Abstract
Recently, a lot of fatigue cracks have been found in steel bridges due to the increase of the large-scale vehicles in number as well as weight. In order to consider a repairing and retrofitting method, it is necessary to estimate whether the crack propagates more or not . In this study, a method to estimate the fatigue crack propagation rate is proposed, which is based on the crack opening width measured by image analysis on a digital image recorded with a portable digital microscope. The accuracy and limitation of the method were investigated through a fatigue test carried out on a fatigue crack in welded joint specimen
