Chapter 7
Explicit Dynamics: Body
Interactions
ANSYS Explicit Dynamics
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Explicit Dynamics: Body Interactions
Body Interactions
Training Manual
• The Body Interactions folder, under
Connections, is used to define global
connection options for Explicit Dynamics
– Contact Detection
• Trajectory (default)
– Formulation
– Shell Thickness Factor
– Tolerence
• Proximity Based
–
–
–
–
Pinball Factor
Timestep Safety Factor
Limiting Timestep Velocity
Edge On Edge Contact
– Body Self Contact
– Element Self Contact
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Explicit Dynamics: Body Interactions
Trajectory Contact
Training Manual
• Recommended option for impact and sliding contact
between Solids, Shells and Beams.
• The trajectory of nodes and faces are tracked during
the computation cycle. If the trajectory of a node and
a face intersects during the cycle a contact event is
detected (see figure).
• Trajectory based contact does not constrain the time
step
– Often provides the most efficient solution.
• Nodes which penetrate into another element at the
start of the simulation will be ignored for contact and
should be avoided.
– To generate duplicate conforming nodes across a
contact interface
a) Use the multi-body part option in DesignModeler and set
the Shared Topology Method to either Imprint or None
b) In Meshing use Arbitrary Match Control (Sweep) or Match
mesh where possible (Patch Independent Tetrahedrons).
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Explicit Dynamics: Body Interactions
Trajectory Contact
Training Manual
• Shell Thickness Factor
– The Shell Thickness Factor (STF) defines the shell (surface body) thickness used for
contact
• A factor of 1.0 takes the true physical shell thickness into account, which means that the contact
surface is positioned at half the true shell thickness on both sides of the shell mid plane.
• A factor of 0.0 means that the shell has no contact thickness and the contact surface is
positioned at the shell mid plane.
• Value must be between 0.0 and 5.0
– For shell node on shell face impacts, the node is always located at the mid-surface of
the shell.
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Explicit Dynamics: Body Interactions
Trajectory Contact
Training Manual
• Formulations
– Penalty Formulation (default, recommended)
•
•
A penalty force is calculated to push a penetrating node back to the face.
Penalty forces are calculated to conserve linear and angular momentum.
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
•
Nodes will be pushed back towards the contact position, but 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 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 penetration at that time point
•
The decomposition response algorithm is more impulsive (in a given cycle) than the penalty method. This
can in some situations give rise to large hourglass energies and energy errors.
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Explicit Dynamics: Body Interactions
Proximity Based Contact
Training Manual
• Alternative option for impact and sliding contact between
Solids, Shells and Beams
• The external faces, edges and nodes of a mesh are
encapsulated by a contact detection zone. If a node
enters this zone, it is repelled using a penalty based force
that conserves linear and angular momentum
• Pinball Factor defines the size of the contact detection
zone (Gap).
– Smallest element size is multiplied by this factor to give the
physical size of the contact detection zone.
– Value must be between 0.1 to 0.5.
– The smaller the fraction the more accurate the solution.
• Initial geometry / mesh must be defined such that there is
a physical gap / separation of at least the contact
detection zone size between interacting nodes and faces
in the model.
F
F
R
R
F
R
R
Gap
– Solver will give error messages if this criteria is not satisfied.
– May not be practical for very complex assemblies.
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Explicit Dynamics: Body Interactions
Proximity Based Contact
Training Manual
– Time step is constrained so that contact nodes can
travel no more than
(contact detection zone size) x (Timestep Safety Factor)
in one cycle.
• Default Timestep Safety Factor is 0.2 (strongly
recommended)
– Increasing to 0.5 (maximum value) may increase time step
and reduce runtimes, but contacts may be missed.
• Time step is computed using the maximum velocity of all
contact nodes, but limited by the Limiting Timestep
Velocity (default very large).
– Use with care or contacts may be misses
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Explicit Dynamics: Body Interactions
Proximity Based Contact
Training Manual
• Edge on Edge Contact
• Edge on Edge contact can be used to extend
contact detection to include edge to edge
contacts (standard detection is node to face).
– All edges of Solid, Shell and Beam elements will
be included in the contact detection.
– This option is numerically intensive and can
significantly increase runtimes.
• Compare results with and without edge contact to
make sure this option is required.
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Explicit Dynamics: Body Interactions
Trajectory and Proximity Based Contact
Training Manual
• Body Self Contact
– By default, the contact detection algorithm will
check for external nodes of a body contacting with
faces of the same body in addition to other bodies.
• Most robust option since all possible external contacts
should be detected.
– If self impact of a body is unlikely, set Body Self
Contact = No
• Reduces the number of possible contact events
• Improve efficiency of the simulation.
• Do not use if a body is likely to fold onto itself during
the simulation e.g. during plastic buckling.
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Explicit Dynamics: Body Interactions
Trajectory and Proximity Based Contact
Training Manual
• Element Self Contact
• This option automatically removes
(erodes) an element if it deforms
such that one of it’s nodes comes
within a tolerance of one of it’s
faces, i.e. before it becomes
degenerate or inverted.
– For Trajectory Contact a Tolerance
factor is specified
• Smallest element size is multiplied
by this factor to give the physical
tolerance.
• Value must be between 0.1 to 0.5.
– For Proximity Based Contact, the
Pinball Factor is used for the
tolerance factor
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Explicit Dynamics: Body Interactions
Body Interaction Objects
Training Manual
• Each Body Interaction object activates a body
interaction for the bodies scoped in the object
– Four Types of Body Interaction
•
•
•
•
Bonded
Frictionless (contact)
Frictional (contact)
Reinforcement
– Contact detection is completely automated.
• Every node of the scoped bodies interacts with every face of
the scoped bodies.
• A default Frictionless interaction is scoped to all bodies
• To improve the efficiency of simulations involving large
number of bodies:
– Suppress the default Frictionless interaction
– Insert new Body Interaction objects which limit interactions
to specific bodies.
• The union of all Frictional / Frictionless body interactions
defines the matrix of possible body interactions during the
simulation.
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Explicit Dynamics: Body Interactions
Body Interaction Objects
Training Manual
• Example
– Body A is travelling towards body B and we
require Frictional contact to take place. A body
Interaction of type frictional scoped only to
Bodies A and B will achieve this.
– Body A will not come close to body C during the
simulation so does not need to be included in
the interaction.
– Body B is bonded to Body C. A body Interaction
of type bonded, scoped to Bodies B and C will
achieve this
– If the bond between bodies B and C breaks
during the simulation. We want frictional contact
to take place between bodies B and C. A body
interaction of type frictional scoped to bodies B
and C will achieve this
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A bonded body interaction of can
be applied in addition to a
frictional / frictionless body
interaction
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Explicit Dynamics: Body Interactions
Body Interaction Objects
Training Manual
• Types of Body Interactions
– Frictionless (default)
• Sliding contact is frictionless.
– Frictional
• Friction Coefficient
– A non-zero value will activate Coulomb type friction between
bodies:
F = μsR
• Dynamic Coefficient, Decay Constant
– Non-zero values will activate dynamic friction where the
relative velocity (v) of sliding interfaces can influence frictional
forces:
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μs =
Fricti
on
Coeff
icient
μd =
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Explicit Dynamics: Body Interactions
Body Interaction Objects
Training Manual
• Types of Body Interactions
– Bonded
• External nodes of bodies included in Bonded
interactions are tied to faces of bodies included in
the interaction if the nodes are within the defined
Maximum Offset distance of a face
– Solver automatically detects bonded nodes / faces
during initialization.
• Bonds can break during a simulation based on a
specified Stress Criteria.
– Effective normal and shear stresses are calculated at
each node involved in a bonded connection.
– If the following criteria is exceeded, the node will be
released from bonded contact.
All released nodes will subsequently be added to the
sliding contact detection algorithm.
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Explicit Dynamics: Body Interactions
Body Interaction Objects
Training Manual
• Types of Body Interactions
– Bonded (continued)
• It is important to select an appropriate value for the
Maximum Offset. The automatic search will bond a node
which is to be tied to a face within the offset distance, the
selected face will be the closest one which is most parallel
to a face to which the tied node belongs.
• The breakable bond criteria works best when equivalent, or
similar, meshes are used on the faces at either side of the
bond.
• Bonded connections must be used with Trajectory Contact
• Bonded connections are only available for Solid and
Surface bodies
• Flexible to Rigid body bonded connections can only be
made using Body Interaction objects
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Explicit Dynamics: Body Interactions
Body Interaction Objects
Training Manual
• Types of Body Interactions
– Reinforcement
• Used to apply discrete reinforcement to solid bodies
– Line body elements scoped to the object, contained within any
solid body in the model, are converted to discrete reinforcement
elements / nodes
– Elements which lie outside solid bodies remain standard beam
elements
– Reinforcing beam nodes are constrained to stay at the same
parametric location within the solid element they reside during
element deformation
• For good modeling, the size of the beam elements should
be similar or less than that of the volume elements
– If volume elements erode, reinforcing nodes tied to them
become free beam nodes
– If reinforcing beam elements erode, and inertia is retained, the
eroded nodes will remain tied to the parametric location of their
solid elements
• Typical applications involve reinforced concrete or reinforced rubber
structures likes tires and hoses
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Explicit Dynamics: Body Interactions
Contact Regions
Training Manual
• Used to define specific face to face contact interactions
• Contact and target faces are scoped to each manual
contact region
– For shell bodies, target shell face option is not respected
for Explicit Dynamics. The nearest shell face is always
selected
• Types
– Bonded
• Bonds can be breakable
– No Separation
• not supported for Explicit Dynamics
– Frictionless
– Rough
• not supported for Explicit Dynamics
– Frictional
• Static and dynamic friction options
• Settings defined under Body Interactions are used for
Body Interaction Object and Contact Region Object
• May reduce Solve times
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Explicit Dynamics: Body Interactions
Spot Welds
Training Manual
• Rigidly connect two discrete points to model welds, rivets,
bolts e.t.c.
–
–
–
–
Points usually belong to two different surfaces
Defined on the geometry (DesignModeler)
Respected by Meshing
Points are connected by a rigid beam element
• Can fail (break) using Breakable Stress Criteria or Force
Criteria.
Where fn and fs are normal and shear interface forces (normal
force is only non-zero for tension), Sn and Ss are the Normal
and Shear Force Limit, and n and s are the Normal and Shear
Stress Exponent (For stress criteria an Effective Diameter is
used to convert the stress limits to equivalent force limits)
• Spot welds of zero length are permitted
– Failure criteria is modified since local normal and shear
directions cannot be defined
where Δf are component force differences across the spot weld
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Explicit Dynamics: Body Interactions
Spot welds
Training Manual
• Connecting shell points have both
translational and rotational degrees of
freedom linked
• Points on solid bodies, have additional
rigid beam elements automatically
generated to enable transfer of rotations
at the spot weld location
• Beam elements are automatically deleted
if weld fails
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