The analysis of aerospace adhesively bonded aluminium plates using electromagnetic acoustic transducers (EMATs)
This Thesis describes techniques used in the analysis of aerospace adhesively bonded sandwich joints using radially polarised shear wave EMATs, together with an analysis of the general performance of the EMAT systems used in the investigation. It is generally accepted that to date there is no satisfactory technique for the detection of defect types commonly encountered in adhesively bonded aerospace samples. This has limited the use of adhesives for joining components on critical parts, where no additional fixing techniques are used. Consequently, if a test procedure could be developed to accurately monitor post production defects and in-service degradation then there would be great benefits in terms of weight saving and strength in joined components which were held together by adhesive bonds alone. Ultrasonics is a technique commonly applied to the assessment of bond quality and readily lends itself to the probing of media which support the propagation of acoustic waves. The principal is that the propagation of sound through a medium gives some indication of the mechanical properties of that medium, and in terms of an adhesive layer could measure the strength of the adhesive bulk itself- cohesion. The efficiency of propagation of sound from one medium to another depends on the physical properties of each medium, and possibly on how well the surfaces of each medium are joined together - adhesion. Adhesion is the more difficult property to monitor as it is not just a case of mechanically keying one medium to another on a nanometre scale, but other interactions occur between the media similar to a Van der Walls type force that also contribute to adhesion. It has been extensively reported in the literature that the most sensitive ultrasonic wave to use would be ashearwave at normal incidence to the adherent-adhesive interface. Radially polarised shear wave EMATs have been used to generate the shear waves that induce a shear stress at an adherent-adhesive interface. Some of the work involved in this project has concentrated on the design, construction and characterisation of these EMATs and the supporting hardware such as current pulser circuits and the necessary low noise-high bandwidth preamplifiers. The experimental work is presented in chapters 3-7. Chapter 3 covers the performance of the EMAT systems, chapter 4 discusses the EMAT operation on birefringent aluminium plates, chapters 5 and 6 cover the results and analysis used in the adhesive bonds and chapter 7 describes a technique to monitor adhesive cure using EMATs which monitor changes at an adherent-adhesive interface and within the adhesive bulk itself.