Contact-Free Ultrasound: The Final Frontier in Non-Destructive Materials Characterization
Adhering to the objectives of non-destructive materials characterization, a contact-less –air-coupled—method of ultrasound is highly desirable. However, exorbitant acoustic impedance mis-match between the coupling air and the test media presents a natural impediment to NCU. This impedance mis-match can be as high as seven orders of magnitude for some materials. Therefore, high frequency Non-Contact Ultrasound (NCU) propagation in materials has been regarded as an impossibility. On the other hand, low frequency ultrasound – 25kHz to 100kHz – can be propagated through materials in non-contact mode. However, these frequencies are practically useless for the interrogation of most materials where the industry demands high resolution and high detectability. Recent advances in high transduction high frequency piezoelectric transducers (200kHz to 5mHz) and an exceptionally high dynamic range nanosecond accuracy ultrasonic system have advanced the NCU practice to levels that rival conventional contact method. In this paper we provide and introduction to NCU and present examples of green and sintered ceramics and composite analysis. Applications of NCU are considered to be truly significant for cost-effective manufacturing of high reliability materials.