implicit-to-explicit sequential solution
DESCRIPTION
Chapter Objectives Upon completion of this chapter, students will be able to perform an Implicit-to-Explicit Sequential Solution 1. Define the meaning of an implicit-to-explicit sequential solution 2. Discuss guidelines for performing an implicit-to-explicit sequential solution 3. Describe typical applications that require implicit-to-explicit solutions 4. Describe in detail the basic steps of an implicit-to-explicit solution March 15, 2001 Inventory #001458 11-2TRANSCRIPT
Implicit-to-Explicit Sequential Solution
Chapter 11
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Training ManualChapter Objectives
• Upon completion of this chapter, students will be able to perform an Implicit-to-Explicit Sequential Solution
1. Define the meaning of an implicit-to-explicit sequential solution2. Discuss guidelines for performing an implicit-to-explicit sequential
solution3. Describe typical applications that require implicit-to-explicit solutions4. Describe in detail the basic steps of an implicit-to-explicit solution
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Implicit-to-Explicit Sequential Solutions• An implicit-to-explicit sequential solution is one for which
stresses are initially determined for a model using an implicit solver. Then, these stresses are applied to a structure before it undergoes an explicit dynamic analysis. The initial stresses, often termed “preloads”, are included in an explicit analysis when they effect the dynamic response of the structure being analyzed.– The implicit-to-explicit solution technique can be applied to any of the
ANSYS/LS-DYNA elements (ELEM160-167) and their corresponding companion implicit elements.
– In the implicit portion of the analysis, elements that will only be used in the explicit solution should be completely constrained.
– Many structures that are to be analyzed using ANSYS/LS-DYNA are pre-stressed. If you are not certain whether a pre-stress will affect a system’s dynamic response, always run an implicit-to-explicit sequential solution.
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Implicit-to-Explicit Solutions - Description• Unlike explicit-to-implicit solutions that are limited to metal
forming processes, implicit-to-explicit procedures can be used for a broad range of engineering applications where a component’s initial stress state effects its dynamic response.
• In an implicit-to-explicit solution, the nodal displacements and rotations from the ANSYS implicit solution are automatically written to the ANSYS/LS-DYNA dynamic relaxation file (drelax).
• The implicit portion of an implicit-to-explicit sequential solution can only incorporate small strains and linear material behavior.
• Before proceeding to a detailed description on how to perform an implicit-to-explicit sequential solution in ANSYS/LS-DYNA, we will outline several typical applications requiring stress pre-loading.
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Training ManualApplications - Rotating Machinery
• Turbines:– Blade-out– Disk burst– Foreign object damage– Mount loads– Bearing loads
• Wheels• Tires
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Training ManualApplications - Pressure Vessels
• Initial hoop stresses are required for subsequent LS-DYNA analysis
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Training ManualApplications - Bolted Joints
• Bolt must be preloaded to evaluate flange dynamic response
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Applications - Parts with Manufactured Preloads• Wound golf balls• Composites with dissimilar materials
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Basic Steps of an Implicit-to-Explicit Sequential Solution• There are eight basic steps required to perform an implicit-to-
explicit sequential solution. These steps include:1. Solve the implicit portion of the analysis2. Change the current job name for the explicit portion of the analysis3. Convert implicit elements to the corresponding explicit ones with their
appropriate attributes (keyopts, real constants, material props., etc.)4. Remove additional constraints added in the implicit analysis5. Write nodal results from the implicit analysis to the drelax file6. Initialize the model’s geometry for the explicit run using the drelax file7. Apply any additional loading conditions for the explicit analysis8. Solve the explicit portion of the analysis
• The following slides will be devoted to describing each of these steps in detail.
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Step 1: Solve the Implicit Portion of the Analysis• When performing the implicit portion of the analysis, there are
several recommended guidelines that should be followed as listed below. – In the implicit portion of the analysis, it is best that elements with
explicit companion pairs be used. These elements include LINK8, BEAM4, SHELL181, SOLID185, COMBIN14, MASS21, and LINK10. Although other elements may be used, the conversion from implicit to explicit is easiest when using the companion elements.
– If non-companion elements are incorporated in the implicit solution, they must have the same node count as the explicit elements they will be converted to. Hence, mid-side node elements should not be used.
– In the implicit solution, all additional nodes and elements to be used in the explicit analysis must be defined. These additional elements, (such as the bird in a birdstrike analysis or a rigid floor in a drop test), should be fully constrained in all DOF’s so that they are not part of the implicit analysis .
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(continued) Step 1: Solve the Implicit Portion of the Analysis
– The implicit analysis should be linear in nature. The element results that will be applied as a preload to the explicit analysis should be small strain.
– The solution of the implicit analysis should be path independent with linear elastic material behavior.
– Temperature results from the implicit solution are not currently used in the explicit analysis.
– Before exiting the implicit analysis, save the database as Jobname1.db
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Step 2: Change the Jobname for the Implicit Run• Change the current Jobname to Jobname2 and then save the database (Jobname2.db). If this is not done, the implicit results file (Jobname1.rst) will be overwritten at the completion of the explicit solution.
Utility Menu: File -> Change Jobname.…
Utility Menu: File -> Save as Jobname.db
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Training ManualStep 3: Convert the Element Types
• As described in the explicit-to-implicit chapter, there are companion explicit and implicit element types. When performing an implicit-to-explicit sequential solution, it is easiest if explicit elements pairs are used in the implicit portion of the analysis. The companion implicit-explicit pairs are:
Implicit Type Explicit Type• LINK8 LINK160• BEAM4 BEAM161• SHELL181 SHELL163• SOLID185 SOLID164• COMBIN14 COMBI165• MASS21 MASS166• LINK10 LINK167
• If all of the elements in the implicit portion of the analysis have a companion pair, they will automatically be converted to explicit elements by executing the ETCHG,ITE command. – Preprocessor: Element Type -> Switch Elem Type....
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(continued) Step 3: Convert the Element Types• Non-companion elements that are used in the implicit analysis will not be
automatically converted with the ETCHG command. These elements must be manually converted using the EMODIF command.
Preprocessor Move/Modify -> -Elements- Modify Attrib
• First Pick the elements that will be modified• Then specify Elem type for the STLOC and the
appropriate new attribute number.
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2. Preprocessor: Move/Modify -> -Elements- Modify Nodes
1. Preprocessor: Create->Nodes->In active CS...
(continued) Step 3: Convert the Element Types• Implicit element types LINK8, LINK10, and BEAM4 are typically
defined with only two nodes yet their explicit companions (LINK160, LINK167, and BEAM161, respectively) require a third orientation node. For companion elements requiring a third node, extra nodes must be manually defined (N command) and incorporated (EMODIF) after the ETCHG command is executed:
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(continued) Step 3: Convert the Element Types• During the element conversion, some element attributes (e.g.,
KEYOPTS) may need to be changed for the explicit portion of the analysis.
• Only linear elastic material properties should be active in the implicit phase of the analysis. Therefore, plasticity material properties may need to be added for some elements in the explicit portion of the analysis.
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Training ManualStep 4: Remove Additional Constraints
• During the implicit solution, additional nodes or elements required in the explicit portion of the analysis are fully constrained. Hence, in order to perform the dynamic portion of the analysis, all additional constraints must be removed using the DDELE command.
Solution: Constraints-> Delete…
• First Select the appropriate nodes
• Then specify that ALL DOFs are to be deleted
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Step 5: Write Nodal Results From the Implicit Analysis• When performing the implicit-to-explicit solution, the results of
the implicit analysis must be written to the LS-DYNA ASCII file drelax . This is accomplished using the REXPORT command. Note that the load step, substep, and filename must be specified. Typically, the filename that should be used is Jobname1.rst.
• Solution: Constraints-> Read Disp....
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Training ManualStep 6: Initialize the Model’s Geometry
• The displacements and rotations contained within the drelax file are prescribed to the structure in the explicit analysis to establish the preload. The EDDRELAX command instructs the LS-DYNA solver to perform a stress initialization using dynamic relaxation. A “static” analysis is conducted over 101 cycles in pseudo time (before TIME=0), in which the kinetic energy is damped out. Only the ANSYS option needs to be selected from the EDDRELAX GUI menu box, as all other fields on this command are ignored for this particular type of sequential solution.
Solution: Analysis Options -> Dynamic Relaxation
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• In the explicit portion of the analysis, it is likely that additional loads will need to be applied to the structure being analyzed. These loads will typically include initial velocities (EDVE) and time dependent loads (EDLOAD). When applying these loads, follow the guidelines presented earlier in this course for general explicit dynamic analyses.
Solution: Initial Velocity->w/Axial Rotate Solution: - Loading Options -> Specify Loads
Step 7: Apply Necessary Loading Conditions
• Use “Transient Only” for Implicit-to-Explicit solutions, even though dynamic relaxation is being used.
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Training ManualStep 8: Perform the Explicit Solution
• Once all of the required loads have been specified, the implicit-to-explicit sequential solution can be executed.
Solution: -SOLVE --> Current LS
To run from the command line: lsdyna57 i=Jobname.k m=drelax
• After a solution has been obtained, the results of the implicit-to-explicit analysis can be viewed and animated using typical ANSYS postprocessing procedures.
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Training ManualFan Blade Containment Exercise
• The exercise for this chapter begins on page E11-1 of Volume II.• The jet engine fan blade containment analysis example is
designed to be run in batch mode. Due to the solution time and disk space requirements, this exercise should be run at a later time. The input file is well documented, so no additional instructions are required.