Fatigue damage in bridges has been identified as a threat to bridge safety. Fatigue cracks in steel structures are well-known to occur at stresses lower than typical design stresses. Short fatigue cracks can be difficult to detect by visual inspection and therefore can be easily overlooked. Since a larger portion of fatigue life in metals is spent on crack nucleation than on propagation, it is vital to develop methods of detecting such damage. We will discuss the initial development work of an in-situ fatigue sensor to detect fatigue damage in steel bridges. The concept of the fatigue sensor is built on the strain-life fatigue analysis method and Miner's damage summation rule. The fatigue sensor is made of electrically conductive material with notched sensor arms with varying stress concentration factors; each arm is designed to fail at different numbers of stress cycles to indicate the accumulation of damage in the structural member to which it is attached. The concept has been verified for constant amplitude cyclic loading in the laboratory. A finite element analysis has been carried out for the sensor and the results are compared. Future work will address the case of variable amplitude loading.
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