Thermal, Magnetic, and Mechanical Modulations of Ultrasonic Diffuse Fields

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

Richard L. Weaver University of Illinois at Urbana-Champaign Proposal # 9988645 Thermal, Magnetic, and Mechanical Modulations of Ultrasonic Diffuse Fields Abstract: This project explores novel modalities for ultrasonic NonDestructive Evaluation and Materials Characterization by investigating the dependence of diffuse ultrasonic signals on parametric variations of temperature, magnetic field, and stress. Conventional (non-diffuse) ultrasound is sensitive to a variety of material properties of engineering significance. It is also sensitive to externally applied parameters. Notable amongst these are temperature, and (in steels) magnetic field. The coefficients of thermal and magnetic variation depend on stress. This implies that ultrasonic measurements of temperature and magnetic field variations should allow inference of stress. Recent demonstrations that ultrasonic diffuse fields are more sensitive (because travel times are much longer than those of conventional ultrasonic fields) and more robust (because demands on apparatus are less) indicators of wave speed change suggest that these variations can be measured with great precision. Diffuse fields are used in this project to measure ultrasonic wave speed as it depends on temperature and magnetic field. The variations are measured with specimens under a variety of static loads, and the correlation (coefficient-of-variation versus stress) assessed. Ultrasound is also affected by low-frequency vibrations and the associated opening and closing of microcracks. In this project the variations of diffuse fields as a specimen is subjected to low-frequency quasi-static strains (e.g. vibrations) are also measured, and correlated with the presence of opening and closing fatigue cracks. The research will lead to new methods for field detection of distributed damage, of individual fatigue cracks, and of residual and applied stress. The corresponding improvements in nondestructive evaluation will tighten assessments of remaining structural life, critical for aging air fleets and civil infrastructure.