Tomographic Imaging of an Ultrasonic Field in a Plane Using a Linear Array

A method is presented based on tomographic reconstruction techniques for the visualization of an ultrasonic field. The tall, narrow piezoelectric elements of a one-dimensional receiving array, rotated about its central axis, provide parallel projection input data for tomographic reconstruction of the field. The technique is demonstrated using simulated ultrasonic beams and experimental measurements of an ultrasonic beam propagated through a stitched woven composite. INTRODUCTION Research is producing increasingly complex materials for use in aerospace and other applications. These advanced material architectures include such developments as multidimensional fiber-reinforced plastics incorporating textiles and stitching [1], materials with embedded sensors and actuators [2], and other multifunctional materials concepts. A feature common to all of these “smart materials” is that they exhibit increased spatial variability of material properties, requiring an increased number of parameters to describe their state. These spatial variations occur on a scale significant to ultrasonic waves at the frequencies used for NDE. Because of this information-rich propagation environment, an interrogating ultrasonic field becomes phase-distorted as it gathers material information along its path. Conventional single-element ultrasonic sensors phase-sensitively average the received ultrasonic field. The phase-distortions within the field thus manifest themselves as a kind of coherent “noise” on the received signal. A conclusion can be reached that increasingly complex materials require increasingly sophisticated NDE. Multiple-element ultrasonic arrays can be used as a means to provide additional degrees of measurement freedom. By exploiting the additional degrees of freedom, it may be possible to measure and interpret the distorted ultrasonic field emerging from a complex material, effectively converting the “noise” into “signal” for more complete quantitative NDE of the material. Within the context of exploiting these additional degrees of freedom, the concept was identified of using a rotated linear array receiver to perform tomographic reconstruction of an ultrasonic beam. † Present affiliation NIST, Boulder, CO