Converting an gANSYS Database to an ANSYS/LS-Dyna/ y

Database

Steve HaleCAE Associates, Inc.

CAE Associates Inc. and ANSYS Inc. Proprietary© 2012 CAE Associates Inc. and ANSYS Inc. All rights reserved.

Step 1: Convert Elements

Convert the Elements from Implicit to Explicit : Convert the Elements from Implicit to Explicit :— If only implicit companion elements were used, issue the ETCHG, ITE

command to convert the implicit elements to their explicit counterparts:Preprocessor > Element Type > Switch Elem Type > Implic to Explic— Preprocessor > Element Type > Switch Elem Type … > Implic to Explic

— NOTE: ANSYS quadratic element types cannot be converted (except for higher-order tet elements (SOLID187))

Implicit LINK8 => Explicit LINK160Implicit BEAM4 => Explicit BEAM161Implicit SHELL181 => Explicit SHELL163Implicit SHELL181 => Explicit SHELL163Implicit SOLID185 => Explicit SOLID164Implicit SOLID187 => Explicit SOLID168Implicit COMBIN14 => Explicit COMBI165Implicit COMBIN14 > Explicit COMBI165Implicit MASS21 => Explicit MASS166Implicit LINK10 => Explicit LINK167

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Step 1: Convert Elements

Most converted element types do not require any modifications However Most converted element types do not require any modifications. However, there are some exceptions.

Experience has shown that the 10-noded tet element type (SOLID168) d t f ll i t tdoes not perform well in contact.

— Does not calculate contact pressure accurately.— Recommendation: Edit the *SECTION_SOLID command in the LS-Dyna input

f f f ( f )file to use element formulation 17 instead of 16 (default).• Change the ELFORM option to type 17 • This formulation calculates contact and applied pressures accurately.

Can also use a 4-noded tet: Tet type 13• Much faster than Solid168s and about 25% slower than Tet type 10• Contact pressures are calculated correctly.• Mesh as Solid164 elements using tet shapes.• Edit the *SECTION_SOLID command in the LS-Dyna input file to include these.

— Change the ELFORM option to define the element type

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Step 1: Convert Elements

The SHELL163 element type should also be The SHELL163 element type should also be evaluated.

— This element type has 20 different element formulations (*SECTION SHELL). Key ones include:( SECTION_SHELL). Key ones include:

— Belytschko-Tsay (BT, KEYOPT(1)=0 or 2, default):• Simple shell element• Very fast (relative cost = 1 0)Very fast (relative cost 1.0)• Used extensively in crashworthiness analyses• Valid for small shear strains, large rotations• Wrong results for warping/torsion – too flexibleWrong results for warping/torsion too flexible• Poor results for in-plane shear deformations > 5%

— Belytschko-Wong-Chiang (BWC, KEYOPT(1)=10):• Relative cost = 1.28 * BT • Correct results for warped shells and torsion• Poor results for in-plane shear deformations > 5%

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Step 1: Convert Elements

General Element Guidelines: General Element Guidelines:

— Avoid small elements whenever possible as they will significantly reduce the time step size, thereby increasing the run time.

Mi i i th f d t l d t i l /t t h d / i l t— Minimize the use of degenerate lower-order triangular/tetrahedron/prism elements. Although these elements are supported, they are not very accurate.

— Minimize the use of Solid168 higher-order tetrahedral elements because they can significantly increase run times AND are not accurate at contact interfaces.significantly increase run times AND are not accurate at contact interfaces.

• Consider using alternate tetrahedral element types if brick meshing is too difficult.

— Avoid acute angled elements and warped shells, as they will degrade the accuracy of the results.

— Fully integrated elements can be defined in regions of a model where hourglass control is needed.

• However, volumetric locking (due to Poisson’s ratios approaching 0.5) and shear locking (e.g., in the bending of a simply supported beam) are possible with fully integrated brick elements.

• Shear locking becomes worse with high aspect ratios.

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Step 2: Assign Hourglass Control

Hourglass modes are nonphysical zero-energy modes of deformation that Hourglass modes are nonphysical, zero-energy modes of deformation that produce zero strain and no stress.

— Hourglass modes typically have no stiffness and give a zigzag deformation appearance to a meshappearance to a mesh.

— Single-point (reduced) integration elements are prone to hourglassing.

— The occurrence of hourglass deformations in an analysis can invalidate results g yand should be minimized.

• Why? Hourglassing will influence stiffness and mass distribution which can lead to inaccuracies in the deformation, stress, and strain solutions.

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Step 2: Assign Hourglass Control

Minimizing hourglassing in ANSYS/LS-DYNA Minimizing hourglassing in ANSYS/LS-DYNA

— Avoid single point loads, which are known to excite hourglass modes. Try to distribute loads over several elements as pressures.

— Avoid poorly-shaped elements and elements with high aspect ratios, which are more susceptible to hourglassing.

— Use hourglass controls (viscous or stiffness):• Solution > Analysis Options > Hourglass Ctrls > Local …

VAL1 3 i ll th VAL1 = 3 is usually the most effective method for “high” speed transients

VAL1 = 5 is usually the ymost effective method for “low” speed transients

VAL1 = 1 is the default

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Step 3: Assign Material Models/Properties

Advanced material modeling capabilitiesS i d

— Metals— Concrete— Foam

Strain-Rate Dependent Properties of Ti-6Al-4V

— Polymers— Rubber— Ceramics— Soil, Sand— Fluid, gas— Others: Biological tissue, wood, airbags, explosives

Material models can include strain-rate dependency, plasticity, equations of state, nonlinear elasticity, and other complex behaviors.LS D t i 150 diff t t i l LS-Dyna contains over 150 different material models.

Many of the material models require numerous parameters

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parameters.

Step 3: Assign Material Models/Properties

Available ANSYS/LS-DYNA material models include: Available ANSYS/LS-DYNA material models include:

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Step 4: Create LS-Dyna Parts

A Part is a group of elements that have the same combination of element type, real g p yp ,constant set, and material reference numbers.

— Typically, a Part is a specific component within a model.

Each Part is given a Part ID number which can be used in various ANSYS/LS DYNA— Each Part is given a Part ID number which can be used in various ANSYS/LS-DYNA commands.

The following are some tasks that can use Part IDs:— Defining or deleting contact (EDCGEN and EDDC)Defining or deleting contact (EDCGEN and EDDC)— Defining rigid body loads, constraints, inertia properties, or assemblies (EDLOAD,

EDCRB, EDIPART, and EDASMP)— Apply initial velocities (EDPV)— Applying damping to a component within a model (EDDAMP)

Example:

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Step 5: Apply Loads and Constraints

Unlike an implicit static analysis an explicit dynamic analysis must Unlike an implicit static analysis, an explicit dynamic analysis must have all loads applied as a function of time. The load step concept of general ANSYS does not apply.

• Because of the time dependence, many standard ANSYS loading commands (e.g., F and SF) are not valid in FORCE ( g )ANSYS/LS-DYNA.

• There is a unique procedure for applying loads in an explicit dynamic analysis

FORCE

using two array parameters. One array is for the time values and the other array is for the loading condition.

TIME

— The D command cannot be used to apply loads because it is not time dependent in nature. It can only be used to apply nodal restraints (fixed zero displacements).

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( p )

Step 5: Apply Loads and Constraints

Define array parameters for time and Define array parameters for time and load:

— Loads are applied over a specific time interval. The times and their corresponding load values are defined in array parameters (*DIM and *SET):

— Utility Menu > Parameters > Array Parameters > Define/Edit > Add

Apply Loads:— Once the nodal component and array

parameters have been defined, the load can be applied to the modelload can be applied to the model using the EDLOAD command:

— Solution > Loading Options > Specify Loads

— Now select the desired load type, the component or part on which to apply the load, and the time/load vectors (parameters):

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Step 5: Apply Loads and Constraints

Initial velocity (EDVEL) cam be used to simulate free motion instead Initial velocity (EDVEL) cam be used to simulate free motion instead of ramping on a load to generate the motion.

— Preprocessor > LS-DYNA Options > Initial Velocity

Fixed Displacements: Fixed Displacements: — Solution > Constraints > Apply > On Nodes (etc.) …— The D command can only be used to apply zero displacements (both

translational and rotational) to nodes As previously noted non zero

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translational and rotational) to nodes. As previously noted, non-zero displacements are applied with the EDLOAD command.

Step 6: Define Contact

The three basic steps in defining contact within ANSYS/LS-DYNA are: The three basic steps in defining contact within ANSYS/LS DYNA are:— Select the contact model that best simulates your physical system— Identify the contact entities (Not required for single surface contact)— Define the contact and any additional input parameters required

Select the contact model and friction:— For most analyses, the automatic single surface (ASSC), automatic general (AG), nodes-to-surface

(NTS), and surface-to-surface (STS) contact models are recommended. They are very robust and cover most applications.pp

• Additional information can be found in the ANSYS/LS-Dyna User’s Guide

— The Surface-to-Surface (STS) contact model is the closest model to the standard surface-to-surface contact type in ANSYS implicit.

• Note: The Automatic Surface-to-Surface contact model is recommended for shell contactNote: The Automatic Surface to Surface contact model is recommended for shell contact.

Identify the contact entities:

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y— Contact and target surfaces can be defined with nodal components or Part numbers.

Step 7: Define Solution Controls

Specify the termination time (analysis transient time): Specify the termination time (analysis transient time):— The termination time is the actual time for which the physical process is being

simulated. In an explicit dynamic analysis, this time is usually of very short duration – often on the order of milliseconds.duration often on the order of milliseconds.

• Solution > Time Controls > Solution Time

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Step 7: Define Solution Controls

Specify the results output and restart frequency: Specify the results output and restart frequency: — Solution > Output Controls > File Output Freq— The frequency for which results and restart files are written is based on the

number of desired data sets or the actual time interval desirednumber of desired data sets or the actual time interval desired— The EDHTIME field specifies how often are written to the time history file

(Jobname.HIS or d3thdt). • This is useful for writing results at a higher frequency for a small subset of the• This is useful for writing results at a higher frequency for a small subset of the

model. The nodal subset for which results are written to this file is specified as follows:

• Solution > Output Controls > Select Component

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