DOUBLE THROUGH-TRANSMISSION BULK WAVE METHOD FOR ULTRASONIC PHASE-VELOCITY MEASUREMENT AND DETERMINATION OF ELASTIC-CONSTANTS OF COMPOSITE-MATERIALS

This paper describes a modification of a nondestructive ultrasonic method for measurements of the phase velocity of bulk waves in arbitrary directions in generally anisotropic materials. In the conventional method the through-transmission technique is used for velocity measurements at a specified angle of incidence. When this angle is changed by rotation of the sample, the transmitted beam changes position, and so the position of the receiving transducer must be changed. This leads to experimental difficulties and loss of precision. In the double-transmission technique, the ultrasonic wave is reflected from reflector plates behind the sample and returns via the same path to the same position on the transmitter/receiver working in pulse-echo mode, which eliminates the necessity of readjusting the receiver position. It is also shown that for arbitrary direction of measurement in anisotropic materials, time-delay measurements give phase velocity regardless of the angle of deviation between phase and group velocities. The physical basis of this phenomenon is explained. An example of velocity measurement and elastic-constant determination is given for unidirectional graphite-epoxy composite. Stability of the nonlinear least-square algorithm used for reconstruction of the elastic constants from the velocity data is demonstrated by computer simulation on a synthetic set of data.