Consider a beam under combined loading. Axial force, bending moment and torsion. I'm interested in determining the principle stress in any stress element on surface of beam. Well as per rule i've to show my attempt so: normal stress=axial load/beam crosssection shear stress=torsion*radius/polar MOI bending stress=moment * radius/ MoI about NA normal stress and bending stress have same line of action so undergo vector addition. Now we have a normal stress and shear stress. We got principal stress and orientation from Mohr's circle. Sounds good... But it didn't work. Where did i go wrong?