A new technique for time-domain ultrasonic NDE of extremely thin plates

The strength of a joint is significantly affected by the thickness of the bond. Over the past quarter-century, a wide variety of ultrasonic techniques have been reported for the measurement of the thickness and wave velocity (or modulus) of a single layer. Among them are pulse-echo [1], resonance testing [2] and pulse interference [3] methods. Evidently as the plate thickness decreases the time interval between two successive echoes, At, decreases and finally the echoes become inseparable. All of the classical methods break down when the successive reflections from the two faces of the layer cannot be separated in the time-domain. In this paper a specimen will be called “thin” if h\u3c3 λ, where λ is the wavelength in the interrogated material at the transducer center-frequency; conversely, it will be called “thick” if h\u3e3 λ. In much of the aerospace applications, the typical adhesive thickness is of the order of 10-2 mm or 10−3 inch. In order to use any of the aforementioned methods, the transducer frequency would have to be larger than 150 MHz. By combining the theory of Fourier transforms with conventional ultrasonic hardware Kinra and Dayal developed a new ultrasonic NDE technique which removed this limitation [4]. Subsequently, this technique has been used for NDE of the properties of an extremely thin plate [5] as well as a three-layered medium (adherend-adhesive-adherend) where the combined thickness of the joint (h) qualifies as thin [6, 7]