In the present study, the distribution of shear strain and warping in a square cross- section shaft generated under a large torsion is investigated using the image analysis based on the Natural Strain theory. The scribe lines are drawn in a grid on a surface of the test pieces made of natural rubber, and the image data of each element in the horizontal direction along the cross-section at the center and upper positions are taken by using a high-pixel camera equipped with macro lens. The distributions for shear strain and warping along the cross-section are obtained from that image data. These measured distributions under large deformation are compared with the distributions based on the conventional torsional theory of a square cross-section shaft, i.e., Saint Venant's theory for torsion. Moreover, taking the effect due to the elongation on the surface of specimens into consideration, the distributions modified elongation of gauge length are compared with experiments, and the validity of the present experimental results are confirmed in this paper.
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