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Theoretical and Applied Mechanics

Dynamics, Vibrations, and Waves
^ Acoustic-Emission Wave Propagation in Plates: The Inverse Source Problem
R. L. Weaver,* N. L. Wolff
Petroleum Environmental Research Forum
r-weaver@uiuc.edu

We are developing algorithms for extracting source distance and type from acoustic-emission waveforms in plates. This study focuses on transient point sources (step forces and double forces corresponding to sources that are smaller than a wavelength in spatial extent) in plates at distances large compared with the plate thickness and at frequencies up through several of the thickness resonances. We determine how best to analyze the resulting waveforms to reveal the character, orientation, and position of the source.

^ Diffuse Ultrasonics and Materials Characterization
R. L. Weaver,* O. I. Lobkis, I. Rozhkov
National Science Foundation, CMS 97-01142
r-weaver@uiuc.edu

The diffuse ultrasonics of polycrystalline materials is studied with a view toward ultimate applications in robust ultrasonic characterization of microstructures and flaw detection in the midst of grain noise. The main focus is on the validation of ultrasonic radiative transfer formulations of multiple diffuse scattering in polycrystals. Of particular interest is the transition from the simple single-scattering limit model used by many, through the complicated regime in which typical rays have scattered a few times, to the once again simple diffusion limit in which typical rays have scattered many times. Theoretical and numerical work is complemented by experimental work done elsewhere.

^ Diffuse Ultrasonics in Strongly Scattering Media
R. L. Weaver,* O. I. Lobkis
National Science Foundation, CMS 97-01142
r-weaver@uiuc.edu

Ultrasound in materials with extremely strong scattering microstructures, with mean free paths of the order of wavelengths, is studied. Examples include solid foams, slurries, and polycrystalline aggregates of crystallites of strong anisotropy. The particular focus is on energy-density fluctuations and energy transport as one approaches the Anderson localization transition.

^ Thermal, Magnetic, and Mechanical Modulations of Ultrasonic Diffuse Fields
R. L. Weaver,* O. I. Lobkis
National Science Foundation,DSC-9988645
r-weaver@uiuc.edu

We explore methods for ultrasonic materials characterization by investigating the dependence of diffuse ultrasonic signals on variations of temperature, magnetic field, and stress. The coefficients of ultrasonic dependence on temperature and magnetic field also depend on stress. Thus measurements of ultrasonic dependence on temperature and magnetic fields should allow inference of stress. Ultrasonic dependence of stress is greatest when caused by opening and closing of microcracks. In this project, the variations of diffuse fields as a specimen is subjected to quasi-static strains, thermal fields, and magnetic fields are measured. The presence of fatigue cracks and contained stress should be inferable.

^ Vibration Measurements of Rail Stress
R. L. Weaver,* V. Damljanovic
Association of American Railroads
r-weaver@uiuc.edu

We are developing and designing methods for measuring longitudinal stress in rails. Rail stress leads to broken and buckled rails, and consequent service delays and derailments. The effect of longitudinal stress on the free vibrations of beams is well understood: compressive forces decrease the flexural frequencies and tensile stresses increase the frequencies. Past efforts attempting to use this dependence for measurements of stress in railroad rails have failed due to an inability to control, or measure adequately, other influences. Here we investigate two new methods that should have minimal sensitivity to these other influences.


Summary of Engineering Research