# Research on the Fatigue Performance of Composite Girders with CSW and ST Truss
## 3 Analysis on Test Results
### Analysis of Fatigue Performance Evolution  
The load-strain relationship at CZ3 and load-displacement relationship at DZ2 up to the peak static load before the commencement of the cyclic fatigue testing are shown in Figure 7a,b, respectively. It was implied that before the cyclic fatigue test started, both strain and displacement of the composite girder showed linear increments with the increase  
Figure 5: Phased array detection: (**a**) phased array; (**b**) detection photo; and (**c**) detection result. Figure 6: Analysis on hot spot stress: (**a**) extrapolation methods; and (**b**) distribution rule. in load, demonstrating that up to the peak load, the composite girder was in an elastic stage, and therefore met the precondition for the fatigue performance test. The changing rule of the strain and displacement at key monitoring points under peak static loading with the increasing fatigue loading cycles is shown in Figure 7c,d, respectively. Figure 8 displayed the time histories of dynamic strain in the intersection zone, and how it changed under the peak static loading with the increasing fatigue loading cycles. The developing process of the fatigue crack is shown in Figure 9a. As shown in Figures 7-9, the evolution of fatigue performance of the test model could generally be divided into three phases, including initiation, propagation, and failure of the fatigue crack. The corresponding number of loading cycles was 0-1,266,000, 1,266,000-1,298,000, and 1,298,000-1,324,000, respectively. The fatigue crack initiated in the intersecting welds between the CSW and the chord, and with the increase of the number of loading cycles, the fatigue crack would develop along the three-dimensional direction of the chord and the CSW. In the fatigue crack initiation phase, the relationships between the fatigue loading cycles and strain and displacement, and dynamic strain time histories were all essentially horizontal lines, i.e., neither strain nor displacement changed substantially with the increasing fatigue loading cycles, indicating that the test model did not experience fatigue cracking and its stiffness was unchanged in the fatigue crack initiation phase. When the fatigue loading cycles reached about 1,266,000, the strain at CZ3 and CY3 began experiencing clear changes with the increasing fatigue loading cycles, but the displacement at the mid-span position changed only slightly. By also considering the phased array detection results, it implied that a fatigue crack initiated near CZ3, and had a length, width, and depth of 1.3 mm, 0.2 mm, and 0.1 mm, respectively. After the number of fatigue loading cycles reached about 1,277,000, the strain changed markedly. At that time, the fatigue crack propagated inside the weld along the chord surface, and its length increased to 2.4 mm. Figure 7: Analysis off results under static load: (**a**) load-strain curves, (**b**) load-displacement curves, (**c**) loading cycles—strain curves, and (**d**) loading cycles—displacement curves. Figure 8: Time histories of dynamic strain: (a) CZ3; and (b) CY3. Figure 9: Cont. When the number of fatigue loading cycles reached about 1,298,000, a sudden drop appeared in the time histories at CZ3. However, the corresponding time histories measured at CY3 continued rising, indicating that after the fatigue crack was initiated, the stress redistribution occurred near the cracked section. In conjunction with the phased array detection results, it implied that the fatigue crack at CZ3 had already propagated to the CSW by spanning the weld, running through the chord thickness, and increasing its length at an accelerated rate. By the comparative analysis on relationships between the loading cycles and displacement, it was illustrated that the stiffness in the mid-span position of the test model developed a clear declining trend. When the loading cycles reached about 1,324,000, the bottom surface of the chord had already been propagated by the fatigue crack, the stiffness was severely reduced, and the flexural bearing capacity was essentially lost.