Structural zooming research and development of an interactive computer graphical interface for stress analysis of cracks

Engineering problems sometimes involve the numerical solution of boundary value problems over domains containing geometric feature with widely varying scales. Often, a detailed solution is required at one or more of these features. Small details in large structures may have profound effects upon global performance. Conversely, large-scale conditions may effect local performance. Many man-hours and CPU-hours are currently spent in modeling such problems. With the structural zooming technique, it is now possible to design an integrated program which allows the analyst to interactively focus upon a small region of interest, to modify the local geometry, and then to obtain highly accurate responses in that region which reflect both the properties of the overall structure and the local detail. A boundary integral equation analysis program, called BOAST, was recently developed for the stress analysis of cracks. This program can accurately analyze two-dimensional linear elastic fracture mechanics problems with far less computational effort than existing finite element codes. An interactive computer graphical interface to BOAST was written. The graphical interface would have several requirements: it would be menu-driven, with mouse input; all aspects of input would be entered graphically; the results of a BOAST analysis would be displayed pictorially but also the user would be able to probe interactively to get numerical values of displacement and stress at desired locations within the analysis domain; the entire procedure would be integrated into a single, easy to use package; and it would be written using calls to the graphic package called HOOPS. The program is nearing completion. All of the preprocessing features are working satisfactorily and were debugged. The postprocessing features are under development, and rudimentary postprocessing should be available by the end of the summer. The program was developed and run on a VAX workstation, and must be ported to the SUN workstation. This activity is currently underway

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

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

(04) State-Based Peridynamic Lattice Modeling of Reinforced Concrete Structures (slides)

Continuum peridynamics provides an alternative to continuum mechanics. However, peridynamics is more general because it allows cracks to emerge. However, peridynamics requires further discretization to be implemented on a computer. Peridynamics assumes the material space is a continuous Cartesian real space. In contrast, in this paper we assume the material space is a discrete Cartesian integer space, defining a regular lattice of material particles, and proceed to develop the state-based peridynamic lattice model (SPLM). With the SPLM, the forces between neighboring particles are characterized by the force state, T, and the stretches between particles are characterized by the deformation state, Y. The material model arises from a peridynamic function relating the force state to the deformation state. With the SPLM, continuous deformations, elasticity, damage, plasticity, cracks, and fragments can be simulated in a coherent and simple manner. With the ongoing increase in computational storage capacity and processing power, the SPLM becomes increasingly competitive with more traditional continuum approaches like the finite element method

(03) State-Based Peridynamic Lattice Modeling of Reinforced Concrete Structures

Walter Gerstle, PhD ‘86, Professor of Civil Engineering, University of New MexicoA Symposium in Honor of Professor Emeritus Anthony R. Ingraffea: Computer Simulation and Physical Testing of Complex Fracturing ProcessesContinuum peridynamics provides an alternative to continuum mechanics. However, peridynamics is more general because it allows cracks to emerge. However, peridynamics requires further discretization to be implemented on a computer. Peridynamics assumes the material space is a continuous Cartesian real space. In contrast, in this paper we assume the material space is a discrete Cartesian integer space, defining a regular lattice of material particles, and proceed to develop the state-based peridynamic lattice model (SPLM). With the SPLM, the forces between neighboring particles are characterized by the force state, T, and the stretches between particles are characterized by the deformation state, Y. The material model arises from a peridynamic function relating the force state to the deformation state. With the SPLM, continuous deformations, elasticity, damage, plasticity, cracks, and fragments can be simulated in a coherent and simple manner. With the ongoing increase in computational storage capacity and processing power, the SPLM becomes increasingly competitive with more traditional continuum approaches like the finite element method.1_t2krk03