## 5 Conclusions and Discussions  
The fatigue behaviors and the fatigue damage detection of headed shear studs in steel-concrete composite beams were experimentally studied by a series of push-out tests. The fatigue failure modes and cyclic strain evolution of test specimens were analyzed. The actual fatigue lives of test specimens were compared with the \(S\)-\(N\) curves of several current  
Figure 11: Ultrasonic crack acting results of shear studs of specimen S-2 after 890,000 loading cycles in the fatigue test: (a) stud No. 1; (b) stud No. 7; (c) stud No. 8.  
Figure 12: Ultrasonic crack testing results of shear studs of specimen S-3 after 133,000 cycles in the fatigue test: (a) stud No. 1; (b) stud No. 2; (c) stud No. 8.  
design codes. The ultrasonic non-destructive testing was also adopted to detect the fatigue crack details of the shear study. With the above investigations, the following conclusions can be drawn:  
1. The root fracture was the main fatigue failure mode for headed shear studs under cyclic fatigue loading. Based on all the three current codes (i.e., AASHTO, Eurocode 4 and BS5400), the safe fatigue life estimation of headed shear studs can be guaranteed only with different safety redundancies.
2. The strain at the good shear stud gradually increased with the fatigue loading cycles until the failure of the specimen. The strain at the shear stud with fatigue damage showed a consistent increasing trend followed by decreasing behavior after reaching the peak value with the loading cycles. The strain measurements were helpful to estimate the fatigue damage to the embedded studs.
3. With the combination of a strain measurement, the ultrasonic non-destructive testing for fatigue crack detection of headed shear studs was feasible. The fatigue crack location of headed shear studs under cyclic loads can be measured.  
The fatigue behaviors, including the failure modes, the fatigue lives, the cyclic strain evolution and the fatigue crack details of headed shear studs, were experimentally investigated. The feasibility of the ultrasonic non-destructive testing with the combination of a strain measurement for shear studs crack detection in push-out tests was proved. However, there is still a lack of detailed monitoring of the fatigue crack growth process of shear studs. Thus, in future studies, the ultrasonic non-destructive testing should be applied to monitor the fatigue crack growth process of headed shear studs in push-out tests or actual in-service composite bridges. With the dynamical inspection of fatigue crack details, the further development of our previous numerical method [32] for the accurate simulation of the fatigue crack propagation process and mechanical principles of failure modes of headed shear studs in steel-concrete composite beams will be conducted.  
Conceptualization, J.X. and H.S.; methodology, J.X. and H.S.; experiments, J.X. and H.S.; investigation, J.X., H.S. and X.G.; writing--original draft preparation, H.S. and J.X.; writing--review and editing, W.C. and X.G.; visualization, X.G.; supervision, W.C.; All authors have read and agreed to the published version of the manuscript.  
The research was financially supported by the scientific research fund of the Yangzhou University through grant no. 137,012,308, for which the corresponding author is most grateful.  
The authors declare no conflict of interest.