Plane section does not remain plane after torsion, which complicates calculation of stress.

Membrane analogy

1. Contour lines give the direction of shear stress

2. Shearing stress at any point is proportional to the slope of the line perpendicular to the contour lines.

3. Torque carried is proportional to twice the volume enclosed by the membrane and the base.

TORSION OF A WIDE RECTANGULAR BAR

Stress is double than a circular shaft of diameter c.

TORSION OF RECTANGULAR BAR – GENERAL CASE

COMPOSITE SECTIONTotal torque T = sum of torques of part 1, 2 and 3.Angle of twist q1 applies to each part as well as the whole.

Max stress occurs at the bar with greatest width,Let this be Part 1.

eliminating q1