High temperature strain measurement method by combining digital image correlation of laser speckle and improved RANSAC smoothing algorithm

Background radiation and airflow disturbance at high temperature reduce the quality of captured images, resulting in a lot of noises in the displacement field obtained by digital image correlation (DIC) and decreased accuracy of strain measurement. A high temperature strain measurement method (IR-DIC) by combining digital image correlation of laser speckle and Improved Random Sample Consensus (IRANSAC) smoothing algorithm for uniform deformation is proposed. In the IR-DIC method, after obtaining the noisy displacement field by the digital image correlation of laser speckle, the IRANSAC algorithm is used to smooth the noisy displacement field by removing noises with an adaptive noise threshold selection for uniform deformation. Also, when fitting the displacement field, instead of using the minimal data model in the classical RANSAC algorithm, a subset with all the remaining effective data points after removing noises are used in the IRANSAC algorithm. An experiment on tensile test of a C/C composite sample at 2000 °C in the inert atmosphere was carried out. The results showed that, the strain curve calculated by the IR-DIC method is basically consistent with the result simultaneously measured by a high temperature contact extensometer. And compared with the least square algorithm, the mean deviation between the strain curve calculated by the IR-DIC method and the strain curve measured by the contact extensometer is reduced about 13.94%.