Abstract
We investigated a method to measure cyclic strain without contact using aluminum foil and laser speckle. In this method, a small piece of the foil is pasted on a specimen and the specimen is loaded cyclically. The laser speckle detects surface change of the foil surface caused by fatigue of the foil. This method allows the estimation of cyclic strain occurred in the base specimen by observing change in the speckle pattern on the foil surface. The foil can be used as a kind of strain gauge. When we use this method, it is necessary to evaluate diffused pattern of the laser speckle accurately under different experimental conditions. In this paper, we derived a method for evaluating light intensity distribution of the speckle pattern. First, the light intensity distribution was smoothed by the row pass filtering and an exponential function was used to approximate the distribution. Broadness of the light intensity distribution was evaluated with the half width of the exponential function. It was found that influence by difference of the intensity of the laser light source and f value of the camera lens is very small using the evaluation method derived in this study.
