Novel analytical and experimental analysis schemes for quantitative infrared thermography for testing and evaluation of solids

Abstract

Non-Destructive Testing and Evaluation (NDT&E) is a reliable process to reveal the surface and sub-surface information of the test objects using various inspection methods without impairing their future usefulness. Among them, InfraRed Thermography (IRT) has been emerging as a reliable NDT&E method due to its fast, non-invasive, whole field, non-contact and quantitative evaluation capabilities. It is based on the principle of mapping the surface infrared emission over the object and subsequently obtaining its sub-surface details. Due to its qualitative and quantitative analysis, IRT can be used in a variety of industrial applications in the field of civil, mechanical, electrical, aerospace, automotive and bio-medical applications. IRT can be broadly classified into two main categories: passive and active thermography. In passive thermography, the natural thermal response is monitored over the test object to reveal its surface and sub-surface information. However, limited depth resolution for deeper sub-surface defects and inability to provide quantitative information limits its applicability. On the other hand, active thermography utilizes an external heat stimulus with pre-defined bandwidth, amplitude and duration to generate thermal waves inside the test object. These pre-defined characteristics of the heat stimulus assists in the quantitative analysis of the sub-surface anomalies. In addition to this, a suitable processing scheme is required to enhance the signal to noise ratio and to facilitate enhanced contrast for extraction of sub-surface details. The conventional frequency domain phase analysis post-processing approach redistributes the total imposed energy into individual frequency components, resulting limited resolution and sensitivity. In order to overcome the limitations of conventional frequency domain phase analysis, time domain matched filtering based pulse compression methodology has been adopted. Pulse compression approach accomplish this task to improves the potential sub-surface defect detection performance by using specialized modulated thermal simulations. The work presented in this thesis highlights a novel analytical approach for investigating the sub-surface features using pulse compression post processing approaches for modulated thermal wave imaging. In the first stage, a linkage is provided between various existing active thermographic methods such as pulsed and mono-frequency excited lock-in thermography and in the next stage a pulse compression based post-processing scheme has been introduced on various pulse compression favorable thermal wave imaging methods. Further, the performance of the proposed analytical approaches has been analyzed and validated with the numerical and experimentation models.