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NASA NTRS1989

A review of some problems in global-local stress analysis

The various types of local-global finite-element problems point out the need to develop a new generation of software. First, this new software needs to have a complete analysis capability, encompassing linear and nonlinear analysis of 1-, 2-, and 3-dimensional finite-element models, as well as mixed dimensional models. The software must be capable of treating static and dynamic (vibration and transient response) problems, including the stability effects of initial stress, and the software should be able to treat both elastic and elasto-plastic materials. The software should carry a set of optional diagnostics to assist the program user during model generation in order to help avoid obvious structural modeling errors. In addition, the program software should be well documented so the user has a complete technical reference for each type of element contained in the program library, including information on such topics as the type of numerical integration, use of underintegration, and inclusion of incompatible modes, etc. Some packaged information should also be available to assist the user in building mixed-dimensional models. An important advancement in finite-element software should be in the development of program modularity, so that the user can select from a menu various basic operations in matrix structural analysis.

Nelson, Richard B.↗

NASA NTRS1983

Wave envelope and infinite element schemes for fan noise radiation from turbofan inlets

Finite element models are presented for the calculation of radiated fan noise in the vicinity of turbofan inlets. The models involve the use of conventional axisymmetric finite elements in an inner region close to the inlet. The far field is represented by infinite elements or wave envelope elements. Theory and results are presented for the case with no mean flow. Comparisons of computed data with analytic solutions and measured values establish the utility of both the infinite element and wave envelope element schemes. The wave envelope scheme is shown to be effective also in the far field. Both schemes use meshes which are much sparser than would be required in conventional numerical discretizations and may be applied to real inlet configurations at modest computational cost.

Astley, R. J.↗

NASA NTRS2019

Shape Sensing for Wings with Spars and Ribs Using Simulated Strain

Active trim shape control can be used to minimize error between target and actual aircraft trim shape during flight. Trim shape sensing for aircraft during flight is not only important for highly flexible aircraft, such as the National Aeronautics and Space Administration (NASA) Helios Prototype remotely piloted flying wing aircraft, but also for a delta-wing type aircraft, such as a supersonic commercial transport aircraft. A two-step theory utilizing distributed strain for a real-time shape sensing of a full three-dimensional structure has been introduced previously. This study focuses on the application of the two-step theory to finite element models of a wing with spars and ribs such as the X-59 QueSST aircraft (Lockheed Martin Corporation, Bethesda, Maryland), a tapered wing, a dihedral/anhedral wing, and a stiffened dihedral/anhedral wing. A finely meshed finite element structural model is desired to capture accurate curvature distributions along the neutral axes of the wing cross sections during pre-test analysis for shape sensing of a wing with ribs and spars. The two-step theory used in this study gives excellent deformation correlation with the MSC/NASTRAN (MSC Software, Newport Beach, California) results along the neutral axis for all test cases used in this study except the X-59 QueSST aircraft.

strain↗

NASA NTRS1994

HRMA calibration handbook: EKC gravity compensated XRCF models

This document, consisting of hardcopy printout of explanatory text, figures, and tables, represents one incarnation of the AXAF high resolution mirror assembly (HRMA) Calibration Handbook. However, as we have envisioned it, the handbook also consists of electronic versions of this hardcopy printout (in the form of postscript files), the individual scripts which produced the various figures and the associated input data, the model raytrace files, and all scripts, parameter files, and input data necessary to generate the raytraces. These data are all available electronically as either ASCII or FITS files. The handbook is intended to be a living document and will be updated as new information and/or fabrication data on the HRMA are obtained, or when the need for additional results are indicated. The SAO Mission Support Team (MST) is developing a high fidelity HRMA model, consisting of analytical and numerical calculations, computer software, and databases of fundamental physical constants, laboratory measurements, configuration data, finite element models, AXAF assembly data, and so on. This model serves as the basis for the simulations presented in the handbook. The 'core' of the model is the raytrace package OSAC, which we have substantially modified and now refer to as SAOsac. One major structural modification to the software has been to utilize the UNIX binary pipe data transport mechanism for passing rays between program modules. This change has made it possible to simulate rays which are distributed randomly over the entrance aperture of the telescope. It has also resulted in a highly efficient system for tracing large numbers of rays. In one application to date (the analysis of VETA-I ring focus data) we have employed 2 x 10(exp 7) rays, a substantial improvement over the limit of 1 x 10(exp 4) rays in the original OSAC module. A second major modification is the manner in which SAOsac incorporates low spatial frequency surface errors into the geometric raytrace. The original OSAC included the ability to use Legendre-Fourier polynomials to describe deviations from the basic optical prescription. To this we have added bicubic splines to address a deficiency in the handling of the sharper deformations in the areas of mirror support pads. SAO has developed software (TRANS-FIT) to translate the most common finite element analysis models into these forms for incorporation into the raytrace program.

Tananbaum, H. D.↗

NASA NTRS1997

Test Verification of the Cassini Spacecraft Dynamic Model

The Cassini spacecraft mission to Saturn will be launched in October 1997, on a Titan IV/Centaur launch vehicle. Cassini is the largest interplanetary spacecraft ever developed. Before launch approval can be obtained, a test verified finite element model of the Cassini spacecraft must be completed and approved by NASA. The correctness of this model is critical to the final verification coupled loads analysis and margin of safety assessments.

Cassini↗

NASA NTRS1992

Hierarchical flux-based thermal-structural finite element analysis method

A hierarchical flux-based finite element method is developed for both a one and two dimensional thermal structural analyses. Derivation of the finite element equations is presented. The resulting finite element matrices associated with the flux based formulation are evaluated in a closed form. The hierarchical finite elements include additional degrees of freedom in the approximation of the element variable distributions by the use of nodeless variables. The nodeless variables offer increased solution accuracy without the need for defining actual nodes and rediscretizing the finite element model. Thermal and structural responses are obtained from a conventional linear finite element method and exact solutions. Results show that the hierarchical flux-based method can provide improved thermal and structural solution accuracy with fewer elements when compared to results for the conventional linear element method.

Polesky, Sandra P.↗

NASA NTRS2002

Effect of Magnetic Fields on g-jitter Induced Convection and Solute Striation During Space Processing of Single Crystals

A 2-D finite element model is presented for the melt growth of single crystals in a microgravity environment with a superimposed DC magnetic field. The model is developed based on the deforming finite element methodology and is capable of predicting the phenomena of the steady and transient convective flows, heat transfer, solute distribution, and solid-liquid interface morphology associated with the melt growth of single crystals in microgravity with and without an applied magnetic field. Numerical simulations were carried out for a wide range of parameters including idealized microgravity conditions, the synthesized g-jitter and the real g-jitter data taken by on-board accelerometers during space flights. The results reveal that the time varying g-jitter disturbances, although small in magnitude, cause an appreciable convective flow in the liquid pool, which in turn produces detrimental effects during the space processing of single crystal growth. An applied magnetic field of appropriate strength, superimposed on microgravity, can be very effective in suppressing the deleterious effects resulting from the g-jitter disturbances.

deGroh, H. C.↗

NASA NTRS2006

Dynamic Crush Characterization of Ice

During the space shuttle return-to-flight preparations following the Columbia accident, finite element models were needed that could predict the threshold of critical damage to the orbiter's wing leading edge from ice debris impacts. Hence, an experimental program was initiated to provide crushing data from impacted ice for use in dynamic finite element material models. A high-speed drop tower was configured to capture force time histories of ice cylinders for impacts up to approximately 100 ft/s. At low velocity, the force-time history depended heavily on the internal crystalline structure of the ice. However, for velocities of 100 ft/s and above, the ice fractured on impact, behaved more like a fluid, and the subsequent force-time history curves were much less dependent on the internal crystalline structure.

Fasanella, Edwin L.↗

NASA NTRS2006

Test and Analysis Correlation of High Speed Impacts of Ice Cylinders

During the space shuttle return-to-flight preparations following the Columbia accident, finite element models were needed that could predict the threshold of critical damage to the orbiter s wing leading edge from ice debris impacts. Hence, an experimental program was initiated to provide crushing data from impacted ice for use in dynamic finite element material models. A high-speed drop tower was configured to capture force time-histories of ice cylinders for impacts up to approximately 100 ft/s. At low velocity, the force-time history depended heavily on the internal crystalline structure of the ice. However, for velocities of 100 ft/s and above, the ice fractured on impact, behaved more like a fluid, and the subsequent force-time history curves were much less dependent on the internal crystalline structure.

Fasanella, Edwin L.↗

NASA NTRS1995

A probabilistic model of a porous heat exchanger

This paper presents a probabilistic one-dimensional finite element model for heat transfer processes in porous heat exchangers. The Galerkin approach is used to develop the finite element matrices. Some of the submatrices are asymmetric due to the presence of the flow term. The Neumann expansion is used to write the temperature distribution as a series of random variables, and the expectation operator is applied to obtain the mean and deviation statistics. To demonstrate the feasibility of the formulation, a one-dimensional model of heat transfer phenomenon in superfluid flow through a porous media is considered. Results of this formulation agree well with the Monte-Carlo simulations and the analytical solutions. Although the numerical experiments are confined to parametric random variables, a formulation is presented to account for the random spatial variations.

Agrawal, O. P.↗

NASA NTRS2012

Modeling of Damage Initiation and Progression in a SiC/SiC Woven Ceramic Matrix Composite

The goal of an ongoing project at NASA Glenn is to investigate the effects of the complex microstructure of a woven ceramic matrix composite and its variability on the effective properties and the durability of the material. Detailed analysis of these complex microstructures may provide clues for the material scientists who `design the material? or to structural analysts and designers who `design with the material? regarding damage initiation and damage propagation. A model material system, specifically a five-harness satin weave architecture CVI SiC/SiC composite composed of Sylramic-iBN fibers and a SiC matrix, has been analyzed. Specimens of the material were serially sectioned and polished to capture the detailed images of fiber tows, matrix and porosity. Open source analysis tools were used to isolate various constituents and finite elements models were then generated from simplified models of those images. Detailed finite element analyses were performed that examine how the variability in the local microstructure affected the macroscopic behavior as well as the local damage initiation and progression. Results indicate that the locations where damage initiated and propagated is linked to specific microstructural features.

Mital, Subodh K.↗

NASA NTRS1977

Low cost silicon solar array project large area silicon sheet task: Silicon web process development

Growth configurations were developed which produced crystals having low residual stress levels. The properties of a 106 mm diameter round crucible were evaluated and it was found that this design had greatly enhanced temperature fluctuations arising from convection in the melt. Thermal modeling efforts were directed to developing finite element models of the 106 mm round crucible and an elongated susceptor/crucible configuration. Also, the thermal model for the heat loss modes from the dendritic web was examined for guidance in reducing the thermal stress in the web. An economic analysis was prepared to evaluate the silicon web process in relation to price goals.

Duncan, C. S.↗

NASA NTRS1985

Computer simulation of plasma electron collection by PIX-II

A wake model was defined for the NASCAP/LEO finite element model for the plasma interaction experiment (PIX-II) launched to study the interaction between high-voltage large solar arrays with the space plasma environment. The cell surface model considers the individual cells, distances between interconnects, and the fraction of surface covered by interconnects. Account is taken of the electrostatic potential around the spacecraft, which travels at 7500 mps, over five times the speed of thermal ions. Ram ions are produced ahead of the array and the wake ion density is described with a geometric shadowing model. The model correctly predicted the currents in high and low bias voltages when compared to orbital data. The panel snapover, however, was projected to occur at 100 V and instead occurred at 300 V, which indicates that the snapover state is bistable. Finally, a low potential was both predicted and measured in the wake.

Mandell, M. J.↗

NASA NTRS1991

An enhanced projection and assembly model reduction methodology

An enhanced projection and assembly (P and A) method employing static correction modes to either the retained mode set of the system or the projected mode sets of the components is presented. The effectiveness of the proposed technique was successfully demonstrated on a 13-DOF mass-spring model and a high-order finite element model of the Galileo spacecraft. When applied to the Galileo cruise model, the system-level augmented P and A method significantly alleviates the zero-mismatch problem found in previous studies.

Lee, Allan Y.↗

NASA NTRS1991

H2-optimal control with generalized state-space models for use in control-structure optimization

Several advances are provided solving combined control-structure optimization problems. The author has extended solutions from H2 optimal control theory to the use of generalized state space models. The generalized state space models preserve the sparsity inherent in finite element models and hence provide some promise for handling very large problems. Also, expressions for the gradient of the optimal control cost are derived which use the generalized state space models.

Wette, Matt↗

NASA NTRS2004

Reduced-Order Modeling and Wavelet Analysis of Turbofan Engine Structural Response Due to Foreign Object Damage (FOD) Events

The development of a wavelet-based feature extraction technique specifically targeting FOD-event induced vibration signal changes in gas turbine engines is described. The technique performs wavelet analysis of accelerometer signals from specified locations on the engine and is shown to be robust in the presence of significant process and sensor noise. It is envisioned that the technique will be combined with Kalman filter thermal/health parameter estimation for FOD-event detection via information fusion from these (and perhaps other) sources. Due to the lack of high-frequency FOD-event test data in the open literature, a reduced-order turbofan structural model (ROM) was synthesized from a finite element model modal analysis to support the investigation. In addition to providing test data for algorithm development, the ROM is used to determine the optimal sensor location for FOD-event detection. In the presence of significant noise, precise location of the FOD event in time was obtained using the developed wavelet-based feature.

Turso, James↗

NASA NTRS1993

Mutual Inductance Problem for a System Consisting of a Current Sheet and a Thin Metal Plate

Rapid inspection of aircraft structures for flaws is of vital importance to the commercial and defense aircraft industry. In particular, inspecting thin aluminum structures for flaws is the focus of a large scale R&D effort in the nondestructive evaluation (NDE) community. Traditional eddy current methods used today are effective, but require long inspection times. New electromagnetic techniques which monitor the normal component of the magnetic field above a sample due to a sheet of current as the excitation, seem to be promising. This paper is an attempt to understand and analyze the magnetic field distribution due to a current sheet above an aluminum test sample. A simple theoretical model, coupled with a two dimensional finite element model (FEM) and experimental data will be presented in the next few sections. A current sheet above a conducting sample generates eddy currents in the material, while a sensor above the current sheet or in between the two plates monitors the normal component of the magnetic field. A rivet or a surface flaw near a rivet in an aircraft aluminum skin will disturb the magnetic field, which is imaged by the sensor. Initial results showed a strong dependence of the flaw induced normal magnetic field strength on the thickness and conductivity of the current-sheet that could not be accounted for by skin depth attenuation alone. It was believed that the eddy current imaging method explained the dependence of the thickness and conductivity of the flaw induced normal magnetic field. Further investigation, suggested the complexity associated with the mutual inductance of the system needed to be studied. The next section gives an analytical model to better understand the phenomenon.

Fulton, J. P.↗

NASA NTRS2004

Integrated Modeling Activities for the James Webb Space Telescope: Structural-Thermal-Optical Analysis

The James Web Space Telescope (JWST) is a large, infrared-optimized space telescope scheduled for launch in 2011. This is a continuation of a series of papers on modeling activities for JWST. The structural-thermal-optical, often referred to as STOP, analysis process is used to predict the effect of thermal distortion on optical performance. The benchmark STOP analysis for JWST assesses the effect of an observatory slew on wavefront error. Temperatures predicted using geometric and thermal math models are mapped to a structural finite element model in order to predict thermally induced deformations. Motions and deformations at optical surfaces are then input to optical models, and optical performance is predicted using either an optical ray trace or a linear optical analysis tool. In addition to baseline performance predictions, a process for performing sensitivity studies to assess modeling uncertainties is described.

Johnston, John D.↗