## 2 Literature Review  
The importance of riveted lap joints in aircraft fuselage panels is reflected in a wide range of scientific and industrial literature dealing with the manufacturing, installation, corrosion and fatigue behavior. Reference [1] provides a good overview of the topic and discusses the dependence of the fatigue behavior of riveted lap joints upon the processing parameters. Some literature explicitly discusses the fatigue performance of riveted lap joints based on numerical simulations [2; 3; 4; 5] as well as experimental studies [6; 7; 8]. However, the well-defined conditions of the scientific studies are rarely fulfilled in the daily aircraft assembling processes [1]. Although the in-service quality assessment of aluminium riveted lap joints is of high interest, it is rarely discussed in scientific publications. Many authors providing overviews of aerospace specific large-scale NDT methods support the common opinion that maintenance of lightweight structures made from composite materials is the new challenge for NDT [9; 10]. This goes in line with many review articles in this field [10; 11; 12; 13; 14]. Even if the share of composite parts in new aircraft is constantly raising, riveted metal-metal and metal-composite joints will remain the most critical structural parts. Taking the labor-intensive work of manual rivet inspections into consideration, it is surprising that only a few publications discuss the automated in-service rivet inspection as a possible application of common NDT methods. Reference [15] discuss Ultrasound Lock-in Thermography as a suitable method for loose rivet detection and conclude that the comparison of aluminium-aluminium and CFRP-aluminium riveted joints shows that only damaged rivets in the latter configuration are detected with ULT. In their work, the authors use unpainted specimens which have significantly different physical parameters compared to a real aircraft fuselage part. Others detect loose contact between riveted lap joints as a consequence of faulty fasteners [16; 17]. However, such a method does not provide enough information for a technician to decide which rivets need to be replaced and which not. Given the recent technological advancements since the aforementioned works [18], it is evident that the investigations on thermal detection of loose rivets with modern technology in software and hardware need to be reopened. ULT belongs to the thermographic methods that measure the heat generated in the defect. Other active thermographic methods like pulse thermography introduce heat at the surface and record the thermal response at the same or the opposite surface. This reveals defects that influence the heat diffusion and therefore cause changes in the measured surface temperature. Although this concept is successfully used for various applications [19], the authors are not aware of any application of this method to detect defected rivet. Another well-established technology to detect surface and near-surface cracks in metallic aeronautical parts is the Eddy current technique [20; 21]. Even though the Eddy current inspection is commonly used for rivet hole inspection [22; 23; 24] and specifically applied to riveted lap joints [25; 26], no work regarding the detection of loose rivets is published. The inspection of riveted lap joints with magneto-optic imaging is described in some work [27; 28] and can be used for the automated detection of rivets [29]. However, only rivet holes with and without surface cracks are distinguished and the authors do not discuss the possibility of characterizing single rivets and their respective state. Another tool which is widely used in the automotive, aerospace and many other industries are measurements with Scanning Laser Doppler Vibrometer (SLDV) [30]. Even if the measurement principle allows only point-wise measurements of the surface vibrations, a sufficient scanning area and a point density plot enables a quasi-full-field measurement which gives a detailed insight into the vibration behavior of various specimens.