Novel pulse compression favorable infrared imaging techniques for non-destructive testing and evaluation of solid materials

Abstract

Non-destructive testing and evaluation is a process adapted to reveal the surface and/or sub-surface defect details of the test specimen without impairing its future usefulness and in-service capabilities. In order to ensure the quality of products, almost all the industries such as aerospace, automotive, building, civil, electrical, electronics, mechanical etc. consider non-destructive evaluation as a mandatory test. Among the widely used non-destructive testing techniques such as ultrasonic testing, radiographic testing, magnetic particle inspection, optical testing, eddy current testing, liquid penetrant inspection, infrared thermography has been emerging as a vital testing method due to its whole-field, remote, safe, fast and quantitative inspection capabilities. It uses the acquired history of infrared emission emanating over the test specimen in order to obtain its surface and/or sub-surface defect details. This can be adopted either in a passive or in an active approach. In passive approach, thermal distribution over the test specimen is recorded at ambient conditions (in the absence of any external heat stimulus). However, due to the limited depth of penetration and inability in providing the quantitative assessment for the surface and sub-surface defects inside the test specimen, limits the applicability of this method for nondestructive testing and evaluation applications. On contrary, to detect defects located deep inside the test specimen with enough thermal contrast, active thermography is preferred. In this, an external heat stimulus with a predefined amplitude, duration and bandwidth is imposed onto the test specimen. These known characteristics of the external stimulus, helps in providing quantitative estimation of the sub-surface anomalies. In addition to the external stimulus, application of a suitable post processing scheme is preferred to enhance the thermal contrast over the test specimen for identifying its sub-surface details with improved signal to noise ratio. Among them, recently proposed pulse compression favorable methodologies accomplish this task and improves the potential surface and sub-surface defect detection performance by using specialized modulated excitations with moderate peak power sources in limited span of experimentation time. The aim of this thesis is to investigate the capabilities of pulse compression favorable aperiodic excitation schemes such as linear frequency modulated thermal wave imaging, digitized frequency modulated thermal wave imaging, Barker coded thermal wave imaging along with the associated post processing methods for improved test resolution and sensitivity of infrared/thermal nondestructive testing. A special focus is given to improve the detection performance through energy concentration rather energy redistribution to the individual involved frequency components.