## 4 Ultrasonic Non-Destructive Testing of Headed Shear Studs
### Crack Detection Results of Headed Shear Studs  
Specimen S-1 was detected by the ultrasonic testing at before loading, 860,000 loading cycles, 1,240,000 loading cycles, 2,200,000 loading cycles, 2,870,000 loading cycles and the final failure time (3,280,000 loading cycles). No obvious structural damage caused by fatigue loading was found for specimen S-1 in the first four times of non-destructive testing. After 2.87 million loading cycles, a fatigue crack was detected in stud No. 2 of specimen S-1, and the rest of the studs were in good condition. In the DAC of stud No. 2 (Figure 10a), the abnormal echo appeared at the downward vertical distance 13.71 mm. At the blue gate position, the peak signal of the reflected wave was different from the material itself. With the deepening of the detection depth, the reflected echo amplitude gradually attenuated until it disappeared. It meant that there were cracks in stud No. 2 at the corresponding depth 13.71 mm. It is worth mentioning that stud No. 2 broke when specimen S-1 finally failed (Figure 6b). Thus, the ultrasonic non-destructive testing was effective in detecting failure for headed shear studs in the fatigue test. In the further analysis, the depth of the abnormal echo was also compared with the detailed size of the test specimen. When using the straight probe, the depth of the abnormal echo (the value of 13.71 mm) was basically consistent with the thickness of the flange plate with a value of 14.00 mm of the H-shaped steel column. The crack appeared at the root of stud No. 2, and the steel flange plate was also damaged. After 3.28 million fatigue loading cycles, specimen S-1 was subjected to the sixth ultrasonic crack testing, and the studs (studs No. 3 to No. 6) at the apparently intact side were inspected. The testing results indicated that studs No. 3 and No. 4 were still intact without fatigue cracks when specimen S-1 reached the ultimate global failure. In the DAC of stud No. 5 (Figure 10b), the abnormal echo appears at the downward vertical distance 9.04 mm and 18.20 mm. The cracks, thus, appeared in the flange plate of the H-shaped steel column and the shear stud shank. The crack in the depth of 9.04 mm would lead to the tear damage to the steel flange plate. Although there was a crack in the stud shank (18.20 mm in depth), the damage in the stud shank was not serious. For stud No. 6, the depth of the abnormal echo was 14.10 mm (Figure 10c), which was consistent with the thickness of the steel flange plate. The crack developed from the root of the shear stud to the steel flange plate. Except for the depth of 14.10 mm, the echo amplitude of stud No. 6 in a deeper position was very small. Thus, there was no obvious fatigue damage in other positions. For specimen S-2, three times ultrasonic crack testing was carried out at before fatigue loading, 700,000 loading cycles and the final failure time (890,000 loading cycles). Before loading and after loading 700,000 cycles, the ultrasonic testing results showed that the shear studs of specimen S-2 had no obvious fatigue damage. After the failure of specimen S-2 (890,000 cycles of fatigue loading), the four studs (studs No. 1, No. 2, No. 7 and No. 8) on the unbroken side were tested by using ultrasonic testing for the third time, as shown in Figure 11. The abnormal echo of stud No. 1 happened at the depth of 9.18 mm and 18.40 mm, which represented that there were cracks in the steel flange plate and the stud  
Figure 10: Ultrasonic crack testing results of shear studs of specimen S-1 in the fatigue test: (**a**) stud No. 2 after 2.87 million cycles; (**b**) stud No. 5 after 3.28 million cycles; (**c**) stud No. 6 after 3.28 million cycles. shank (Figure 11a). Comparatively, for stud No. 1, the fatigue damage in the steel flange plate was more serious, while the crack in the stud shank was very lighter. There was no abnormal echo in the DAC of stud No. 2, which proved that it was intact in the fatigue test. In the DAC of stud No. 7 (Figure 11b), there was an obvious abnormal echo at the depth of 15.97 mm.