Abstract
Process of flow forming has been for a long time used for manufacturing of pipes from low workability alloys. In research on this process special attention was paid to workability (technological plasticity) of the pipe material. In a majority of published research prediction of the workability was made by various fracture criteria, which use stresses and strains calculated by finite element (FE) method. On the other hand, during manufacturing of thick wall tubes cyclic deformation involves repeating of the material loading and unloading. In consequence, residual stresses occur and are accumulated in the tube. These are compressive stresses at the outer surface and tensile stresses at the inner surface. Magnitude of these stresses is comparable with those occurring during deformation. It is expected that using FE program with constitutive laws, which do not account for the unloading phase, may lead to erroneous predictions of conditions of crack appearance as well as of localization of cracks. To prove this thesis two models of the flow forming process for thick wall tubes were developed. Constitutive law in the first model accounted for both elastic-plastic deformation and elastic unloading. The second simplified model used the rigid-plastic flow rule. The material flow stress model was the same in both solutions, with initial part representing elastic deformation. The models were implemented in the Qform8 software. Obtained results show that in the first case large tensile stresses at the inner surface were predicted, what means an increased risk of fracture. These stresses occur not only in the deformation zone but also along the tube. In the second solution, which did not account for the unloading, maximum stresses occur directly in the deformation zone. Distribution of the fracture criterion parameters shows differences in the localization of fracture for these two solutions. The first variant predicted fracture at the inner surface of the tube, while the second variant at the outer surface. The former was confirmed by experimental results. Necessity of accounting for unloading cycle in simulations of the flow forming for thick tubes was confirmed.