# Influence of Fatigue Crack Formation and Propagation on Reliability of Steel Members
###### Abstract  
During the years of bridge exploitation, many degradation processes and external influences attack its structure. Therefore, bridge reliability and durability is decreasing in time. On the other hand, the traffic load remains almost the same or even higher than in the past. However, bridges should not to become the limiting component of communication capacity and traffic reliability. Regarding to reliability, bridges should be assessed from the viewpoint of the Ultimate Limit States (ULS) and Serviceability Limit States (SLS). Within the ULS, cross-sections and members are verified for various types of stressing and their combinations, and also for fatigue at the same time. The cross-sectional verification, e.g., for bending stresses and fatigue, is done independently according to corresponding criteria of the ULS determined for strength verification a fatigue assessment separately. The presented article deals with the steel railway plate girder bridge with bottom member deck, in which there is an effort to prove the effect of the crack in tension bottom flange due to fatigue stressing on the change of bending resistance over  
the damage equivalent stress range \(\Delta\sigma_{\rm E2}\) to the fatigue strength given in [25] in accordance with the corresponding bridge structural detail. Damage equivalent stress range should be calculated using the reference stress range \(\Delta\sigma_{\rm p}\) and the damage equivalence factor \(\lambda\), which determination is given in [24] for railway and road bridges separately. The verification is done for determined bridge lifetime, so that this assessment does not reflect the change of structural member resistance in time due to crack growth.  
The fatigue cracks on tension flange of steel structure have been an issue since the beginning of steel bridge design activities. The objective has been to use such preventive activities that would eliminate gradual growth and propagation of cracks resulting in considerable deterioration of reliability of the bridge. Though more and more advanced modifications and control methods have been introduced, the cracks still appear. The standard [25] makes it possible to use a damage tolerance method as an alternative under certain conditions. Methods proposed include the observation of a \"critical\" crack [26]. That term, however, has not been defined for real welded structural elements. It is hardly possible in practice to use a small test sample for direct application [27; 28].  
In connection with the fatigue damage of components, the design method is very important. If the ULS for fatigue are applied when designing elements in terms of design effects of the traffic load, the verification is done for the real traffic load throughout the designed service life. The acceptable size of the fatigue crack should be such so that the bridge superstructure might, at the end of its service life, still provide the required level of reliability. The tension flanges are the most stressed elements of the superstructure. Therefore, the standardized level of reliability for fatigue is determined for the tension elements.  
The paper is focused on the influence of crack formation and propagation due to fatigue on the change of steel member's reliability from the viewpoint of the Ultimate Limit States (ULS) in time. Because the work is focused on member where a measurable crack has already appeared and has propagated (grew), the linear elastic fracture mechanics (LEFM) was considered.