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.