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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Tension fracture of laminates for transport fuselage. Part 1: Material screening

Transport fuselage structures are designed to contain pressure following a large penetrating damage event. Applications of composites to fuselage structures require a database and supporting analysis on tension damage tolerance. Tests with 430 fracture specimens were used to accomplish the following: (1) identify critical material and laminate variables affecting notch sensitivity; (2) evaluate composite failure criteria; and (3) recommend a screening test method. Variables studied included fiber type, matrix toughness, lamination manufacturing process, and intraply hybridization. The laminates found to have the lowest notch sensitivity were manufactured using automated tow placement. This suggests a possible relationship between the stress distribution and repeatable levels of material inhomogeneity that are larger than found in traditional tape laminates. Laminates with the highest notch sensitivity consisted of toughened matrix materials that were resistant to a splitting phenomena that reduces stress concentrations in major load bearing plies. Parameters for conventional fracture criteria were found to increase with crack length for the smallest notch sizes studied. Most material and laminate combinations followed less than a square root singularity for the largest crack sizes studied. Specimen geometry, notch type, and notch size were evaluated in developing a screening test procedure. Traitional methods of correcting for specimen finite width were found to be lacking. Results indicate that a range of notch sizes must be tested to determine notch sensitivity. Data for a single small notch size (0.25 in. diameter) was found to give no indication of the sensitivity of a particular material and laminate layup to larger notch sizes.

Walker, T. H.↗
NASA NTRS1938

Load Tests on a Stiffened Circular Cylindrical Shell

The present report describes tests in which the stress distribution may be determined in a stiffened circular cylindrical shell loaded longitudinally at four symmetrically situated points. As being of particular importance are the cases investigated of groups of bending and arching or convexing forces, respectively. From the stress measurements on the longitudinal stiffeners, the shear stresses and the bulkhead ring stresses in the skin could be evaluated. These measurements showed that the "simple shear field" used in theoretical computations in which all normal stresses in the skin are neglected, must be extended by the addition of the transverse or circumferential stresses if the bulkhead rings are not riveted to the skin.

Schapitz, E↗
NASA NTRS1988

Thermo-viscoelastic analysis of composite materials, volume 1

Advanced composite materials, especially graphite/epoxy, are being applied to aircraft structures in order to improve performance and save weight. An important consideration in composite design is the residual strength of a structure containing holes, delaminations, or interlaminar damage when subjected to compressive loads. Recent studies have revealed the importance of viscoelastic effects in polymer-based composites. The viscoelastic effect is particularly significant at elevated temperature/moisture conditions since the matrix material is strongly affected by the environment. The solution of viscoelastic problems in composites was limited to special cases which can be solved by classical lamination theory. A finite element procedure is presented for calculating time-dependent stresses and strains in composite structures with general configurations and complicated boundary conditions. Using this procedure the in-plane and interlaminar stress distributions and histories in notched and unnotched composites were obtained for mechanical and thermal loads. Both two-dimensional and three-dimensional viscoelastic problems are analyzed. The effects of layup orientation and load spectrum on creep response and stress relaxation were also studied.

Lin, K. Y.↗
NASA NTRSDate not supplied

On-Orbit Glass Stress Measurement Tool Development

Exposed window surfaces on-orbit are vulnerable to a variety of damage including impacts. These impacts impart stress into the panes and reduce the remaining life of the window. Spacecraft windows are evaluated before and after flight to determine the minimum strength of the glass prior to launch. Currently, no method exists to accurately evaluate the windows on-orbit. An on-orbit optical system is under development to evaluate spacecraft windowpane material for remaining flight life by directly measuring the stress in the glass. This optical system builds on the industry-standard practice of calculating the amount of impact-induced stress through birefringence measurement. In industry, the birefringence is imaged through the pane with a detector and light source on opposite sides. On-orbit, there is only access to one side of the pane. This system has been designed to image birefringence by reflecting the light off the pane to a detector on the same side of the glass. Previous studies have shown a correlation between material stress distribution and the associated K-value. This system is designed to determine low strength areas which can be caused by defects due to impact damage. Glass samples will be imaged and broken in order to correlate impact-induced stress to the fracture strength of the material. Once this correlation is determined, a quantitative assessment of the stress areas in spacecraft windows will occur prior to and during mission operations to reliably determine the overall strength of the glass.

Max J Schneiderman↗
NASA NTRS2006

Solid Modeling of Crew Exploration Vehicle Structure Concepts for Mass Optimization

Parametric solid and surface models of the crew exploration vehicle (CEV) command module (CM) structure concepts are developed for rapid finite element analyses, structural sizing and estimation of optimal structural mass. The effects of the structural configuration and critical design parameters on the stress distribution are visualized, examined to arrive at an efficient design. The CM structural components consisted of the outer heat shield, inner pressurized crew cabin, ring bulkhead and spars. For this study only the internal cabin pressure load case is considered. Component stress, deflection, margins of safety and mass are used as design goodness criteria. The design scenario is explored by changing the component thickness parameters and materials until an acceptable design is achieved. Aluminum alloy, titanium alloy and an advanced composite material properties are considered for the stress analysis and the results are compared as a part of lessons learned and to build up a structural component sizing knowledge base for the future CEV technology support. This independent structural analysis and the design scenario based optimization process may also facilitate better CM structural definition and rapid prototyping.

Mukhopadhyay, Vivek↗
NASA NTRS1990

A macro-micromechanics analysis of a notched metal matrix composite

A macro-micromechanics analysis was formulated to determine the matrix and fiber behavior near the notch tip in a center-notched metal matrix composite. Results are presented for a boron/aluminum monolayer. The macro-level analysis models the entire notched specimen using a three dimensional finite element program which uses the vanishing-fiber-diameter model to model the elastic-plastic behavior of the matrix and the elastic behavior of the fiber. The micro-behavior is analyzed using a Discrete Fiber-Matrix (DFM) model containing one fabric and the surrounding matrix. The dimensions of the DFM model were determined by the ply thickness and the fiber volume fraction and corresponded to the size of the notch-tip element in the macro-level analysis. The boundary conditions applied to the DFM model were determined from the macro-level analysis. Stress components within the DFM model were calculated and stress distributions are presented along selected planes and surfaces within the DFM model, including the fiber-matrix interface. Yielding in the matrix was examined at the notch tip in both the macro- and micro-level analyses. The DFM model predicted higher stresses (24 percent) in the fiber compared to the global analysis. In the notch-tip element, the interface stresses indicated that a multi-axial criterion may be required to predict interfacial failure. The DFM analysis predicted yielding to initiate in the notch-tip element at a stress level 28 percent lower than predicted by the global analysis.

Bigelow, C. A.↗
NASA NTRS1992

Scale effects on the transverse tensile strength of graphite epoxy composites

The influence of material volume on the transverse tensile strength of AS4/3501-6 graphite epoxy composites was investigated. Tensile tests of 90 degree laminates with 3 different widths and 5 different thicknesses were conducted. A finite element analysis was performed to determine the influence of the grip on the stress distribution in the coupons and explain the tendency for the distribution of failure locations to be skewed toward the grip. Specimens were instrumented with strain gages and extensometers to insure good alignment and to measure failure strains. Data indicated that matrix dominated strength properties varied with the volume of material that was stressed, with the strength decreasing as volume increased. Transverse strength data were used in a volumetric scaling law based on Weibull statistics to predict the strength of 90 degree laminates loaded in three point bending. Comparisons were also made between transverse strength measurements and out-of-plane interlaminar tensile strength measurements from curved beam bending tests. The significance of observed scale effects on the use of tests for material screening, quality assurance, and design allowables is discussed.

Obrien, T. Kevin↗
NASA NTRS1961

Stress Concentrations in Filamentary Structures

Theoretical analyses are made of the stress distributions in a sheet of parallel filaments which carry normal loads and are imbedded in a matrix which carries only shear. In all cases, uniform loading at infinity is assumed and small-deflection elasticity theory is used. Static and dynamic stress-concentration factors due to one or more filaments being broken are determined. Particular attention is paid the dynamic overshoot resulting when the filaments are suddenly broken. The dynamic-response factor increases from 1.15 to 1.27 as the number of broken filaments is increased from one to infinity. A somewhat lower dynamic-response factor is obtained when a hole is suddenly caused in the filament sheet.

Hedgepeth, John M.↗
NASA NTRS1984

Thermal and thermomechanical effects in dry sliding

Developments in the study of interrelated thermal and mechanical phenomena in sliding systems are reviewed. The topics reviewed include mechanisms of frictional heating and the distribution of heat during sliding friction, the experimental measurement and analysis of surface and near-surface temperatures resulting from frictional heating, thermal deformation around sliding contacts and the changes in contact geometry caused by thermal deformation and thermoelastic instability, and the thermomechanical stress distribution around the frictionally heated and thermally deformed contact spots. The influence of the thermal and thermomechanical contact phenomena on friction and wear, surface melting, softening, chemical deterioration, and thermocracking are discussed. The phenomena have important implications in the design and application of sliding or sliding-rolling mechanical components such as dynamic seals, brakes, clutches, plastic bearings, solid or boundary-lubricated bearings, and gears.

Kennedy, F. E., Jr.↗
NASA NTRS1975

An investigation of the edge-sliding mode in fracture mechanics

A boundary collocation procedure has been applied to the Williams stress function to determine the elastic stress distribution for the crack tip region of a finite, edge-cracked plate subjected to mode II loading at the crack tips. The asymmetric specimen selected was particularly suitable for the determination of plane strain fracture toughness for mode II loading. Numerical solutions for stress intensity factors for the edge-sliding mode obtained by the boundary collocation method were in close agreement with values obtained from photoelastic experiments. Fracture tests of several compact shear specimens of 2024-T4 aluminum were conducted in order to experimentally investigate the behavior of the edge-sliding mode. In each case a brittle shear failure was observed and mode II fracture toughness values were obtained. The average value for K-IIc obtained from two tests was 39.5 ksi radical (in). No K-Ic data for 2024-T4 were available for comparison purposes; however, K-Ic values for a similar alloy, 2024-T351, have been reported as 34 ksi radical (in) which is only about 15 per cent below the corresponding K-IIc value.

Jones, D. L.↗
NASA NTRS1992

An approximate solution for interlaminar stresses in laminated composites: Applied mechanics program

An approximate solution for interlaminar stresses in finite width, laminated composites subjected to uniform extensional, and bending loads is presented. The solution is based upon the principle of minimum complementary energy and an assumed, statically admissible stress state, derived by considering local material mismatch effects and global equilibrium requirements. The stresses in each layer are approximated by polynomial functions of the thickness coordinate, multiplied by combinations of exponential functions of the in-plane coordinate, expressed in terms of fourteen unknown decay parameters. Imposing the stationary condition of the laminate complementary energy with respect to the unknown variables yields a system of fourteen non-linear algebraic equations for the parameters. Newton's method is implemented to solve this system. Once the parameters are known, the stresses can be easily determined at any point in the laminate. Results are presented for through-thickness and interlaminar stress distributions for angle-ply, cross-ply (symmetric and unsymmetric laminates), and quasi-isotropic laminates subjected to uniform extension and bending. It is shown that the solution compares well with existing finite element solutions and represents an improved approximate solution for interlaminar stresses, primarily at interfaces where global equilibrium is satisfied by the in-plane stresses, but large local mismatch in properties requires the presence of interlaminar stresses.

Rose, Cheryl A.↗
NASA NTRS1977

NASTRAN finite element idealization study

The investigation of the effects of variations of mesh refinement and mesh pattern were conducted using a basic rectangular mesh pattern. When employing the constant strain TRMEM element, the basic rectangular pattern was subdivided into triangles. This subdivision employs two different triangular patterns and allows results to be obtained which demonstrate the effect of modelling bias. Errors in tip deflection, direct stress, and shearing stress as a function of mesh size and element aspect ratio were obtained as well as mid-span stress distributions. All problems were solved on an IBM 360/95 computer using MacNeal-Schwendler Version MSC-38 Rigid Format-1 of the NASTRAN computer program. While NASTRAN uses double precision arithmetic for the solution of the global equations for displacements, subsequent computations to obtain element stresses are carried out in single precision. This suggests that some improvement in stress recovery might be expected when using the higher precision DCD machines.

Case, W. R.↗
NASA NTRS1984

Analysis of a unidirectional, symmetric buffer strip laminate with damage

A method for predicting the fracture behavior of hybrid buffer strip laminates is presented in which the classical shear-lag model is used to represent the shear stress distribution between adjacent fibers. The method is demonstrated by applying it to a notched graphite/epoxy laminate, and the results show clearly the manner in which the most efficient combination of buffer strip properties can be selected in order to arrest the crack. The ultimate failure stress of the laminate is plotted vs the buffer strip width. It is shown that in the case of graphite-epoxy and S-glass epoxy laminates, the optimum buffer strip spacing to width ratio should be about four to one.

Dharani, L. R.↗
NASA NTRS1975

Rough-to-smooth transition of an equilibrium neutral constant stress layer

Purpose of research on rough-to-smooth transition of an equilibrium neutral constant stress layer is to develop a model for low-level atmospheric flow over terrains of abruptly changing roughness, such as those occurring near the windward end of a landing strip, and to use the model to derive functions which define the extent of the region affected by the roughness change and allow adequate prediction of wind and shear stress profiles at all points within the region. A model consisting of two bounding logarithmic layers and an intermediate velocity defect layer is assumed, and dimensionless velocity and stress distribution functions which meet all boundary and matching conditions are hypothesized. The functions are used in an asymptotic form of the equation of motion to derive a relation which governs the growth of the internal boundary layer. The growth relation is used to predict variation of surface shear stress.

Logan, E., Jr.↗
NASA NTRS1980

Stress analysis of bonded plates and joints

The paper presents a general numerical method of solution to the transverse normal stress and longitudinal shear stress distributions in the adhesive layers of bonded stiffener plates, double lap joints, and strap joints, all of dissimilar orthotropic adherends. The bending deformations and the transverse shear deformations are included in the analytical model. The present influence of bending deformations of adherends on both the normal and shear stresses in adhesive layers is discussed in detail for practical cases.

Yuceoglu, U.↗
NASA NTRS1973

Analysis of stresses at the bore of a drilled ball operating in a high-speed bearing

Three-dimensional stress distributions were calculated for both a regular drilled ball with a stiffening web. The balls were 20.6 mm (0.8125 in.) in diameter and had a 12.6 mm (0.496 in.) diameter concentric hole. The stiffening web was 1.5 mm (0.06 in.) thick. The calculations showed that a large reversing tangential stress at the hole bore was reduced by one-half by the addition of the web.

Coe, H. H.↗
NASA NTRS1992

Diffusive crack growth at a bimaterial interface

The diffusional microcrack growth behavior in a bimaterial system is investigated with an aim at estimating service life of advanced ceramic composites under creep-rupture conditions. The crack is assumed to grow via a coupled surface and grain-boundary diffusion under steady state conditions. The tensile stress distribution along the interface ahead of the moving crack tip is solved, and it is found that a new length parameter exists as a scaling factor for which the solution becomes identical to the single phase case when plotted on the nondimensional physical plane. In contrast to the elastic stress solution which shows singularity at the tip, together with oscillatory character away from the tip, the creep stresses have a peak value away from the tip due to a wedging effect and interfacial sliding eliminates stress oscillation resulting in a decoupling between mode I and mode II loading. This solution ties the far-field loading parameter to the crack tip conditions in terms of the unknown crack velocity to give a specific V-K(1) relationship. It is shown that an exponent of 12 in the conventional crack growth power law emerges at the higher applied stress range.

Chuang, Tze-Jer↗
NASA NTRS1973

Experimental studies of turbulent aircraft wake

Review of wind tunnel test data obtained for tip vortex studies on a square-tipped rectangular wing. The results include wing surface pressure distributions, three-dimensional velocity components in the wake, and principal vortex characteristics such as peak tangential velocity and core size distributions. The wind tunnel measurements are compared with flight test data. These comparisons show that the magnitudes of circumferential velocities, normalized by flight speed and lift coefficient, as well as the vortex core radius, normalized by wing span, are in close agreement. The data obtained make possible the calculation of turbulence stress distributions and the formulation of models for the prediction of downstream flow fields.

Chigier, N. A.↗