The orientation of the neutral axis in a cross section may be determined by setting

=0. Then, if (x*,y*) are the coordinates of points that lie on the neutral axis,

(My/Iy )x* -( Mx/Mx )y*=0

is the equation of the neutral axis in the x-y plan. Let the angles

and

be defined by:

tan

= My/Mx , tan

= y*/x*

as illustrated in fig in left. That is, the moment vector is oriented at angle

measured clockwise from the positive x-axis, and the neutral axis is oriented at angle

measured clockwise from the positive x-axis. Then: tan

=( Ix/Iy) tan

If you want make neutral axis parallel to the axis x. you must change applied moment’s orientation.

Where

is angle of the principal axis orientation, and Iyp, Ixp are the principal moments of inertia about x and y axes. The applied bending moments are:

Mx=M