The highest strength according to (2) corresponds to the optimal winding angle α * (or the optimum ratio of applied stresses tgβ=σθ/σz) [4, 5,   The use of piecewise linear approximation (2) -(3) in computer calculations is more difficult than using a single expression for the strength criterion. In the biaxial tension of the tubes under stresses σz and σθ, the experimental **data** in the σz-σθ coordinates are well described by the ellipse section (5 in Fig. 2). The ellipse passing through points on the axes (σz(α) and σθ(α)) corresponding to the strengths under uniaxial loading, and through a point on the optimal loading line (4) from the origin to the length associated with the strength (3) along the fibers σz * (0): 
z z z z z t t t t                     (5)
where σz * (0), σz * (45) are the tensile strengths of tubes with windings (0) and (±45 O ); t=tgα.