*Article* **A New and Direct R-Value Measurement Method of Sheet Metal Based on Multi-Camera DIC System**

**Siyuan Fang <sup>1</sup> , Xiaowan Zheng 1,2 , Gang Zheng 1 , Boyang Zhang 1 , Bicheng Guo <sup>1</sup> and Lianxiang Yang 1, \***


**Abstract:** More and more attention has been given in the field of mechanical engineering to a material's R-value, a parameter that characterizes the ability of sheet metal to resist thickness strain. Conventional methods used to determine R-value are based on experiments and an assumption of constant volume. Because the thickness strain cannot be directly measured, the R-value is currently determined using experimentally measured strains in the width, and loading directions in combination with the constant volume assumption, to determine the thickness strain indirectly. This paper provides an alternative method for determining the R-value without any assumptions. This method is based on the use of a multi-camera DIC system to measure strains in three directions simultaneously. Two sets of stereo-vision DIC measurement systems, each comprised of two GigE cameras, are placed on the front and back sides of the sample. Use of the double-sided calibration strategy unifies the world coordinate system of the front and back DIC measurement systems to one coordinate system, allowing for the measurement of thickness strain and direct calculation of R-value. The Random Sample Consensus (RANSAC) algorithm is used to eliminate noise in the thickness strain data, resulting in a more accurate R-value measurement.

**Keywords:** R-value; thickness strain; digital image correlation; multi-camera DIC; non-destructive testing
