Last modified: 2016-05-23
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 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 the finite element (FE) method.
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.
The process investigated in the present paper concerns thick wall tubes. In this process the deformation is carried out by three rolls and transverse cracks appear at the inner surface of the tubes. Due to this, the following new factors influencing the flow forming process have to be considered:
1. During the deformation of thick-wall tubes significantly inhomogeneous deformation occurs. Under these conditions, alternating loading and unloading leads to residual stresses, which cannot be neglected in the analysis of the process.
2. Alternating stages of loading and unloading lead to a large number of load cycles, which involve significant tensile residual stresses at the inner surface of the tube. Due to this, the process of failure has to be considered from the perspective of the theory of low-cycle loading.
This paper discusses the numerical model of the reverse flow forming of the pipe from Inconel 718 alloy using three roll machine. Additionally, two models of rigid-viscoplastic theory of deformation and the elastic-plastic theory are compared. The analysis of the applicability of these two types of fracture theories, based on critical deformation and low-cycle fracture theory, is shown. The fracture models were implemented in the QForm8 software using Lua script language.
The following results were obtained.
1. Occurrence of residual stresses cannot be neglected in simulations of the flow forming of thick-walled pipes. Tensile residual stresses at the inner surface of the pipe are the result of an alternative occurrence of the active deformation and unloading. The value of tensile residual stress at the inner tube surface is comparable to the yield stress. For this reason, they can significantly affect the fracture.
2. Application of the rigid-plastic theory of plasticity does not allow to simulate the effect of unloading and the occurrence of residual stress. For this reason, this approach is not suitable for simulation of the flow forming of thick-walled tubes.
3. A large number of deformation cycles allow considering the flow forming from the viewpoint of low-cycle theory of fracture. This approach allows predicting correctly the localization of fracture. Commonly used failure criteria based on the critical strain give wrong prediction of the localization of the failure in the flow forming of thick-walled tubes.