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.