State of the art in monitoring crack patching
For the “analysis, design and assessment of composite repairs” Jones
et al. recommend among others the method of IrT as one NDI technique
that is capable to picture damages. [7] Various studies on the
potential capacity of IrT in the damage analysis of crack patched
structures can be found in literature. The applicability of IrT methods
for the assessment of bonded repair patches was shown for instance in
[9], [10], and [11]. Avdelidis et al. analysed the
capability of active thermography methods in detecting cracks and patch
delamination [9] and Paipetis et al. showed the capability of LT to
detect artificially introduced defects in the adhesive and between the
patch layers [10]. Similar studies focussing on the repair
assessment were carried out by Grammatikos et al. in [14] and
Pavlopoulou et al. in [12]. Grammatikos et al. examined LT to assess
the off-line and online characteristics for maintenance activities,
while Pavlopoulou et al. compared LT to other NDI techniques as a
possible structural health monitoring technique assessing the repair
integrity under loading conditions. The ability using IrT methods for
the analysis of the damage process was shown for instance by Avdelidis
et al. [9, 13]. Grammatikos et al. further showed in [14] and
[15] that LT can be used as a qualitative and quantitative approach
to analyse the patch disbond process. Aiming on improving the
predictability of the repair performance under service conditions, also
the overall degradation process is of great interest. A preceding study,
performed at the institute for structural mechanics and lightweight
design (SLA), RWTH Aachen University, showed the great potential of
using IrT for the analysis of subsurface damage propagation behaviour
under mechanical loading.[8] The methods of passive IrT and optical
LT were examined with respect to their ability to monitor the adhesive
damage propagation process under quasi-static and fatigue loading.
Results show that both thermography methods are able to picture the
subsurface disbond propagation under loading conditions. The qualitative
information given by the thermal images can be used to prove or disprove
assumptions concerning the made during the design process. The method of
passive IrT turned out to be much easier to handle. Therefore, also in
this study mainly the method of passive IrT is used. The idea is to
visualize the heterogeneous subsurface damage procedures of crack
patched components under mechanical loading. The goal is to find out
whether the different failure mechanisms can be distinguished on the
thermal images and how the use of IrT can contribute to improve the
prediction quality by an improved understanding of the damage processes.