**7. Conclusions**

The boundary value problem for the flow of the orthotropic material, resulting from the problem formulated in Section 2 and illustrated in Figure 1, has been solved with the resulting solution being in closed form. The stress field has been determined up to an arbitrary constant (*K*1 in Equation (20)). Emphasized are the qualitative features of the solution. In particular, if the friction law demands that the friction stress at sliding is less than the shear yield stress referred to in the principal axes of anisotropy then:


If the friction law demands that the friction stress at sliding is equal to the shear yield stress referred to the principal axes of anisotropy then:


The effect of plastic anisotropy on the solution is controlled by the constitutive parameter *c* and *c* = 0 for isotropic material. Even though the qualitative features of the solution are independent of the value of *c*, the quantitative effect may be quite significant. For example, the values of two critical angles, *αcr* and *αs* (*<sup>α</sup>s* is introduced in (38)), are sensitive to the value of c, and these angles control the overall structure of the solution.

**Author Contributions:** Conceptualization, S.A.; closed form solution, E.L.; statement of the boundary value problem P.C., project administration

**Funding:** This research was funded by the Russian Foundation for Basic Research (Project RFBR-19-51-52003) and Vietnam Academy of Science and Technology (Project QTRU01.05/18-19).

**Acknowledgments:** EL and PC acknowledge support from grants RFBR-19-51-52003 (Russia) and QTRU01.05/18-19 (Viet Nam).

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