Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

Oasys LS-DYNA Environment: User Guide (Version 8.1)
model of interest is *MAT_POWER_LAW_PLASTICITY which allows the commonly used
parameters n & K to be used directly; these are often available from the material supplier, and
n is critical in assessing the forming limit of the material. This model also includes strain rate
sensitivity (but see discussion below on scaling tool velocity). As with all material models in LS-
DYNA, work hardening data must always be based on true stress vs. true strain values.
Generally, splits in a blank occur due to a strain limit defined in the plane of the sheet by a
forming limit curve. It is most convenient to assess splitting by post-processing, comparing
values with the forming limit curve (e.g in Oasys D3PLOT) rather than attempt to include failure
in the analysis.
TOOLS
The tooling surfaces (punch, die, upper and lower rings, etc.) are usually represented by rigid
elements, typically shells, although in some cases where flexibility of the blank holder ring is
important it is worth considering modelling the ring with elastic solids (with obvious cpu
penalties); Rigid to Deformable switching can sometimes be useful here. The rules on mesh
generation for tool surfaces are more flexible than for deformable parts; the mesh does not have
to be continuous and can use high aspect ratio elements for example, but this may put excessive
demands on the contact surface. It is important, however, that the intended geometry is
accurately represented. Small radii require small elements and at least five elements around a
90b radius are recommended. Automatic mesh generation techniques that modify the element
size according to the geometry are very useful for creating tool models; eta\DYNAFORM has
a powerful tool mesh generator.
LS-DYNA offers the option to use CAD data directly to represent the tools; both IGES and VDA
databases are supported. The VDA option has generally been the most effective mostly because
of the wide variation in IGES definitions. Use of CAD data gives the most accurate
representation of the tooling surfaces as no discretisation of the geometry is required (a coarse
mesh is generated automatically for ease of post-processing - this is not used in the calculation).
Springback prediction will also be improved as stresses are more accurate. This option offers
significant time savings in model creation but does require high quality, fully trimmed surface
definition with no overlaps and very small gaps. It is also often more demanding on cpu. The
file containing the location CAD data must be identified in the Oasys submission shell; an alias
must be defined on each *PART card to identify which file is the die, which the punch etc.
Options in LS-DYNA associated with use of CAD data include the ability to translate the surface
to its initial position and a function to create one tool from another by a normal offset.
Implementation is by means of aliases on each *MAT_RIGID card together with a file
identifying which surfaces are associated with which alias (this file can be specified using the
Additional Files option in the Oasys submission shell). The syntax for this file is given in the
user manual; see Appendix I for more details.
TOOL MOTION
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