2. Explicit Dynamics Workflow
2.9. Apply Mesh Controls/Preview Mesh
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.
Solid, Surface, and Line bodies can be present in an Explicit Dynamics analysis.
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
• Supports: Displacement, Fixed Support and 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
(2.2)
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.
Factor (p. 16) described below. For analyses involving high velocities, the time step used in the analysis is often controlled by the contact algorithm.