Development of measurement procedures and analysis of data from non-destructive tests for the survey of materials and components

Abstract Infrared thermography is a fast, clean and safe technology that is used in a wide variety of applications. Thermography has a number of approaches differing in experiment setup, thermographic technique applied and the way the collected data are processed. Infrared thermography is highly dependent on the sensor selection and the experimental setup. Adequate setup and testing procedures are necessary to avoid questionable results in the thermographic investigation. This thesis addresses the problems of making the right choices for each specifically application. Infrared thermography is based on the study of temperature distribution and its evolution with time on the surface of the sample of interest. Although almost any thermal sensing device is applicable for use in thermographic experiments, modern thermography is generally performed with the use of thermal imagers (cameras). Thermal cameras allow one to obtain two-dimensional images showing the spatial distribution of temperature on the visible surface of the sample under analysis. Modern thermal cameras allow for the fast acquisition of such images, which creates possibilities for a large spectrum of potential experiments. The second component of a thermographic set up is the source of thermal influence. The primary function of this component is to bring the object of interest out of thermal equilibrium and initiate heat flows and temperature re-distribution in the sample. The choice of the source is highly dependent on the nature of the sample to be studied. Modern thermographic techniques utilize fast and high precision devices for collection of data about the surface thermal distribution. At the same time, advanced computational processing of collected data is utilized in many modern thermographic approaches for extraction of qualitative and quantitative information on structure and integrity of studied objects. Even if the measuring chain is almost the same in real applications, each individual part of the measuring chain affects the relation between the source of the infrared radiation and the output signal of the measuring system. The technological requirements regarding size, design, optical conditions, thermal and spatial resolution and many other framework conditions have to be diversified with respect to a selected experiment. This results in very complex issues that users must solve when trying to design optimum measuring arrangements or conditions. There are no simple rules and the problems cannot be solved without a basic understanding of the correlations. Besides, different algorithms have to be specifically used to extract the right information from the collected data. In this thesis, the feasibilities of experimentation using thermography are then introduced including the experimental setup. Software and data collection and processing methods used in the current work are introduced and the importance of various variables are studied. From this investigation knowledge of the importance of the experimental parameters is obtained and used in the remainder of experimental work. In Part I, the background of the material included in this thesis has been provided. Chapter 1 motivates the research and presentes the included publications. Chapter 2 gives an introduction to the principles of thermal infrared imaging and its applications. It also gives an in depth review of the main types of thermography that use the addition of energy from light sources is then presented. Chapter 3 gives an overview of the radiation detector principles used in thermal imaging systems: the background knowledge of the operation principles, the limiting factors for the detector performance, and the imaging systems. The acquisition procedure and the processing algorithms have been also introduced for interpretation of the thermal data to extract the features of interest and discussed. The results are presented in the papers in Part II. Discussions of the case studies are summarized in Section 4.1 and conclusions are provided in Section 4.2. This thesis has implemented infrared thermography in the important fields of non-destructive testing. The principles and essential theoretical background in this field have been reviewed. This background information are useful in a better understanding of the problems. Infrared thermography has experienced a great evolution in a relatively short time. Important improvements were achieved in different fields. However, there is a variety of limitations that need to be considered. Infrared thermography is highly dependent on the sensor selection and the experimental setup. It may be affected by the instrumentation and by the environment. These problems can be minimized, but only with adequate setup, testing procedures and processing algorithms, which mostly depend on the operator’s skill. Infrared thermography is a mature technique for non-destructive testing. The case studies show that, with an opportune choice of the experimental parameters, this technology can be employed to detect many types of defects and investigates many type of materials.