Infrared thermography is a two-dimensional, non-contact inspection technique for surface temperature mapping. It has been widely used for quality assurance of manufacturing processes and for non-destructive evaluation of engineering products. In this study, two infrared approaches, active and passive thermography, were adopted to acquire the temperature distribution from which internal defect in metals, composite structures and electronic circuit boards can be identified. Abnormal temperature distributions due to invisible artificial defects in the specimens were observed in thermal images when the specimens were transiently heated up by external or internal heat sources. To evaluate the accuracy of the inspection results, radiography was also used so that comparison could be made. The system was applied to metal and composite samples with defects of flat-bottom holes in varying diameters and at different depths. Furthermore, Finite Element Modelling (FEM) for steady-state heat conduction was performed to study the defect-induced thermal and mechanical behaviours of the structures. All these have provided insights into the defect detection capability of thermographic imaging techniques.
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