Improved model for the prediction of the residual stress field in autofrettaged cylinders

The stress-strain curve in a loading-unloading process of high-strength steels can be significantly affected by the previous stress-strain history. In particular, if the material has accumulated plastic strain, the elastic modulus can decrease and the Bauschinger effect can impact the plastic response. The combined effect of the reduction of the elastic modulus and the Bauschinger effect can affect the residual stress field in autofrettaged cylindrical pressure vessels. In particular, the value of the compressive residual stress at the bore is, at the same time, the one most impacted by the loss of either strength or stiffness and the one that bottle-necks the pressure vessel performances. An experimental campaign has been performed to accurately measure the constitutive behavior for the high-strength steel of an autofrettaged component. Uniaxial tests were performed on specimens extracted from the above mentioned autofrettaged component. A complete 3-D incremental elastic-plastic constitutive model has been tuned to reproduce the observed behavior. This model, implemented in the ANSYS® commercial Code via User Programmable Features, has then been used to simulate the autofrettage process. The residual stress field, obtained via the proposed FE simulation, was compared with the distribution measured on the component by means of the slitting method. The comparison was used to assess the validity of the proposed constitutive model and its numerical implementation