## 3 Methods
### In-Plane Heatwave Thermography  
As mentioned, active thermography based on a time-varying amount of heat flux at the specimen surface is not known as a successful method for defect rivet detection. Most of the applications of this method are on structures with homogeneous surfaces containing internal heterogeneities parallel to the surface. In this work, however, the thermal diffusion perpendicular to the specimen surface is of major interest. In general, thermal diffusion happens in all spatial directions and cannot be \"steered\". However, heat diffusion in plate-like structures experiences boundary conditions similar to \"guided sonic waves\" which are regularly used in ultrasonic inspections. Far from the heat source, the heat diffusion is mainly parallel to the plate surface. Hence, measuring the surface temperature reveals information of cracks, inhomogeneities or thermal obstacles perpendicular to the surface since they disturb the heat diffusion in the plate. This principle is successfully applied by for example, Reference [35] which detected thin (1 um to 25 um) infinite as well as finite cracks in stainless steel plates [35; 36]. To detect such small cracks, the aforementioned studies applied a contactless and periodic Laser-excitation of the specimen in combination with lock-in post-processing.  
This work proposes a similar method called In-plane Heatwave Thermography (IPHWT) which is based on the temperature change at the surface generated by a local heat pulse. A challenging point is the signal evaluation because the relevant small signal change must be identified within a large general temperature increase. This is due to a different thermal excitation and requires a significantly enhanced post-processing for the feature extraction. The thermal excitation documented in this paper is realized with a simple pulse excitation by a Peltier element attached to the surface of the fuselage panel. Although a contactless heat excitation as performed by Reference [35] has some advantages, it is not an option in the framework of this work. The authors want to highlight the common practice of the term \"thermal wave\" in thermal NDT.  
The authors want to comment shortly on the term \"wave\" in the expression \"In-plane Heatwave Thermography\". The space-time behavior of the temperature is described by a diffusion equation and not a wave equation. Especially for pulse excitation, there is no periodicity of the temperature in time and/or space. Nevertheless, the term \"heat wave\" is widely accepted in the NDT community for periodic [37] as well as pulse excitation [19]. This is partly justified, because for periodic excitation the solution has the form of a strongly damped and dispersive wave [37] and because the pulse excitation solution can be Fourier decomposed in a series of periodic excitations.  
For the feature extraction of the recorded thermal heatwave propagation, two methods are proposed. One is based on a local and the other one on a global pixel comparison. The first method is sensitive to a local spatial gradient of the temporal surface temperature while the latter method is sensitive to variations of pixel behavior compared to a wider range of pixels in the video. Below, both methods will be described separately.