Strain distribution around a crack tip in high temperature environment using picture processing

Abstract

A new optical strain measuring method developed by the authors is described. Here, we employ a simple method which uses point recognition picture processing and the least square approximation method. The surface of the specimen for which we are going to obtain the strain distribution is marked with points using a punch or a paint marker beforehand. At each loading stage, the digital data of the black and white picture of the specimen's marked surface are taken with the television camera under the control of the computer to which the data are sent. From these data, the locations of the marked points at each loading stage are obtained by the picture processing technique. Using these locations decided, we obtain the least square approximation of the displacement distribution, and then the strain distribution. To show the effectiveness of the present technique, the method is applied to the strain measurement near the crack tip in the CT specimen of Type 304 stainless steel. Because of the high ductility of this material, the crack tip blunting is notable so that the crack tip opening displacement observed by this method is as large as 2.5 mm at the specimen surface before initiation. Next, the strain distributions of a center-notched panel made of Type 304 stainless steel under high temperature creep condition are obtained. It is found from the above two experiments that the present strain measuring technique is powerful for the measurement of high strain, say more than 1%, whereas it is unsuitable to apply to the case of small strain at the present developing stage.