Civil Engineering Reference
In-Depth Information
FIGURE 4.9
Simulation of injection forging of an annular flange.
due to inaccurate modeling. It is therefore necessary to test different meshing-schemes with reference to
experimental results before selection of a suitable scheme. The stress analysis of a structure usually requires
the use of high order elements to achieve the required accuracy of analysis; this is particularly true for
the analysis of stress concentrations at particular locations. However, due to the fact that the mesh
distortion invariably occurs in the simulation of large-scale material-flow in injection forging, lower
order elements are recommended, as these are less sensitive to mesh distortion. The computational
precision can be improved by using a fine mesh. Fine tolerances and large numbers of iterations should
also be used for the simulation of unstable deformation. Numerical ''uncertainty'' is inevitable in the
simulation of buckling and material folding, which usually occurs in the form of hunting between two
contact point-pairs. Increasing the number of the iterations would also increase the chances of arresting
hunting, which would also be affected by increasing the number of contact elements at key sections of
the model.
Billet buckling, mesh distortion, and excessive use of contact elements may not be the only factors to
cause the numerical uncertainty that may result in non-convergence of the solution; friction character-
istics that are incorporated into contact analysis also have a significant influence on the convergence of
the solution. Whether or not a friction model causes non-converging of contact analysis usually depends
on the algorithm or formulation of the friction model that is to be incorporated in the FE software; this,
incidentally, is an additional criterion for evaluation of the robustness of FE software. In ABAQUS,
Amonton's Law is used for the contact analysis; the use of a constant coefficient of friction did not result
in any numerical difficulty in the simulation of the injection forging. In order to use user-defined friction
models, a user subroutine was designed to link with ABAQUS. The simulations showed that contact
analyses were sufficiently stable to converge in the prescribed iteration steps, which may suggest that the
