Experimental and theoretical investigation of ultrasound propagation in materials containing void inclusions

The non-destructive characterization of multiple embedded void inclusions forms an important part of the assessment of the integrity and quality of materials. In this report, we present an ultrasonic sensing method for detecting void inclusions. The idea is based upon the arrival probability of ultrasonic rays. When incident rays impinge on voids, they are diffracted around the voids to increase propagation time. We analyzed this phenomenon from a probabilistic view point, and found that the probability with which ultrasonic rays will impinge on pores plays an important role in determining both sound velocity and waveform of arrival rays. On the basis of this theory, we derived the relational equations between void distributions (volume fraction and mean size) and ultrasonic characteristics (velocity and pulse width). Although this theory is very simple, it gives a reasonable explanation of the changes in both velocity and waveform. In order to examine the theory for its validity, we carried out ultrasonic tests using aluminum specimens with multiple drilled holes and ceramic specimens with artificial voids. The experimental results demonstrated the validity of this theory. We propose this method as a smart sensing technique for evaluating the integrity of materials containing multiple defects.