1. The problem statement, all variables and given/known data I want to understand why in noncicular members in torsion book tells me that surface of a member is free and so no shearing stresses there. This is from wikipedia for shafts: Note that the highest shear stress occurs on the surface of the shaft, where the radius is maximum. Why then for noncircular members it isn't so? 2. Relevant equations From the book: Consider a small cubic element located at a corner of the cross section of a square bar in torsion and select coordinate axes parallel to the edges of the element. Since the face of the element perpendicular to the y axis is part of the free surface of the bar, all stresses on this face must be zero. 3. The attempt at a solution On wikipedia i've read that free surface is a notion relative to fluids not solid members. And i can't understand why we have free surface on the edges of a bar not at other parts of its surface.