Relationship among the surface of the membrane and distribution of shearing stresses:

The relationship among the surface of the membrane and distribution of shearing stresses in the section of twisted bar may be stated as under:

 (a) The tangent to a contour line at any particular point of the deflected membrane gives the direction of the shearing stress at the equivalent point in the cross-section of the twisted bar.

 (b) The maximum slope of the membrane at any point is equal to the magnitude of the shearing stress at the corresponding point in the twisted bar.

 (c) The torque on the twisted bar is equal to two times the volume included between the membrane and plane of its outline.