Characterisation of the design and of the shear horizontal mode excitation of a shear transducer for ultrasonic guided wave applications

Guided waves are currently being used to inspect pipelines, aircraft, tanks, bridges and rail structures, for the purpose of assessing their structural integrity. However, the multimodal and dispersive nature of guided waves require the selection of a reliable excitation system. Piezoelectric transducers based on Lead Zirconate Titanate have been extensively employed to excite guided waves. In particular, thickness-shear piezoelectric transducers have found widespread use in industrial applications for plate-like and tubular structures.

Research has been continuously carried for shear transducers in the last two decades to improve sensitivity and resolution. Nonetheless, enhancements have been mainly based on post signal processing techniques or the design of arrays to cancel out unwanted modes through backward cancellation. Research in the literature has suggested that the design of the transducer might be introducing unexpected wave modes along with the desired ultrasonic response.

The aim of the thesis is to assess the effect of the design of the transducer on ultrasonic guided wave excitation. This is achieved by comparing the output of the transducer in a plate-like structure with the expected design based on the ideal point source hypothesis. The thesis then seeks to provide explanations for the differences between the ideal and real behaviour, based on physical explanations of the phenomena involved. Guidelines are also elaborated to meliorate the existing state of the art on shear transducers.

The first theme of the thesis is then the characterisation of a single shear transducer in a plate-like structure. In chapter 4 an experimental analysis based on 3D LDV Vibrometry is combined with a numerical finite element approach where the geometry of the transducer and the electromechanical nature of excitation are both taken into account. A mesh sensitivity analysis is carried out to ensure the convergence of the model. Both experimental and numerical results indicate that an unexpected out-of-plane mode is propagating in the plate, compromising the mode selectivity and purity of the transducer in terms of shear horizontal mode.

The second theme is the interpretation of the physical phenomena described in chapter 4. In Chapter 5 the validated numerical model is exploited to provide qualitative and quantitative evidence on the transducer behaviour. The vibration of the piezoelectric element, of the wear-plate and of the backing mass are assessed and discussed. A physical interpretation of the effect of the design on the ultrasonic response is provided. It is proposed that the electrical layout and the geometry heavily influences the propagation of guided waves. An alternative is numerically studied to show the improvement in shear horizontal mode purity with a different electrical configuration.

The third theme is the development of alternative designs to provide evidence of possible improvement. Thus, in chapter 6 two prototypes with different electrode layouts are developed and tested according to the procedure developed in Chapter 4. A definite improvement in shear horizontal mode purity is found when an alternative electrical design is considered. However, the effect of the geometry on the excitation remains to be assessed.

The fourth theme is the creation of a general methodology to study the influence of the geometrical parameters. In chapter 7 the fundamentals requirement of the transducer are formulated mathematically as design criteria and an objective function is established. The combination of features and the heuristic search of an adequate objective function are described. The integration between a finite element software and the genetic algorithm optimisation approach is also fully discussed. Both a two-dimensional and a three dimensional geometrical optimisation are carried out. The importance of the geometrical parameters and their interdependence in terms of ultrasonic excitation is also discussed. Indications in terms of future design changes to be implemented in industrial applications are provided.