A new method for fatigue life estimation of steels

A contactless method has been developed for evaluating fatigue damage of steels using laser speckle sensors. When steel specimens are subjected to cyclic loading, slip-bands are produced on the specimen surface by fatigue. The density of these slip-bands increases with the advance of the fatigue process and initial cracks appear within the persistent slip-bands. Some of these cracks emit light and lead to final fracture. When laser light is directed onto the surface of the fatigued steel, the light intensity distribution of the laser speckle pattern caused by the reflected light changes with the change of surface properties invoked by the slip-bands. It has been found that the width of the speckle pattern widens in relation to the spatial frequency distribution of the surface, and thus it is presumed that the speckle pattern broadens with increase in slip-band density. This means that fatigue damage can be evaluated by observing and assessing the laser speckle pattern. Changes in speckle pattern were observed during fatigue loading. Empirical equations have been derived in order to express the relation between the rate of change of the speckle pattern corresponding to the loading cycles and the stress amplitude. A method is proposed for estimating the fatigue life by observing the change in speckle patterns based on these equations.