An evaluation of 1-3 connectivity composite transducers for air-coupled ultrasonic applications

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 1996
Thesis identifier
  • T9429
Qualification Level
Qualification Name
Department, School or Faculty
  • Conventional ultrasonic non-destructive evaluation techniques require some form of liquid coupling between the piezoelectric transducer and the sample under inspection to facilitate mechanical energy transfer. Under automated conditions, maintaining a consistent couplant layer can constitute a problem and is manifest in slower scanning rates. To improve scanning speed the ultrasound should ideally propagate across an intermediate airgap. This possibility has been inhibited by the lack of a suitable transducer technology to overcome the practical difficulties inherent in the air medium. The large acoustic impedance mismatch, between a piezoelectric element and air, and the large air attenuation factor ensures both poor sensitivity and resolution characteristics. This Thesis advances transducer technology, enabling these problems to be challenged effectively.Initially, the Thesis describes the creation of a wideband characterisation methodology for aircoupled ultrasonic transducers operating in the range 100kHz - 2MHz. The proposed technique utilises piezopolymer transducers in both transmission and reception modes and despite the obvious measurement difficulties, good agreement between theory and experiment is illustrated. Results are presented on the characterisation of both electrostatic and 1-3 connectivity piezocomposite devices, in which the inherent narrowband nature of the piezocomposite transducer demonstrates considerable potential for airborne applications.Subsequently, the performance of air-coupled 1-3 connectivity piezocomposite transducers is investigated using both uni-dimensional and finite element modelling techniques. Experimental verification of these techniques enables the finite element approach to be used as the benchmark for evaluation of practical transducer configurations. Consequently, a series of design guidelines are presented for practical air-coupled composite transducer systems. For many applications, the conventional method to improve system resolution, through attachment of a backing block, produces an unacceptably low system sensitivity. Hence, an alternative approach is proposed, utilising strong coupling between the fundamental thickness mode and the first lateral resonance in a 1-3 piezocomposite. Finite element analysis has been employed in the design of a laterally-coupled composite receiver and a -6dB bandwidth of 150% at a centre frequency of 1 MHz is presented. Finally, the new transducer design techniques were applied to three different airborne non-destructive testing applications. Successful detection of various defects in carbon-fibre reinforced composite plates is demonstrated.
Resource Type
Date Created
  • 1996
Former identifier
  • 995485533402996