Distortion-Induced Fatigue Crack Growth

Out-of-plane distortion of transverse cross-bracing systems in bridge superstructures has been known to cause localized fatigue cracking. The cracking is a result of the differential displacement of adjacent bridge girders, which causes high stress concentrations in unstiffened web-gap regions at the interface of girder and stiffener connection plates. Fatigue cracking within the web gap can occur because of cyclical loading scenarios. Rehabilitation efforts have shown to be unpredictable in preventing crack propagation once through-thickness crack profiles have developed. Therefore, understanding the mechanisms causing fatigue crack propagation in such detail is essential in developing effective repair methods. In this study, numerical analyses of typical web-gap connections are conducted using the extended FEM to characterize crack directionality and growth rate. Simulations varying the web-gap length, stress range, loading ratio, and initial crack length were used to determine the primary variables effecting crack growth in the web-gap region. Increasing the web-gap length was noted to stabilize crack propagation rates, resulting in stable growth, which can be correlated to the Paris Law. Elongation of horizontal initial crack lengths resulted in reorientation of crack growth. In stiffer web gaps, a combined high stress ratio and long initial crack length resulted in crack profiles propagating into the flange-web interface with eventual crack arrestment. The study suggests that horizontal extension of initial cracks has the potential to cause the crack to arrest, after stable propagation, without adversely compromising local structural integrity of bridge superstructures.