Fig. 3: Geometry preliminary test
One layer of a biaxial CFRP prepreg material is mounted on the metallic substrate to simulate the repair patch. The defect in the metallic plate is simulated by a drilled hole of 6 mm in diameter. Two different patch configurations are tested, one with an extra adhesive layer (DuploTEC 10400, Lohmann) and the other using the epoxy itself joining the components. A small square of newsprint is intentionally worked in the specimen between the patch and the metal to simulate local patch delamination.
For LT, the measurement depth is dependent on the lock-in frequency as well as the number of lock-in periods. The pure delamination defect on the right-hand side of the specimen is used to calibrate the camera for the defect depth. The second defect is a combination of local patch detachment and a defect in the metallic part simulated by a drilled hole underneath.
Fig. 4 shows the thermal images for the lock-in frequencies of\(f=0.1\) Hz, \(f=0.2\) Hz and \(f=1\) Hz.