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At least 109 records · Page 6

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NASA NTRS2004

Modeling the Nonlinear, Strain Rate Dependent Deformation of Shuttle Leading Edge Materials with Hydrostatic Stress Effects Included

An analysis method based on a deformation (as opposed to damage) approach has been developed to model the strain rate dependent, nonlinear deformation of woven ceramic matrix composites, such as the Reinforced Carbon Carbon (RCC) material used on the leading edges of the Space Shuttle. In the developed model, the differences in the tension and compression deformation behaviors have also been accounted for. State variable viscoplastic equations originally developed for metals have been modified to analyze the ceramic matrix composites. To account for the tension/compression asymmetry in the material, the effective stress and effective inelastic strain definitions have been modified. The equations have also been modified to account for the fact that in an orthotropic composite the in-plane shear response is independent of the stiffness in the normal directions. The developed equations have been implemented into LS-DYNA through the use of user defined subroutines (UMATs). Several sample qualitative calculations have been conducted, which demonstrate the ability of the model to qualitatively capture the features of the deformation response present in woven ceramic matrix composites.

Goldberg, Robert K.↗

NASA NTRS1977

Comparison of linear inversion methods by examination of the duality between iterative and inverse matrix methods

Linear numerical inversion methods applied to atmospheric remote sounding generally can be categorized in two ways: (1) iterative, and (2) inverse matrix methods. However, these two categories are not unrelated; a duality exists between them. In other words, given an iterative scheme, a corresponding inverse matrix method exists, and conversely. This duality concept is developed for the more familiar linear methods. The iterative duals are compared with the classical linear iterative approaches and their differences analyzed. The importance of the initial profile in all methods is stressed. Calculations using simulated data are made to compare accuracies and to examine the dependence of the solution on the initial profile.

Fleming, H. E.↗

NASA NTRS1989

Composite blade structural analyzer (COBSTRAN) user's manual

The installation and use of a computer code, COBSTRAN (COmposite Blade STRuctrual ANalyzer), developed for the design and analysis of composite turbofan and turboprop blades and also for composite wind turbine blades was described. This code combines composite mechanics and laminate theory with an internal data base of fiber and matrix properties. Inputs to the code are constituent fiber and matrix material properties, factors reflecting the fabrication process, composite geometry and blade geometry. COBSTRAN performs the micromechanics, macromechanics and laminate analyses of these fiber composites. COBSTRAN generates a NASTRAN model with equivalent anisotropic homogeneous material properties. Stress output from NASTRAN is used to calculate individual ply stresses, strains, interply stresses, thru-the-thickness stresses and failure margins. Curved panel structures may be modeled providing the curvature of a cross-section is defined by a single value function. COBSTRAN is written in FORTRAN 77.

Aiello, Robert A.↗

NASA NTRS1993

Analysis of delamination in cross-ply laminates initiating from impact induced matrix cracking

Two-dimensional finite element analyses of (02/90(8)/02) glass/epoxy and graphite/epoxy composite laminates were performed to investigate some of the characteristics of damage development due to an impact load. A cross section through the thickness of the laminate with fixed ends, and carrying a transverse load in the center, was analyzed. Inclined matrix cracks, such as those produced by a low-velocity impact, were modeled in the 90 deg ply group. The introduction of the matrix cracks caused large interlaminar tensile and shear stresses in the vicinity of both crack tips in the 0/90 and 90/0 interfaces, indicating that matrix cracking may give rise to delamination. The ratio of Mode I to total strain energy release rate, G(I)/G(total), at the beginning of delamination, calculated at the two (top and bottom) matrix crack tips was 60 and 28 percent, respectively, in the glass/epoxy laminate. The corresponding ratio was 97 and 77 percent in the graphite/epoxy laminate. Thus, a significant Mode I component of strain energy release rate may be present at the delamination initiation due to an impact load. The value of strain energy release rate at either crack tip increased due to an increase in the delamination length at the other crack tip and may give rise to an unstable delamination growth under constant load.

Salpekar, S. A.↗

NASA NTRS1990

Integrated modeling and analysis of a space-truss article

MSC/NASTRAN is being used in the Controls-Structures Interaction (CSI) program at NASA Langley Research Center as a key analytical tool for structural analysis as well as the basis for control law development, closed-loop performance evaluation, and system safety checks. Guest investigators from academia and industry are performing dynamics and control experiments on a flight-like deployable space truss called Mini-Mast to determine the effectiveness of various active-vibration control laws. MSC/NASTRAN was used to calculate natural frequencies and mode shapes below 100 Hz to describe the dynamics of the 20-meter-long lightweight Mini-Mast structure. Gravitational effects contribute significantly to structural stiffness and are accounted for through a two-phase solution in which the differential stiffness matrix is calculated and then used in the eigensolution. Reduced modal models are extracted for control law design and evaluation of closed-loop system performance. Predicted actuator forces from controls simulations are then applied to the extended model to predict member loads and stresses. These pre-test analyses reduce risks associated with the structural integrity of the test article, which is a major concern in closed-loop control experiments due to potential instabilities.

Alan E Stockwell↗

NASA NTRS2005

NASALife-Component Fatigue and Creep Life Prediction Program and Illustrative Examples

NASALife is a life prediction program for propulsion system components made of ceramic matrix composites (CMC) under cyclic thermo-mechanical loading and creep rupture conditions. Although, the primary focus was for CMC components the underlying methodologies are equally applicable to other material systems as well. The program references data for low cycle fatigue (LCF), creep rupture, and static material properties as part of the life prediction process. Multiaxial stresses are accommodated by Von Mises based methods and a Walker model is used to address mean stress effects. Varying loads are reduced by the Rainflow counting method. Lastly, damage due to cyclic loading (Miner s rule) and creep are combined to determine the total damage per mission and the number of missions the component can survive before failure are calculated. Illustration of code usage is provided through example problem of a CMC turbine stator vane made of melt-infiltrated, silicon carbide fiber-reinforced, silicon carbide matrix composite (MI SiC/SiC)

Murthy, Pappu L. N.↗

NASA NTRS2000

Equivalent Skin Analysis of Wing Structures Using Neural Networks

An efficient method of modeling trapezoidal built-up wing structures is developed by coupling. in an indirect way, an Equivalent Plate Analysis (EPA) with Neural Networks (NN). Being assumed to behave like a Mindlin-plate, the wing is solved using the Ritz method with Legendre polynomials employed as the trial functions. This analysis method can be made more efficient by avoiding most of the computational effort spent on calculating contributions to the stiffness and mass matrices from each spar and rib. This is accomplished by replacing the wing inner-structure with an "equivalent" material that combines to the skin and whose properties are simulated by neural networks. The constitutive matrix, which relates the stress vector to the strain vector, and the density of the equivalent material are obtained by enforcing mass and stiffness matrix equities with rec,ard to the EPA in a least-square sense. Neural networks for the material properties are trained in terms of the design variables of the wing structure. Examples show that the present method, which can be called an Equivalent Skin Analysis (ESA) of the wing structure, is more efficient than the EPA and still fairly good results can be obtained. The present ESA is very promising to be used at the early stages of wing structure design.

Liu, Youhua↗

NASA NTRS1986

Ultrasonic determination of the elastic constants of the stiffness matrix for unidirectional fiberglass epoxy composites

The elastic constants of a fiberglass epoxy unidirectional composite are determined by measuring the phase velocities of longitudinal and shear stress waves via the through transmission ultrasonic technique. The waves introduced into the composite specimens were generated by piezoceramic transducers. Geometric lengths and the times required to travel those lengths were used to calculate the phase velocities. The model of the transversely isotropic medium was adopted to relate the velocities and elastic constants.

Marques, E. R. C.↗

NASA NTRS1988

A hybrid-stress finite element for linear anisotropic elasticity

Standard assumed displacement finite elements with anisotropic material properties perform poorly in complex stress fields such as combined bending and shear and combined bending and torsion. A set of three dimensional hybrid-stress brick elements were developed with fully anisotropic material properties. Both eight-node and twenty-node bricks were developed based on the symmetry group theory of Punch and Atluri. An eight-node brick was also developed using complete polynomials and stress basis functions and reducing the order of the resulting stress parameter matrix by applying equilibrium constraints and stress compatibility constraints. Here the stress compatibility constraints must be formulated assuming anisotropic material properties. The performance of these elements was examined in numerical examples covering a broad range of stress distributions. The stress predictions show significant improvement over the assumed displacement elements but the calculation time is increased.

Fly, Gerald W.↗

NASA NTRS2018

Influence of Fiber Content, Thermal Cycles and Creep on Electrical Resistivity Measurements of SiCf/SiC Minicomposites

Fiber-reinforced Ceramic Matrix Composites (CMCs) are currently being used in high-temperature applications such as jet engine components. In such extreme environment applications, materials are exposed to many mechanical and thermal loading cycles during the life of the component. This exposure can degrade the component, and thus, components need to be monitored and inspected during service. The electrical resistivity nondestructive evaluation technique has been shown to be very sensitive to damage in CMCs, and although its performance is highly dependent on temperature, this temperature-dependence is still not well understood. Therefore, this current study aims to establish the influence of thermal cycles, creep, CVI-SiC matrix cracks and fiber volume fraction on the electrical resistance of Hi Nicalon and Hi-Nicalon Type S SiC fiber-reinforced minicomposites with BN interphases. Minicomposites with 97% CVI-SiC matrix volume fraction were used to characterize the electrical resistivity of the CVI-SiC matrix. Next, models were constructed to calculate fiber resistivity and onset and evolution of damage during tensile testing at room-temperature. The thermal cycles were achieved by heating as-received and precracked specimens to 1200Â°C and then cooling them to room temperature, with or without applied stress on fibers.

creep↗

NASA NTRS1982

Mechanics aspects of NDE by sound and ultrasound

Nondestructive evaluation (NDE) is considered as a means to detect the energy release mechanism of defects and the interaction of microstructures within materials with sound waves and/or ultrasonic waves. Ultrasonic inspection involves the frequency range 20 kHz-1 GHz with amplitudes depending on the sensitivity of the test instrumentation. Pulse echo systems are most frequently used in NDE. Information is extracted from the signals through measurements of the signal velocity, attenuation, the acoustic emission when stress is applied, and calculation of the acoustoelastic coefficients. Fracture properties, tensile and shear strengths, the interlaminar shear strength, the cohesive strength, yield and impact strengths, the hardness, and the residual stress can be assayed by ultrasonic methods. Finally, attention is given to analytical treatment of the derived data, with mention given to transition matrix, integral equation, and eigenstrain approaches.

Fu, L. S.↗

NASA NTRS1972

On fracture phenomena in advanced fiber composite materials.

The extension of linear elastic fracture mechanics (LEFM) from metallic alloys to advanced fiber composite laminates is considered. LEFM is shown to be valid for both isotropic and anisotropic homogeneous continua; the applicability of LEFM to advanced fiber composites is thus dependent on the validity of a homogeneous model of such materials. An experimental program to determine the validity of such a model for graphite/epoxy laminates is reviewed. Such laminates are found to have an apparent fracture toughness, from which it is inferred that a homogeneous material model is valid for the particular specimen geometry and composite laminates considered. Strain energy release rates are calculated from the experimentally determined fracture toughness of the various laminates. These strain energy release rates are found to lie in one of two groups, depending upon whether crack extension required fiber failure or matrix failure. The latter case is further investigated. It is concluded that matrix failure is governed by the tensile stress normal to the crack path.

Konish, H. J., Jr.↗

NASA NTRS1988

On the viscoplastic response of a composite wafer

In the present treatment of a viscoplastic composite wafer formed from a viscoplastic matrix that is reinforced by elastic or viscoplastic fibers attached to its plane surfaces, one constitutive equation is established by considering the viscoplastic behavior of the matrix as determined by an integral-type constitutive law whose intrinsic time-measure is pertinent to endochronic viscoplasticity. Attention is given to asymptotic cases where fiber stiffnesses and the hydrostatic modulus of the wafer are much larger than the latter's shear modulus. An explicit calculation is used when the stress field is uniaxial.

Valanis, K. C.↗

NASA NTRS1986

Temperature dependent nonlinear metal matrix laminae behavior

An analytical method is described for computing the nonlinear thermal and mechanical response of laminated plates. The material model focuses upon the behavior of metal matrix materials by relating the nonlinear composite response to plasticity effects in the matrix. The foundation of the analysis is the unidirectional material model which is used to compute the instantaneous properties of the lamina based upon the properties of the fibers and matrix. The unidirectional model assumes that the fibers properties are constant with temperature and assumes that the matrix can be modelled as a temperature dependent, bilinear, kinematically hardening material. An incremental approach is used to compute average stresses in the fibers and matrix caused by arbitrary mechanical and thermal loads. The layer model is incorporated in an incremental laminated plate theory to compute the nonlinear response of laminated metal matrix composites of general orientation and stacking sequence. The report includes comparisons of the method with other analytical approaches and compares theoretical calculations with measured experimental material behavior. A section is included which describes the limitations of the material model.

Barrett, D. J.↗

NASA NTRS1991

Triangular Element For Analyzing Elasticity Of Laminates

Flat triangular element developed for use in finite-element analyses of stress and strain in laminated plates made of such materials as plywood or advanced fiber/epoxy composite materials. Has multiple layers, each of which can have different isotropic or orthotropic elastic properties. Many such elements used in finite-element mesh to calculate stiffness of plate. Formulation of element straight-forward, and calculation of its stiffness matrix simple and fast.

Martin, C. Wayne↗

NASA NTRS1990

Micro-mechanical analysis of damage growth and fracture in discontinuous fiber reinforced metal matrix composites

The near-crack-tip stresses in any planar coupon of arbitrary geometry subjected to mode 1 loading may be equated to those in an infinite center-cracked panel subjected to the appropriate equivalent remote biaxial stresses (ERBS). Since this process can be done for all such mode 1 coupons, attention may be focused on the behavior of the equivalent infinite cracked panel. To calculate the ERBS, the constant term in the series expansion of the crack-tip stress must be retained. It is proposed that the ERBS may be used quantitatively to explain different fracture phenomena such as crack branching.

Goree, James G.↗

NASA NTRS1989

Isothermal life prediction of composite lamina using a damage mechanics approach

A method for predicting isothermal plastic fatigue life of a composite lamina is presented in which both fibers and matrix are isotropic materials. In general, the fatigue resistances of the matrix, fibers, and interfacial material must be known in order to predict composite fatigue life. Composite fatigue life is predicted using only the matrix fatigue resistance due to inelasticity micromechanisms. The effect of the fiber orientation on loading direction is accounted for while predicting composite life. The application is currently limited to isothermal cases where the internal thermal stresses that might arise from thermal strain mismatch between fibers and matrix are negligible. The theory is formulated to predict the fatigue life of a composite lamina under either load or strain control. It is applied currently to predict the life of tungsten-copper composite lamina at 260 C under tension-tension load control. The calculated life of the lamina is in good agreement with available composite low cycle fatigue data.

Abuelfoutouh, Nader M.↗

NASA NTRS1989

Isothermal life prediction of composite lamina using a damage mechanics approach

Abuelfoutouh, Nader Mohamed↗