To formulate the constitutive equation for nonproportional cyclic loading, the influence of preloading on subsequent loading should be investigated experimentally. In this paper, both the stress-strain relation and the equi-plastic strain surface of subsequent loading after cyclic preloading have been examined carefully from combined tension-compression and torsion test with SUS304 stainless steel. The equiplastic strain surface could be represented by the quadratic form of stress when it was probed from the current center of the yield surface after cyclic preloading. Then, all the stress-strain curves of subsequent radial loading from the center were confirmed to be represented by the modified Ramberg-Osgood law. As a result, the modified Ramberg-Osgood law obtained shows availability to derive the constitutive equations of cyclic plasticity.

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