## 3 Fatigue Resistance of the Riveted Stringer-to-Cross-Grider Connection
### Fatigue Test Results  
The crack initiation was considered the failure criterion during the tests. Since the rivet holes were covered by the connecting angles, the initiation of the fatigue crack was difficult to follow. However, extensive plasticization around the rivet holes and subsequent crack initiation caused significant deformations of the free end of the monitored sample where the alternating force was acting. Then, it was a problem to ensure the set vibration mode of the sample, and the test had to be stopped. Thus, fatigue failure was defined indirectly as the inability of the specimen to sustain the applied load. Typical failures of the investigated detail are presented in Figure 15. The failures themselves were recorded only after the samples were disassembled when the tests were completed. The picture on the left side shows the out-of-roundness of the hole diameter due to material cyclic plasticity, after which, the global deflection of the beam increased rapidly. The right figure shows the  
Figure 14: Illustration photos from fatigue tests. Figure 13: Visualisation of numerical shell FEM models of the test samples. final failure of one sample induced by static loading only after finishing the fatigue test itself. Although a total of eleven fatigue tests were performed, we could finally use the results of only ten to determine the fatigue detail category, as in one case, the fatigue crack did not appear in the web of the stringer (Figure 15) but in the connecting angle (Figure 16). The shape of the growing fatigue crack is somewhat different from the shape that appears on real stringers (Figure 1). The difference is due to the different static scheme of the test specimens, which was chosen as a cantilever beam for both practical and economic reasons. The actual stringer with the connection to the cross girder according to Figure 1 can be considered at the beginning of its service life as a beam with a semirigid connection to the cross girder on both sides, albeit with relatively low bending stiffness. In the case of the fatigue crack origin, the bending stiffness of the cross-section at the connection point gradually decreased with the gradual crack growth, which made the static scheme of the stringer closer and closer to a double-hinged beam. The result is a gradual decrease in the bending moment in the connection and the related redistribution of stresses in the stringer. It is reflected by the gradual curvature of the shape of the crack growing in the direction perpendicular to the main tensile stresses, as well as by slowing down or even stopping the crack growth. On the other hand, in the case of the cantilever, which is a statically determinate structure regardless of the actual stiffness of the joint (higher than zero, of course), the bending moment at a constant loading force is constant even in the case of fatigue crack development, and the direction of the main tensile stresses does not change much. However, this discrepancy in the static schemes does not have a fundamental impact on the recorded design life of the investigated detail because the design life (in terms of fatigue) is generally related to the crack initiation, and it is reasonable to assume that the crack initiation period will be the same for both static schemes. Figure 16: Fatigue crack in the connecting angle. Figure 15: Fatigue cracks in the web of stringer. The results of fatigue tests in the form of normal stress ranges and the corresponding numbers of load cycles to the fatigue failure are summarized in Table 1. The stress range values determined from the measurements during the experiments had to be modified, as the mean stress value was always a little different from zero, but the standard S-N curves of fatigue resistance are derived for fully reversed loading.