Ultrasound field measurement and modelling for non-destructive testing

Ultrasonic Testing (UT) is one of most important methods of Non-destructive Testing (NDT) and ultrasonic waves can be generated and detected by means of numeric methods. This thesis focuses on piezoelectric transducers and Electromagnetic Acoustical Transducer (EMAT). For testing with piezoelectric transducer (also called a probe), couplant has to be applied between the test material and the probe allowing for ultrasonic waves generated in the probe by an active piezoelectric crystal to propagate into the testing material. The couplant can be water, mineral oil, or gel, dependent on the applications concerned and material compatibility, for example, water for immersion tests or automatated inspections. No two piezoelectric transducer designs are identical in terms of their frequency range, beam propagation characteristics and directionality. The shape, dimensions, backing and matching of the transducer to the pulse generator together play a major role in the generation of the ultrasonic waves. Furthermore, the quantitative interpretation of pulse echo data obtained when such transducers are used in nondestructive evaluation (NDE) requires a complete knowledge of the ultrasonic field transmitted. Ultrasonic fields of circular probes have been studied experimentally using a miniature probe and theoretically with models developed to predict ultrasonic field of such a probe. Good agreement has been observed. In a fluid, the ultrasonic field generated by a circular piezoelectric transducer can be described in terms of a combination of locally plane waves that radiate in the geometric region straight ahead of the active transducer element and edge waves radiating from the rim of the element. When a piezoelectric transducer is directly mounted onto a solid material, the ultrasonic field includes locally plane longitudinal waves, edge longitudinal waves and mode-converted edge shear waves. Both cases can be studied using miniature piezoelectric probes. For electrically conductive materials, EMATs can be used for generation by means of Lorentz force or magnetostriction or both, and detection. EMAT technique is non-contact and couplant free and can work at high temperature. These attributes make it ideal for inspection in extreme conditions, such as high temperature, high speed, rough surface, etc. This thesis focuses on Lorentz force generation. The main disadvantages of an EMAT detector are its lower sensitivity compared to a piezoelectric probe and it is not straightforward to miniaturise the device to operate as a point sensor for the range of wavelengths of interest here. Therefore, optimal EMAT design is extremely important for successful EMAT application. Ultrasound may be generated without presence of external magnetic field as excitation electric current provides magnetic field as it induces eddy currents in the material under test, which creates Lorentz forces for ultrasonic generation. Where external magnetic field is applied, EMATs have to be designed correctly to achieve enhanced efficiency. As an example, Rayleigh wave EMAT generation has been studied. It is found that where external magnetic field is applied, constructive or destructive effects have been observed, which is understood dependent on direction of the external magnetic fields applied relevant to electric current direction. Optical interferometer to measure the true normal displacement of the solid surface with a resolution in the order of nanometres, but it is much more complex than an EMAT and a piezoelectric probe and requires an optically flat surface. The receiving EMAT detector measures particle velocity. By careful design, in-plane or out-of-plane (or both) velocities can be chosen for detection. This capability is very useful for the detection of longitudinal waves, shear waves, Rayleigh waves or Lamb waves efficiently. The ultrasonic pulse-echo technique has been widely used in ultrasonic NDT. Ultrasonic pulse-echo responses and ultrasonic field signals are not the same. Typically, edge waves are rarely seen in a pulse echo response because the plane waves that are normal to the major face of the active crystal of the same probe are nearly in phase to constructively result in a significant signal whilst edge waves arrive at the active crystal in different directions and different phases cancelling each other and destructively producing only a small signal that is barely observable. As an example of ultrasonic pulse-echo application, weak bond evaluation, has been performed. Weak bond evaluation has always been a challenge. As an example of practical applications, this study has evaluated Integrated Circuit packaging in electronic industry using scanning acoustical microscopy. The relationship among resulting ultrasonic C-scan images, destructive mechanical failure measurement, degradation cycles have been observed. The result is promising indicating the SAM is a very useful tool for weak bond evaluation. Ultrasonic field measurement using a miniature probe and specially design EMAT is very important to characterize and standardize a probe. Such a technique can also find its applications in defect detection and categorization, which has not been considered in this study.