An enhanced plasticity model for material characterization at large strain

An experimental campaign on some isotropic steels for pipeline applications has been put forth. It was based on tests with different stress states: tension on smooth and notched geometries, torsion, three point bending, plane strain, and combined tension-torsion. The aim was the characterization of the material elasto-plastic behavior up to large strain and the calibration of a ductile damage model for failure estimation. Results from tension and torsion were to be used for plasticity behavior description: from both of them, it is possible to retrieve the material true-stress true-strain curve until final failure. Unexpected differences were found. A critical interpretation led to the hypothesis that the ordinary isotropic J2-plasticity modelization could not predict all experimental evidences, starting from medium deformations, with increasing errors when plastic strain builds up. To approach this issue, an enhanced plasticity model has been developed and implemented into FEM code. It is a modification of a formulation widely used for geomaterials, which can take into account the influence of triaxiality and deviatoric effects on plastic accumulation. In addition, it allows a progressive transition from the Von Mises criterion, which demonstrated to be accurate for small strains, to a more complex formulation. A preliminary calibration of the model has been provided. This has been accomplished using a multiple-target inverse approach, exploiting experimental global data and FEA, by means of a dedicated optimization procedure. Appreciable improvements have been observed in terms of experimental-numerical match. © The Society for Experimental Mechanics Inc. 2014.