Stresses Analysis of Round Long Straight Bar under Free Torsion

There are shear stresses as well as normal stresses,which can be neglected in the cases of small deformation,on the section of round straight bar under free torsion.The conclusion that there are not compress and tension stresses on the section is not true during the experiment due to the deformation is no longer small.Furthermore,the device to carry the experiment is not proper to provide the free torsion boundary.The purpose of the article is to try to demonstrate the stresses on the section of round straight bar under free torsion and give a reasonable explanation.