Abstract:

Based on a series of uniaxial ratcheting tests of 304 stainless steel performed, this paper investigated the influence of four stresses (mean, amplitude, peak and valley) on saturated ratcheting (SR) strain. The results show that there exists a relevant stress threshold for each of the four stresses so that the ratcheting deformation happens and its SR strain increases or decreases in a line like law with the stress as it gets over or below its threshold. An important discovery is given in this paper that there exists a unique threshold for peak stress and SR strain varies monotonously with peak stress in a parabola law closed to a line without influence from the other three stresses if SR strain is positive. So, peak stress is the essential reason that leads to positive ratcheting deformation of materials. According to this principle, this paper gives concepts of ratcheting threshold and ratcheting stress, presents a saturated ratcheting model (SRM) used to describe the constitutive relationship between SR strain and ratcheting stress and develops a ratcheting evolution model (REM) used to predict evolution of ratcheting strain for SR materials under uniaxial cyclic stressing. Correlative coefficients of SRM and REM to regress a series of relevant data of tests carried out all are larger than 0.98. A universal ratcheting model (URM) is obtained from these two models and can be used to predict satisfactorily ratcheting strain under cyclic stressing with no previous load history. URM is easy to be built based on ratcheting tests of only 4~6 uniaxial specimens and be available for engineering design on ratcheting control of materials. Specially, a single specimen test method to build SRM and to gain threshold of ratcheting stress is discussed.

Key words: stainless steel, plasticit y, ratcheting, cyclic stress, threshold, predicting methods, strain evolution equation