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At least 847 records · Page 47

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NASA NTRS1989

Mean stress and the exhaustion of fatigue-damage resistance

Mean-stress effects on fatigue life are critical in isothermal and thermomechanically loaded materials and composites. Unfortunately, existing mean-stress life-prediction methods do not incorporate physical fatigue damage mechanisms. An objective is to examine the relation between mean-stress induced damage (as measured by acoustic emission) and existing life-prediction methods. Acoustic emission instrumentation has indicated that, as with static yielding, fatigue damage results from dislocation buildup and motion until dislocation saturation is reached, after which void formation and coalescence predominate. Correlation of damage processes with similar mechanisms under monotonic loading led to a reinterpretation of Goodman diagrams for 40 alloys and a modification of Morrow's formulation for life prediction under mean stresses. Further testing, using acoustic emission to monitor dislocation dynamics, can generate data for developing a more general model for fatigue under mean stress.

Berkovits, Avraham↗

NASA NTRS1989

Plasma stress hormones in resting rats - Eighty four day study

The effects of a repeated mild stress of handling and placing rats temporarily into unfamiliar cages on the blood-plasma concentration of the stress hormones (corticosterone, ACDH, and prolactin) were investigated in male Sprague-Dawley rats subjected to this type of stress once every week during a period of three months. Results showed that repeated mild stress of handling (as well as repeated blood sampling) did not affect the plasma stress-hormone concentrations in these animals.

Popovic, Vojin↗

NASA NTRS1989

X-ray diffraction measurements of residual stresses in SiC/Ti composites

Residual stresses are developed in metal matrix composites during consolidation because of the difference in the coefficient of thermal expansion of the fiber and matrix constituents. X-ray diffraction techniques were used to measure tensile residual stresses in the matrix component of two different titanium alloy composites reinforced with continuous silicon carbide fibers. The titanium alloy matrices were a beta-stabilized alloy Ti-15V-3Al-3Cr-3Sn, and an ordered multiphase (alpha + beta) alloy, Ti-14Al-21Nb. Residual tensile stresses as high as 62 percent of the yield of Ti-15V-3Al-3Cr-3Sn and 70 percent of the yield of Ti-14Al-21Nb were measured in the longitudinal direction on unidirectionally reinforced composites. By measuring residual stresses in four different composite lay-ups, the composite lay-up was determined to have a significant effect on the residual stress state near the surface.

Brown, K. M.↗

NASA NTRS1990

Effects of material non-linearity on the residual stresses in a dendritic silicon crystal ribbon

Thermal stresses developed in a dendritic silicon crystal ribbon have been shown to cause plastic deformation and residual stresses in the ribbon. This paper presents an implementation of a numerical model proposed for thermoelastoplastic behavior of a material. The model has been used to study the effects of plasticity of silicon on the residual stresses. The material properties required to implement this model are all assumed, and the response of the material to the variations in these assumed parameters of the constitutive law and in the finite element mesh is investigated. The steady state growth process is observed to be periodic with nonzero residual stresses. Numerical difficulties are also encountered in the computer solution process, resulting in sharp jumps and large oscillations in the stress responses.

Ray, Sujit K.↗

NASA NTRS1989

A dynamic model of stress and sustained attention

Arguments are presented that an integrated view of stress and performance must consider the task demanding a sustained attention as a primary source of cognitive stress. A dynamic model is developed on the basis of the concept of adaptability in both physiological and psychological terms, that addresses the effects of stress on vigilance and, potentially, a wide variety of attention-demanding performance tasks. The model provides an insight into the failure of an operator under the driving influences of stress and opens a number of potential avenues through which solutions to the complex challenge of stress and performance might be posed.

Hancock, P. A.↗

NASA NTRS1990

Elastic/plastic analyses of advanced composites investigating the use of the compliant layer concept in reducing residual stresses resulting from processing

High residual stresses within intermetallic and metal matrix composite systems can develop upon cooling from the processing temperature to room temperature due to the coefficient of thermal expansion (CTE) mismatch between the fiber and matrix. As a result, within certain composite systems, radial, circumferential, and/or longitudinal cracks have been observed to form at the fiber-matrix interface. The compliant layer concept (insertion of a compensating interface material between the fiber and matrix) was proposed to reduce or eliminate the residual stress buildup during cooling and thus minimize cracking. The viability of the proposed compliant layer concept is investigated both elastically and elastoplastically. A detailed parametric study was conducted using a unit cell model consisting of three concentric cylinders to determine the required character (i.e., thickness and material properties) of the compliant layer as well as its applicability. The unknown compliant layer mechanical properties were expressed as ratios of the corresponding temperature dependent Ti-24Al-11Nb (a/o) matrix properties. The fiber properties taken were those corresponding to SCS-6 (SiC). Results indicate that the compliant layer can be used to reduce, if not eliminate, radial and circumferential residual stresses within the fiber and matrix and therefore also reduce or eliminate the radial cracking. However, with this decrease in in-plane stresses, one obtains an increase in longitudinal stress, thus potentially initiating longitudinal cracking. Guidelines are given for the selection of a specific compliant material, given a perfectly bonded system.

Arnold, Steven M.↗

NASA NTRS1991

Plate and butt-weld stresses beyond elastic limit, material and structural modeling

Ultimate safety factors of high performance structures depend on stress behavior beyond the elastic limit, a region not too well understood. An analytical modeling approach was developed to gain fundamental insights into inelastic responses of simple structural elements. Nonlinear material properties were expressed in engineering stresses and strains variables and combined with strength of material stress and strain equations similar to numerical piece-wise linear method. Integrations are continuous which allows for more detailed solutions. Included with interesting results are the classical combined axial tension and bending load model and the strain gauge conversion to stress beyond the elastic limit. Material discontinuity stress factors in butt-welds were derived. This is a working-type document with analytical methods and results applicable to all industries of high reliability structures.

Verderaime, V.↗

NASA NTRS1991

Local delamination in laminates with angle ply matrix cracks. Part 1: Tension tests and stress analysis

Quasi-static tension tests were conducted on AS4/3501-6 graphite epoxy laminates. Dye penetrant enhanced x-radiography was used to document the onset of matrix cracking and the onset of local delaminations at the intersection of the matrix cracks and the free edge. Edge micrographs taken after the onset of damage were used to verify the location of the matrix cracks and local delamination through the laminate thickness. A quasi-3D finite element analysis was conducted to calculate the stresses responsible for matrix cracking in the off-axis plies. Laminated plate theory indicated that the transverse normal stresses were compressive. However, the finite element analysis yielded tensile transverse normal stresses near the free edge. Matrix cracks formed in the off-axis plies near the free edge where in-plane transverse stresses were tensile and had their greatest magnitude. The influence of the matrix crack on interlaminar stresses is also discussed.

Obrien, T. Kevin↗

NASA NTRS1990

Global/local stress analysis of composite panels

A method for performing a global/local stress analysis is described and its capabilities are demonstrated. The method employs spline interpolation functions which satisfy the linear plate bending equation to determine displacements and rotations from a global model which are used as 'boundary conditions' for the local model. Then, the local model is analyzed independently of the global model of the structure. This approach can be used to determine local, detailed stress states for specific structural regions using independent, refined local models which exploit information from less-refined global models. The method presented is not restricted to having a priori knowledge of the location of the regions requiring local detailed stress analysis. This approach also reduces the computational effort necessary to obtain the detailed stress state. Criteria for applying the method are developed. The effectiveness of the method is demonstrated using a classical stress concentration problem and a graphite-epoxy blade-stiffened panel with a discontinuous stiffener.

Ransom, J. B.↗

NASA NTRS1991

Nonlinear Reynolds stress model for turbulent shear flows

A nonlinear algebraic Reynolds stress model, derived using the renormalization group, is applied to equilibrium homogeneous shear flow and fully developed flow in a square duct. The model, which is quadratically nonlinear in the velocity gradients, successfully captures the large-scale inhomogeneity and anisotropy of the flows studied. The ratios of normal stresses, as well as the actual magnitudes of the stresses are correctly predicted for equilibrium homogeneous shear flow. Reynolds normal stress anisotropy and attendant turbulence driven secondary flow are predicted for a square duct. Profiles of mean velocity and normal stresses are in good agreement with measurements. Very close to walls, agreement with measurements diminishes. The model has the benefit of containing no arbitrary constants; all values are determined directly from the theory. It seems that near wall behavior is influenced by more than the large scale anisotropy accommodated in the current model. More accurate near wall calculations may well require a model for anisotropic dissipation.

Barton, J. Michael↗

NASA NTRS1991

Eddy damping, backscatter, and subgrid stresses in subgrid modeling of turbulence

The Brief Report demonstrates the relationship of eddy-viscosity models to subgrid stresses. A formula that determines the relative importance of the cross stress and the Reynolds stress for the net eddy-damping and backscatter contributions is derived. The cross-stress term with sharp-cut filtering is identified as an important quantity to model. These concepts could prove useful as a basis for constructing specific models for the Reynolds and cross stresses.

Zhou, YE↗

NASA NTRS1991

Temporal versus spatial variation in leaf reflectance under changing water stress conditions

Leaf reflectance changes associated with changes in water stress were analyzed in two separate experiments. Results indicate that the variation in reflectance among collections of leaves of a given species all at the same level of water stress is at least as great as the variation in reflectance associated with changes in water stress for a given leaf collection of that species. The implications is that results from leaf reflectance-water stress studies have only limited applicability to the remote sensing of plant canopy water stress.

Cohen, Warren B.↗

NASA NTRS1991

Traction free finite elements with the assumed stress hybrid model

An effective approach in the finite element analysis of the stress field at the traction free boundary of a solid continuum was studied. Conventional displacement and assumed stress finite elements were used in the determination of stress concentrations around circular and elliptical holes. Specialized hybrid elements were then developed to improve the satisfaction of prescribed traction boundary conditions. Results of the stress analysis indicated that finite elements which exactly satisfy the free stress boundary conditions are the most accurate and efficient in such problems. A general approach for hybrid finite elements which incorporate traction free boundaries of arbitrary geometry was formulated.

Kafie, Kurosh↗

NASA NTRS1991

C deg continuity elements by Hybrid Stress method

An intensive study of the assumed variable distribution necessary for the Assumed Displacement Formulation, the Hellinger-Reissner Formulation, and the Hu-Washizu Formulation is made in a unified manner. With emphasis on physical explanation, a systematic method for the Hybrid Stress element construction is outlined. The numerical examples use four and eight node plane stress elements and eight and twenty node solid elements. Computation cost study indicates that the hybrid stress element derived using recently developed Uncoupled Stress Formulation is comparable in CPU time to the Assumed Displacement element. Overall, main emphasis is placed on providing a broader understanding of the Hybrid Stress Formulation.

Kang, David Sung-Soo↗

NASA NTRS1992

Gear tooth stress measurements of two helicopter planetary stages

Two versions of the planetary reduction stages from U.S. Army OH-58 helicopter main rotor transmissions were tested at NASA Lewis. One sequential and one nonsequential planetary were tested. Sun gear and ring gear teeth strains were measured, and stresses were calculated from the strains. The alternating stress at the fillet of both the loaded and unloaded sides of the teeth and at the root of the sun gear teeth are reported. Typical stress variations as the gear tooth moves through mesh are illustrated. At the tooth root location of the thin rimmed sun gear, a significant stress was produced by a phenomenon other than the passing of a planet gear. The load variation among the planets was studied. Each planet produced its own distinctive load distribution on the ring and sun gears. The load variation was less for a three planet, nonsequential design as compared to that of a four planet, sequential design. The reported results enhance the data base for gear stress levels and provide data for the validation of analytical methods.

Krantz, Timothy L.↗

NASA NTRS1990

Effects of uniaxial stress on magnetoacoustic emission and magnetoacoustic responses in steel

Results of magnetoacoustic emission (MAE) and magnetoacoustic (MAC) measurements performed with an unembrittled HY-80 steel specimen are presented. The MAE measurements were obtained by applying an ac magnetic field of 20 Hz parallel to the uniaxial stress axis. The effects of tensile stress were shown to monotonically decrease the peak amplitude of the MAE burst, and they were consistent with the predicted trend based on the tensile stress-induced domain alignment. The effects of compressive stress were found to increase the peak amplitude of the MAE burst initially and to eventually decrease as the magnitude of compressive stress increased.

Namkung, M.↗

NASA NTRS1990

Determination of the stress distributions in a ceramic, tensile specimen using numerical techniques

Finite element analyses (FEA) were used to determine the stress distributions in a ceramic, tensile specimen with two types of button-head gripping systems. The FEA revealed stress raisers at both the button-head and the transition from the gage section to the shank. However, the stress field within the bulk of the gage section is uniform and uniaxial. The stress ratio, kt, between the button-head and gage section stresses varied from 0.35 to 0.72 for the tapered collet or the straight collet systems, respectively. Previous empirical tests confirm these results whereby the tapered collet system, compared to the straight collet system, sustained over twice the average load before failure at the button-head.

Jenkins, M. E.↗

NASA NTRS1991

An approximate analytical solution for interlaminar stresses in angle-ply laminates

An improved approximate analytical solution for interlaminar stresses in finite width, symmetric, angle-ply laminated coupons subjected to axial loading is presented. The solution is based upon statically admissible stress fields which take into consideration local property mismatch effects and global equilibrium requirements. Unknown constants in the admissible stress states are determined through minimization of the complementary energy. Typical results are presented for through-the-thickness and interlaminar stress distributions for angle-ply laminates. It is shown that the results represent an improved approximate analytical solution for interlaminar stresses.

Rose, Cheryl A.↗