Ultrasonic Material Characterization System Employing an Array of Receiving Transducers

In a typical ultrasonic material characterization system, the output voltage of the transducer is recorded as a function of the relative position of the transducer and the specimen. The velocity and attenuation of the leaky surface acoustic waves and the reflectance function for the immersion liquid–specimen interface can be obtained from the recorded data. The experimental arrangements with longitudinal movement of the transducer (the V(z) scheme) and lateral translation of the receiving transducer (the V(x) scheme) are well known systems. The disadvantages of these methods are obvious: the mechanical scanning of the transducer is associated with slow data acquisition, and precision mechanics is required. In this work, we proposed a new measurement method based on recording the ultrasonic field distribution of the scattered wave with an array of the receiving transducers. The parameters of the leaky waves can be obtained by processing the set of the output waveforms. To experimentally confirm the proposed method, a new material characterization system has been developed. The relative position of the transmitting focused transducer and the receiving linear array of small transducers were constant in the experimental setup. The system was successfully tested on several materials with known acoustical properties. Introduction: Measurements of phase velocity and propagation attenuation of Leaky Surface Acoustic Waves (LSAW) are widely used for material characterization. In most of these quantitative ultrasonic material characterization (UMC) systems, the output signal is recorded as a function of the relative position of the transducers and the specimen immersed in a liquid. In the V(z) technique, the point–focused or line–focused transducer is translated perpendicular to the surface of the specimen (Fig. 1). Many modifications of the V(z) technique employing various types of focused transducers, processing algorithms, and electronic equipment have been proposed. Typically, the phase velocity and propagation attenuation of the leaky guided waves, such as leaky Rayleigh, Lamb, skimming longitudinal waves, etc., as well as a reflectance function for the specimen–water interface can be obtained from the recorded V(z) data [1–5]. Recently, several ultrasonic material characterization systems based on lateral scanning of the receiving transducer along x axis have been developed [6-9]. In comparison with the V(z) system, the V(x) scheme potentially possesses better angular resolution and temperature stability. The disadvantages of these methods are obvious: the mechanical scanning of the transducer is associated with slowness of the data acquisition, and the accuracy of the measurements depends on the precision of the mechanical movement. Fig. 1. The schemes of the V(z) and V(x) ultrasonic material characterization systems. We propose a material characterization system based on the receiving of the reflected wave with an immovable ultrasonic array. The scheme of the proposed UMC system is shown in Fig. 2. The LSAW is generated in a local area of the immersed specimen by the single focused transducer T. The leaky wave propagates along the surface of the specimen reradiating back in the immersion liquid, and the reflected wave is received by the linear ultrasonic array tilted at the angle θ0. Fig. 2. The UMC system with the array of the receiving transducers. The output waveforms of the elements of the array Vn(t) represent the spatio–temporal distribution of the reflected waves in the plane of the array. The velocity and attenuation factor of the LSAW can be obtained by processing the Vn(t) data. Based on the geometry of the problem and ray model of the system, it can be shown that the relative time delay ∆t between LSAW responses received by two neighbor elements of the array is related to the critical angle of the leaky wave θR by the following equation: ) sin( 0 θ θ − = ∆ R C p t , (1) where C is the sound velocity in the liquid, and p is the pitch of the array. Using the measured value of ∆t, the phase velocity of the non dispersive leaky wave CR can be calculated according to Snell’s law. Let us consider now the amplitudes of the responses of the neighbor elements. For a particular frequency f0, the ratio of these amplitudes η can be written as: } cos ) sin ) cos( exp{( 0 0