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.