used to calculate the design factor and margin of safety for that component, a critical part of the design verification process.
Mollmann, Catherine↗
Analysis of hypervelocity perforation of a visco-plastic solid including the effects of target-material yield strength
Hypervelocity perforation of viscoplastic solid - effects of target material yield strength
HYPERVELOCITY IMPACT↗
Plane strain crack toughness testing of high strength metallic materials.
Book on plain strain crack toughness testing of high strength metallic materials
Brown, W. F., Jr.↗
Solar-array-materials passive LDEF experiment (A0171)
The objective of this experiment is to evaluate the synergistic effects of the space environment on various solar-array materials, including solar cells, cover slips with various antireflectance coatings, adhesive, encapsulants, reflector materials, substrate strength materials, mast and harness materials, structural composites, and thermal control treatments. The experiment is passive and consists of an arrangement of material specimens mounted in a 3-in.-deep peripheral tray. The effects of the space environment on the specimens will be determined by comparison of preflight and postflight measurements of mechanical, electrical, and optical properties.
Whitaker, A. F.↗
Mechanical Tests For Monitoring Aging Of Composites
Experimental study addresses related topics of thermo-oxidative aging of PMR-15 (polyimide matrix/graphite-fiber composite materials) strengths of interfacial bonds, effects of various surface treatments, and relative merits of mechanical tests yielding measures of strengths of materials.
Bowles, Kenneth J.↗
Some Factors Affecting Fatigue of Aircraft Structures
Several points of interest to the operator of aircraft are reviewed. Rates of crack propagation are shown to be high, and residual static strength is shown to decrease more significantly in high strength materials than in lower strength materials. Ground-air-ground and other negative loadings produce much more dn.mage than previously recognized or predicted. In first approximation, fatigue life is used in units corresponding to the number of flights, rather than to miles or hours as is commonly thought. Regular inspection for cracks is the only real way to assure safe operation of flight structures.
Hardrath, Herbert F.↗