Enhancement of strain measurement accuracy using optical extensometer by application of dual-reflector imaging

摘要

At present, the accuracy of strain measurement using a common optical extensometer with 2D digital image correlation is not sufficient for experimental applications due to the effect of out-of-plane motion. Therefore, this paper proposes a dual-reflector imaging method to improve the accuracy of strain measurement when using a common optical extensometer, with which the front and rear surfaces of a specimen can be simultaneously recorded in the sensor plane of a digital camera. By averaging the strain in two optical extensometers formed on the front and rear surfaces of a specimen, the effect of any slight out-of-plane motion can be eliminated and therefore the strain measurement accuracy can also be improved. Uniaxial tensile tests with an Al-alloy specimen, including static loading and continuous loading, were conducted to validate the feasibility and reliability of the proposed method. The strain measurement results obtained with the proposed method and those obtained with an electrical-resistance strain gauge were found to be in good agreement. The average errors of the proposed method for the two continuous loading tests were found to be 8 +/- 10 mu epsilon and -6 +/- 8 mu epsilon. Given that no correction sheet or compensation specimen is required, the proposed method is easy to implement and thus especially suitable for determining the mechanical properties of brittle materials due to the high level of accuracy with which strain can be measured.