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

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Oasys LS-DYNA Environment: User Guide (Version 8.1) Associated with springback is the *DATABASE_SPRING_FORWARD option to output a nodal force file used in an iterative approach to die correction for springback. Please contact Oasys for more information on LS-NIKE3D and its use in springback calculations. (Note that LS-NIKE3D may also be considered for simulating the blank holder warp stage of the process - this is still under development at LSTC). BLANK The undeformed blank should be meshed using shell elements as use of solids would be prohibitively costly due to the large number of elements and the small timestep required. The default B-T shell is generally acceptable but the number of integration points should be increased to five or more for increased accuracy in the springback calculation. Membrane elements can be used where cpu usage is critical but care must be exercised in interpreting the results. Wellconditioned quadrilateral elements are preferred as triangles and “chevron” elements can give local changes in stiffness. Element size in the blank should be as small as reasonable for the draw phase, although larger elements can be used in the gravity and blank holder wrap. Adaptive re-meshing is very helpful especially where there are tight radii in the tooling surfaces. If an element on the blank is too large at a tight radius excess, force will be developed leading to excess constraint or even a locked condition. Adaptivity allows the mesh to be refined locally only as required in the calculation and is more efficient than using a very fine uniform mesh. It is also much easier to prepare than a structured mesh manually refined in selected areas. The option to output the final adapted mesh without displacements is a useful way to carry out sensitivity analyses. The last version of LS-DYNA can adapt the mesh prior to contact with the tooling. This “look ahead” capability gives the most efficient and reliable implementation of adaptive remeshing (see the comments below on Control Cards for more details). In order to represent the elastic-plastic deformation of the blank, the standard material models applicable to shells may be used but there are also several models provided specifically for sheet metal forming simulation. The manufacturing process for sheet metal means that the material properties are no longer isotropic. Rolling introduces anisotropy in the through-thickness direction such that the ratio of width strain to thickness strain, r, is not unity. For most steel r > 1 giving improved drawability. In addition, r can vary along or across the sheet leading to definition of r 00, r 45 and r 90 to attempt to represent the anisotropy in more detail; this is normally required to capture the behaviour of aluminium alloys. Anisotropy is included in a number of models. *MAT_TRANSVERSELY_ANISOTROPIC_ELASTIC_PLASTIC is based on Hill’s 1948 quadratic yield surface taking into account normal anisotropy (the ‘r’ value); this model equates to Von Mises with r = 1. (*MAT_FLD_TRANSVERSELY_ANISOTROPIC is identical but includes definition of a forming limit diagram to calculate an additional parameter used in certain post-processors; Oasys D3PLOT does not require this variable). *MAT_3- PARAMETER_BARLAT is based on Barlat’s three parameter yield surface model using the planar anisotropy parameters r 0, r 45 and r 90 (a full implementation of Barlat is also available). This should always be used for aluminium (Hill should not be used in this case). One other Page 17.2
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 Page 17.3
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Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

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