Strain measurement based on laser mark automatic tracking line mark on specimen

Abstract

Conventional video extensometers, using a measurement mark on specimen to obtain material strain, have a problem with deformation of the measurement mark. Therefore, the accurate position of the measurement mark is difficult to evaluate, and measurement accuracy is limited. To solve this problem, a strain measurement method based on a laser mark automatically tracking a line mark on the specimen is proposed. This method is using an undeformed laser mark to replace the line mark to calculate the specimen strain and eliminates the measurement error induced by the deformation of specimen marks. The positions of the laser mark and the line mark are achieved by using digital image processing. Automatic tracking is realized by means of an intelligent motor control. Also, the strain of the specimen is obtained by analyzing the movement trace of the laser mark. A video extensometer experimental setup based on the proposed method is constructed. Two experiments were carried out. The first experiment verified the validity and the repeatability of the method via tensile testing of the specimens of low-carbon steel and cast iron. The second one demonstrated the high measurement accuracy of the method by comparing with a clip-on extensometer.