Tools for ultrasonic characterization of layered media

Abstract: Many industries are dealing with composite materials and multi-layered structures of various materials. Errors in the manufacturing process may lead to defects in the final product, and hence, avoiding imperfections is crucial. The importance of repeated quality assurance online is therefore essential during the process. For quality assurance the industry needs accurate and cost-effective diagnostic methods. Ultrasonic measurement techniques are familiar to most people from their medical applications, such as looking for the fetus in the mother's womb or imaging tissue anomalies in order to detect e.g. tumours. However, applications for ultrasound are widely used in the industry today, as a nondestructive evaluation technique for many different media. In the aircraft industry for example, components are inspected before they are assembled into the aircraft and also periodically inspected throughout their useful life, by using for example ultrasonic techniques. Ultrasonic inspection is extensively used to locate tiny cracks and to measure the thickness of the aircraft skin from the outside. The objectives of this thesis is to address the advancement of methods for ultrasonic characterization of layered media. The research problem addressed is stated as: How can methods for Nondestructive Evaluation of layered media using ultrasound be developed or improved? To easier approach this question it can be divided into smaller parts which are addressed separately and in combinations. The approach to answer the research question and the project objectives is chosen to be in the field of ultrasound, because of its superior penetrating properties in solid materials. When using ultrasound and ultrasonic measurement techniques we are restricted to measure frequency dependent phase velocity and attenuation. However, these properties can be used to calculate material properties of interest in the investigated medium. The focus of the work in this thesis is on the development of a parametric model for multi-layered materials. The model is used to describe ultrasonic signals reflected from a multi-layered structure in a successive way. Results show that the proposed model can be used to deal with the research problem in this thesis. The thesis is divided into two parts. The first part contains an introduction to the research area together with a summary of thecontributions, and the second part is a collection of four papers describing the research.