###### Abstract  
Many in-service bridges with steel-concrete composite beams are currently aging and experiencing performance deterioration. Under long-term cyclic loads from traffic on bridges, headed shear stands in steel-concrete composite beams are vulnerable to fatigue damage. The comprehensive understanding of fatigue behaviors and the feasible detection of fatigue damage of headed shear tends is, thus, crucial for the accurate numerical simulation of the fatigue crack propagation process. The paper, thus, experimentally investigates the fatigue behaviors of headed shear study through push-out tests of three specimens. The fatigue failure modes and cyclic strain evolution of specimens are analyzed. The fatigue lives of headed shear studs are compared with the \(S\)-\(N\) curves of the AASHTO, Eurocode 4 and BS5400 codes. The fatigue crack details of shear studs in push-out tests are then detected using the ultrasonic non-destructive testing. The results show that the root fracture is the main fatigue failure mode of shear studs under fatigue loading. The fatigue life estimations based on the three current codes (i.e., AASHTO, Eurocode 4 and BS5400) can be safely guaranteed only with different safety redundancies. The strain at the shear stud with fatigue damage shows a consistent increasing trend followed by decreasing behavior after reaching the peak value with the loading cycles. Moreover, the feasibility of the ultrasonic non-destructive testing with the combination of a strain measurement for fatigue crack details detection of headed shear studs in composite beams is proved. s 20  
the 1960s. Generally, there are two experimental methods to study the fatigue performance of headed shear studs, i.e., the push-out test and the beam test. With advantages of low cost, convenient loading and short testing period, push-out tests are widely adopted in the experimental studies of the fatigue characteristics of headed shear studs. At first, the research emphasis was the fatigue life evaluation. [PERSON] and [PERSON] [4] carried out 44 fatigue tests on shear studs under constant amplitude stress cycles and gave a stud fatigue-life evaluation equation. They also observed two failure modes in headed shear studs, one in the root of the studs (Figure 1a) and the other in the root of the weld collar (Figure 1b). [PERSON] [5] also conducted push-out tests on the shear studs specimens and observed the third failure mode, which was started from the root of weld collar and propagated to the flange of beam (Figure 1c). Based on a number of push-out tests, [PERSON] and [PERSON] [6] studied the fatigue-induced stud stiffness degradation. They found that there was a linear relationship between the stud stiffness degradation and the fatigue life. [PERSON] et al. [7] studied the bi-directional cyclic fatigue behavior of headed shear studs. Their tests confirmed that for a given range of load, the fatigue life of connectors subjected to bi-directional cyclic loading was normally longer than those subjected to unidirectional loads only. [PERSON] and [PERSON] [8; 9] reported a series of experimental works with push-out specimens to determine the fatigue life and a possible reduction in the static strength of the headed shear studs subjected to unidirectional cyclic loading. Moreover, [PERSON] and [PERSON] [10] and [PERSON] and [PERSON] [11] performed similar studies. However, all the above research focused on the fatigue behaviors of a single shear stud with a diameter ranging from 13 mm to 22 mm. With the continuous development of engineering construction, the conventional headed studs cannot meet the increasing strength requirements. Thus, some researchers then performed push-out tests on large shear studs (with a stud diameter over 22 mm) [12; 13] and grouped shear studs [14; 15; 16; 17] to investigate the corresponding fatigue lives. The results showed that the fatigue lives of the large shear studs and the grouped studs tended to be lower.