CLOSED-FORM SOLUTION OF THE FINITE TORSION PROBLEM BY USE OF THE lambda FORM OF THE STRAIN-ENERGY FUNCTION.

A closed-form solution for the stress distribution in a cylindrical elastic rod under finite torsion is given involving use of the logarithmic lambda form of the strain-energy function proposed by Valanis and Landel. Experimental torque data on natural rubber which give support to the present solution over a large range of angles of twist are shown. It is also shown how torsion data may be used to determine the lambda form for deformations that lie outside the range of applicability of the logarithmic function if certain conditions apply. A solution, also in closed form, is given by using another lambda form proposed by Dickie and Smith on the basis of experiments on styrene - butadiene rubber.