**7. Conclusions**

An accurate rigid plastic solution for a general strain-hardening viscoplastic material model has been found. The solution describes the plane strain process of bending under tension due to large strains. The elastic portion of the strain tensor has been neglected. However, the effect of elasticity on the accuracy of solutions is negligible except at the beginning of the process [34].

The numerical solution reduces to an ordinary differential equation in a non-standard form. A special numerical procedure is required to solve this equation. The procedure is described in Section 5. It is used to determine the stress field and the bending moment in the pure bending of a sheet made of a material obeying Equation (46). This numerical solution is presented and discussed in detail in Section 6.

The solution presented may be considered a starting point for designing experiments to identify constitutive equations. It is advantageous in this respect that the general solution is not restricted to a specific function or even a class of functions but is valid for an arbitrary dependence of the yield stress on the equivalent strain and the equivalent strain rate.

The solution can be used as a benchmark problem for verifying numerical codes. This necessary step applies to different constitutive equations, including equations used in metal forming applications [35–37].

**Author Contributions:** Conceptualization and writing, S.A.; formal analysis and writing E.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was made possible by grant 20-69-46070 from the Russian Science Foundation.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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

#### **References**

