QUANTITATIVE ULTRASONIC IMAGING FOR MATERIALS CHARACTERIZATION

This thesis addresses the physical basis of the propagation and detection of ultrasound for quantitative imaging. The expanding use of ultrasonic evaluations has focused attention on techniques used in making quantitative measurements. Experimental methods for imaging in inhomogeneous material are presented which address the problems of the frequency dependent ultrasonic beamwidth and of characterizing the field at an aperture. Improvements gained with these methods are demonstrated by a study of fatigue and impact damage in composites using quantitative images. Images which show the altered ultrasonic properties as a result of fatigue or impact or both are presented.;Characterizing the field at an aperture is considered for both phase sensitive and phase insensitive detection methods. For measurements in which waves with distorted phasefronts are to be received, phase insensitive techniques have been shown to be advantageous. We present the response characteristics of a phase insensitive hybrid acoustoelectric receiver which exhibits increased sensitivity over that obtained with single crystal CdS-based devices.;As an alternate method of characterizing the ultrasonic field we examine the applicability of sampling an aperture with an array transducer and calculating the spatial moments of the ultrasonic intensity distribution over the aperture. The spatial moments provide information concerning average properties of the field, such as intensity or beamwidth. Simulations are carried out to calculate the pressure for several geometries of interest, and spatial moments of these fields are calculated for various element densities. Experimental verification is carried out by measuring the spatial moments over a sampled aperture for sound transmitted through an excised dog heart.;Using experimental techniques which promote accurate front-end treatment of the propagation and detection of ultrasound, we have demonstrated substantial improvement in quantitative imaging of inhomogeneous materials including graphite-epoxy composite laminates.;Broadband measurements have been shown previously to yield accurate estimates of the ultransonic properties of materials. Such measurements introduce distortion in images as a result of the finite, frequency dependent beamwidth. We examine three digital filters for deconvolving the effects of the finite beamwidth and demonstrate improvements obtained with the Wiener filter.