The problems of development of high-temperature ultrasonic transducers for modern science and technology applications are analysed. More than 10 piezoelectric materials suitable for operation at high temperatures are overviewed. It is shown that bismuth titanate based piezoelectric elements are most promisable. Bonding methods of piezoelectric elements to a protector and backing are discussed. Thermosonic gold-to-gold bonding is most modern and possesses unique features. Our achievements in this field are analysed in the context of world progress. In sol-gel and chemical vapour deposition technology bonding and sometimes coupling problems are avoided at all. Design peculiarities of the transducers are reviewed. Commercial sensors in the meanwhile are often characterized with poor performance in extreme conditions.In conclusions recommendations for high temperature ultrasonic transducers design are formulated.
COMPOSITESThe objective of this study was to adapt ultrasonic and radiographic techniques for the inspection of wind turbine blades and to compare the obtained results. The measurements performed show that radiographic techniques are capable of reliably detecting a number of structural defects within the blade. The adapted air-coupled ultrasonic technique, using Lamb waves, proved to be the most promising in terms of implementation as it only requires access to one side. However, the novel combination of contact pulse-echo and immersion techniques using a moving container of water identified the shape and size of defects better. Through comparisons of images obtained using both radiographic and ultrasonic techniques, different defect properties can be identified. Hence, the best results are achieved when both techniques are combined together.
Air-coupled ultrasonic techniques are being increasingly used for material characterization, non-destructive evaluation of composite materials using guided waves as well as for distance measurements. Application of those techniques is mainly limited by the big losses of ultrasonic signals due to attenuation and mismatch of the acoustic impedances of ultrasonic transducers and air. One of the ways to solve this problem is by application of novel more efficient piezoelectric materials like lead magnesium niobate-lead titanate (PMN-PT) type crystals. The objective of this research was the development and investigation of low frequency (<50 kHz) wide band air-coupled ultrasonic transducers and arrays with an improved performance using PMN-32%PT crystals. Results of finite element modelling and experimental investigations of the developed transducers and arrays are presented. For improvement of the performance strip-like matching elements made of low acoustic impedance, materials such as polystyrene foams were applied. It allowed to achieve transduction losses for one single element transducer −11.4 dB, what is better than of commercially available air-coupled ultrasonic transducers. Theoretical and experimental investigations of the acoustic fields radiated by the eight element ultrasonic array demonstrated not only a good performance of the array in a pulse mode, but also very good possibilities to electronically focus and steer the ultrasonic beam in space.
Ultrasonic waveguide sensor for measurement of viscosity of highly viscous fluids has been developed. The measurement principle is based on application of guided shear-horizontal SH0 mode of the Lamb waves propagating in an aluminium planar waveguide immersed in a viscous liquid. Attenuation of the guided wave depends on viscosity of the surrounding liquid and is used for viscosity estimation. The developed sensor is mechanically robust and may be used for in-line process control of viscous liquids.
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