1. Introduction
Metallic components exposed to cyclically varying loading tend to fatigue cracking. Adhesively bonded patches made of fibre reinforced plastics (FRP) are one repair solution that can significantly enhance the fatigue life of cracked metallic components. The repair basically consists of three major components, which are the cracked metallic structure, the FRP repair patch and the adhesive layer joining patch and the cracked component. The patch bridges the crack and loads that are applied to the metallic structure are transferred into the patch via the adhesive zone. The patch thereby reduces stresses at the crack tip which reduces crack growth. Additionally, due to the stiffening further crack opening is reduced, which again retards the crack growth. Even though crack growth can be theoretically be completely inhibited by a properly designed patch, see e.g. [1, 2, 3], cyclic mechanical loading and environmental influences cause material degradation resulting in further crack propagation during service [2, 4, 5]. But, even when the full patch functionality cannot be guaranteed, crack growth rates can still be reduced. However, for a repair with impaired functionality, the crack growth behaviour differs from theoretical performance predictions. To improve the prediction quality, it is essential to understand the damage processes under loading conditions.
For a patch repair, different damage mechanisms can occur. The two major damage mechanisms are metallic crack growth and local failure of the adhesive zone resulting in patch disbond and thereby reduced repair functionality. Depending on the repair configuration (stiffness ratio, application process, crack length, etc.) the driving damage mechanism can vary. The identification of the driving mechanism and possible mutual influences of the different damage mechanisms can be a significant factor to improve the patch design process, especially the quality of the performance prediction. As damage propagation is mainly a subsurface process, the use of non-destructive inspection (NDI) methods offers the ability to get an insight into the damage procedures of a crack patched structure under cyclic loading conditions. Optical infrared thermography (IrT) is one NDI technique commonly used in the detection of subsurface defects [6], also showing great applicability in the analysis of the damage behaviour of crack patched structures [7, 8]. The aim of this paper is to examine how IrT can help monitoring the cyclic heterogeneous damage propagation of crack patched structures. The method used is passive IrT supported by optical lock-in thermography (LT), when necessary.