Institution of Civil Engineers (ICE) Publication: Fatigue testing of wires from the Forth Road Suspension Bridge
ESR Technology has been involved in various suspension bridge main cable inspections both in the UK and abroad, including the Forth Road, Severn Crossing and Tsing Ma in Hong Kong. The Forensic Engineering and Materials team have a vast amount of experience relating to the examination and testing of high strength steel wires on both bridges and in other applications.
As part of the Forth Road suspension bridge main cable intrusive inspection performed in 2012 a series of fatigue tests were performed on wire specimens removed from the cable. The precise process of wire fracture within suspension bridge main cables still remains a phenomenon and information is still being collected from inspections performed on other bridges around the globe. Due to the limited amount of knowledge in this area material, mechanical testing such as fatigue testing provides vital information for understanding the potential cause of failure and aids in predicting the remaining life of cables. Further information regarding the findings from wire inspections can be found here.
A total of 65 specimens were taken from several locations on both the East and West cables. The cables consist of 11,618 strands of high strength 5mm galvanised steel wires. Only specimens classified as Stage 3 and 4, which in accordance with NCHRP 534 Guidelines possess indications of corrosive attack, were tested in order to determine the fatigue resistance properties and propensity to wire fracture under cyclic loading.
Analysis of the fatigue test data along with fracture mechanism examination using Environmental Scanning Electron Microscopy (ESEM) and Energy Dispersive X-ray Analysis (EDX) was the subject of a journal paper recently accepted for publication by the Institution of Civil Engineers. The purpose of the work was to determine possible links with wires fractured under accelerated fatigue testing compared with fracture wires removed directly from the bridge. Although some theories exist that dismiss the possibility of fatigue acting upon main cable wires, some common material characteristics exist, suggesting that this mechanism should not be readily discounted.
The test program revealed a linear relationship between the maximum crack depths and the number of fatigue cycles at failure; which indicates a steady state growth rate. However, like wires found fractured within the main cable, the size of the crack at failure showed variations. This is most likely to be due to several factors which include; crack tip geometry, possible hydrogen embrittlement mechanism, a reduction in fracture toughness and the amount of localised residual stress.
Despite the number of unknown factors involved in the examination of suspension bridge main cables, inspection has shown that the newly installed dehumidification system on the Forth Road Bridge appears to have slowed down the rate of corrosion, which in turn should reduce the cracking in individual wires. However, best practice will demand that the Forth Road Bridge main cables will continue to require internal inspections and strength evaluations for the remainder of the operational life of the bridge.
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