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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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All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

The Mode 1 mechanical crack tip stress field in hyperelastic and incompressible materials

The finite deformation field of a plane strain Mode 1 crack in a hyperelastic and incompressible material was examined under the assumptions of small scale nonlinearity. Finite element analyses were performed for two different material laws, a Neo-Hookean material and a third order invariant of a Rivlin material. The numerical results for both materials were compared to the appropriate theoretical asymptotic solution. A local cavitation locus surrounding the crack tip was identified for the Neo-Hookean material. For the third order invariant Rivlin material, maximum values of the dominant stress component were found close to the surface of the crack, above and below the deformed crack tip.

Quigley, Claudia J.↗
NASA NTRS1995

Buckling Analysis of Debonded Sandwich Panel Under Compression

A sandwich panel with initial through-the-width debonds is analyzed to study the buckling of its faceskin when subject to an in-plane compressive load. The debonded faceskin is modeled as a beam on a Winkler elastic foundation in which the springs of the elastic foundation represent the sandwich foam. The Rayleigh-Ritz and finite-difference methods are used to predict the critical buckling load for various debond lengths and stiffnesses of the sandwich foam. The accuracy of the methods is assessed with a plane-strain finite-element analysis. Results indicate that the elastic foundation approach underpredicts buckling loads for sandwich panels with isotropic foam cores.

Sleight, David W.↗
NASA NTRS1997

ZIP2DL: An Elastic-Plastic, Large-Rotation Finite-Element Stress Analysis and Crack-Growth Simulation Program

ZIP2DL is a two-dimensional, elastic-plastic finte element program for stress analysis and crack growth simulations, developed for the NASA Langley Research Center. It has many of the salient features of the ZIP2D program. For example, ZIP2DL contains five material models (linearly elastic, elastic-perfectly plastic, power-law hardening, linear hardening, and multi-linear hardening models), and it can simulate mixed-mode crack growth for prescribed crack growth paths under plane stress, plane strain and mixed state of stress conditions. Further, as an extension of ZIP2D, it also includes a number of new capabilities. The large-deformation kinematics in ZIP2DL will allow it to handle elastic problems with large strains and large rotations, and elastic-plastic problems with small strains and large rotations. Loading conditions in terms of surface traction, concentrated load, and nodal displacement can be applied with a default linear time dependence or they can be programmed according to a user-defined time dependence through a user subroutine. The restart capability of ZIP2DL will make it possible to stop the execution of the program at any time, analyze the results and/or modify execution options and resume and continue the execution of the program. This report includes three sectons: a theoretical manual section, a user manual section, and an example manual secton. In the theoretical secton, the mathematics behind the various aspects of the program are concisely outlined. In the user manual section, a line-by-line explanation of the input data is given. In the example manual secton, three types of examples are presented to demonstrate the accuracy and illustrate the use of this program.

Deng, Xiaomin↗
NASA NTRS1997

Determination of Stress Intensity Factor Distributions for "Interface" Cracks in Incompressible, Dissimilar Materials

The present study was undertaken in order to develop test methods and procedures for measuring the variation of the stress intensity factor through the thickness in bimaterial specimens containing cracks within and parallel to the bond line using the frozen stress photoelastic method. Since stress freezing materials are incompressible above critical temperature, and since thick plates are to be employed which tend to produce a state of plane strain near the crack tip, the interface near tip fracture equations reduce to the classic form for homogeneous materials. Moreover, zero thickness interfaces do not exist when materials are bonded together. It was decided early on that it would be important to insure a uniform straight and accurate crack tip region through the thickness of the body to reduce scatter in the SIF distribution through the thickness. It was also observed that rubberlike materials which were desired to be modeled exhibited significant tip blunting prior to crack extension and that some blunting of the tip would provide a more realistic model. It should be noted that, in normal stress freezing photoelastic work, it is considered good practice to avoid utilizing data near bond lines in photoelastic models due to the bond line stresses which inevitably develop when two parts are bonded together. Thus, the present study involves certain exploratory aspects in deviating from standard practice in stress freezing work. With the above ideas in mind, several different test methods were investigated and are described in the following sections and appendices. The geometry selected for the program was a thick, edge cracked specimen containing a bond line.

Smith, C. W.↗
NASA NTRS1999

Testing and Analysis of Composite Skin/Stringer Debonding Under Multi-Axial Loading

Damage mechanisms in composite bonded skin/stringer constructions under uniaxial and biaxial (in-plane/out- of-plane) loading conditions were examined. Specimens consisted of a tapered composite flange bonded onto a composite skin. Tests were performed under monotonic loading conditions in tension, three-point bending, and combined tension/bending . For combined tension/bending testing, a unique servohydraulic load frame was used that was capable of applying both in-plane tension and out-of-plane bending loads simultaneously. Specimen edges were examined on the microscope to document the damage occurrence and to identify typical damage patterns. The observations showed that, for all three load cases, failure initiated in the flange, near the flange tip, causing the flange to almost fully debond from the skin. A two-dimensional plane-strain finite element model was developed to analyze the different test cases using a geometrically nonlinear solution. For all three loading conditions, principal stresses exceeded the transverse strength of the material in the flange area. Additionally, delaminations of various lengths were simulated in two locations where delaminations were observed. The analyses showed that unstable delamination propagation is likely to occur in one location at the loads corresponding to matrix ply crack initiation for all three load cases.

Krueger, Ronald↗
NASA NTRS1999

Technology for Elevated Temperature Tests of Structural Panels

A technique for full-field measurement of surface temperature and in-plane strain using a single grid imaging technique was demonstrated on a sample subjected to thermally-induced strain. The technique is based on digital imaging of a sample marked by an alternating line array of La2O2S:Eu(+3) thermographic phosphor and chromium illuminated by a UV lamp. Digital images of this array in unstrained and strained states were processed using a modified spin filter. Normal strain distribution was determined by combining unstrained and strained grid images using a single grid digital moire technique. Temperature distribution was determined by ratioing images of phosphor intensity at two wavelengths. Combined strain and temperature measurements demonstrated on the thermally heated sample were DELTA-epsilon = +/- 250 microepsilon and DELTA-T = +/- 5 K respectively with a spatial resolution of 0.8 mm.

Thornton, E. A.↗
NASA NTRS1998

Sand Shear Band Thickness Measurements by Digital Imaging Techniques

Digital imaging analysis was used to study localized deformations in granular materials tested under plane strain condition. Two independent techniques were applied and compared. In the first, the digitized optical images of a grid printed on the latex membrane were used to measure the shear band orientation angle and thickness, and were found to be 54.5' and 3.01 mm respectively. The second technique involved introducing an ultra-low viscosity resin into the specimen in preparation for thin- sectioning and microscopic study of the internal fabric. A total of 24 microscopic images obtained from four thin sections were analyzed and void ratio variation was measured. The shear band thickness measurements from images located along the shear band axis (at two locations) were equal to 3.19 mm and 3.29 mm which are very close to the average value obtained from surface analysis. The study was then extended to investigate the effects of sand grain-size and properties, specimen density, and confining pressure on shear band thickness. It was found that the normalized shear band thickness decreases as grain-size and confining pressure increase and as density decreases. Finally, shear band thickness is highly influenced by the specimen dilatancy.

Alshibli, Khalid A.↗
NASA NTRS2000

Residual Strength Analyses of Riveted Lap-Splice Joints

The objective of this paper was to analyze the crack-linkup behavior in riveted-stiffened lap-splice joint panels with small multiple-site damage (MSD) cracks at several adjacent rivet holes. Analyses are based on the STAGS (STructural Analysis of General Shells) code with the critical crack-tip-opening angle (CTOA) fracture criterion. To account for high constraint around a crack front, the "plane strain core" option in STAGS was used. The importance of modeling rivet flexibility with fastener elements that accurately model load transfer across the joint is discussed. Fastener holes are not modeled but rivet connectivity is accounted for by attaching rivets to the sheet on one side of the cracks that simulated both the rivet diameter and MSD cracks. Residual strength analyses made on 2024-T3 alloy (1.6-mm thick) riveted-lap-splice joints with a lead crack and various size MSD cracks were compared with test data from Boeing Airplane Company. Analyses were conducted for both restrained and unrestrained buckling conditions. Comparison of results from these analyses and results from lap-splice-joint test panels, which were partially restrained against buckling indicate that the test results were bounded by the failure loads predicted by the analyses with restrained and unrestrained conditions.

Seshadri, B. R.↗
NASA NTRS2000

Testing and Analysis of Composite Skin/Stringer Debonding Under Multi-Axial Loading

A consistent step-wise approach is presented to investigate the damage mechanism in composite bonded skin/stringer constructions under uniaxial and biaxial (in-plane/out-of-plane) loading conditions. The approach uses experiments to detect the failure mechanism, computational stress analysis to determine the location of first matrix cracking and computational fracture mechanics to investigate the potential for delamination growth. In a first step, tests were performed on specimens, which consisted of a tapered composite flange, representing a stringer or frame, bonded onto a composite skin. Tests were performed under monotonic loading conditions in tension, three-point bending, and combined tension/bending to evaluate the debonding mechanisms between the skin and the bonded stringer. For combined tension/bending testing, a unique servohydraulic load frame was used that was capable of applying both in-plane tension and out-of-plane bending loads simultaneously. Specimen edges were examined on the microscope to document the damage occurrence and to identify typical damage patterns. For all three load cases, observed failure initiated in the flange, near the flange tip, causing the flange to almost fully debond from skin. In a second step, a two dimensional plane-strain finite element model was developed to analyze the different test cases using a geometrically nonlinear solution. For all three loading conditions, computed principal stresses exceeded the transverse strength of the material in those areas of the flange where the matrix cracks had developed during the tests. In a third step, delaminations of various lengths were simulated in two locations where delaminations were observed during the tests. The analyses showed that at the loads corresponding to matrix ply crack initiation computed strain energy release rates exceeded the values obtained from a mixed mode failure criterion in one location, Hence. Unstable delamination propagation is likely to occur as observed in the experiments.

Krueger, Ronald↗
NASA NTRS2001

A Review of the CTOA/CTOD Fracture Criterion: Why it Works

The CTOA/CTOD fracture criterion is one of the oldest fracture criteria applied to fracture of metallic materials with cracks. During the past two decades, the use of elastic-plastic finite-element analyses to simulate fracture of laboratory specimens and structural components using the CTOA criterion has expanded rapidly. But the early applications were restricted to two-dimensional analyses, assuming either plane-stress or plane-strain behavior, which lead to generally non-constant values of CTOA, especially in the early stages crack extension. Later, the non-constant CTOA values were traced to inappropriate state-of-stress (or constraint) assumptions in the crack-front region and severe crack tunneling in thin-sheet materials. More recently, the CTOA fracture criterion has been used with three-dimensional analyses to study constraint effects, crack tunneling, and the fracture process. The constant CTOA criterion (from crack initiation to failure) has been successfully applied to numerous structural applications, such as aircraft fuselages and pipelines. But why does the "constant CTOA" fracture criterion work so well? This paper reviews the results from several studies, discusses the issues of why CTOA works, and discusses its limitations.

Newman, J. C., Jr.↗
NASA NTRS2002

Reliability Testing of NASA Piezocomposite Actuators

NASA Langley Research Center has developed a low-cost piezocomposite actuator which has application for controlling vibrations in large inflatable smart space structures, space telescopes, and high performance aircraft. Tests show the NASA piezocomposite device is capable of producing large, directional, in-plane strains on the order of 2000 parts-per-million peak-to-peak, with no reduction in free-strain performance to 100 million electrical cycles. This paper describes methods, measurements, and preliminary results from our reliability evaluation of the device under externally applied mechanical loads and at various operational temperatures. Tests performed to date show no net reductions in actuation amplitude while the device was moderately loaded through 10 million electrical cycles. Tests were performed at both room temperature and at the maximum operational temperature of the epoxy resin system used in manufacture of the device. Initial indications are that actuator reliability is excellent, with no actuator failures or large net reduction in actuator performance.

Wilkie, W.↗
NASA NTRS2002

3D Finite Element Analysis of Particle-Reinforced Aluminum

Deformation in particle-reinforced aluminum has been simulated using three distinct types of finite element model: a three-dimensional repeating unit cell, a three-dimensional multi-particle model, and two-dimensional multi-particle models. The repeating unit cell model represents a fictitious periodic cubic array of particles. The 3D multi-particle (3D-MP) model represents randomly placed and oriented particles. The 2D generalized plane strain multi-particle models were obtained from planar sections through the 3D-MP model. These models were used to study the tensile macroscopic stress-strain response and the associated stress and strain distributions in an elastoplastic matrix. The results indicate that the 2D model having a particle area fraction equal to the particle representative volume fraction of the 3D models predicted the same macroscopic stress-strain response as the 3D models. However, there are fluctuations in the particle area fraction in a representative volume element. As expected, predictions from 2D models having different particle area fractions do not agree with predictions from 3D models. More importantly, it was found that the microscopic stress and strain distributions from the 2D models do not agree with those from the 3D-MP model. Specifically, the plastic strain distribution predicted by the 2D model is banded along lines inclined at 45 deg from the loading axis while the 3D model prediction is not. Additionally, the triaxial stress and maximum principal stress distributions predicted by 2D and 3D models do not agree. Thus, it appears necessary to use a multi-particle 3D model to accurately predict material responses that depend on local effects, such as strain-to-failure, fracture toughness, and fatigue life.

Shen, H.↗
NASA NTRS2004

Measuring Permeability of Composite Cryotank Laminants

This paper describes a test method developed to identify whether certain materials and material systems are suitable candidates for large pressurized reusable cryogenic tanks intended for use in current and future manned launch systems. It provides a quick way to screen numerous candidate materials for permeability under anticipated loading environments consistent with flight conditions, as well as addressing reusability issues. cryogenic tank, where the major design issue was hydrogen permeability. It was successfully used to evaluate samples subjected to biaxial loading while maintaining test temperatures near liquid hydrogen. After each sample was thermally preconditioned, a cyclic pressure load was applied to simulate the in-plane strain. First permeability was measured while a sample was under load. Then the sample was unloaded and allowed to return to ambient temperature. The test was repeated to simulate reusability, in order to evaluate its effects on material permeability.

Oliver, Stanley T.↗
NASA NTRS2001

A Curved, Elastostatic Boundary Element for Plane Anisotropic Structures

The plane-stress equations of linear elasticity are used in conjunction with those of the boundary element method to develop a novel curved, quadratic boundary element applicable to structures composed of anisotropic materials in a state of plane stress or plane strain. The curved boundary element is developed to solve two-dimensional, elastostatic problems of arbitrary shape, connectivity, and material type. As a result of the anisotropy, complex variables are employed in the fundamental solution derivations for a concentrated unit-magnitude force in an infinite elastic anisotropic medium. Once known, the fundamental solutions are evaluated numerically by using the known displacement and traction boundary values in an integral formulation with Gaussian quadrature. All the integral equations of the boundary element method are evaluated using one of two methods: either regular Gaussian quadrature or a combination of regular and logarithmic Gaussian quadrature. The regular Gaussian quadrature is used to evaluate most of the integrals along the boundary, and the combined scheme is employed for integrals that are singular. Individual element contributions are assembled into the global matrices of the standard boundary element method, manipulated to form a system of linear equations, and the resulting system is solved. The interior displacements and stresses are found through a separate set of auxiliary equations that are derived using an Airy-type stress function in terms of complex variables. The capabilities and accuracy of this method are demonstrated for a laminated-composite plate with a central, elliptical cutout that is subjected to uniform tension along one of the straight edges of the plate. Comparison of the boundary element results for this problem with corresponding results from an analytical model show a difference of less than 1%.

Smeltzer, Stanley S.↗
NASA NTRS2004

Residual Strength Characterization of a Curved Integrally-Stiffened Panel

Over the years, Finite-element fracture simulation methodology has been very well established at NASA Langley to predict the residual strength of damaged aircraft structures. This methodology has been experimentally verified at NASA Langley for structures ranging from laboratory coupons up to full-scale built-up structural components with single and multiple-site damage cracking. The methodology uses the critical crack-tip-opening-angle (CTOA) fracture criterion to characterize the fracture behavior of the material. The CTOA fracture criterion assumes that stable crack growth occurs when the crack-tip angle reaches a constant critical value. The use of the CTOA criterion requires an elastic-plastic, finite-element analysis. The critical CTOA value is determined by simulating fracture behavior in laboratory specimens, such as a compact specimen, to obtain the angle that best fits the observed test behavior. The critical CTOA value appears to be independent of loading, crack length, and in-plane dimensions. However, it is a function of material thickness and local crack-front constraint. Modeling the local constraint requires either a three-dimensional analysis or a two-dimensional analysis with an approximation to account for the constraint effects. In recent times as the aircraft industry is leaning towards monolithic structures with the intension of reducing part count and manufacturing cost, there has been a consistent effort at NASA Langley to extend critical CTOA based numerical methodology in the analysis of integrally-stiffened panels. In this regard, a series of fracture tests were conducted on curved aluminum-alloy integrally-stiffened panels. These curved panels were subjected to uniaxial tension and pressure loading. During the test, applied load-crack extension, out-of-plane displacements and local deformations around the crack tip region were measured. Compact and middle-crack tension specimens were tested to determine the critical angle (psi(sub c) using three-dimensional code (ZIP3D) and the plane-strain core height (h(sub c)) using two-dimensional code (STAGS).

Seshadri, B. R.↗
NASA NTRS2000

Testing and Analysis of Composite Skin/Stringer Debonding under Multi-Axial Loading

A consistent step-wise approach is presented to investigate the damage mechanism in composite bonded skin/stringer constructions under uniaxial and biaxial (in-plane/out-of-plane) loading conditions. The approach uses experiments to detect the failure mechanism, computational stress analysis to determine the location of first matrix cracking and computational fracture mechanics to investigate the potential for delamination growth. In a first step, tests were performed on specimens, which consisted of a tapered composite flange, representing a stringer or frame, bonded onto a composite skin. Tests were performed under monotonic loading conditions in tension, three-point bending, and combined tension/bending to evaluate the debonding mechanisms between the skin and the bonded stringer. For combined tension/bending testing, a unique servohydraulic load frame was used that was capable of applying both in-plane tension and out-of-plane bending loads simultaneously. Specimen edges were examined on the microscope to document the damage occurrence and to identify typical damage patterns. For all three load cases, observed failure initiated in the flange, near the flange tip, causing the flange to almost fully debond from the skin. In a second step, a two-dimensional plane-strain finite element model was developed to analyze the different test cases using a geometrically nonlinear solution. For all three loading conditions, computed principal stresses exceeded the transverse strength of the material in those areas of the flange where the matrix cracks had developed during the tests. In a third step, delaminations of various lengths were simulated in two locations where delaminations were observed during the tests. The analyses showed that at the loads corresponding to matrix ply crack initiation computed strain energy release rates exceeded the values obtained from a mixed mode failure criterion in one location. Hence, unstable delamination propagation is likely to occur as observed in the experiments.

Krueger, Ronald↗
NASA NTRS2010

Space Shuttle Orbiter Wing-Leading-Edge Panel Thermo-Mechanical Analysis for Entry Conditions

Linear elastic, thermo-mechanical stress analyses of the Space Shuttle Orbiter wing-leading-edge panels is presented for entry heating conditions. The wing-leading-edge panels are made from reinforced carbon-carbon and serve as a part of the overall thermal protection system. Three-dimensional finite element models are described for three configurations: integrated configuration, an independent single-panel configuration, and a local lower-apex joggle segment. Entry temperature conditions are imposed and the through-the-thickness response is examined. From the integrated model, it was concluded that individual panels can be analyzed independently since minimal interaction between adjacent components occurred. From the independent single-panel model, it was concluded that increased through-the-thickness stress levels developed all along the chord of a panel s slip-side joggle region, and hence isolated local joggle sections will exhibit the same trend. From the local joggle models, it was concluded that two-dimensional plane-strain models can be used to study the influence of subsurface defects along the slip-side joggle region of these panels.

Knight, Norman F., Jr.↗
NASA NTRS2010

Fracture Mechanics Analyses of Reinforced Carbon-Carbon Wing-Leading-Edge Panels

Fracture mechanics analyses of subsurface defects within the joggle regions of the Space Shuttle wing-leading-edge RCC panels are performed. A 2D plane strain idealized joggle finite element model is developed to study the fracture behavior of the panels for three distinct loading conditions - lift-off and ascent, on-orbit, and entry. For lift-off and ascent, an estimated bounding aerodynamic pressure load is used for the analyses, while for on-orbit and entry, thermo-mechanical analyses are performed using the extreme cold and hot temperatures experienced by the panels. In addition, a best estimate for the material stress-free temperature is used in the thermo-mechanical analyses. In the finite element models, the substrate and coating are modeled separately as two distinct materials. Subsurface defects are introduced at the coating-substrate interface and within the substrate. The objective of the fracture mechanics analyses is to evaluate the defect driving forces, which are characterized by the strain energy release rates, and determine if defects can become unstable for each of the loading conditions.

Raju, Ivatury S.↗