Civil Engineering Reference
In-Depth Information
The analysis may be conducted to achieve “optimal” friction with reference to specific requirements,
such as stability of the workpiece, property of the deformed material, component-form error, and
forming force requirements.
Tool-defect prediction and life analysis
—The reduction of tool wear and the prevention of fracture of
tools are two major aspects which are invariably considered for improving tool life. Prediction of the
pressure on the tool-surface can be determined by FE analysis; reduction of the amplitude of the pressure may
be enabled or its distribution may be optimized by optimizing the geometry of the tool to improve the
flow of work-material. FE analysis is also able to provide detailed information of stresses and strains in
the tool; the information may be used to initiate fracture mechanics analyses to predict the development
of the cracks. For example, by evaluating the stresses and strains around a crack-tip, the fracture mechanical
values such as
-integral can be calculated and the yielding crack growth rate and the crack
direction can be predicted. Based on these, die working-surface or tooling construction may be optimized
with respect to the life considerations.
K
-factor or
J
A forming process may be regarded as operations con-
ducted in a system which includes several elements such as machine, tooling, workpiece, and auxiliary
media. Static and dynamic behavior of the machine would influence the behavior of tooling and
consequently influence the component-form, such as influencing the concentricity of component and
dimensional error. A forming operation may be influenced by tool, particularly by parameters such
as die-cavity-geometry, tool material property, and tool surface coating and wear. The accurate pre-
diction of tool-life and component-form error can only be achieved with reference to the deflection
of machine, tool, and work-material. While this would be difficult to achieve by using any analytical
method, FEM is an efficient tool of the modeling of the forming system; FE modeling of machine,
tool and work-material as integrated models is currently time-consuming, but promises scope for
more complete analyses of forming processes.
Although evidence indicates that FEM is an efficient tool for analysis and design of metal forming
operations, the applications of FE have not been as popular and as efficient as expected. It has been
proven that both efficient application of commercial FE codes and programming of a new FE software
requires a high level of expertise. Although effort has been expended to improve the use of the FE software
by continuously adding updated pre- and post-processing functions to the software, FE codes may not
be treated as a “black box.” Computational failures could occur at any stage of FE modeling and com-
putation. The successful modeling of a complex forming-process relies, to a large extent, on previous
experience and universal rules may not be derived to enable easy access for the beginner. Further, modeling
technologies are required for the simulation of particular forming processes and process-conditions,
particularly the simulation of the dynamic characteristics of development of flaws. Innovative modeling
techniques, based on available FE software, may resolve some of the current difficulties while others may
require the further development of FE-theory and the method of numerical-solution.
Forming-machine/tooling system analysis
—
4.3
FE Modeling of Injection Forging
Modeling Requirement
Early attempts to develop a predictive model of material flow in injection upsetting relied on upper-bound
techniques [46]. The model was based on the following assumptions: the material flow within the die-cavity
was in a steady-state throughout injection, the transmission of punch pressure along the injection chamber
was effected with insignificant friction losses and the dimension of the upsetting cavity in the thickness
direction was treated as a continuous variable. Another attempt at modeling treated the flange as an
expanding thick-walled tube, the plastic deformation of which provided a definition of the energy
requirement [1]. However, the hardness test of a formed flange by injection upsetting [47] showed that the
strain-field was not as uniform as would have been the case for the expansion of a thick-walled tube;
the forming of the flange may not be assumed to result from the uniform radial flow of material.
