By methods of homogeneous solutions and the spectral theory of operators, the construction of solutions of the Saint-Venant problems of tension-torsion of a cylindrical tube with helical anisotropy is reduced to integration of boundary-value problems for ordinary differential equations with variable coefficients. The solutions are constructed by analytical and numerical methods. Elements of the stiffness matrix and the stress-strain state are analyzed, depending on the problem parameter.The present paper describes the construction of solutions of the Saint-Venant problem of tension and torsion of a cylinder possessing helical rhombohedral anisotropy. By methods of the spectral theory of operators [1-6], the problems are reduced to integration of boundary-value problems for ordinary differential equations. An analytical solution is obtained for a particular case of cylindrical rhombohedral anisotropy. The solutions of problems for a cylinder with helical rhombohedral anisotropy are constructed by two methods: for small values of the dimensionless parameter τ 0 = aτ , where a is the cylinder radius and τ is the "torsion" (characteristic of helical anisotropy), the analytical solution is constructed by the method of the small parameter; for arbitrary values of τ , the solution is obtained by means of numerical integration of appropriate boundary-value problems.
CONSTITUTIVE RELATIONS OF THE ELASTICITY THEORY AND FORMULATION OF BOUNDARY-VALUE PROBLEMS
Formulation of the Problem. Let us consider a cylindrical solid occupying the volume V = S ×[0, L](S is the cylinder cross section and L is its length). We denote the side surface by Γ = ∂S × [0, L], where ∂S is the boundary of the cross section S. We align the origin of the Cartesian coordinate system Ox 1 x 2 x 3 with the geometric center of gravity of one of the end faces of the cylinder and direct the Ox 3 axis along the cylinder centerline. This coordinate system will be called the basic coordinate system. To describe helical anisotropy, we introduce a helical cylindrical coordinate system (r, θ, z) related to the basic coordinate system by the expressionswhere τ = const. The transition to the cylindrical coordinate system is made because the main attention will be paid below to solving the problem of a cylinder with a ring-shaped cross section S = [r 1 , r 2 ] × [0, 2π] (r 1 and r 2 are the inner and outer radii of the cylinder, respectively).At r = const and θ = const, relations (1.1) are parametric equations of the helical line, with τ = 2π/h (h is the helical pitch). The radius-vector of the points of the helical line is presented in the form R = re 1 + ze 3 ,