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At least 703 records · Page 39

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NASA NTRS1988

Stress-intensity factors for corner cracks in rectangular bars

Stress intensity factors are presented for a wide range of quarter-elliptical corner cracks in a rectangular bar. The bars were subjected to remote tension, out-of-plane bending, or in-plane bending loading, and the stress intensity factors were calculated from a three-dimensional finite element analysis using the nodal-force method. For all loadings considered, smaller width bars produce higher normalized stress intensity factors than the larger width bars. For tensile loading, quarter-circular corner cracks give higher stress intensity factors near the free surfaces and lower values in the interior. The results are compared with data in the literature for remote tensile loading.

Raju, I. S.↗

NASA NTRS1987

Stress corrosion evaluation of powder metallurgy aluminum alloy 7091 with the breaking load test method

The stress corrosion behavior of the P/M aluminum alloy 7091 is evaluated in two overaged heat treatment conditions, T7E69 and T7E70, using an accelerated test technique known as the breaking load test method. The breaking load data obtained in this study indicate that P/M 7091 alloy is highly resistant to stress corrosion in both longitudinal and transverse orientations at stress levels up to 90 percent of the material yield strength. The reduction in mean breaking stress as a result of corrosive attack is smallest for the more overaged T7E70 condition. Details of the test procedure are included.

Domack, Marcia S.↗

NASA NTRS1989

Weibull crack density coefficient for polydimensional stress states

A structural ceramic analysis and reliability evaluation code has recently been developed encompassing volume and surface flaw induced fracture, modeled by the two-parameter Weibull probability density function. A segment of the software involves computing the Weibull polydimensional stress state crack density coefficient from uniaxial stress experimental fracture data. The relationship of the polydimensional stress coefficient to the uniaxial stress coefficient is derived for a shear-insensitive material with a random surface flaw population.

Gross, Bernard↗

NASA NTRS1990

Micromechanics analysis of space simulated thermal deformations and stresses in continuous fiber reinforced composites

Space simulated thermally induced deformations and stresses in continuous fiber reinforced composites were investigated with a micromechanics analysis. The investigation focused on two primary areas. First, available explicit expressions for predicting the effective coefficients of thermal expansion (CTEs) for a composite were compared with each other, and with a finite element (FE) analysis, developed specifically for this study. Analytical comparisons were made for a wide range of fiber/matrix systems, and predicted values were compared with experimental data. The second area of investigation focused on the determination of thermally induced stress fields in the individual constituents. Stresses predicted from the FE analysis were compared to those predicted from a closed-form solution to the composite cylinder (CC) model, for two carbon fiber/epoxy composites. A global-local formulation, combining laminated plate theory and FE analysis, was used to determine the stresses in multidirectional laminates. Thermally induced damage initiation predictions were also made.

Bowles, David E.↗

NASA NTRS1989

Stressed Ge:Ga photoconductors for space-based astronomy. (Is there life beyond 120 micron)

Information is given in viewgraph form. Information is given on the characteristics of stressed Ge:Ga, a spring type stress cavity, mounting hardware, materials parameters affecting dark current, and the behavior of low dark current stressed Ge:Ga. It is concluded that detectors exist today for background-limited detection at 200 microns, that researchers are narrowing in on the significant parameters that effect dark current in stressed photoconductors, that these findings may be applied to other photoconductor materials, and that some creative problem solving for an ionizing effect reset mechanism is needed.

Beeman, J. W.↗

NASA NTRS1988

Relationship between fatigue life in the creep-fatigue region and stress-strain response

On the basis of mechanical tests and metallographic studies, strainrange partitioned lives were predicted by introducing stress-strain materials parameters into the Universal Slopes Equation. This was the result of correlating fatigue damage mechanisms and deformation mechanisms operating at elevated temperatures on the basis of observed mechanical and microstructural behavior. Correlation between high temperature fatigue and stress strain properties for nickel base superalloys and stainless steel substantiated the method. Parameters which must be evaluated for PP- and CC- life are the maximum stress achievable under entirely plastic and creep conditions respectively and corresponding inelastic strains, and the two more pairs of stress strain parameters must be ascertained.

Berkovits, A.↗

NASA NTRS1989

Analysis of thermal stresses in polymer matrix composites

In the present micromechanics study of the thermally-induced stress field which emerges at very low temperatures in the matrix and fibers of continuous fiber-reinforced polymer composites, the results obtained by a specialized FEM analysis are compared to an analytical solution of the composite cylinder model for several composite materials. Attention is given to the influence of microstructural geometry and constituent properties; it is established that the distributions and magnitudes of the thermally-induced stresses are affected by the assumed microstructural geometry of the sample. While matrix stresses are not a strong function of fiber properties, the temperature dependence of the matrix properties significantly affects the magnitudes of thermally-induced matrix stresses.

Bowles, David E.↗

NASA NTRS1991

Software Models Impact Stresses

Generalized Impact Stress Software designed to assist engineers in predicting stresses caused by variety of impacts. Program straightforward, simple to implement on personal computers, "user friendly", and handles variety of boundary conditions applied to struck body being analyzed. Applications include mathematical modeling of motions and transient stresses of spacecraft, analysis of slamming of piston, of fast valve shutoffs, and play of rotating bearing assembly. Provides fast and inexpensive analytical tool for analysis of stresses and reduces dependency on expensive impact tests. Written in FORTRAN 77. Requires use of commercial software package PLOT88.

Hanshaw, Timothy C.↗

NASA NTRS1991

Fully-Stressed-Design Algorithm For Plate/Shell Structures

Modification of basic stress-ratio algorithm enhances convergence. In conjunction with reliable stress-analysis algorithm, stress-ratio algorithm used to compute fully stressed state to obtain efficient distribution of materials in structure.

Miura, Hirokazu↗

NASA NTRS1991

The effect of stress on hydrogen uptake and desorption by A-286

The uptake and desorption of hydrogen by A-286 as a function of stress was studied using electrochemical methods. It was found that the apparent surface hydrogen concentration, the mean hydrogen concentration, and the hydrogen distribution uniformity all increased up to a stress level 50 percent of yield and decreased thereafter. The value of the hydrogen diffusion coefficient was relatively unaffected by stress while the percent of trapped hydrogen appeared to decrease with increasing stress.

Danford, Merlin D.↗

NASA NTRS1991

Thermal stress tectonics on the satellites of Saturn and Uranus

Thermal stress histories of the Saturnian and Uranian satellites are investigated. To this end, the thermal evolution of an icy satellite subjected to accretional and radiogenic heating, thermal conduction, and solid-state convection is modeled, and changes in the internal stress that occur during satellite evolution are examined. Results show that internal temperature changes that occur during normal evolution of many of the satellites of Saturn and Uranus can be expected to generate large extensional stresses in the satellites' outer regions. These stresses arise from three sources: (1) radiogenic warming, causing thermal expansion of materials in the satellite's deep interior; (2) radiogenic warming in larger satellites that can induce a phase transition from ice II to ice I and to produce a volume increase in the deep interior; and (3) accretional heating depositing heat in the satellite'e outer regions.

Hillier, John↗

NASA NTRS1991

Variation of crack-opening stresses in three-dimensions - Finite thickness plate

A 3D elastic-plastic finite-element analysis is conducted to study crack-growth behavior of thin and thick center-cracked specimens under constant-amplitude loading conditions. The numerical analysis and the specimen configuration and loading are described for both the thin and thick conditions. Stabilized crack-opening stresses of interior and exterior regions are given as are the closure and opening profiles of the crack-surface plane after the tenth cycle. The effect of thickness is discussed with respect to the crack-opening stress levels and the plastic zones of the interior and exterior regions. A load-reduced-displacement technique allows the calculation of the crack-opening stresses at three locations on the crack surface plane. The constraint effect related to thickness gives a lower stabilized crack-opening stress level for the thick specimens.

Chermahini, R. G.↗

NASA NTRS1992

Ultrasonic Dynamic Vector Stress Sensor

Stress inferred from measurements in specimens rather than in bonded gauges. Ultrasonic dynamic vector stress sensor (UDVSS) measures changes in dynamic directional stress occurring in material or structure at location touched by device when material or structure put under cyclic load. Includes phase-locked loop, synchronous amplifier, and contact probe. Useful among manufacturers of aerospace and automotive structures for stress testing and evaluation of designs.

Heyman, Joseph S.↗

NASA NTRS1992

Influence of engineered interfaces on residual stresses and mechanical response in metal matrix composites

Because of the inherent coefficient of thermal expansion (CTE) mismatch between fiber and matrix within metal and intermetallic matrix composite systems, high residual stresses can develop under various thermal loading conditions. These conditions include cooling from processing temperature to room temperature as well as subsequent thermal cycling. As a result of these stresses, within certain composite systems, radial, circumferential, and/or longitudinal cracks have been observed to form at the fiber matrix interface region. A number of potential solutions for reducing this thermally induced residual stress field have been proposed recently. Examples of some potential solutions are high CTE fibers, fiber preheating, thermal anneal treatments, and an engineered interface. Here the focus is on designing an interface (by using a compensating/compliant layer concept) to reduce or eliminate the thermal residual stress field and, therefore, the initiation and propagation of cracks developed during thermal loading. Furthermore, the impact of the engineered interface on the composite's mechanical response when subjected to isothermal mechanical load histories is examined.

Arnold, Steven M.↗

NASA NTRS1992

A compressible Navier-Stokes solver with two-equation and Reynolds stress turbulence closure models

This report outlines the development of a general purpose aerodynamic solver for compressible turbulent flows. Turbulent closure is achieved using either two equation or Reynolds stress transportation equations. The applicable equation set consists of Favre-averaged conservation equations for the mass, momentum and total energy, and transport equations for the turbulent stresses and turbulent dissipation rate. In order to develop a scheme with good shock capturing capabilities, good accuracy and general geometric capabilities, a multi-block cell centered finite volume approach is used. Viscous fluxes are discretized using a finite volume representation of a central difference operator and the source terms are treated as an integral over the control volume. The methodology is validated by testing the algorithm on both two and three dimensional flows. Both the two equation and Reynolds stress models are used on a two dimensional 10 degree compression ramp at Mach 3, and the two equation model is used on the three dimensional flow over a cone at angle of attack at Mach 3.5. With the development of this algorithm, it is now possible to compute complex, compressible high speed flow fields using both two equation and Reynolds stress turbulent closure models, with the capability of eventually evaluating their predictive performance.

Navier-Stoke solver↗

NASA NTRS1992

Stress concentrations for straight-shank and countersunk holes in plates subjected to tension, bending, and pin loading

A three dimensional stress concentration analysis was conducted on straight shank and countersunk (rivet) holes in a large plate subjected to various loading conditions. Three dimensional finite element analysis were performed with 20 node isoparametric elements. The plate material was assumed to be linear elastic and isotropic, with a Poisson ratio of 0.3. Stress concentration along the bore of the hole were computed for several ratios of hole radius to plate thickness (0.1 to 2.5) and ratios of countersink depth to plate thickness (0.25 to 1). The countersink angles were varied from 80 to 100 degrees in some typical cases, but the angle was held constant at 100 degrees for most cases. For straight shank holes, three types of loading were considered: remote tension, remote bending, and wedge loading in the hole. Results for remote tension and wedge loading were used to estimate stress concentration for simulated rivet in pin loading. For countersunk holes only remote tension and bending were considered. Based on the finite element results, stress concentration equations were developed. Whenever possible, the present results were compared with other numerical solutions and experimental results from the literature.

Shivakumar, K. N.↗

NASA NTRS1992

Determination of stress intensity factors for interface cracks under mixed-mode loading

A simple technique was developed using conventional finite element analysis to determine stress intensity factors, K1 and K2, for interface cracks under mixed-mode loading. This technique involves the calculation of crack tip stresses using non-singular finite elements. These stresses are then combined and used in a linear regression procedure to calculate K1 and K2. The technique was demonstrated by calculating three different bimaterial combinations. For the normal loading case, the K's were within 2.6 percent of an exact solution. The normalized K's under shear loading were shown to be related to the normalized K's under normal loading. Based on these relations, a simple equation was derived for calculating K1 and K2 for mixed-mode loading from knowledge of the K's under normal loading. The equation was verified by computing the K's for a mixed-mode case with equal and normal shear loading. The correlation between exact and finite element solutions is within 3.7 percent. This study provides a simple procedure to compute K2/K1 ratio which has been used to characterize the stress state at the crack tip for various combinations of materials and loadings. Tests conducted over a range of K2/K1 ratios could be used to fully characterize interface fracture toughness.

Naik, Rajiv A.↗

NASA NTRS1991

Polarized-cathodoluminescence study of uniaxial and biaxial stress in GaAs/Si

The strain-induced splitting of the heavy-hole (hh) and light-hole (lh) valence bands for 4-microns thick GaAs/Si is examined on a microscopic scale using linear polarized-cathodoluminescence imaging and spectroscopy. The energies and intensities of the hh- and lh-exciton luminescence are quantitatively analyzed to determine spatial variations in the stress tensor. The results indicate that regions near and far from the microcracks are primarily subject to uniaxial and biaxial tensile stresses, respectively. The transition region where biaxial stress gradually converts to uniaxial stress is analyzed, and reveals a mixing of lh and hh characters in the strain-split bands.

Rich, D. H.↗