Abstract: In this paper, on the base of damage mechanics, theexpression of torque threshold is deduced. The method of reloading afterunloading is adopted to carry out the stiffness degradation experiment undercertain twisting angles, and the residual torque is obtained, and then thethreshold of the shear strain is calculated correspondent to everyreloading. According to the experiment, the threshold of the shear strain isincreased in certain degree under loading after unloading for many times.This conclusion can be used to increase the threshold of structures made ofsimilar material which need higher threshold in engineering application.