CHARACfERIZA nON OF POROSITY IN THICK COMPOSITES USING ULTRASONIC WAVE VELOCITY MEASUREMENTS LN. Komsky, LM. Daniel, and S.C. Wooh Northwestern University, Evanston, IL 60208 INTRODUCTION Determination of content and distribution of porosity introduced in composites during fabrication, is an important issue in characterizing composite materials, especially thick ones. Dispersed or discrete elongated pores, located at the fiber-matrix or ply interfaces, can act as nuclei of further damage growth during loading of the composite laminate. Both velocity and attenuation measurements of ultrasonic waves are widely used for nondestructive evaluation of composites. However, attenuation measurements investigated in [1-3] are susceptible to large scatter in the results due to non uniformity of the surface of the composite material and a high signal loss. These measurements usually require acquisition of multiple echoes that is also limited by signal loss. On the other hand, measurements of absolute value or dispersion of ultrasonic wave velocity are not so sensitive to the different surface conditions. Velocity measurements in composites have been carried out using single transmission of broadband pulses with center frequencies of 5 and 10 MHz in an immersion tank [4]. According to this technique the absolute value of the wave velocity in a composite can be determined only for a sample of known thickness measured separately. The technique described in the present paper extends the area of application of velocity measurements for characterization of porosity in composite materials. This technique makes it possible to measure the absolute value and dispersion of ultrasonic wave velocity in thick composite laminates independently of the variation of composite thickness using non-contact transducers. The technique was applied to evaluation of porosity in unidirectional 200-ply graphite/ epoxy composite. Experimental results were compared with theoretical calculations and then correlated with void content measurements made by image analysis of photomicrographs of specimen cross-sections. MEASUREMENT OF ULTRASONIC WAVE VELOCITY An ultrasonic technique for velocity measurement was developed for cases with high signal loss caused by attenuation and from the specimen surface. This technique utilizing only the through-transmission signals is illustrated in Fig. 1. The air-coupled transmitter generates an ultrasonic wave which being repeatedly reflected by the front face of the specimen and the face of transmitter provides a train of signals received by a contact transducer on the opposite side of the specimen. The time-of-flight for a directly transmitted signal (#1) can be expressed as Review of Progress in Quantitative Nondestructive Evaluation, Vol. 12 Edited by D.O. Thompson and D.E. Chimenti, Plenum Press, New York, 1993 1273