1. Explicit-to-Implicit Sequential Solution
Chapter 10
Explicit-to-Implicit Sequential Solution

2. Chapter Objectives
Upon completion of this chapter, students will be able to perform an Explicit-to-Implicit Sequential Solution.
1. Define the meaning of and reasons for a sequential solution
2. Describe an explicit-to-implicit sequential solution
3. Define springback
4. Describe in detail the basic steps of an explicit-to-implicit solution
5. Given step-by-step guidance, a springback analysis will be performed
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3. Sequential Solution - Definition
A sequential solution is an analysis technique that uses a combination of implicit (general ANSYS) and explicit (ANSYS/LS-DYNA) solution methods. In problems that require a sequential solution, the results of an explicit analysis are imported into an implicit model (or vice-versa) for the purpose of obtaining a final solution.
Reasons for Performing Sequential Solution
Some engineering processes are very complex and contain both dynamic and static phases (e.g., initial hoop stress in a pressure vessel before a drop test or the linear springback after sheet metal forming).
The explicit technique is geared towards solving nonlinear dynamic-impact problems and is not robust for solving static phases of physical phenomena.
The implicit method is best suited for solving static or quasi-static problems.
Combining the ANSYS implicit and ANSYS/LS-DYNA explicit solvers is an extremely powerful tool for allowing simulations of otherwise intractable engineering problems.
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4. Explicit-to-Implicit Sequential Solutions
An explicit-to-implicit sequential solution is one for which a dynamic solution is first obtained for a geometry using the ANSYS/LS-DYNA program. Then, the deformed geometry and stresses are imported into an ANSYS implicit analysis where a subsequent solution is obtained through the application of appropriate boundary conditions.
The explicit-to-implicit solution technique currently can only be applied to SHELL 163 and SOLID 164 elements.
The results of an explicit analysis can be imported into SHELL181 or SOLID 185 implicit elements.
Appropriate constraints must be specified in the implicit analysis to prevent rigid body motion.
Explicit-to-implicit sequential solutions are predominantly intended for simulating springback in sheet metal forming.
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5. Springback – Definition
In sheet metal forming processes, springback is defined as the dimensional change of the formed part (i.e., blank), from that of the die, which occurs from linear elastic unloading. In sheet metal forming, elastic energy is stored in the blank material while it is in contact with the die. When the forming pressure is removed, the elastic energy is released, causing the blank to deform or springback towards its original geometry. A simplified forming process with springback is depicted below: V V
PUNCH D
BLANK
DIE
Simplified Stamping Geometry
Forming of Blank V
Springback of Blank During loading
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6. Basic Steps of an Explicit-to-Implicit Sequential Solution
There are nine basic steps required to perform an explicit-to-implicit sequential solution. These steps include:
1. Solve the explicit portion of the analysis
2. Enter the implicit solver and change the current jobname
3. Convert explicit elements to implicit with the appropriate attributes
4. Turn off implicit element shape checking
5. Update the geometry of the implicit elements to the deformed shape of the explicit solution
6. Un-select unnecessary elements for the implicit solution (retain only non-rigid body SHELL181 and SOLID 185 elements)
7. Redefine the boundary conditions
8 . Import the true stresses and shell element thicknesses from the explicit analysis
9. Solve the implicit portion of the analysis
The following slides will be devoted to describing each of these steps in detail.
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7. Step 1: Solve the Explicit Portion of the Analysis
When performing the explicit portion of the analysis, follow the recommendations given in the previous chapters of the course notes. In addition, there are several other guidelines that should be considered when investigating springback.
SHELL163 elements must be used to model the blank in forming simulations that will consider springback effects.
Make sure that the shell element thicknesses for the blank are realistic.
Increase the punch velocity to speed up the simulation time.
SOLID 164 elements can also be used.
Verify the results of the explicit analysis before proceeding to the implicit solution.
Ensure that undesirable dynamic effects (i.e. vibrations) are not left in the blank at the completion of the explicit solution using Time History Postprocessing
Before exiting the explicit analysis, save the database as Jobname1.db
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8. Step 2: Change the Jobname for the Implicit Run
Change the current jobname to Jobname2 and save the database (Jobname2.db).
If this is not done, the explicit results file (Jobname1.rst) will be overwritten at the completion of the implicit solution.
Utility Menu: File -> Change Jobname....
Utility Menu: File -> Save as Jobname.db
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9. Step 3: Convert the Element Types
In ANSYS, there are companion explicit and implicit element types. When performing a sequential solution, all elements being analyzed must be converted to their companion element to obtain a final solution. The companion explicit-implicit pairs are:
Explicit Type Implicit Type
LINK160 LINK8
BEAM161 BEAM4
SHELL163 SHELL181
SOLID164 SOLID185
COMBI165 COMBIN14
MASS166 MASS21
LINK167 LINK10
All explicit elements are automatically converted to implicit elements by executing the ETCHG,ETI command.
Preprocessor: Element Type -> Switch Elem Type
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10. (continued) Step 3: Convert the Element Types
During the element conversion, some element attributes (i.e., KEYOPTS) may need to be changed for the implicit portion of the analysis.
Typically, Real Constants such as element thicknesses (SHELL181) do not need to be redefined - they will actually be read in from the explicit results (see Step 8).
Only linear elastic material properties can be active in the implicit phase of the analysis. Therefore, the plasticity material properties used for the blank in the explicit portion of the analysis must be deleted.
Preprocessor: Material Props -> Material Models -> Edit -> Delete
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11. Step 4: Turn Off Element Shape Checking
During the explicit solution, it is likely that the elements of the blank have undergone significant deformations during the forming process. Since single-point integration explicit elements are more robust for large deformations than implicit elements, element shape checking should be turned off to obtain a solution.
Element shape checking is turned off with the SHPP,OFF command
Preprocessor: Checking Controls -> Shape Checking
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12. Step 5: Update the Geometry of the Implicit Elements
The starting point for the implicit springback analysis is the final deformed geometry of the explicit solution.
In order to transfer the explicit deformed geometry of the blank to the implicit analysis, the UPGEOM command is used. In the UPGEOM command, the explicit results filename and the corresponding load step and substep must be specified to update the geometry.
Preprocessor: Update geometry ...
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13. Step 6: Remove Unnecessary Elements
In most forming analyses, solid or rigid elements are used for the punch and dies during the explicit portion of the solution. As these elements are not required in the springback portion of the analysis (and may even cause convergence difficulties), they should be unselected before proceeding to the implicit solution.
Since most entities within an explicit analysis are likely to have a PARTID with a unique material number, it is usually easiest to un-select the elements using material attributes.
Utility Menu: Select -> Elements -> By Attributes
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14. Step 7: Redefine the Boundary Conditions
During the explicit portion of the analysis, no constraints are required on the blank during forming.
In an implicit solver, however, constraints are needed in all directions to restrain rigid body motion.
Therefore, in order to implicitly model springback, additional constraints are required for each of the six degrees of freedom for a shell element.
Typically, two or three nodes of each model component should be fully constrained in the implicit portion of the analysis.
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15. Step 8: Import the Stresses
Along with the updated geometry (Step 5), input true stresses and shell thicknesses need to be imported from the explicit solution using the RIMPORT command.
It is important to note that the deformed integration point thicknesses are averaged before being imported and will override thicknesses defined by the real constants for shell elements.
As with the UPGEOM command, the RIMPORT command requires the explicit results filename, load step, and substep to be specified.
Solution: -Loads-Apply-> -Structural-Other -> Import Stress...
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16. Step 9: Perform the Implicit Solution
Before solving the implicit springback solution, it is important to turn on nonlinear geometry effects because it is likely that the blank has a highly deformed geometry at the beginning of the implicit solution.
It is also important to make sure that the Analysis Options are set to Large Displacement Static
Once the nonlinear geometry has been turned on, the explicit-to- implicit sequential solution is ready to be solved.
Solution: -SOLVE- -> Current LS
Solution: -Analysis Type -> Sol’n Control->Analysis Options
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17. Springback Exercise
The exercise for this chapter begins on page E10-1 of Volume II.
A metal plate is first stamped (explicit analysis) and then allowed to elastically springback (implicit analysis).
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