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?
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.
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.