The fracture mechanisms of main cable wires from the forth road suspension

The mechanism of galvanised wire strand fracture in suspension bridge main cables is still a subject which is not fully understood and is often the focus of research studies. This current study presents findings from the third cable inspection performed on the Forth Road Bridge in Scotland. An extensive test programme which included metallographic wire examination, tensile testing and cyclic fatigue testing was carried out. From the findings a discussion of the possible mechanisms contributing to wire failure is provided. The results showed that tensile strength reduction is due to extremely localised corrosion, which propagates perpendicular cracks within individual wires, and was not directly related to the general corrosion severity classification used in the industry. The required depth of propagation to cause fracture was shown to range from as low as 150 μm, up to the radius of the wire. Possible mechanisms of crack initiation and propagation are considered and include; pitting corrosion, hydrogen assisted cracking, branched cracking and corrosion-fatigue.