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
