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At least 1,549 records · Page 86
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Application of NASTRAN for stress analysis of left ventricle of the heart

Knowing the stress and strain distributions in the left ventricular wall of the heart is a prerequisite for the determination of the muscle elasticity and contractility in the process of assessing the functional status of the heart. NASTRAN was applied for the calculation of these stresses and strains and to help in verifying the results obtained by the computer program FEAMPS which was specifically designed for the plane-strain finite-element analysis of the left ventricular cross sections. Adopted for the analysis are the true shape and dimensions of the cross sections reconstructed from multiplanar X-ray views of a left ventricle which was surgically isolated from a dog's heart but metabolically supported to sustain its beating. A preprocessor was prepared to accommodate both FEAMPS and NASTRAN, and it has also facilitated the application of both the triangular element and isoparameteric quadrilateral element versions of NASTRAN. The stresses in several crucial regions of the left ventricular wall calculated by these two independently developed computer programs are found to be in good agreement. Such confirmation of the results is essential in the development of a method which assesses the heart performance.

Pao, Y. C.↗
NASA NTRS1974

Effects of shot-peening residual stresses on the fracture and crack-growth properties of D6AC steel

The fracture strength and cyclic crack-growth properties of surface-flawed, shot-peened D6AC steel plate were investigated. For short crack lengths (up to 1.5 mm) simple linear elastic fracture mechanics - based only on applied loading - did not predict the fracture strengths. Also, Paris' Law for cyclic crack growth did not correlate the crack-growth behavior. To investigate the effect of shot-peening, additional fracture and crack-growth tests were performed on material which was precompressed to remove the residual stresses left by the shot-peening. Both tests and analysis show that shot-peening residual stresses influence the fracture and crack-growth properties of the material. This report presents the analytical method of compensating for residual stresses and the fracture and cyclic crack-growth test results and predictions.

Elber, W.↗
NASA NTRS1975

Effect of shape and size of lung and chest wall on stresses in the lung

To understand better the effect of shape and size of lung and chest wall on the distribution of stresses, strains, and surface pressures, we analyzed a theoretical model using the technique of finite elements. First we investigated the effects of changing the chest wall shape during expansion, and second we studied lungs of a variety of inherent shapes and sizes. We found that, in general, the distributions of alveolar size, mechanical stresses, and surface pressures in the lungs were dominated by the weight of the lung and that changing the shape of the lung or chest wall had relatively little effect. Only at high states of expansion where the lung was very stiff did changing the shape of the chest wall cause substantial changes. Altering the inherent shape of the lung generally had little effect but the topographical differences in stresses and surface pressures were approximately proportional to lung height. The results are generally consistent with those found in the dog by Hoppin et al (1969).

Vawter, D. L.↗
NASA NTRS1975

Biogenic amines and acute thermal stress in the rat

A study is summarized which demonstrates that depletion of the biogenic amines 5-hydroxytryptamine (5-HT) or norepinephrine (NE) alters the normal thermoregulatory responses to acute temperature stress. Specifically, NE depletion caused a significant depression in equilibrium rectal temperature at 22 C and a greater depression in rectal temperature than controls in response to cold (6 C) stress; NE depletion also resulted in a significantly higher rectal temperature response to acute heat (38 C) stress. Depletion of 5-HT had less severe effects. It remains unclear whether the primary site of action of these agents is central or peripheral.

Williams, B. A.↗
NASA NTRS1975

Stress analysis of adhesive bonded stiffener plates and double joints

The general problem of adhesive bonded stiffener plates and double joints of dissimilar orthotropic adherends with transverse shear deformations are analyzed. Adhesive layers are assumed to be of an isotropic, elastic and relatively flexible material. It is shown that the stress distributions in the adhesive layers are very much dependent on the bending deformations in adherends. Also, it is found that, in the adhesive layer, maximum transverse normal stress is, in many cases, larger than the longitudinal shear stress and that both occur at the edge of the joint. The general method of solution developed is applied to several practical examples.

Yuceoglu, U.↗
NASA NTRS1974

Changes in the performance characteristics of a GaAs near infrared light emitting diode when exposed to various current and thermal stresses

The changes that occurred in the optical and electrical characteristics of a near infrared, GaAs light emitting diode, when operated under various levels and combinations of current and thermal stresses are discussed. A total of forty parts were operated for two thousand hours under eight different sets of dc current and ambient temperature conditions. Degradation in the radiant optical power of these devices was thirty-four percent when operated at their rated current and an ambient temperature of 298K (25 C). Derating the current and/or the thermal stress reduced the degradation of this parameter in approximately a linear manner. All degraded devices behaved similarly, exhibiting rapid nonlinear degradation followed by a gradual linear degradation and finally a period of stable operation. An attempt was made to correlate initial device condition to degradation during stress testing, but met with little success.

Thomas, E. F., Jr.↗
NASA NTRS1975

Yield stress of cemented tungsten carbide

Cemented tungsten carbide yields plastically at room temperature in the presence of a large hydrostatic pressure component. By approximate analysis of the state of stress in supported opposed anvils and by measurement of the pressure at which the anvil tips exhibit a permanent deviation from planarity, we have obtained the yield stress of such materials. Our value for the yield stress of a 3% cobalt cemented tungsten carbide is 86 kbar.

Ruoff, A. L.↗
NASA NTRS1975

Residual thermal stresses in a solid sphere cast from a thermosetting material

Expressions are developed for the residual thermal stresses in a solid sphere cast from a chemically hardening thermosetting material in a rigid spherical mold. The description of the heat generation rate and temperature variation is derived from a first-order chemical reaction. Solidification is described by the continuous transformation of the material from an inviscid liquidlike state into an elastic solid, with intermediate properties determined by the degree of chemical reaction. Residual stress components are obtained as functions of the parameters of the hardening process and the properties of the hardening material. Variation of the residual stresses with a nondimensionalized reaction rate parameter and the relative compressibility of the hardened material is discussed in detail.

Levitsky, M.↗
NASA NTRS1976

Residual stresses in polymer matrix composite laminates

Residual stresses in composites are induced during fabrication and by environmental exposure. The theory formulated can describe the shrinkage commonly observed after a thermal expansion test. Comparison between the analysis and experimental data for laminates of various material systems indicates that the residual stress-free temperature can be lower than the curing temperature, depending on the curing process. Effects of residual stresses on ply failure including the acoustic emission characteristics are discussed.

Hahn, H. T.↗
NASA NTRS1976

On a 3-D singularity element for computation of combined mode stress intensities

A special three-dimensional singularity element is developed for the computation of combined modes 1, 2, and 3 stress intensity factors, which vary along an arbitrarily curved crack front in three dimensional linear elastic fracture problems. The finite element method is based on a displacement-hybrid finite element model, based on a modified variational principle of potential energy, with arbitrary element interior displacements, interelement boundary displacements, and element boundary tractions as variables. The special crack-front element used in this analysis contains the square root singularity in strains and stresses, where the stress-intensity factors K(1), K(2), and K(3) are quadratically variable along the crack front and are solved directly along with the unknown nodal displacements.

Atluri, S. N.↗
NASA NTRS1976

The effect of several intact or broken stringers of the stress intensity factor in a cracked sheet

The stringers which are continuously attached to the plate and placed perpendicular to the crack may be partially debonded due to high stress concentrations. Since the stringers may even break under excessive loading conditions, both intact and broken stringers are considered to investigate the effect of rupture. The continuity of displacements along the bond lines leads to an integral equation which is solved to give the shear stress distribution in the adhesive and the stress intensity factors at the crack tips.

Arin, K.↗
NASA NTRS1977

Fatigue criterion for the design of rotating shafts under combined stress

A revised approach to the design of transmission shafting which considers the flexure fatigue characteristics of the shaft material under combined cyclic bending and static torsion stress is presented. A fatigue failure relation, corroborated by published combined stress test data, is presented which shows an elliptical variation of reversed bending endurance strength with static torsional stress. From this elliptical failure relations, a design formula for computing the diameter of rotating solid shafts under the most common condition of loading is developed.

Loewenthal, S. H.↗
NASA NTRS1976

Interlaminar stresses in composite laminates: A perturbation analysis

A general method of solution for an elastic balanced symmetric composite laminate subject to a uniaxial extension was developed based upon a perturbation analysis of a limiting free body containing an interfacial plane. The solution satisfies more physical requirements and boundary conditions than previous investigations, and predicts smooth continuous interlaminar stresses with no instabilities. It determines the finite maximum intensity for the interlaminar normal stress in all laminates, provides mathematical evidences for the singular stresses in angle-ply laminates, suggests the need for the experimental determination of an important problem parameter, and introduces a viable means for solving related problems of practical interest.

Hsu, P. W.↗
NASA NTRS1977

Three-dimensional elastic stress and displacement analysis of finite geometry solids containing cracks

The line method of analysis is applied to the Navier-Cauchy equations of elastic equilibrium to calculate the displacement fields in finite geometry bars containing central, surface, and double-edge cracks under extensionally applied uniform loading. The application of this method to these equations leads to coupled sets of simultaneous ordinary differential equations whose solutions are obtained along sets of lines in a discretized region. Normal stresses and the stress intensity factor variation along the crack periphery are calculated using the obtained displacement field. The reported results demonstrate the usefulness of this method in calculating stress intensity factors for commonly encountered crack geometries in finite solids.

Kring, J.↗
NASA NTRS1977

Influence of two-dimensional hygrothermal gradients on interlaminar stresses near free edges

Interlaminar stresses are determined for mechanical loading, uniform hygrothermal loading, and gradient moisture loading through implementation of a finite element computer code. Nonuniform two-dimensional hygroscopic gradients are obtained from a finite difference solution of the diffusion equation. It is shown that hygroscopic induced stresses can be larger than those resulting from mechanical and thermal loading, and that the distribution of the interlaminar normal stress may be changed significantly in the presence of a two-dimensional moisture gradient in the boundary layer of a composite laminate.

Farley, G. L.↗
NASA NTRS1976

Aqueous stress-corrosion cracking of high-toughness D6AC steel

The crack growth behavior of D6AC steel as a function of stress intensity, stress and corrosion history, and test technique, under sustained load in filtered natural seawater, 3.3 per cent sodium chloride solution, and distilled water, was investigated. Reported investigations of D6AC were considered in terms of the present study with emphasis on thermal treatment, specimen configuration, fracture toughness, crack-growth rates, initiation period, and threshold. Both threshold and growth kinetics were found to be relatively insensitive to these test parameters. The apparent incubation period was dependent on technique, both detection sensitivity and precracking stress intensity level.

Gilbreath, W. P.↗
NASA NTRS1977

Boundary layer shear stress in subsonic and supersonic flow

A wide range of shear stress distributions for turbulent boundary layers is examined. A solution for the shear stress in terms of the mean flow is obtained for the limiting case of large Reynolds numbers. Attention is given to turbulent boundary layer shear stress, zero pressure gradient flow, increasing pressure gradient flow, and decreasing pressure gradient flow.

Sandborn, V. A.↗
NASA NTRS1977

Nonlinear stress waves in a perfectly flexible string

This paper discusses nonlinear stress-wave propagation in a perfectly flexible string obeying a quasilinear (rate-dependent) constitutive equation. Wave speeds and compatibility relations valid along various families of characteristics were determined. It was shown that the compatibility relations associated with the transverse as well as the longitudinal waves readily yield a physical interpretation when they are expressed in suitable variables and in vector form. Coding based on the present information was completed for the machine solution of a class of mixed initial- and boundary-value problems of practical interest. Computer simulation of the stress-wave interaction in the 40-foot lanyard in the Arcas 'Rocoz' system during deployment was carried out using a stress-strain relation for nylon at the strain rate of 30/second. A method for estimating the maximum tension and strain in a string during the initial loading phase is proposed.

Fan, D.-N.↗