Walter H. Gerstle

Non-Linear Fracture Models for Discrete Crack Propagation

The proper fracture mechanics to be applied to crack propagation in concrete is determined by scale effects.

SAR-Based Vibration Estimation Using the Discrete Fractional Fourier Transform

A vibration estimation method for synthetic aperture radar (SAR) is presented based on a novel application of the discrete fractional Fourier transform (DFRFT). Small vibrations of ground targets introduce phase modulation in the SAR returned signals. With standard preprocessing of the returned signals, followed by the application of the DFRFT, the time-varying accelerations, frequencies, and displacements associated with vibrating objects can be extracted by successively estimating the quasi-instantaneous chirp rate in the phase-modulated signal in each subaperture. The performance of the proposed method is investigated quantitatively, and the measurable vibration frequencies and displacements are determined. Simulation results show that the proposed method can successfully estimate a two-component vibration at practical signal-to-noise levels. Two airborne experiments were also conducted using the Lynx SAR system in conjunction with vibrating ground test targets. The experiments demonstrated the correct estimation of a 1-Hz vibration with an amplitude of 1.5 cm and a 5-Hz vibration with an amplitude of 1.5 mm.

Crack Growth in Flexural Members--A Fracture Mechanics Approach

ACI 318 contains several empirically based provisions regarding cracking and minimum flexural reinforcement of concrete beams and slabs. These provisions can be significantly improved by application of fracture mechanics principles. Rational analysis methods for concrete beams, which demonstrate a size effect, are developed using a simplification of the Hillerborg fictitious crack model. Using fracture mechanics concepts, a method is developed to predict not only crack widths and lengths, but also the strength and cracking stability of reinforced concrete beams. Design equations and charts are presented.

Finite and boundary element modeling of crack propagation in two and three dimensions

Computer modeling of mixed-mode crack propagation has rarely been attempted. This is because of the difficulty in updating the geometrical description to represent the changing crack geometry. The development of two interactive, graphical fracture propagation systems is described here. The Finite Element Fracture Analysis Program—Graphical (FEFAP-G) is a two-dimensional fracture propagation system. The BEM3D is a three-dimensional boundary element fracture propagation system.In addition, the implementation of the BEM3D analysis program in a configuration formed by an FPS-264 processor attached to a VAX-11/750 used as host computer is described. The results show that a realistic three-dimensional boundary element analysis of crack propagation is not only feasible with the aid of attached processors, but it can have its total time reduced by factors of the order of hundreds when compared to VAX alone statistics.In an example problem concerning fatigue crack propagation in a stiffened wing skin, both FEFAP-G and the BEM3D are employed to illustrate the utility of the fracture propagation systems.

Justification of ACI 446 Proposal for Updating ACI Code Provisions for Shear Design of Reinforced Concrete Beams

In order to ensure adequate safety margins, the size effect for designing reinforced concrete beams against shear failure must be incorporated into American Concrete Institute (ACI) code provisions. A purely empirical approach is impossible because the available test data, mostly obtained on small beams, need to be extrapolated to much larger beams for which tests are scant or nonexistent. This paper summarizes arguments for an improved code formulation, and reviews verification by a database compiled by Joint ACI-ASCE Committee 445 which shows that the current code is unconservative for large beams. Currently, concrete design experts are debating which of several alternative formulas for size effect provides the safest extrapolation for very large beams.

Reduction of Vibration-Induced Artifacts in Synthetic Aperture Radar Imagery

Target vibrations introduce nonstationary phase modulation, which is termed the micro-Doppler effect, into returned synthetic aperture radar (SAR) signals. This causes artifacts, or ghost targets, which appear near vibrating targets in reconstructed SAR images. Recently, a vibration estimation method based on the discrete fractional Fourier transform (DFrFT) has been developed. This method is capable of estimating the instantaneous vibration accelerations and vibration frequencies. In this paper, a deghosting method for vibrating targets in SAR images is proposed. For single-component vibrations, this method first exploits the estimation results provided by the DFrFT-based vibration estimation method to reconstruct the instantaneous vibration displacements. A reference signal, whose phase is modulated by the estimated vibration displacements, is then synthesized to compensate for the vibration-induced phase modulation in returned SAR signals before forming the SAR image. The performance of the proposed method with respect to the signal-to-noise and signalto-clutter ratios is analyzed using simulations. Experimental results using the Lynx SAR system show a substantial reduction in ghosting caused by a 1.5-cm 0.8-Hz target vibration in a true SAR image.

Three-dimensional fatigue crack propagation analysis using the boundary element method

Abstract The boundary element method and interactive computer graphics together make possible the analysis of general, three-dimensional linear elastic crack propagation. A program called the Fracture Editor has been developed for this purpose. The pre-processing, post-processing and fracture prediction capabilities of this program make up a general purpose fracture propagation processor. The function of the Fracture Editor is described in detail. These functions include file management, fracture propagation prediction, automatic crack front updating, interactive remeshing, attribute editing, mesh inspection, and post-processing of results. An example problem, fatigue crack propagation in a crane runway girder, is included. Predictions concerning crack front shape and size as a function of number of cycles of applied load are made with the Fracture Editor.

Demonstration of target vibration estimation in synthetic aperture radar imagery

Synthetic-aperture radar (SAR) can be used to remotely estimate ground target vibrations by exploiting the Doppler in the returned signals. Recent studies suggest that time-frequency signal-processing tools can retrieve the vibration signature from the returned SAR signals. A vibration estimation method based on the fractional Fourier transform (FRFT) was reported earlier and it was tested on simulated SAR data. In this paper, a first-time demonstration of the FRFT-based vibration estimation method is reported using real SAR data collected by the Lynx (Ku-band) SAR system. The vibrating target is an aluminum triangular trihedral with lateral length of 15 inches. The FRFT-based algorithm is shown to successfully retrieve a 3 mm peak-to-peak amplitude, 5 Hz vibration of the target from real SAR data.

An Object-Oriented Framework for Multidisciplinary, Multi-Physics, Computational Mechanics

Abstract.This paper presents the design and development of an object-oriented framework for computational mechanics. The framework has been designed to address some of the major deficiencies in existing computational mechanics software packages. The framework addresses the deficiencies of existing computational mechanics software packages by (a) having a sound design using the state of the art in software engineering, and (b) providing model manipulation features that are common to a large set of computational mechanics problems.The framework provides features that are essential to a large set of computational mechanics problems. The domainspecific features provided by the framework are a geometry sub-system specifically designed for computational mechanics, an interpreted Computational Mechanics Language (CML), a structure for management of analysis projects, a comprehensive data model, model development, model query and analysis management. The domain independent features provided by the framework are a drawing subsystem for data visualization, a database server, a quantity subsystem, a simple GUI and an online help server.It is demonstrated that the framework can be used to develop applications that can: (a) extend or modify important parts of the framework to suit their own needs; (b) use CML for rapid prototyping and extending the functionality of the framework; (c) significantly ease the task of conducting parametric studies; (d) significantly ease the task of modeling evolutionary problems; (e) be easily interfaced with existing analysis programs; and (f) be used to carry out basic computational mechanics research. It is hoped that the framework will substantially ease the task of creating families of software applications that apply existing and upcoming theories of computational mechanics to solve both academic and real world interdisciplinary simulation problems.

Structural Design of a Unique Passive Telescope

Most telescopes designed today assume that, as the telescope is pointed at different locations on the celestial sphere, there will be some interactive human or computationally-activated mechanical adjustment for defocus or misalignment during routine use. As part of a collaboration between the University of New Mexico and the University of Texas at Austin, design of the second iteration of the CCD/Transit Instrument (CTI-II) is underway. With a one degree square scientific focal plane mosaic of charged coupled devices (CCDs) operated in the time-delay and integrate readout mode, the stationary, 1.8-m telescope will accomplish a multi-bandpass photometric and astrometric imaging survey of more than 300 square degrees of the sky. The structural design goal for CTI-II is to make the telescope as passive as possible, so that little or no physical human or computationally-activated mechanical adjustment is required during routine use. Because the telescope will never be repositioned during normal operations, structural deformations due to the earth’s gravity will not vary during operation. We are designing the telescope so that its optical behavior will be as nearly as possible thermally invariant, and we are also attempting to minimize vibrations due to wind and ambient seismicity. The structural design philosophy, methodology, and provisions for thermally and optically tuning the telescope (hopefully an infrequent operation) are discussed.