High frequency focused imaging for ultrasonic probe integrity inspection

Most ultrasonic probes present a layered structure that may comprise delay lines, acoustic lens, wear plates, matching layers, electrodes, piezoelectric layers, and a backing block. Probe integrity can be compromised by different defects, such as deficient bonding, presence of air bubbles or porosity, lack of electrical conductivity in electrodes, and deviations in the layers' acoustic thicknesses, impedances or attenuation values. Conventional testing procedures are based on the integral properties of the probe, such as acoustic directivity, sensitivity, and electrical impedance. An alternative technique for the non-destructive inspection of the probe's integrity is presented in this study. The proposed technique is based on imaging of the internal structure using a higher frequency focused ultrasound. Scanning the probe's surface using this focused beam provides two imaging modes: reflection and transmission. In the reflection mode, the higher intensity of the echoes indicates the presence of the defects, such as disbonds, poor acoustic matching or porosity. The main novelty of the proposed technique is in the transmission mode: the probe's electric response to the high frequency focused beam is used to get a 2D distribution of transduction efficiency. Moreover, the probe's response can be gated to separate, both front and back side, responses in the time domain, so the local integrity of an electroacoustic structure is evaluated. To verify the proposed technique, a focused 20 MHz transducer with a focal spot diameter of 0.35 mm was mounted on an ultrasonic scanner and immersed in water above the surface of the probe to be tested. Signals were gated by the layers' thickness, giving the C-scan images corresponding to the different material interfaces inside the tested probes: front face, matching layers, piezoelement and backing. It was demonstrated that the resulting C-scan images are informative in evaluating probe's integrity and layers' acoustic thickness. Comparison of the C-scan images obtained in the reflection and transmission modes shows a good agreement in the structure deficiencies evaluation. The technique proposed is attractive due to its simplicity. It can be useful in several applications: i) the probes already in use can be verified and if any deficiencies are detected, the in- formation obtained can be used to evaluate and guide repair; ii) fabrication quality can be verified by confirming the adhesion of layers and parameters in 2D; and iii) the development of new fabrication routes can be guided, where adherence, compatibility, homogeneity, etc. of new materials or new technologies can be evaluated.