Fig. 2: Specimen geometries
For the fatigue specimens hand sawn cuts of 2 mm length each and placed perpendicular to the load direction act as a crack starter. Specimen preparation is kept simple since the focus lies on the monitoring methods and not on the patch performance. After cleaning the metal sheets with acetone, patches are directly bonded without using an extra adhesive layer. Specimens are heated in a temperature chamber (\(T=100\)°C, \(t=2\) h) placed on a metallic sheet and covered by a plastic foil. A vacuum pump is applied throughout the heating process in order to create a proper bonding.
2.3. Preliminary estimations
At first, a metallic plate without mechanical loading is examined using optical LT. The aim of this preliminary test is to qualitatively find out whether or not it is possible to detect a crack in the metallic plate lying underneath a local defect in the adhesive layer. The specimen design can be seen in Fig. 3.