**4. Conclusions**

A new R-value measurement method based on the use of two stereo-DIC systems has been presented in this paper. This method provides a new possibility for direct and simultaneous measurement of strains in the length, width, and thickness directions, thus allowing for direct measurement of the R-value. During the measurement process, it was found that DP980 and aluminum alloy 6061 do not follow the constant volume assumption in the necking stage. It was also found that a polymer material does not follow this assumption during the entire tensile test. The test results have certain significance for the research of different materials. Compared with the traditional R-value measurement method, this method has the advantages of direct measurement without assumption. The constant volume assumption has certain limitations, especially in the necking stage of material tensile deformation and polymer tensile deformation. Thus, the R-value obtained by the direct measurement method has more research value.

A main challenge to measuring strains in the third direction is that the strain data in this direction has more noise than other two strains due to the small value of the thickness. A standard DIC algorithm cannot be applied for such a measurement without using a special noise-removing algorithm, such as RANSAC presented in this paper. Some application limitations include the cost of four cameras and the large space requirement of the front and rear camera groups. Nevertheless, this paper provides an alternative method that directly measures R-value. The results for direct R-value measurement, as well as the comparison with the indirect R-value method, show that the method and the algorithm proposed is feasible and effective.

This paper has directly and indirectly measured the R-value of different materials, compared the results of the two measurement methods, and shown the accuracy of the

direct measurement method. However, these measurement results are based on uniaxial tensile tests. Thus, it cannot completely reveal the reasons and limitations of the constant volume assumption. Future work may be necessary to study the microstructure of different materials before and after uniaxial tensile tests and determine the scope of application of the constant volume assumption through the analysis of the material microstructure.

**Author Contributions:** Conceptualization, S.F. and L.Y.; methodology, S.F., X.Z., B.Z.; validation, S.F., G.Z. and B.Z.; formal analysis, B.G.; investigation, G.Z.; resources, S.F., X.Z., B.Z.; data curation, S.F., G.Z.; writing—original draft preparation, S.F.; writing—review and editing, L.Y.; visualization, S.F., G.Z.; supervision, L.Y.; project administration, L.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing not applicable.

**Acknowledgments:** The authors would like to express their sincere thanks to Bernard Sia, who made careful correction of the manuscript as well as the English.

**Conflicts of Interest:** The authors declare no conflict of interest.
