Investigation of Infrared Thermography NDE Techniques for Use in Power Station Environments

Abstract

Three active thermal methods capable of detecting surface breaking cracks in metals are considered in this Thesis. The three thermal methods exploit different means of excitation, each with practical advantages and varying abilities to detect specific types of crack morphology. Thermosonics uses a broadband, high power ultrasonic input to vibrate the test-piece. Defects damp the vibrational energy into heat which is imaged by a thermal camera. Laser-spot thermography uses a short laser pulse to spot heat the surface of the test-piece, and the subsequent radial heat diffusion is then observed. Defects can cause both increased emission of infrared and localised increases in thermal impedance, both effects causing distortion of the radial heat diffusion. Eddy-current induced thermography uses a high power magnetic field to induce a flow of current inside the test-piece. Defects create a localised increase in electrical impedance, diverting the electric field around the defect. This diversion of current flow causes neighbouring regions of high and low current density, the corresponding Joule heating imaged by a thermal camera. In this Thesis the three methods are explored experimentally. For laser-spot thermography and eddy-current induced thermography the physical phenomena are characterised and experimental best-practice for short pulse excitation determined. The effect of crack opening on each of the three methods is found to give insight into which applications the methods are most suited. It was found that the relationship between crack opening and detectability was complex for thermosonics, relatively linear for laser-spot thermography, and that eddy-current induced thermography is largely insensitive to crack opening. The methods are tested for the feasibility of detecting cracks in Inconel buried beneath metallic and ceramic coatings typical of gas turbine blades, with thermosonics and eddy-current induced thermography found to be viable methods. A study of the detectability of a large number of cracks in steel, titanium and Waspaloy by eddy-current induced thermography is detailed, and from this data the probability of detection is established. Eddy-current thermography is shown to be an extremely sensitive method capable of detecting fatigue cracks of approximately 0.25 mm in steel and 0.50-0.75 mm in titanium and Waspaloy. The practicality of the thermal methods is discussed, and the methods put into the context of the wider field of NDE. Based on the works in this Thesis it was found that for most applications eddy-current induced thermography is the most appealing thermal method since it is highly sensitive, rapid, non-contacting and relatively easy to validate. However, both thermosonics and laser-spot thermography remain useful alternative inspections for more niche applications.