SUMMARYConstitutive relations in elastoplasticity may be formulated in a variety of ways, and different update algorithms may be employed to solve the resulting equations. Several implicit integration schemes, although some not widely used, have been suggested in the last years. Among them, the closest point projection method (CPPM) has proven to be an effective and robust integration scheme. In order to gain maximum control of the stress projection, a two-level CPPM iteration scheme is proposed. The hardening variables are fixed during the stress projection onto consequently fixed yield surfaces, and after the stress projection, new values of the hardening variables are calculated defining new yield surfaces. The update of the hardening parameters which, in general, may be highly nonlinear functions, may be determined by a combination of a Picard Iteration (PI) on the hardening variables and an adaptative order inverse interpolation (AOII) on the difference of subsequent iterations of the hardening variables.The integration scheme has been implemented in a general constitutive driver which has been formulated independent of the selected constitutive model and easily linked to finite element codes. A third stress invariant dependent, cone-cap elastoplastic constitutive model, referred to as the MRS-Lade, with a highly non-linear hardening function has been used to show the applicability of the proposed iteration scheme. Error analyses and accuracy assessment are presented along with some representative test results.
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