Fig. 4: Results preliminary test
For all frequencies tested, the square defect on the right-hand side of each picture can clearly be seen. On the left-hand sides, the square damage as well as the hole in the metal can be detected. This means, that the detection of both defect types is possible within a certain range of lock-in frequencies. It can further be seen that the outlines of the defects are more clearly visible for the lower configuration without an additional adhesive layer than for the upper one with an adhesive layer.
Nevertheless, for both configurations the defects are visible. As a conclusion one can say, that LT is generally capable to detect a damage in the metallic part of a patch repair that is lying underneath a local defect in the adhesive layer. Since a hole of 0.6 mm is not representable in dimension for a fatigue crack, the applicability of this method to detect small cracks thus needs to be further analysed. Statements concerning the influence of patch thickness cannot be made either so far.