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1. Explicit Dynamics Analysis Guide Overview ... 1
2. Explicit Dynamics Workflow ... 3
2.1. Introduction ... 3
2.2. Create the Analysis System ... 4
2.3. Define Engineering Data ... 4
2.4. Attach Geometry ... 4
2.5. Define Part Behavior ... 6
2.6. Define Connections ... 7
2.6.1. Spot Welds in Explicit Dynamics Analyses ... 8
2.6.2. Body Interactions in Explicit Dynamics Analyses ... 9
2.6.2.1. Properties for Body Interactions Folder ... 11
2.6.2.1.1. Contact Detection ... 11
2.6.2.1.2. Formulation ... 13
2.6.2.1.3. Shell Thickness Factor and Nodal Shell Thickness ... 14
2.6.2.1.4. Body Self Contact ... 14
2.6.2.1.5. Element Self Contact ... 14
2.6.2.1.6. Tolerance ... 15
2.6.2.1.7. Pinball Factor ... 15
2.6.2.1.8. Time Step Safety Factor ... 16
2.6.2.1.9. Limiting Time Step Velocity ... 16
2.6.2.1.10. Edge on Edge Contact ... 16
2.6.2.2. Interaction Type Properties for Body Interaction Object ... 16
2.6.2.2.1. Frictionless Type ... 16
2.6.2.2.2. Frictional Type ... 17
2.6.2.2.3. Bonded Type ... 18
2.6.2.2.4. Reinforcement Type ... 19
2.6.2.3. Identifying Body Interactions Regions for a Body ... 21
2.7. Setting Up Symmetry ... 21
2.7.1. Explicit Dynamics Symmetry ... 21
2.7.1.1. General Symmetry ... 21
2.7.1.2. Global Symmetry Planes ... 22
2.7.2. Symmetry in an Euler Domain ... 22
2.8. Define Remote Points ... 23
2.8.1. Explicit Dynamics Remote Points ... 23
2.8.2. Explicit Dynamics Remote Boundary Conditions ... 24
2.8.3. Initial Conditions on Remote Points ... 24
2.8.4. Constraints and Remote Points ... 24
2.9. Apply Mesh Controls/Preview Mesh ... 27
2.10. Establish Analysis Settings ... 29
2.10.1. Analysis Settings for Explicit Dynamics Analyses ... 33
2.10.1.1. Explicit Dynamics Step Controls ... 33
2.10.1.2. Explicit Dynamics Solver Controls ... 37
2.10.1.3. Explicit Dynamics Euler Domain Controls ... 41
2.10.1.4. Explicit Dynamics Damping Controls ... 42
2.10.1.5. Explicit Dynamics Erosion Controls ... 43
2.10.1.6. Explicit Dynamics Output Controls ... 44
2.10.1.7. Explicit Dynamics Data Management Settings ... 47
2.10.1.8. Recommendations for Analysis Settings in Explicit Dynamics ... 47
2.10.1.9. Explicit Dynamics Analysis Settings Notes ... 51
2.12. Apply Loads and Supports ... 52
2.12.1. Impedance Boundary ... 53
2.12.2. Detonation Point ... 56
2.13. Solve ... 60
2.13.1. Solving from Time = 0 ... 60
2.13.2. Resume Capability for Explicit Dynamics Analyses ... 60
2.13.2.1. Load and Constraint Behavior when Extending Analysis End Time ... 61
2.13.3. Explicit Dynamics Performance in Parallel ... 62
2.14. Postprocessing ... 63
2.14.1. Solution Output ... 63
2.14.2. Result Trackers ... 63
2.14.2.1. Point Scoped Result Trackers for Explicit Dynamics ... 64
2.14.2.2. Body Scoped Result Trackers for Explicit Dynamics ... 69
2.14.2.3. Spring Result Trackers for Explicit Dynamics ... 73
2.14.2.4. Viewing and Filtering Result Tracker Graphs for Explicit Dynamics ... 73
2.14.2.5. Force Reaction Result Trackers for Explicit Dynamics ... 74
2.14.3. Review Results ... 75
2.14.4. Eroded Nodes in Explicit Dynamics Analyses ... 76
2.14.5. Euler Domain in Explicit Dynamics Analyses ... 77
2.14.6. User Defined Results for Explicit Dynamics Analyses ... 80
3. Transforming an Implicit Model to run in Explicit Dynamics ... 87
3.1. When Implicit Models Can be Run in Explicit ... 87
3.2. When to Consider an Explicit Analysis ... 88
3.2.1. Incorrect Model Setup ... 88
3.2.2. Large Deformations ... 89
3.2.3. Large Contact Models ... 90
3.2.4. Rigid Body Deformations ... 91
3.3. Setting up the Explicit Dynamics Analysis ... 92
3.3.1. Attaching an Explicit Dynamics System to an Existing Static Structural System ... 92
3.3.2. Materials ... 93
3.3.3. Meshing ... 93
3.3.3.1. Uniform Mesh Works Best ... 94
3.3.3.2. Midside Nodes not Used ... 94
3.3.3.3. Hex/Rectangular Mesh Elements most Effective ... 95
3.3.4. Contact/Connections ... 95
3.3.4.1. Contacts Tab ... 95
3.3.4.2. Body Interactions Tab ... 96
3.3.5. Boundary Conditions ... 96
3.3.5.1. Adjusting Load Cases for Reasonable Run Times ... 96
3.3.5.2. Missing Boundary Conditions from Explicit Dynamics ... 97
3.3.5.3. Application of Boundary Conditions Using Steps ... 97
3.3.5.4. Avoiding Conflicting Boundary Conditions ... 97
3.3.5.5. Initial Conditions ... 97 3.3.6. Analysis Settings ... 98 3.3.6.1. End Time ... 98 3.3.6.2. Output Controls ... 98 3.4. Solver ... 99 3.4.1. Timestep Controls ... 99
3.4.2. Analysis Setting Preference ... 101
3.4.3. Solution Stability ... 101
3.4.3.1. Mass Scaling ... 101
3.4.3.3. Damping ... 103 3.4.3.4. Restarting an Analysis ... 103 3.4.4. Solution Information ... 103 3.5. Postprocessing ... 104 3.5.1. Result Trackers ... 105 3.5.2. Result Sets ... 105
3.5.3. Improving your Simulation ... 106
4. Applying Pre-Stress Effects for Explicit Analysis ... 107
4.1. Recommended Guidelines for Pre-Stress Explicit Dynamics ... 107
4.2. Pre-Stress Object Properties ... 109
5. Using Explicit Dynamics to Define Initial Conditions for Implicit Analysis ... 111
5.1. Transfering Explicit Results to MAPDL ... 111
6. Explicit Dynamics Theory Guide ... 115
6.1. Why use Explicit Dynamics? ... 115
6.2. What is Explicit Dynamics? ... 115
6.2.1. The Solution Strategy ... 116
6.2.2. Basic Formulations ... 116
6.2.2.1. Implicit Transient Dynamics ... 117
6.2.2.2. Explicit Transient Dynamics ... 117
6.2.3. Time Integration ... 118
6.2.3.1. Implicit Time Integration ... 118
6.2.3.2. Explicit Time Integration ... 118
6.2.3.3. Mass Scaling ... 120 6.2.4. Wave Propagation ... 120 6.2.4.1. Elastic Waves ... 121 6.2.4.2. Plastic Waves ... 121 6.2.4.3. Shock Waves ... 121 6.2.5. Reference Frame ... 122
6.2.5.1. Lagrangian and Eulerian Reference Frames ... 122
6.2.5.2. Eulerian (Virtual) Reference Frame in Explicit Dynamics ... 123
6.2.5.3. Key Concepts of Euler (Virtual) Solutions ... 125
6.2.5.3.1. Multiple Material Stress States ... 126
6.2.5.3.2. Multiple Material Transport ... 128
6.2.5.3.3. Supported Material Properties ... 128
6.2.5.3.4. Known Limitations of Euler Solutions ... 128
6.2.6. Explicit Fluid Structure Interaction (Euler-Lagrange Coupling) ... 128
6.2.6.1. Shell Coupling ... 130 6.2.6.2. Sub-cycling ... 130 6.3. Analysis Settings ... 131 6.3.1. Step Controls ... 131 6.3.2. Damping Controls ... 132 6.3.3. Solver Controls ... 136 6.3.4. Erosion Controls ... 144
6.4. Model Size Limitations in Explicit Dynamics ... 145
6.5. References ... 146
7. Material Models Used in Explicit Dynamics Analysis ... 149
7.1. Introduction ... 149
7.2. Explicit Material Library ... 151
7.3. Density ... 157
7.4. Linear Elastic ... 157
7.4.1. Isotropic Elasticity ... 157
7.4.3. Viscoelastic ... 158
7.5. Test Data ... 159
7.6. Hyperelasticity ... 159
7.7. Plasticity ... 164
7.7.1. Bilinear Isotropic Hardening ... 165
7.7.2. Multilinear Isotropic Hardening ... 165
7.7.3. Bilinear Kinematic Hardening ... 166
7.7.4. Multilinear Kinematic Hardening ... 166
7.7.5. Johnson-Cook Strength ... 166
7.7.6. Cowper-Symonds Strength ... 168
7.7.7. Steinberg-Guinan Strength ... 169
7.7.8. Zerilli-Armstrong Strength ... 170
7.8. Brittle/Granular ... 172
7.8.1. Drucker-Prager Strength Linear ... 172
7.8.2. Drucker-Prager Strength Stassi ... 173
7.8.3. Drucker-Prager Strength Piecewise ... 174
7.8.4. Johnson-Holmquist Strength Continuous ... 175
7.8.5. Johnson-Holmquist Strength Segmented ... 177
7.8.6. RHT Concrete Strength ... 179 7.8.7. MO Granular ... 184 7.9. Equations of State ... 185 7.9.1. Background ... 185 7.9.2. Bulk Modulus ... 186 7.9.3. Shear Modulus ... 186
7.9.4. Ideal Gas EOS ... 186
7.9.5. Polynomial EOS ... 187
7.9.6. Shock EOS Linear ... 189
7.9.7. Shock EOS Bilinear ... 190
7.9.8. JWL EOS ... 192
7.10. Porosity ... 194
7.10.1. Porosity-Crushable Foam ... 194
7.10.2. Compaction EOS Linear ... 197
7.10.3. Compaction EOS Non-Linear ... 198
7.10.4. P-alpha EOS ... 200
7.11. Failure ... 203
7.11.1. Plastic Strain Failure ... 205
7.11.2. Principal Stress Failure ... 205
7.11.3. Principal Strain Failure ... 206
7.11.4. Stochastic Failure ... 207
7.11.5. Tensile Pressure Failure ... 208
7.11.6. Crack Softening Failure ... 209
7.11.7. Johnson-Cook Failure ... 211
7.11.8. Grady Spall Failure ... 212
7.12. Strength ... 213
7.13. Thermal Specific Heat ... 214
7.14. Rigid Materials ... 214
7.15. References ... 214
8. Using ANSYS LS-DYNA for an Explicit Dynamics Analysis ... 217
8.1. How to use the Explicit Dynamics LS-DYNA System ... 217
8.2. Supported LS-DYNA Keywords ... 217
8.3. LS-DYNA General Descriptions ... 246
ANSYS Explicit Dynamics is a transient explicit dynamics Workbench application that can perform a variety of engineering simulations, including the modeling of nonlinear dynamic behaviour of solids, fluids, gases and their interaction. Additionally, the Explicit Dynamics (LS-DYNA Export) system is available to export the model in LS-DYNA .k file format for subsequent analysis with the LS-DYNA solver.
A typical simulation consists of setting up the model, interactions and the applied loads, solving the model's nonlinear dynamic response over time for the loads and interactions, then examining the details of the response with a variety of available tools.
The Explicit Dynamics application has objects arranged in a tree structure that guide you through the different steps of a simulation. By expanding the objects, you expose the details associated with the object, and you can use the corresponding tools and specification tables to perform that part of the simulation. Objects are used, for example, to define environmental conditions such as contact surfaces and loadings, and to define the types of results you want to have available for review.
The following sections describe in detail how to use the Explicit Dynamics application to set up and run a simulation:
• Explicit Dynamics Workflow (p. 3)
• Transforming an Implicit Model to run in Explicit Dynamics (p. 87)
• Applying Pre-Stress Effects for Explicit Analysis (p. 107)
• Using Explicit Dynamics to Define Initial Conditions for Implicit Analysis (p. 111)
• Explicit Dynamics Theory Guide (p. 115)
• Material Models Used in Explicit Dynamics Analysis (p. 149)
The following section discusses how to solve an Explicit Dynamics (LS-DYNA Export) system to produce the LS-DYNA keyword file:
To learn how to perform an analysis, see Create Analysis System in the ANSYS Mechanical User's Guide. Note that the features available may differ from one solver to another.
To perform analyses that are beyond those available using Workbench, you can insert a Commands object in the tree.
This chapter contains the following topics:
2.1. Introduction
2.2. Create the Analysis System 2.3. Define Engineering Data 2.4. Attach Geometry
2.5. Define Part Behavior 2.6. Define Connections 2.7. Setting Up Symmetry 2.8. Define Remote Points
2.9. Apply Mesh Controls/Preview Mesh 2.10. Establish Analysis Settings
2.11. Define Initial Conditions 2.12. Apply Loads and Supports 2.13. Solve
2.14. Postprocessing
2.1. Introduction
You can perform a transient explicit dynamics analysis in the Mechanical application using an Explicit Dynamics system. Additionally, the Explicit Dynamics (LS-DYNA Export) system is available to export the model in LS-DYNA .k file format for subsequent analysis with the LS-DYNA solver. Unless specifically mentioned otherwise, this section addresses both the Explicit Dynamics and Explicit Dynamics (LS-DYNA Export) systems. Special conditions for the Explicit Dynamics (LS-DYNA Export) system are noted where pertinent.
An explicit dynamics analysis is used to determine the dynamic response of a structure due to stress wave propagation, impact or rapidly changing time-dependent loads. Momentum exchange between moving bodies and inertial effects are usually important aspects of the type of analysis being conducted. This type of analysis can also be used to model mechanical phenomena that are highly nonlinear.
Nonlinearities may stem from the materials, (for example, hyperelasticity, plastic flows, failure), from
contact (for example, high speed collisions and impact) and from the geometric deformation (for example, buckling and collapse). Events with time scales of less than 1 second (usually of order 1 millisecond) are efficiently simulated with this type of analysis. For longer time duration events, consider using a
Transient analysis system.
The time step used in an explicit dynamics analysis is constrained to maintain stability and consistency via the CFL condition, that is, the time increment is proportional to the smallest element dimension in the model and inversely proportional to the sound speed in the materials used. Time increments are
usually on the order of 1 microsecond and therefore thousands of time steps (computational cycles) are usually required to obtain the solution.
An explicit dynamics analysis typically includes many different types of nonlinearities including large deformations, large strains, plasticity, hyperelasticity, material failure etc.
This section contains the following topics:
An explicit dynamics analysis can contain both rigid and flexible bodies. For rigid/flexible body dynamic simulations involving mechanisms and joints you may wish to consider using either the Transient Structural Analysis or Rigid Dynamics Analysis options.
Note
The intent of this document is to provide an overview of an explicit dynamics analysis. Consult our technical support department to obtain a more thorough treatment of this topic.
2.2. Create the Analysis System
For general information about creating an analysis system see Create Analysis System in the ANSYS
Mechanical User's Guide.
From the Toolbox drag an Explicit Dynamics or an Explicit Dynamics (LS-DYNA Export) template to the Project Schematic.
Note
Explicit dynamics analyses only support the mm, mg, ms solver unit system. The Explicit Dynamics solver is double precision.
2.3. Define Engineering Data
For general information about defining engineerng data, see Define Engineering Data in the ANSYS
Mechanical User's Guide
Material properties can be linear elastic or orthotropic. Many different forms of material nonlinearity can be represented including hyperelasticity, rate and temperature dependant plasticity, pressure de-pendant plasticity, porosity, material strength degradation (damage), material fracture/failure/fragment-ation. For a detailed discussion on material models used in Explicit Dynamics, refer to Material Models Used in Explicit Dynamics Analysis (p. 149).
Density must always be specified for materials used in an explicit dynamics analysis. Data for a range of materials is available in the Explicit material library.
2.4. Attach Geometry
For general information about attaching a geometry to a system, see Attach Geometry in the ANSYS
Mechanical User's Guide.
Only symmetric cross sections are supported for line bodies in Explicit Dynamics analyses, except for the Explicit Dynamics (LS-DYNA Export) systems. The following cross sections are not supported: T-Sections, L-T-Sections, Z-T-Sections, Hat sections, Channel Sections. For I-T-Sections, the two flanges must have the same thickness. For rectangular tubes, opposite sides of the rectangle must be of the same thickness. For LS-DYNA Export systems all available cross sections in DesignModeler will be exported for analysis with the LS-DYNA solver. However there are some limitations in the number of dimensions that the LS-DYNA solver supports for the Z, Hat and Channel cross sections. For more information consult the LS-DYNA Keywords manual.
To prevent the generation of unnecessarily small elements (and long run times) try using DesignModeler to remove unwanted “small” features or holes from your geometry.
Thickness can be specified for selected faces on a surface body by inserting a thickness object. Constant, tabular, and functional thickness are all supported.
Symmetry is not supported when exporting to the LS-DYNA .k file.
Stiffness Behavior
Flexible behavior can be assigned to any body type.
Rigid behavior can be applied to Solid, Surface, and Line bodies.
Coordinate System
Local Cartesian coordinate systems can be assigned to bodies. These will be used to define the material directions when using the Orthotropic Elasticity property in a material definition. The material directions 1, 2, 3 will be aligned with the local x, y and z axes of the local coordinate system.
Note
Cylindrical coordinate systems assigned to bodies are not supported for Explicit Dynamics systems. Cylindrical coordinate systems are only supported to define rotational displacement or velocity constraints.
Reference Temperature
This option defines the initial (time=0.0) temperature of the body.
Reference Frame
Available for solid bodies when an Explicit Dynamics system is part of the solution; the user has the option of setting the Reference Frame to Lagrangian (default) or Eulerian (Virtual). If Stiffness Behavior is defined as Rigid, Eulerian is not a valid setting.
Rigid Materials
For bodies defined to have rigid stiffness, only the Density property of the material associated with the body will be used. For Explicit Dynamics systems all rigid bodies must be discretized with a Full Mesh or the Rigid Body Behavior must be defined as Dimensionally Reduced. The Full Mesh option will be specified by default for the Explicit meshing physics preference.
The mass and inertia of the rigid body will be derived from the elements and material density for each body.
By default, a kinematic rigid body is defined and its motion will depend on the resultant forces and moments applied to it through interaction with other Parts of the model. Elements filled with rigid materials can interact with other regions via contact.
Constraints can only be applied to an entire rigid body. For example, a fixed displacement cannot be applied to one edge of a rigid body, it must be applied to the whole body.
Note
• 2-D Explicit Dynamics analyses are supported for Plane Strain and Axisymmetric behaviors. • Only symmetric cross-sections are supported for line bodies
• Flexible and rigid bodies cannot be combined in Multi-body Parts. Bonded connections can be applied to connect rigid and flexible bodies
• The Thickness Mode and Offset Type fields for surface bodies are not supported for Explicit Dynamics systems
• Initial over-penetrations of nodes/elements of different bodies should be avoided or minimized if sliding contact is to be used. There are several methods available in Workbench to remove initial penetration
2.5. Define Part Behavior
For general information about defining parts, see Define Part Behavior in the ANSYS Mechanical User's
Guide.
Nonlinear effects are always accounted for in explicit dynamics analysis.
Parts may be defined as rigid or flexible. In the solver, rigid parts are represented by a single point that carries the inertial properties together with a discretized exterior surface that represents the geometry. Rigid bodies should be meshed using similar Method mesh controls as those used for flexible bodies. The inertial properties used in the solver will be derived from the discretized representation of the body and the material density and hence may differ slightly from the values presented in the properties of the body in the Mechanical application GUI.
At least one flexible body must be specified when using the ANSYS Autodyn solver. The solver requires this in order to calculate the time-step increments. In the absence of a flexible body, the time-step be-comes underdefined. The boundary conditions allowed for the rigid bodies with explicit dynamics are: • Connections
– Contact Regions: Frictionless, Frictional and Bonded.
– Body Interactions: Frictionless, Frictional and Bonded. Bonded body interactions are not supported for LS-DYNA Export.
– For ANSYS Autodyn, rigid bodies may not be bonded to other rigid bodies. • Initial Conditions: Velocity, Angular Velocity
• Loads: Pressure and Force. Force is not supported for ANSYS Autodyn.
For an Explicit Dynamics analysis, the following postprocessing features are available for rigid bodies: • Results and Probes: Deformation only - that is, Displacement, Velocity.
• Result Trackers: Body average data only.
If a multibody part consists only of rigid bodies, all of which share the same material assignment, the part will act as a single rigid body, even if the individual bodies are not physically connected.
2.6. Define Connections
For general information about defining connections, see Define Connections in the ANSYS Mechanical
User's Guide
Line body to line body contact is possible if:
• Contact Detection should be set to Proximity Based in the Body Interactions Details view. • Edge on Edge is set to Yes in the Body Interactions Details view.
• The Interaction Type is defined as Frictional or Frictionless – bonded interactions and connections are not supported for line bodies.
• LS-DYNA Export systems export the *CONTACT_AUTOMATIC_GENERAL and *CONTACT_AUTOMAT-IC_SINGLE_SURFACE keywords when a friction or frictionless Body Interaction is scoped to geometry that contains line bodies. The keywords handle contacts between line bodies only, and line bodies to other body types respectively. In the case where the Body Interaction is scoped to only line bodies, then only the *CONTACT_AUTOMATIC_GENERAL keyword is exported.
Reinforcement body interaction should be supported in the case when only line bodies are scoped to a Body Interaction of Type = Reinforcement. The line bodies will then be tied to any solid body that they intersect. Reinforcement body interactions are not supported for LS-DYNA Export systems or for 2D Explicit Dynamics analyses. However utilizing Keyword Snippets under Contact Region objects should provide a suitable alternative.
Body Interactions (p. 9),Contact and Spot Welds are all valid in explicit dynamics analyses. Frictional, Frictionless and Bonded body interactions and contact options are available. Conditionally bonded contact can be simulated using the breakable property of each bonded region. Spot Welds can also be made to fail using the breakable property.
Joints and Beam connections are not supported for explicit dynamics analyses. Springs are not supported for Explicit Dynamics (LS-DYNA Export) analyses. The Contact Tool is also not applicable to explicit dy-namics analyses.
For Explicit Dynamics (LS-DYNA Export) systems, bonded body interactions are not supported. Also, Contact Region objects with Auto Asymmetric Behavior or just Asymmetric Behavior are treated the same. Symmetric Behavior will create a _SURFACE_TO_SURFACE keyword for the contact and an Asymmetric Behavior will create a _NODES_TO_SURFACE keyword.
For Explicit Dynamics (LS-DYNA Export) systems, contacts between line bodies and solids can be imple-mented using the Keyword Snippets facility available under the Manual Contact Region objects.
Bonded contact is not supported in an explicit dynamics analysis for bodies that have their Reference Frame set to Eulerian (Virtual). A solver warning is shown to let the user know that such bodies will be ignored for bonds. Bonded contact is not support in a 2D explicit dynamics analysis.
To avoid hourglassing problems, remote points should be used instead of bonded contact in certain situations.
Bonds are not recommended for joining tetrahedral meshes. Use multibodied parts or remote points instead.
By default, a Body Interaction object will be automatically inserted in the Mechanical application tree and will be scoped to all bodies in the model. This object activates frictionless contact behavior between all bodies that come into proximity during the analysis.
2.6.1. Spot Welds in Explicit Dynamics Analyses
Spot welds provide a mechanism to rigidly connect two discrete points in a model and can be used to represent welds, rivets, bolts, etc. The points usually belong to two different surfaces and are defined on the geometry (see DesignModeler help).
During the solver initialization process, the two points defining each spot weld will be connected by a rigid beam element. Additionally, rigid beam elements will be generated on each surface to enable transfer of rotations at the spot weld location (see figure below). If the point of the spot weld lies on a shell body, both translational and rotational degrees of freedom will be linked at the connecting point. If the point of the spot weld lies on a surface of a solid body, additional rigid beam elements will be generated to enable transfer of rotations at the spot weld location.
Spot welds can be released during a simulation using the Breakable Stress or Force option. If the stress criteria is selected the user will be asked to define an effective cross sectional area. This is used to convert the defined stress limits into equivalent force limits. A spot weld will break (release) if the fol-lowing criteria is exceeded
(2.1)
Where:
fn and fs are normal and shear interface forces
Sn and Ss are the maximum allowed normal and shear force limits
n and s are user defined exponential coefficients
Note that the normal interface force fn is non-zero for tensile values only.
After failure of the spot weld the rigid body connecting the points is removed from the simulation. Spot welds of zero length are permitted. However, if such spot welds are defined as breakable the above failure criteria is modified since local normal and shear directions cannot be defined. A modified criteria is used with global forces
Where, are the force differences across the spot weld in the global coordinate system.
Note
A spot weld is equivalent to a rigid body and as such multiple nodal boundary conditions cannot be applied to spot welds.
2.6.2. Body Interactions in Explicit Dynamics Analyses
Within an explicit dynamics analysis, the body interaction feature represents contact between bodies and includes settings that allow you to control these interactions. If the geometry you use has two or more bodies in contact, a Body Interactions object folder appears by default under Connections in the tree. Included in a Body Interactions folder are one or more Body Interaction objects, with each object representing a contact pair.
You can also manually add these two objects:
• To add a Body Interactions folder, highlight the Connections folder and choose Body Interactions from the toolbar. A Body Interactions folder is added and includes one Body Interaction object.
• To add a Body Interaction object to an existing Body Interactions folder, highlight the Connections folder, the Body Interactions folder, or an existing Body Interaction object, and choose Body Interaction from the toolbar.
General Notes
Each Body Interaction object activates an interaction for the bodies scoped in the object. With body interactions, contact detection is completely automated in the solver. At any time point during the
analysis any node of the bodies scoped in the interaction may interact with any face of the bodies scoped in the interaction. The interactions are automatically detected during the solution.
The default frictionless interaction type that is scoped to all bodies activates frictionless contact between any external node and face that may come into contact in the model during the analysis.
To improve the efficiency of analyses involving large number of bodies, you are advised to suppress the default frictionless interaction that is scoped to all bodies, and instead insert additional Body Inter-action objects which limit interInter-actions to specific bodies. The union of all frictional/frictionless body interactions defines the matrix of possible body interactions during the analysis.
For example, in the model shown below:
• Body A is traveling towards body B and we require frictional contact to occur. A frictional body interaction type scoped only to bodies A and B will achieve this. Body A will not come close to body C during the ana-lysis so it does not need to be included in the interaction.
• Body B is bonded to body C. A bonded body Interaction type, scoped to bodies B and C will achieve this. • If the bond between bodies B and C breaks during the analysis, we want frictional contact to take place
between bodies B and C. A frictional body interaction type scoped only to bodies B and C will achieve this.
A bonded body interaction type can be applied in addition to a frictional/frictionless body interaction. A reinforcement body interaction type be can be applied in addition to a frictional/frictionless body interaction.
Object property settings are included in the Details view for both the Body Interactions folder and the individual Body Interaction objects. Refer to the following sections for descriptions of these properties.
2.6.2.1. Properties for Body Interactions Folder
2.6.2.2. Interaction Type Properties for Body Interaction Object 2.6.2.3. Identifying Body Interactions Regions for a Body
2.6.2.1. Properties for Body Interactions Folder
All properties for the Body Interactions folder are included in an Advanced category and define the global properties of the contact algorithm for the analysis. These properties are applied to all Body Interaction objects and to all frictional and frictionless manual contact regions.
This section includes descriptions of the following properties for the Body Interactions folder:
2.6.2.1.1. Contact Detection 2.6.2.1.2. Formulation
2.6.2.1.3. Shell Thickness Factor and Nodal Shell Thickness 2.6.2.1.4. Body Self Contact
2.6.2.1.5. Element Self Contact 2.6.2.1.6. Tolerance
2.6.2.1.7. Pinball Factor
2.6.2.1.8. Time Step Safety Factor 2.6.2.1.9. Limiting Time Step Velocity 2.6.2.1.10. Edge on Edge Contact
2.6.2.1.1. Contact Detection
The available choices are described below.
Trajectory
The trajectory of nodes and faces included in frictional or frictionless contact are tracked during the computation cycle. If the trajectory of a node and a face intersects during the cycle a contact event is detected.
The trajectory contact algorithm is the default and recommended option in most cases for contact in Explicit Dynamics analyses. Contacting nodes/faces can be initially separated or coincident at the start of the analysis. Trajectory based contact detection does not impose any constraint on the analysis time step and therefore often provides the most efficient solution.
Note that nodes which penetrate into another element at the start of the simulation will be ignored for the purposes of contact and thus should be avoided. To generate duplicate conforming nodes across a contact interface:
1. Use the multibody part option in DesignModeler and set Shared Topology to Imprint.
2. For meshing, use Contact Sizing, the Arbitrary match control or the Match mesh Where Possible option of the Patch Independent mesh method.
Proximity Based
The external faces, edges and nodes of a mesh are encapsulated by a contact detection zone. If during the analysis a node enters this detection zone, it will be repelled using a penalty based force.
Note
• An additional constraint is applied to the analysis time step when this contact detection algorithm is selected. The time step is constrained such that a node cannot travel through a fraction of the contact detection zone size in one cycle. The fraction is defined by the Time Step Safety
Factor (p. 16) described below. For analyses involving high velocities, the time step used in the analysis is often controlled by the contact algorithm.
• The initial geometry/mesh must be defined such that there is a physical gap/separation of at least the contact detection zone size between nodes and faces in the model. The solver will give error messages if this criteria is not satisfied. This constraint means this option may not be prac-tical for very complex assemblies.
2.6.2.1.2. Formulation
This property is available if Contact Detection is set to Trajectory. The available choices are described below.
Penalty
If contact is detected, a local penalty force is calculated to push the node involved in the contact event back to the face. Equal and opposite forces are calculated on the nodes of the face in order to conserve linear and angular momentum.
Trajectory based penalty force,
Proximity based penalty force,
Where:
D is the depth of penetration
M is the effective mass of the node (N) and face (F)
Δt is the simulation time step
Note
• Kinetic energy is not necessarily conserved. You can track conservation of energy in contact using the Solution Information object, the Solution Output, or one of the energy summary result trackers.
• The applied penalty force will push the nodes back towards the true contact position during the cycle. However, it will usually take several cycles to satisfy the contact condition.
Decomposition Response
All contacts that take place at the same point in time are first detected. The response of the system to these contact events is then calculated to conserve momentum and energy. During this process, forces are calculated to ensure that the resulting position of nodes and faces does not result in further penet-ration at that time point.
Note
• The decomposition response algorithm cannot be used in combination with bonded contact regions. The formulation will be automatically switch to penalty if bonded regions are present in the model.
• The decomposition response algorithm is more impulsive (in a given cycle) than the penalty method. This can give rise to large hourglass energies and energy errors.
2.6.2.1.3. Shell Thickness Factor and Nodal Shell Thickness
These properties are available if the geometry includes one or more surface bodies and if Contact De-tection is set to Trajectory.
The Shell Thickness Factor allows you to control the effective thickness of surface bodies used in the contact. You can specify a value between 0.0 and 1.0.
• A value of 0.0 means that contact will ignore the physical thickness of the surface body and the contact surface will be coincident with the mid-plane of the shell
• A value of 1.0 means that the contact shell thickness will be equal to the physical shell thickness. The contact surface will be offset from the mid-plane of the shell by half the shell thickness (on both sides of the shell) Nodal Shell Thickness is only active when the Shell Thickness Factor value is not zero (0). It allows you to obtain the most accurate shell to shell contact by improving on the Shell Thickness Factor ap-proach.
• When set to Yes, contact between shells is improved by eliminating the inherent small overlap that may occur even when the Shell Thickness Factor is set to 1.0. Essentially this setting (along with a thickness factor of 1.0) will provide the most accurate shell thickness contact behaviour.
• When set to No, the contact shell thickness will be determined by the value of the Shell Thickness Factor and the nodal shell thickness will not have any effect.
When set to Program Controlled, the behavior of nodal shell thickness is determined by the Analysis Settings Preference Type (p. 47).
2.6.2.1.4. Body Self Contact
When set to Yes, the contact detection algorithm will check for external nodes of a body contacting with faces of the same body in addition to other bodies. This is the most robust option since all possible external contacts should be detected.
When set to No, the contact detection algorithm will only check for external nodes of a body contacting with external faces of other bodies. This setting reduces the number of possible contact events and can therefore improve efficiency of the analysis. This option should not be used if a body is likely to fold onto itself during the analysis, as it would during plastic buckling for example.
When set to Program Controlled, the behavior of self contact is determined by the Analysis Settings Preference Type (p. 47).
Presented below is an example of a model that includes self impact.
2.6.2.1.5. Element Self Contact
When set to Yes, automatic erosion (removal of elements) is enabled when an element deforms such that one of its nodes comes within a specified distance of one of its faces. In this situation, elements
are removed before they become degenerated. Element self contact is very useful for impact penetration examples where removal of elements is essential to allow generation of a hole in a structure.
When set to Program Controlled, the behavior of self contact is determined by the Analysis Settings Preference Type (p. 47).
2.6.2.1.6. Tolerance
This property is available if Contact Detection is set to Trajectory and Element Self Contact is set to Yes.
Tolerance defines the size of the detection zone for element self contact when the trajectory contact option is used. (see Element Self Contact (p. 14)). The value input is a factor in the range 0.1 to 0.5. This factor is multiplied by the smallest characteristic dimension of the elements in the mesh to give a physical dimension. A setting of 0.5 effectively equates to 50% of the smallest element dimension in the model.
Note
The smaller the fraction the more accurate the solution.
2.6.2.1.7. Pinball Factor
This property is available if Contact Detection is set to Proximity Based.
The pinball factor defines the size of the detection zone for proximity based contact. The value input is a factor in the range 0.1 to 0.5. This factor is multiplied by the smallest characteristic dimension of the elements in the mesh to give a physical dimension. A setting of 0.5 effectively equates to 50% of the smallest element dimension in the model.
Note
The smaller the fraction the more accurate the solution. The time step in the analysis could be reduced significantly if small values are used (see Time Step Safety Factor (p. 16)).
2.6.2.1.8. Time Step Safety Factor
This property is available if Contact Detection is set to Proximity Based.
For proximity based contact, the time step used in the analysis is additionally constrained by contact such that in one cycle, a node in the model cannot travel more than the detection zone size, multiplied by a safety factor. The safety factor is defined with this property and the recommended default is 0.2. Increasing the factor may increase the time step and hence reduce runtimes, but may also lead to missed contacts. The maximum value you can specify is 0.5.
2.6.2.1.9. Limiting Time Step Velocity
This property is available if Contact Detection is set to Proximity Based.
For proximity based contact, this setting limits the maximum velocity that will be used to compute the proximity based contact time step calculation.
2.6.2.1.10. Edge on Edge Contact
This property is available if Contact Detection is set to Proximity Based.
By default, contact events in Explicit Dynamics are detected by nodes impacting faces. Use this option to extend the contact detection to include discrete edges impacting other edges in the model.
Note
This option is numerically intensive and can significantly increase runtimes. It is recommended that you compare results with and without edge contact to make sure this feature is required. A model with edge on edge contact cannot be run in parallel.
2.6.2.2. Interaction Type Properties for Body Interaction Object
This section includes descriptions of the interaction types for the Body Interaction object:
2.6.2.2.1. Frictionless Type 2.6.2.2.2. Frictional Type 2.6.2.2.3. Bonded Type 2.6.2.2.4. Reinforcement Type
2.6.2.2.1. Frictionless Type
Setting Type to Frictionless activates frictionless sliding contact between any exterior node and any exterior face of the scoped bodies. Individual contact events are detected and tracked during the ana-lysis. The contact is symmetric between bodies (that is, each node will belong to a master face impacted by adjacent slave nodes; each node will also act as a slave impacting a master face).
Supported Connections
Explicit Dynamics Line Shell Volume Connection Geometry Yes Yes Yes VolumeLine Shell Volume Connection Geometry Yes Yes Yes Shell *Yes Yes Yes Line
*Only for Contact Detection = Proximity Based and Edge on Edge Contact = Yes (This option switches on contact between ALL lines / bodies / edges, that is, there is no dependence on the scoping selection of body interactions.)
Explicit Dynamics (LS-DYNA Export)
Line Shell Volume Connection Geometry No Yes Yes Volume No Yes Yes Shell No No No Line
2.6.2.2.2. Frictional Type
Descriptions of the following properties are also addressed in this section: • Friction Coefficient
• Dynamic Coefficient • Decay Constant
Setting Type to Frictional activates frictional sliding contact between any exterior node and any exter-ior face of the scoped bodies. Individual contact events are detected and tracked during the simulation. The contact is symmetric between bodies (that is, each node will belong to a master face impacted by adjacent slave nodes, each node will also act as a slave impacting a master face).
Friction Coefficient: A non-zero value will activate Coulomb type friction between bodies (F = μR). The relative velocity (ν) of sliding interfaces can influence frictional forces. A dynamic frictional formu-lation for the coefficient of friction can be used.
(2.3)
where
= friction coefficient
= dynamic coefficient of friction β = exponential decay coefficient
ν = relative sliding velocity at point of contact
Non-zero values of the Dynamic Coefficient and Decay Constant should be used to apply dynamic friction.
Supported Connections
Explicit DynamicsLine Shell Volume Connection Geometry Yes Yes Yes Volume Yes Yes Yes Shell *Yes Yes Yes Line
*Only for Contact Detection = Proximity Based and Edge on Edge Contact = Yes (This option switches on contact between ALL lines / bodies / edges, that is, there is no dependence on the scoping selection of body interactions.)
Explicit Dynamics (LS-DYNA Export)
Line Shell Volume Connection Geometry No Yes Yes Volume No Yes Yes Shell No No No Line
2.6.2.2.3. Bonded Type
Descriptions of the following properties are also addressed in this section: • Maximum Offset
• Breakable – Stress Criteria
→ Normal Stress Limit → Normal Stress Exponent → Shear Stress Limit → Shear Stress Exponent
External nodes of bodies included in bonded interactions will be tied to faces of bodies included in the interaction if the distance between the external node and the face is less than the value (tolerance) defined by the user in Maximum Offset. The solver automatically detects the bonded nodes/faces during the initialization phase of the analysis.
Note that it is important to select an appropriate value for the Maximum Offset (tolerance). The auto-matic search will bond everything together which is found within this value (tolerance).
Use the custom variable BOND_STATUS to check bonded connections in Explicit Dynamics. The variable records the number of nodes bonded to the faces on an element during the analysis. This can be used not only to verify that initial bonds are generated appropriately, but also to identify bonds that break during the simulation.
Maximum Offset defines the tolerance used at initialization to determine whether a node is bonded to a face.
Breakable = Stress Criteria implies that the bond may break (or be released) during the analysis. The criteria for breaking a bond is defined as:
(2.4)
where
= Normal Stress Limit n = Normal Stress Exponent
= Shear Stress Limit m = Shear Stress Exponent
The options in the Advanced section are all currently ignored by the Explicit solver, including the Ad-vanced > Pinball region option. The tolerance must be defined via the Maximum Offset value and is only used at initialization.
Supported Connections
Explicit Dynamics Line Shell Volume Connection Geometry Yes Yes Yes Volume Yes Yes Yes Shell Yes Yes Yes LineNote
Bonded body interactions and contact are not supported for 2D Explicit Dynamics analyses.
Explicit Dynamics (LS-DYNA Export)*
Line Shell Volume Connection Geometry No Yes Yes Volume No Yes Yes Shell No Yes Yes Line
*The above matrix is valid only for Contact Regions. Bonded body interactions are not supported at all.
2.6.2.2.4. Reinforcement Type
This body interaction type is used to apply discrete reinforcement to solid bodies. All line bodies scoped to the object will be flagged as potential discrete reinforcing bodies in the solver. On initialization of the solver, all elements of the line bodies scoped to the object which are contained within any solid body in the model will be converted to discrete reinforcement. Elements which lie outside all volume bodies will remain as standard line body elements.
The reinforcing beam nodes will be constrained to stay at the same initial parametric location within the volume element they reside during element deformation. Typical applications involve reinforced concrete or reinforced rubber structures likes tires and hoses.
If the volume element to which a reinforcing node is tied is eroded, the beam node bonding constraint is removed and becomes a free beam node.
On erosion of a reinforcing beam element node, if inertia is retained, the node will remain tied to the parametric location of the volume element. If inertia is not retained, the node will also be eroded
Note
Volume elements that are intersected by reinforcement beams, but do not contain a beam node, will not be experiencing any reinforced beam forces. Good modeling practice is therefore to have the element size of the beams similar or less than that of the volume ele-ments.
Table 2.1: Example: Drop test onto reinforced concrete beam
Note that the target solid bodies do not need to be scoped to this object – these will be identified automatically by the solver on initialization.
Supported Connections
Explicit Dynamics Line Shell Volume Connection Geometry *Yes No No Volume No No No Shell No No *Yes Line*Only the line body needs to be included in the scope. The ANSYS Autodyn solver automatically detects which volume bodies that the line body passes through.
Note
Reinforcement body interactions are not supported for 2D Explicit Dynamics analyses.
Explicit Dynamics (LS-DYNA Export)
Line Shell Volume Connection Geometry No No No Volume No No No Shell No No No Line
2.6.2.3. Identifying Body Interactions Regions for a Body
See the description for Body Interactions for Selected Bodies in the section Correlating Tree Outline Objects with Model Characteristics in the ANSYS Mechanical User's Guide.
2.7. Setting Up Symmetry
For general information about setting up symmetry see Symmetry in the Mechanical Application in the
ANSYS Mechanical User's Guide.
2.7.1. Explicit Dynamics Symmetry
Symmetry regions can be defined in explicit dynamics analyses. Symmetry objects should be scoped to faces of flexible bodies defined in the model. All nodes lying on the plane, defined by the selected face are constrained to give a symmetrical response of the structure.
Note
• Anti-symmetry, periodicity and anti-periodicity symmetry regions are not supported in Explicit Dynamics systems.
• Symmetry cannot be applied to rigid bodies.
• Only the General Symmetry interpretation is used by the solver in 2D Explicit Dynamics analyses.
Symmetry conditions can be interpreted by the solver in two ways:
2.7.1.1. General Symmetry 2.7.1.2. Global Symmetry Planes
2.7.1.1. General Symmetry
In general, a symmetry condition will result in degree of freedom constraints being applied to the nodes on the symmetry plane. For volume elements, the translational degree of freedom normal to the sym-metry plane will be constrained. For shell and beam elements, the rotational degrees of freedom in the plane of symmetry will be additionally constrained.
For nodes which have multiple symmetry regions assigned to them (for example, along the edge between two adjacent faces), the combined constraints associated with the two symmetry planes will be enforced.
Note
• Symmetry regions defined with different local coordinate systems may not be combined, unless they are orthogonal with the global coordinate system.
• General symmetry does not constrain eroded nodes. Thus, if after a group of elements erodes, a “free” eroded node remains, the eroded node will not be constrained by the symmetry condition. This can be resolved in certain situations via the special case of Global symmetry, described in the next section.
2.7.1.2. Global Symmetry Planes
If a symmetry object is aligned with the Cartesian planes at x=0, y=0 or z=0, and all nodes in the model are on the positive side of x=0, y=0, or z=0, the symmetry condition is interpreted as a special case termed Global symmetry plane. In addition to general symmetry constraints:
• If a symmetry plane is coincident with the YZ plane of the global coordinate system (X=0), and no parts of the geometry lie on the negative side of the plane, then a symmetry plane is activated at X=0. This will prevent any nodes (including eroded nodes) from moving through the plane X=0 during the analysis. • If a symmetry plane is coincident with the ZX plane of the global coordinate system (Y=0), and no parts of
the geometry lie on the negative side of the plane, then a symmetry plane is activated at Y=0. This will prevent any nodes (including eroded nodes) from moving through the plane Y=0 during the analysis. • If a symmetry plane is coincident with the XY plane of the global coordinate system (Z=0), and no parts of
the geometry lie on the negative side of the plane, then a symmetry plane is activated at Z=0. This will prevent any nodes (including eroded nodes) from moving through the plane Z=0 during the analysis.
Note
Global symmetry planes are only applicable to 3D Explicit Dynamics analyses.
2.7.2. Symmetry in an Euler Domain
There are additional considerations if an Euler Domain is defined for an analysis. For symmetry to be applied to an Euler Domain, symmetry will have to be defined with the global coordinate system, not a local one, and it will need to be applied on geometry faces which lie on the global coordinate system planes.
• If the symmetry is not defined with the global coordinate system, it is ignored and a warning is shown in the messages window saying that such symmetry will be ignored but the analysis continues to solve. • If the symmetry is not applied on faces which lie on the global coordinate system planes then an error
In the case where symmetry is valid for use with Euler Domains, if the boundary of the Euler Domain which is parallel to the symmetry plane is below the symmetry plane, then that boundary will be moved to lie on the symmetry plane if the following conditions are true:
• The Euler Domain Size Definition option in the Analysis settings is set to Program Controlled. • The Euler body is on the positive side of the global coordinate axis.
2.8. Define Remote Points
The algorithm in the Explicit Dynamics solver is different from the Implicit solver in the way it handles rigid bodies. For general information about how to use remote points, see Specifying Remote Points in the Mechanical Application and Remote Boundary Conditions in the ANSYS Mechanical User's Guide. The following topics are available:
2.8.1. Explicit Dynamics Remote Points
2.8.2. Explicit Dynamics Remote Boundary Conditions 2.8.3. Initial Conditions on Remote Points
2.8.4. Constraints and Remote Points
2.8.1. Explicit Dynamics Remote Points
A remote point in Explicit Dynamics consists of a:
• Location - The position where a remote boundary condition can be applied • Scoped region - A group of rigid body nodes
• Boundary condition (optional)
The Explicit Dynamics solver does not support Deformable Behavior using remote points.
The group of rigid body nodes is treated as a regular rigid body by the solver. For example, if the scoped region consists of two faces from separate parts, the solver determines the center of mass and inertia properties for the combined group of nodes making up the two faces. This calculation creates a rigid connection between the two parts.
In the solution, the forces acting on the group of rigid body nodes are assembled for each time step. This calculation determines the rigid body motion and therefore the motion of the nodes belonging to the remote point. The group of rigid body nodes is unable to deform: the elements in the solid part may be flexible, but the scoped side face will not deform, although it may rotate or translate. Due to this restriction, it is important to have a sufficient number of nodes in the scoped area if the solid part is flexible.
Note
When using Remote Points in Explicit Dynamics analyses:
• Remote Points only work with the Explicit Dynamics system, not the Explicit Dynamics (LS-DYNA Export) system.
• The Behavior field must be set to Rigid. If it is set to Deformable the solution will terminate and an error will be generated.
• Currently, only the remote displacement boundary condition is supported for Remote Points in Explicit Dynamics analyses.
• Commands are not supported for Remote Points in Explicit Dynamics analyses. • Remote Points are not supported for 2D Explicit Dynamics analyses.
2.8.2. Explicit Dynamics Remote Boundary Conditions
Currently, the only remote boundary conditions available in the Explicit Dynamics solver are remote displacements.
The Explicit Dynamics solver treats a remote displacement as follows:
• It tracks the motion of the scoped group of nodes specified by the remote point. • It tracks the velocity of the location of the remote point.
• The actual translation and rotation of the remote point are a combination of the imposed boundary constraints of the remote displacement definition and the forces acting on the group of nodes scoped to the remote point. Therefore, the translation and rotation of the remote point and scoped nodes are determined simul-taneously and are enforced with the use of a single corrective force and moment.
2.8.3. Initial Conditions on Remote Points
Initial conditions are scoped to geometric parts in the model. Effectively this means that the initial condition is scoped to a set of elements. However, remote points are scoped to the underlying nodes in the model. This may result in different initial conditions on the same node in a remote point definition. This section describes the behavior in such instances.
Initial condition on a flexible part:
Initial conditions can be scoped to a subset of or all elements in a flexible part. It is not necessary to scope an initial condition to all the nodes in the remote point definition, as long as there is only one initial con-dition defined for the nodes that participate in the remote point definition.
Initial condition on a rigid body part:
The remote point definition will automatically include all the nodes in a rigid part. Therefore the initial condition (or multiple identical initial conditions) should be scoped to all the elements in the rigid part. The scoped nodes of the remote point will follow the initial condition of the scoped rigid body. If the flexible scoped nodes of the remote point contain their own initial condition, this will be ignored.
2.8.4. Constraints and Remote Points
It is possible to over-constrain bodies by having an incorrect mix of boundary conditions (loads and supports) and Remote Points applied. Remote Points effectively make a face act as rigid, and it is im-portant to remember that remote points are defined per model and therefore may conflict when shared with another analysis type with different constraint requirements. Remote displacements are boundary conditions but are applied to remote points, and for the purpose of this document are not considered as constraining boundary conditions.
Fixed Support Constraining boundary conditions (Restricted Use)
Velocity Simply Supported Fixed Rotation Displacement Gravity Hydrostatic Pressure Detonation Point Pressure
Examples of permitted boundary conditions (Unrestricted Use)
Acceleration Force
Symmetry Planes
Euler Boundary Flow Out Line Pressure
Remote Displacement (treated as a Velocity)
Remote point applied boundary conditions
The following rules apply when applying constraints and Remote Points to Flexible and Rigid Bodies in an Explicit Dynamics analysis. If incompatible conditions are applied, the pre-solve checks will identify the problem and inform the user prior to starting the Solve.
FLEXIBLE BODY
Allowed? + Notes Conditions
Example
Yes Remote Point applied to one face.
Yes Remote Point and Remote Displacement applied to one face.
No Remote Point applied to two adjacent faces.
The 2 faces share common nodes along one edge.
FLEXIBLE BODY
Allowed? + Notes Conditions
Example
Yes Remote Point applied to two faces that do not
share any nodes.
Yes Remote Point applied to two faces that do not
share any nodes, with Remote Displacement applied to one of the Remote Points.
No Remote Point on one face with Remote
Displacement applied. Constraining boundary
condition applied to adjacent face. The boundary
condition scope shares nodes with the scope of the Remote
Displacement. No
Remote Point on one face. Constraining boundary condition applied to adjacent face.
The boundary
condition scope shares nodes with the scope of the Remote Point. Yes
Remote Point on one face. Constraining boundary condition on another but with no common scoped nodes.
Yes Remote Point on one face with Remote
Displacement applied. Constraining boundary condition on another but with no common scoped nodes. RIGID BODY Allowed? + Notes Conditions Example Yes Remote Point applied to one face.
This is largely superfluous as the body is rigid already so making the face rigid does not make any difference.
RIGID BODY
Allowed? + Notes Conditions
Example
Yes Remote Point and Remote
Displacement applied to one face.
Yes Remote Point applied to two
adjacent faces.
Yes Remote Point applied to two
faces that do not share any
nodes. This is largely superfluous as
the body is rigid already so making the face rigid does not make any difference. Yes
Remote Point applied to two faces that do not share any nodes, with Remote Displacement applied to one of the Remote Points.
Yes Remote Point on one face.
Constraining boundary condition on body.
No Remote Point on one face with
Remote Displacement applied.
Two constraining boundary conditions on a Rigid body are not allowed.
Constraining boundary condition on body.
2.9. Apply Mesh Controls/Preview Mesh
For general information about how to apply mesh controls and preview the mesh, see Apply Mesh Controls and Preview Mesh in the ANSYS Mechanical User's Guide
All mesh methods available in the Workbench meshing application can be utilized in Explicit Dynamics systems.
• Swept Volume Meshing
• Patch Dependant Volume Meshing • Hex Dominant Meshing
• Patch Independent Tetrahedral Meshing • Multizone Volume Meshing
• Patch dependant shell meshing • Patch independent shell meshing
A smooth uniform mesh should be sought in the regions of interest for the analysis. Elsewhere,
coarsening of the mesh may help to reduce the overall size of the problem to be solved. Use the Explicit
meshing preference (set by default) to auto-assign the default mesh controls that will provide a mesh well suited for Explicit Dynamics analyses. This preference automatically sets the Rigid Body Behavior mesh control to Full Mesh. The Full Mesh setting is only applicable to Explicit Dynamics analyses. Other physics preferences can be used if better consistency is desired between implicit and explicit models.
Consideration should be given to the number of elements in the model and the quality of the mesh to give larger resulting time steps and therefore more efficient simulations. A coarse mesh can often be used to gain insight into the basic dynamics of a system while a finer mesh is required to investigate nonlinear material effects and failure.
Swept/multi-zone meshes are preferred in Explicit Dynamics analyses so geometry slicing, combined with multibody part options in DesignModeler are recommended to facilitate hexahedral meshing. Al-ternatively use the patch independent tetrahedral meshing method to obtain more uniform element sizing and take advantage of automatic defeaturing.
Define the element size manually to produce more uniform element size distributions especially on surface bodies.
Midside nodes should be dropped from the mesh for all elements types (solids, surface and line bodies). Error/warning messages are provided if unsupported (higher order) elements are present in the mesh. Pyramid elements are not supported in Explicit Dynamics analyses. Any elements of this type are con-verted into two tetrahedral elements, and will warrant a warning in the message window of the Mech-anical application.
An Explicit Dynamics model with fewer elements than the number of slave processes specified cannot be run in parallel.
For Explicit Dynamics (LS-DYNA Export) systems, only the element types listed below are supported (partly due to LS-DYNA limitations). Any parts with a mesh containing unsupported elements will be excluded from the exported mesh. A warning is displayed specifying excluded parts.
• Shells
– 1st Order: triangles, quadrilaterals – 2nd Order: none
• Solids
– 2nd Order: tetrahedrons
Note
Pyramids are not recommended for LS-DYNA. A warning is issued if such elements are present in the mesh.
When performing an implicit static structural or transient structural analysis to an Explicit Dynamics analysis, the same mesh is required for both the implicit and explicit analysis and only low order elements are allowed. If high order elements are used, the solve will be blocked and an error message will be issued.
2.10. Establish Analysis Settings
For general information about how to establish analysis settings, see Establish Analysis Settings in the
ANSYS Mechanical User's Guide
The basic analysis settings for explicit dynamics analyses (p. 33) are:
• Step Controls - The required input for step control is the termination time for the analysis. This should be set to your best estimate of the solution time required to simulate the event being modeled. You should normally allow the solver to determine its own time step size based on the smallest CFL condition in the model. The efficiency of the solution can be increased with the help of mass scaling options. Use this feature with caution. Too much mass scaling can give rise to non-physical results.
An explicit dynamics solution may be started, interrupted and resumed at any point in time. For ex-ample, an existing solution that has reached its End Time may be extended to continue to review the progression of the mechanical phenomena simulated. The Resume From Cycle option enables you to select which Restart file you would like the Solve to resume the analysis from. See Resume Capability for Explicit Dynamics Analyses (p. 60) for more information. Explicit dynamics analyses are always solved in a single analysis step.
Step Control options
– Resume from cycle (option not available in LS-DYNA)
– Maximum Number of Cycles in ANSYS Autodyn is replaced by Maximum time steps in LS-DYNA – Reference energy cycle (option not available in LS-DYNA)
– The Maximum Element Scaling and Update frequency (options not available in LS-DYNA)
• Solver Controls – These advanced controls allow you to control a range of solver features including element formulations and solution velocity limits. The defaults are applicable to wide range of applications.
– Shell thickness update, shell inertia update, density update, minimum velocity, maximum velocity and radius cutoff options can only be set in ANSYS Autodyn.
– Full shell integration and a selectable Unit System are available only in the LS-DYNA Export system. • Euler Domain Controls – There are three sets of parameters that are necessary to define the Euler Domain:
(Domain Resolution Definition), and the type of boundary conditions to be applied to the edges of the domain.
Note
Euler capabilities are not supported for the Explicit Dynamics (LS-DYNA Export) system.
The domain size can be defined automatically (Domain Size Definition = Program Controlled) or manually (Domain Size Definition = Manual). For both the automatic and manual options, the size is defined from a 3D origin point and the X, Y, and Z dimensions of the domain.
For the automatic option, specify the Scope of the Domain Size Definition so that the origin and X, Y, and Z dimensions are set to create a box large enough to include all bodies in the geometry (Scope = All Bodies) or the Eulerian Bodies only (Scope = Eulerian Bodies Only). The automatically determ-ined domain size can be controlled with three scaling parameters, one for each direction (X Scale Factor, Y Scale Factor, Z Scale Factor).
The size of the domain is affected by the scale factors according to the following equations:
(2.5) (2.6) (2.7)
where
lx, ly, lz are the lengths of the unscaled domain in the x, y, and z directions respectively. These para-meters are obtained automatically from the mesh.
l'x, l'y, l'z are the lengths of the scaled domain in the x, y, and z directions respectively. Fx, Fy, Fz are the scale factors for the x, y, and z directions respectively.
For the Manual option of the Domain Size Definition, specify the origin of the Euler Domain (Minimum X Coordinate, Minimum Y Coordinate, Minimum Z Coordinate) and the dimension in each direction (X Dimension, Y Dimension, Z Dimension).
The domain resolution specifies how many cells should be created in the X, Y, and Z directions of the domain. Use the Domain Resolution Definition field to specify how to determine the resolution: either the cell size (Cell Size), the number of cells in each of the X, Y, and Z directions (Cells per Component), or the total number of cells to be created (Total Cells).
– For the Cell Size option, specify the size of the cell in the Cell Size parameter. The value specified is the dimension of the cell in each of the X, Y, and Z directions. The units used for the cell size follow the ones specified in the Mechanical application window and are displayed in the text box.
The number of the cells in each direction of the domain are then determined from this cell size and the size of the domain with the following equations:
(2.8)
