ULTRASONIC SIZING OF VOIDS USING AREA FUNCTIONS J. Yang and L. J. Bond* Department of Mechanical Engineering University College London (UCL) Torrington Place London WCIE 7JE, UK * Now on leave from UCL at National Institute of Technology and Standards and University of Colorado at Boulder Colorado, 80309-0427, USA INTRODUCTION We present a simple technique for determining the size of voids by the inversion of backscattered ultrasonic signals using the area function formula. The formulation of this method is based on the Born approximation, which is a weak scattering approximation, but the method works weIl for voids. The area function has been widely used as a method for determining the position of the flaw centroid to assist implementation of some inversion algorithms. The method has been reported in [6]. Here, we report some further studies, and more experimental results in detail. AREA FUNCTION Fig. 1 shows a simple backscattering geometry, where Rj(t) is the backscattered longitudinal-to-Iongitudinal impulse response from the flaw, and Set) corresponds to the cross-sectional area of the flaw intersected by a wave plane defined by the time, t, or by the position, z. Note that in the backscattering geometry, t and z have a simple relation, ct Z=-- 2 where c is the longitudinal wave velocity. According to the time domain Born approximation, the backscattered impulse response, R1(t), from a flaw has a simple relationship to the cross-sectional area, Set), of the flaw [1] R (t) = -E!.... d 2 S( t) 1 c2 dt 2 Review of Progress in Quantitative Nondestructive Evaluation, Vol. 11 Edited by D.O. Thompson and D.E. Chimenti, Plenum Press, New York, 1992 (1) (2) 849