Experimental Investigation of Retrofitting Techniques
Fatigue is a mode of failure that affects 80-90% of all steel structures. With more steel bridges reaching the end of their design life each year, fatigue failure is prominent and many steel bridges must undergo repair if they’re to remain in service. The state of New South Wales in Australia has nearly 5000 steel bridges with 17% being older than 50 years old Henderson. Although the risk of fatigue failure generally increases proportionally to the number of years steel bridges are in service, this is not always the case. Many studies have been conducted showing that poor detailing, increased loadings, imperfections, weld defects and holes can significantly reduce the fatigue life of the structure. This means that any structure is susceptible to fatigue whether the materials are old or new. Over the last century an increase in loading especially in the road and railway transportation industry has caused a significant impact to the design life of steel bridges.
The increased loads experienced now were not accounted for in initial designs. There are three phases that take place to produce fatigue failure: crack initiation, crack propagation and fracture. Crack initiation occurs from cyclic loading to which the material goes from its initial condition to the development of a macro-crack. The crack propagation phase is the stable growth of the crack from the crack initiation phase. The final phase is fracture where the unstable crack growth ultimately leads to the failure of the material. Fatigue can be further categorised into low cycle fatigue or high cycle fatigue dependant on the magnitude of stress and the number of cycles until failure.
When a structural component survives a large number of cycles between 104 and 108 , high cycle fatigue is used to describe the situation. Low cycle fatigue typically causes failure in the structural component in less than 104 cycles. The primary difference between high and low cycle fatigue is the stress regime in which they are loaded. The stress regime associated with high cycle fatigue typically operates within the linear elastic region of the stress-strain curve. In contrast the stress regime associated with low cycle fatigue may enter the inelastic region of the stress-strain curve occasionally..
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