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The measure of safety with constant M/N is performed in perfect analogy to that developed for uniaxial bending. Point S is a representative point of the external design forces ⎨NSd, MξSd ,MηSd⎬ , the measure of safety is given (see figure) from the ratio between the length of segments OR and OS measured along the straight line r belonging to plane Π passing through the axis N and forming a constant angle β with axis Mξ:
β = arctan (MηSd / MξSd )
The check consists to find the particular direction α of neutral axis whose corresponding spatial curve of interaction Ψ intersects the plane of interaction Π just in the point R (intersection of line r and the interaction surface. As the point R belongs to Π plane , its components MηRd, MξRd form , the, the same β angle.

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