Lock-in thermography for non-destructive testing

This wave moves into the object in and with increasing distance of the surface is absorbed and negatively phase shifted. If the wave reaches areas of the object, within which the thermophysical properties change (e.g. at delaminations or inclusions), it is partly reflected. The reflected part interferes with the wave created at the surface, whereby an interference pattern in the local surface temperature and thus in the surface radiation is caused, which oscillates with the frequency of the thermal wave. By evaluating the amplitude and the phase of the local surface temperatures one gets information about the internal structure of test object. The thermal wave at the surface of test object is not only influenced by reflections at the inner structure. Some further object parameters, e.g. porosity, have effects on the temporal and local behavior of the temperature oscillation. Thereby it is possible to compare objects with each other in relation to a variation of these properties. A substantial advantage of this method is in the fact that by the use of a periodic stimulation detailed examinations can be executed with a relatively small energy input into the object. This permits the examination of thermally sensitive components and the use of relatively simple heat sources.