Fatigue Crack Growth Rate of Cable-Stayed Portion of Runyang Bridge: Part II – Steel Wire Crack Growth Due to Disproportionate Cable Tightening/Loosening and Traffic Loading

Having completed an investigation in Part I that deals with the fatigue crack growth in the cables of the Runyang cable-stayed bridge based the design data for the cables, Part II of this work is concerned with the fatigue crack growth of the steel wires being the constituents of the cables. The objective is not only to check the compatibility of the fatigue crack growth behavior of the cable and wire but also the independent design fatigue limits imposed on both the cable and wire. In particular, assumptions are made to relate the mechanical and fatigue properties of the cable in relation to those for the steel wire. The mean stress and stress amplitude are accounted for simultaneously such that the increase and decrease of the initial cable tension on the fatigue cracking of steel wires can be assessed. The increase use of higher strength wire materials calls for consideration of damage at both the micro- and macro-scale. That is more advanced material can absorbed more energy at the microscopic scale in contrast to energy dissipated by macrocracking. A dual scale fatigue crack model is therefore required to address the micro/macro interaction for the mechanical and fatigue properties of the material and their influence on crack growth.

Under traffic and an increase of the initial wire tension of α=1.6 with α=1 being the design reference for the cable, a defect of 0. 1mm in the wire can extend more than 60% across a 5mm diameter wire after 9×10⁵ cycles which is about 45% of the fatigue life of the wire. According to the cable design specification, the defect should extend only 0.1mm after 2×10⁶ cycles. This shows that the fatigue life of wires is sensitive to initial defects and to the initial wire tension. These conditions are not considered in the present design of the cable nor the wire. What this means is that inspection and maintenance procedures are not thought of at the design stage; they are established during the life span of the bridge. Even more of a disadvantage is that there are no basis for comparing the monitored data with wire and/or cable damage analyses in order to gain an understanding of the failure mechanisms of wire and cable.