Implementing Realistic, Nonlinear, Material Stress–Strain Behavior in ANSYS for the Autofrettage of Thick-Walled Cylinders

High-pressure vessels are autofrettaged to introduce favorable, compressive, residual stresses around their inner diameters. The efficacy of autofrettage is limited by a phenomenon called the Bauschinger effect, which describes the early onset of nonlinearity during unloading in a material that has previously been subjected to initial deformation. The degree of prestressing achieved determines the fatigue life of the vessel, hence, high fidelity prediction of the stress field developed is essential for accurate prediction of fatigue life. This requires precise representation of material behavior within the autofrettage model used. This paper describes the adaption and development of USERMAT, a user programmable feature within ANSYS (ANSYS Finite Element Program, ANSYS, Inc., Southpointe, 275 Technology Drive, Canonsburg, PA), to create a framework to represent realistic behavior of candidate gun steels. A number of materials including A723 were modeled to investigate and validate the framework. A723 was then used in simulations of both a uni-axial test and hydraulic autofrettage. These results are compared with spreadsheet data from the material-fit equations and equivalent results from the Hencky program, respectively. Close agreement was observed between the results in both cases, indicating the model is an effective means of representing the considerable variation in behavior between loading and unloading in candidate steels.