Expermental and analytical study of thermographic techniques for stress analysis and materials characterization

The mechanical designer is often called upon to solve problems of components and even very complex structures subjected to both static and dynamic loads; in this framework, the capability to quantify in a reliable and not invasive way the mechanical and physical characteristics of materials and components is fundamental. In this respect, the development and optimization of increasingly efficient techniques for stress analysis and material characterization is a topic of strong engineering interest. The Infra Red (IR) Thermography is the basis of a family of such the techniques, precisely thermographic techniques, which over recent years attracted the attention of both the academic and industrial sector thanks to the peculiarities that make them competitive with other characterization techniques (such as strain gauges, X-ray, photoelasticity, classical destructive tests ecc..), which, even if well established, have limits of practical application or instrumentation costs or the quality and quantity of the information acquired. The potential fields and methods of use of thermographic techniques are different, the versatility of these techniques lies in the possibility of using the temperature signal as a sentinel of physical phenomena that depend on the characteristics under study. The motivation behind this research study is the important impact in the academic and industrial fields that the implementation of innovative thermographic techniques can have. Hence the need to investigate the application potential offered in the field of experimental mechanics The main goal is to explore this potential through the study, development, optimization and validation of IR thermography-based procedures for the characterization of materials, aiming at increasingly accuracy, reliability and precision. In particular, the study was focused on four main promising application, which are: 1)The evaluation of the superficial stress field of components and structures by means of the Thermoelastic Stress Analysis (TSA), 2) The identification of residual stress exploiting the mean load effect on the Thermoelastic signal, 3) the fracture mechanics characterization of components, which included the combination of TSA equations with the linear elastic theory to find the Stress Intensity Factor and the identification of the locally plasticized area around the crack tip through the interpretation of the thermal dissipation footprint, 4) The temperature measurement in aerospace applications implementing the Dual Color technique, that is a free-emissivity temperature measurement technique suitable to characterize the spaceships Thermal Protection System performance in extreme environmental conditions during atmospheric re-entry tests. Even if different levels have been reached, in relation to the point of maturity of the application itself, each of them was addressed with a systematic approach which saw a first phase of study of the state of the art on the solutions proposed for the particular problem with an examination of their validity, limits and potential; the analysis therefore focused on the physical principles underlying all the phenomena involved (with particular attention to heat transfers and including sources of noise), aiming at an analytical description of the system that was as close to reality as possible and identifying particular cases of interest that allow simplifications of the most complex relation. Once defined the analytical relations between the observed phenomena and the thermal response, the model validation involved the planning and implementation of experimental campaigns on sample specimens whose response can be known or measurable with an alternative validated comparison technique. In this phase was important to identify the main variables acting on the system. It was not a one-way path, but an iterative process in which experimental detection can provide evidence of relationships and phenomena that should be involved in the analytical description, until reaching the desired goal of a stable and precise prediction. In this regard, a further step was to propose a Robust Design-like approach to optimize the measurement performances of thermographic techniques. In fact, even if the availability of complex analytical models linking signal and measurement gives the chance to assess the capability of an experimental technique to obtain the required measurement with the available sensors, the complexity of relations and the effect of random noise variables make the implementation of statistical methods a useful tool to study the applicability of the technique.