# Fatigue Performance Test and Numerical Analysis of Composite Girders with CSW-CFST Truss Chords
## 3 Analysis of Fatigue Performance Results
### Fatigue Failure Process  
The test results of key strain and displacement measurement points in the composite girder with CSW-CFST truss chords, after different numbers of reversed loading cycles, are presented in Figures 10 and 11, while Figure 12 shows the time histories of strains at key measurement points. In Figure 10, the strain is the maximum value corresponding to the peak load when experiencing different numbers of reversed loading cycles. The evolution of fatigue damage using Figures 10-12 was divided into three stages: fatigue crack initiation, propagation, and failure. During the initial 2,045,000 reversed loading cycles, both strain and the displacement of any key measurement point experienced no obvious changes. Figure 11: Fatigue loading cycles-displacement curves: (**a**) WL2; and (**b**) WR2. Figure 9: Distribution and comparison of hot-spot stress. When the number of reversed loading cycles reached 2,045,000, both strain at XL3 and displacement at WL2 gradually started increasing with increase in the reversed loading cycles. By combining the observation with the phased array nondestructive detection, as shown in Figure 13, where \(C_{\rm L}\) and \(h_{\rm L}\) represent the length and the depth of the fatigue crack in the left side of the section, \(C_{\rm R}\) and \(h_{\rm R}\) represent the length and the depth of the fatigue crack in the right side of the section, and the red line represents the fatigue crack, it can be concluded that the girder started cracking at XL3, with the length, width and depth of fatigue crack being 0.8 mm, 0.1 mm, and 0.1 mm, respectively. The fatigue life consumed before fatigue crack initiation accounted for 93.0% of the total reversed fatigue loading cycles. Compared to that of the composite girder with CSW-ST truss chords reported in [11], concrete in the chord lengthened the service life of the composite girder with CSW-ST truss chords by about 61.5% under the action of the same fatigue load. When the reversed loading cycles were between 2,045,000 and 2,145,000, the strain at XL3 and displacement at WL2 grew rapidly. The time history of strain at XL3 had a point of inflection and then exhibited a stepped decline when the reversed loading cycles  
Figure 12: Time histories of strain: (**a**) XL3; and (**b**) XR3. Figure 13: Fatigue crack detection results: (**a**) initiation; (**b**) propagation; and (**c**) failure. reached about 2,145,000, indicating that the fatigue crack propagated through the chord wall thickness until it penetrated the wall completely. After the reversed loading cycles exceeded 2,145,000, the fatigue crack propagated further along the circumferential direction of the corrugated steel web and the chord. After about 2,194,000 reversed loading cycles, the length of the fatigue crack reached about 269.7 mm, and it crossed the very bottom of the chord. At that time, the displacement of the composite girders also reached a maximum value (15.2 mm), which exceeded 1/600 of the calculated span length, namely 14.6 mm, indicating that the composite girder essentially lost its loading capacity and entered the fatigue failure stage. The propagation rates of fatigue cracks in different directions can be further compared. The relationships between fatigue crack propagation depth or length and the fatigue loading cycles of the composite girder with CSW-CFST and CSW-ST truss chords are shown together in Figure 14.  
It can be seen in Figure 14a that the fatigue crack propagation depth-loading cycle curves of the composite girder with CSW-CFST truss chords were all above those of the composite girder with CSW-ST truss chords, indicating that, for the same fatigue load and same fatigue crack propagation depth, the reversed fatigue loading cycles borne by the composite girders with CSW-CFST truss chords were greater than that borne by the composite girder with CSW-ST truss chords.