PEMP
AME2510
Contacts in LS-Dyna
Session delivered by:
Mr.Suman M.L.J.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
PEMP
AME2510
Session Topics
•Types of Contacts
•Interaction between parts
•Brief discussion about how contacts works?
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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LS-DYNA'S CONTACT ALGORITHMS
•Flexible body contact
•Flexible body to rigid body contact
•Rigid body to rigid body contact
•Edge-to-edge contact
•Eroding contact
•Tied surfaces
•Rigid walls
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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CONTACT TYPES
•Single surface
•Nodes to surface
•Surface to surface
CONTACT OPTIONS
•Normal
•Automatic
•Rigid
•Tied
•Tied with failure
•Eroding
•Edge
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
CONTACT MODELING IN LS-DYNA
•Contact provides a way of treating interaction
between different parts or disjoints parts. It is
useful for impacting one part over other part.
•Contact forms an integral part of many largedeformation problems.
•Accurate modeling of contact interfaces between
bodies is crucial for the prediction capability of
the finite element simulations.
•Contact in LS-DYNA is defined using contact
surfaces. Contact occurs when one segment of a
model’s outer surface penetrates another
segment.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
INTERACTION BETWEEN PARTS
CONTACT INTERFACES
• Penalty stiffness: Determines the size of the force
applied on the nodes in contact.
High penalty  - smaller penetrations and
- larger risk for instability
Penalty stiffness depends on:
- element volume
- contact segment area
- element stiffness
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
• Definition of segments:
1. Segment by segment
2. By property sets. Example:
master - part1, part2
slave - part3, part4
Contact entities (analytical surfaces) can be attached to rigid
bodies, and interact with deformable parts.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
INTERACTION BETWEEN PARTS
Contact Interfaces
Master
Slave
F = f.k.dp
f - Penalty Scale factor
k- Proportionality const.
dp - Penetration
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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PEMP
AME2510
• Single-Surface Contact:
-The single surface contact algorithm
establishes contact when an external
surface of one body contacts itself or
the external surface of another body
- Single surface contact is the most
general type of contact used because
in LS-DYNA program automatically
searches all of the external surfaces
within a model to determine if
penetration has occurred.
- Since all of the external surfaces are
included, no contact or target surface
definitions are required.
Consider when a part
contacts itself
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
•Most impact and crash applications require single surface contact to be
defined.
• Unlike Implicit modeling, where over-defining contact will dramatically
increase CPU time,where as in single surface contact in LS-DYNA will
cause minor CPU time increase
•The valid contact types used are
Single Surface, Automatic Single
Surface, Automatic General,
Eroding Single Surface and
Single Edge.
•Single surface contact can be
very powerful for self contact or
large deformation problems
where areas of contact are not
known beforehand.
Crush box
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
Example for single surface contact:
Side impact simulation using MDB
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
•Nodes to Surface Contact
-Node to surface contact algorithm establishes contact when a
contacting node penetrates a target surface. It is the fastest algorithm
because it is asymmetric. Here only the contact nodes impacting the
target surface is considered
-Node to surface contact is very robust for problems where the contact
area is relatively small and the contact area is known beforehand.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
Nodes to surface cont…
The following guidelines should be used with nodes to surface type
of contact:
Flat or convex surfaces should be the target surface(Master)
while concave surfaces should be contact nodes (Slave).
Coarse meshes should be target surface and finer meshes
should be contact surfaces.
• One part will be MASTER, another part will be Slave
• A node may belong to several parts
• An interface node can be in rigid body
• Breaks down with bad or disjointed mesh
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
Nodes to surface cont…
Algorithm
1. For each slave node finds
the closest master node or
master segment
2. Check if slave node has
penetrated the master
segment
3. Find contact point
4. Compute penetration
5. Apply forces to reduce
penetration
Detection of Closest master node
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
Nodes to surface cont…
Search for master node with minimum distance to slave node
Node
S4
S5
M2
S0
S2
S6
M1
S1
S3
•S0 - Previous closest
segment to N at time t1
•M1 -closest node to N
belonging to S0
•S0, S1, S2, S3 Segments
connected to M1
•M2 - New closest master
node
•S2 ,S4, S5, S6 New possible
closest master segment
Detection of Closest master node
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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PEMP
AME2510
Nodes to surface cont…
Example for Node to surface contact:
Drop test of a Liquid tank
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
• Surface to Surface Contact
-Surface to Surface contact algorithm
establishes contact when the surface of one
body penetrates the surface of another
Master - slave
contacts
-Surface to Surface contact is fully
symmetric so that the choice of contact and
target surfaces are arbitrary.
-For Surface to Surface contact ,nodal
components are required for the contact and
target surfaces. Nodes may belong to
multiple contact surfaces.
-Surface to Surface contact is a general
algorithm and is commonly used for bodies
that have large contact areas and the contact
surfaces are known.
Fm3  f s  k  d ps
fs = penalty scale factor
k = proportional constant
dp3=penetration at node
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
Surface to Surface Contact cont…
-Surface to Surface contact algorithm records the overall
resultant contact forces in the ASCII rcforc file.
-Surface to Surface contact is most efficient for bodies that
experience large amounts of relative sliding, such as a block
sliding on a plane.
- Simulate impact between two surfaces
- Works good when both are convex
- On the master side segments must be with shell or Solid elements
- Not affected by bad mesh
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
Surface to Surface Contact cont…
Example for Surface to Surface Contact
Impact of a two bar
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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•Automatic Contact
- Automatic contact allows contact to occur on both sides of
shell elements
- Automatic contact is most commonly used
- Difference in automatic orientation of surface
- Checks are made in both sides of shell
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
• General Contact
- General contact does not consider shell thickness on
contact force calculations.
- General contact is Simple and widely used
- Advantage is extremely fast and robust
- Concern is surface orientation
- Orientation will occur, only when no penetration
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
• Eroding Contact
-The purpose of defining
eroding contact is to allow
contact to occur with the
remaining elements, after
the elements originally
forming the outer surface
have failed.
-By using these type of
contact
where
solid
elements should fail after
an impact. As it has to
mimic the real world
conditions
Impact of a projectile to a plate
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
•Tied Contact
- Gluing together of contact surface to
target surface. May be of interest
when the meshes do not fit together
- Both should be coplanar initially
- Thereafter contact nodes are forced
to maintain the iso-parametric position
- Master can deform , and slave will
follow
- Coarser mesh will be master
(i.e,mesh2 )
- Often used to model bolted parts
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
• Edge contact
- Edge contact will be used when the surface normal are orthogonal
to the impact direction.
- For sheet metal forming
Impact of a corrugated plates
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
General Contact Guidelines
•Initial penetrations between contact surfaces are not allowed.
If LS-DYNA detects initial penetration between surfaces, it will
automatically move the overlapping surfaces out of contact.
•Always use realistic material property and shell thickness
values.The material properties and geometry of contacting surfaces
are used to determine k.
•Do not make multiple contact definitions between the same parts.
•Use single surface contact if the exact contact behavior is not
known beforehand.
•List the defined contact surfaces prior to solution to ensure that
contact has been properly defined.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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PEMP
AME2510
Controls in LS-DYNA
Session delivered by:
Mr.Suman M.L.J.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
PEMP
AME2510
Session Topics
Basic solution control options in LS-Dyna.
 LS-Dyna Binary and ASCII output files.
 Simulation control
 Solution Control
Card Format and creation
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
Basic solution control options
The basic parameters to be specified in an explicit solution are:
• Time
-The actual time for which the physical process is being
simulated.
-Here actual solution time should be of very short duration,
often in milliseconds.
• Critical Time step
-The time step for an explicit analysis is determined as the
minimum stable time steps in any deformable finite element in
the mesh.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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Critical Time step cont…..
-This is determined by the CFL condition (Courant-Friedrichs-Lewy
criterion) that determines the stable time step in an element as
characteristic length divided by the acoustic wave
l
t 
c
Where, t is the time needed for the wave to propagate
through the length l
l
is the characteristic length
c
is the acoustic wave
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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PEMP
AME2510
Critical Time step cont…..
T
cri

L
c
E

Where, E = Young’s modulus
 = Density
- To ensure stability maximum timestep is normally multiplied by
a scale factor (TSSFAC) of 0.9, but could reduce to as low as 0.66.
This is used to decrease the time step.
- LS-Dyna solver automatically calculates the minimum time step
size of each element based on the length and density.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
Critical Time step cont…..
- The actual time step size used by LS-Dyna is the smallest of
these values
- Elements resulting in the 100 smallest timesteps are reported in
the d3hsp file for debugging during solution
- Timesteps may also be plotted in LS-Pre/Post for debugging
- 1 ms – a commonly used minimum timestep for
crashworthiness problems
Note: The critical Time step size for explicit time integration depends on
element length and material properties (sonic speed)
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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PEMP
AME2510
TIME INTEGRATION
For nonlinear problem, only numerical solutions are possible. LSDYNA uses the explicit central difference method to integrate the
equation of motion
The semi-discrete equations of motion at time n is given as
Man = Pn- F n + Hn
Where, M is the diagonal mass matrix,
pn accounts for external and body force loads,
Fn is the stress divergence vector, and
Hn is the hourglass resistance.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
Comparision of time integration techniques
Explicit
 predicts the solution at time
t+dt by using the solution at
time t.
 The displacement of node n2 at
time level t+Δt is equal to known
values of the displacement at
nodes n1, n2, and n3 at time level
t.
Implicit
 solves the equation at time t+dt
based on itself, and also using
the solution which has been
found for time t.
 The displacement of node n2 at
time level t+Δt is equal to known
values of displace-ment at nodes n1,
n2, and n3 at time level t, and also
the unknown displacements of
nodes n1 and n3 at time level t+Δt.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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PEMP
AME2510
TIME INTEGRATION
• TERMINATION TIME, TIME STEP
tn-1 tn tn+1..
0
Tt
Time step
size t
The shorter time step, the more time increments needed to reach
the termination time.
stiffness
density
Element size
Speed of sound
TIME
STEP
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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PEMP
AME2510
TIME INTEGRATION Cont…
•
Direct methods to increase the time step:
- increase the element size (merge small elements)
- increase the density
- decrease the stiffness
•
Special methods to increase the time step:
- Mass scaling
• increases density in elements to get larger time step
• increases the total mass
•
Methods to decrease the total CPU-time:
- Reduce the number of elements
- Choose a simpler element type, formulation
- Reduce termination time
- Speed up loading process
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
GLOBAL DATA
• INITIAL VELOCITIES:
Initial velocities can be applied to the complete system or to
parts.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
GLOBAL DATA cont…
• BODY FORCES:
Body forces or “ base acceleration” is a force acting on a volume
or certain mass. The unit for body forces is N/kg or m/s2.
Examples:
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OUTPUT CONTROL
• OUTPUT DATA
- Output data for animations
- Output data for plotting
• ANIMATIONS:
Position, displacement, velocities, acceleration, stresses
and strains are stored in binary files with an interval
specified by the user.
• PLOT DATA:
Various variables can be stored and are normally stored
in different ASCII files depending on type of data
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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OUTPUT CONTROL cont..
• OTHER COMMON CONTROL OPTIONS:
- Time step computation method
- Standard formulations for beams and shells
- Global penalty for sliding interfaces
- Input method for materials and parts
- Global system damping
- Number of CPU’s for execution
- Coupling options (MYDAMO)
- Mass scaling
- Dynamic relaxation
- ALE options
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
POST-PROCESSING
• Output File - d3hsp File (ASCII)
– Errors and warnings
– Echo of input data
– Critical contact time step - must not be smaller than the
actual time step (search for surface timestep)
– Initial contact penetrations (search for penetration)
– Masses, COGs, and Inertias (search for mass of body)
– Mass scaling information (search for added mass).
Should not be larger than 5% of the total mass.
– 100 most critical time steps (search for 100 smallest
timesteps). If just a few elements spoil the time step, try
to repair the mesh or use mass scaling.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
POST-PROCESSING Cont..
• Animation Data - d3plot Files
– Animation
– Gives displacements, velocities, accelerations, stresses,
strains, …)
– Translate in HyperMesh .res file using hmdyna or use
HyperView for direct input
– Check for validity (contacts, penetrations, overall
deformation)
– Check for failing regions (plastic strain)
– Shell results are given on the different planes of the
shells
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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POST-PROCESSING Cont..
• Plotting Data - ASCII Files
– xy plotting
– glstat: Energies
• watch kinetic energy if mass scaling is used
• watch hourglass energy to be < 5% of internal energy
• watch interface energy
• check how kinetic and internal energy behave to each
other
• stability of solution (no jumps in energy, or noise)
– nodeout: Displacements (calculate intrusions, relative
displacements, distances)
– rcforc, secforc, rwforc, …: Forces, accelerations (very
noisy, need filtering to obtain essential information)
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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POST-PROCESSING cont..
• Filtering (low pass filter)
fc - cutoff frequency
0dB
pass region
block region
fc
Frequency [Hz]
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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POST-PROCESSING cont..
• SAE Filter
– Low pass filter
– Filter classes by frequency: 60, 180, 600, 1000 Hz (1/s)
– Recommended usage:
Vehicle structural analysis
Collision simulation input
60
Integration for velocity and displacement
180
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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POST-PROCESSING cont..
• Using Filters
–
–
–
–
Always state how the result was filtered
Be sure how the filter affects the result
Be careful about the beginning and the end of the curve
If results are compared between physical and numerical
experiment use the same filter class
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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AME2510
SIMULATION CONTROL
• Sense Switch controls allow the user to interrupt the solution
process and to check for the actual state.
• To use Sense Switch controls Type in the LS-Dyna solver as
shown below
‘Control-C’ Interrupt LS-Dyna solver and prompts for a sense
switches
sw1. A restart file is written and LS-Dyna terminates.
sw2. LS-dyna responds with time and cycle numbers.
sw3. A restart file is written and LS-Dyna continues calculations
sw4. A plot state is written and LS-Dyna continues calculations.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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SOLUTION CONTROL
• The LS-Dyna solver writes all important messages (errors, warnings,
failed elements, contact problems) to the LS-Dyna output window and
to the file d3hsp
• The First estimation of CPU time is usually too high. So by using
CTRL-C to interrupt the solver and by typing sw2 (Sense Switch
controls) for the actual values of the global statistics.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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General Approach involved in solving LS-Dyna
•Create FE Model
•Choose Material Model and Properties
•Assign Material and Property
•Assign loads and boundary condition
•Specify control parameters
•Create “.k” input file
•Solve the .k file in LS-Dyna solver to get
“d3plot” output file
•Post process the d3plot file in LS-Dyna post
processor
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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Approach to modeling – things to remember,
tips,
pitfalls
1.Components
•
•
•
All material model : Elastic modulus , density and
Poisson’s ratio are common input require
All additional parameters are material model dependent
Unit consistency needs to be carefully examined for
various material models while providing numerical values
for various material model parameters.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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•All stress-strain data must be provided in the form of true stresstrue strain data
•Time step should not be less then critical time step otherwise
computational error will grownup and instability will develop in
the solution
2.Interfaces/Contacts
1. While creating contact slave and master segments are
necessary for one way contact.
2. If necessary then only specify the cards value other wise
go with default values.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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INPUT FORMAT
•Input decks for analysis consist of data blocks, each consisting of
a keyword followed by data pertaining to the keyword
•Data blocks are either free- or fixed-format
•Input can change from free- to fixed-format anywhere within the
input deck, but not within a data block
•Input is order independent except for the *END keyword at the
end of the input deck
•Keywords are left justified and start with a ‘*’ in the first column
•A ‘$’ sign in the first column indicates a comment line
•Input is case insensitive
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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COMMANLY USED KEYWORDS
*KEYWORD
*SECTION
*BOUNDARY
*TITLE
*EOS
*INTERFACE
*DATABASE
*HOURGLASS
*RIGIDWALL
*CONTROL
*SET
*NODE
*DEFINE
*ELEMENT
*CONTACT
*MAT
*CONSTRAINED
*PART
*INITIAL
*INCLUDE
*LOAD
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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BASIC STRUCTURE
Area
Geometry
Material
Contact
Component
Keyword
Nodes
*NODE
Elements
*ELEMENT_
Part
*PART
Section
*SECTION_
Material
*MAT_
EOS
*EOS_
Hourglass
*HOURGLASS
Contact Definition
*CONTACT_
Rigidwall Definition *RIGIDWALL_
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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BASIC STRUCTURE Cont…
Area
Loads and BCs
Constraints
Component
Keyword
Loads
*LOAD_
BCs
*BOUNDARY_SPC_
Load Curves
*DEFINE_CURVE
Constrained Nodes
*CONSTRAINED_N
ODE_SET
*CONSTRAINED_S
POT_WELD
*CONSTRAINED_RI
VETS
Welds
Rivets
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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BASIC STRUCTURE Cont…
Area
Output Control
Termination
Component
Keyword
Various
*DATABASE_
Default
*CONTROL_OUTPUT
Time
*CONTROL_TERMINATION
CPU
*CONTROL_CPU
DOF
*TERMINATION_NODE
Contact
*TERMINATION_CONTACT
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SAMPLE DECK
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SAMPLE DECK cont….
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SAMPLE DECK cont….
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SAMPLE DECK cont….
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CONTROL CARDS
• Control cards means setting of importand parameters and Input
options
• Default settings are acceptable but the following settings should
be defined by the user
*CONTROL_ENERGY
Hourglass Energy Calculation - optional (HGEN = 2)
*CONTROL_PARALLEL
Controls the number of CPU‘s required (Default = 1 CPU)
 M.S. Ramaiah School of Advanced Studies, Bengaluru
PEMP
AME2510
*CONTROL_TERMINATION
Sets the end time
*CONTROL_TIMESTEP
Sets the analysis time step in two ways
(1) Time step is determined automatically (default)
(2) Constant time setep determined by the user
(Paramter DT2MS).
The mass is scaled for elements that violate the time step
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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DATABASE CONTROL
•
•
•
Database control is used for setting result output
Default is just the output of the d3hsp file, the protocol file
(ASCII)
The result output is defined by the *DATABASE_ cards
*DATABASE_BINARY_D3PLOT
•
•
Sets the frequency of the displacement and stress output
Writes the d3plot files (binary)
*DATABASE_BINARY_D3DUMP
•
•
•
Sets the output frequency for the restart file
Write the d3dump files (binary)
Needed for restart after system failure
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*DATABASE_GLSTAT
• Sets the frequency of the energy output.
• Writes the glstat file (ASCII)
*DATABASE_MATSUM
• Sets the frequency of the material-wise result output
• Writes the matsum file (ASCII)
• Contents are for example the internal energy of a
component
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*BOUNDARY_PRESCRIBED_MOTION
• Defines prescribed displacement or velocity over time for
a node or node set.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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*CONSTRAINED_EXTRA_NODES
• Nodes that are not a direct part of a Rigid Body can be
defined as associated with a Rigid Body.
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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*CONSTRAINED_JOINT_OPTION
• Joints can only be defined between two Rigid Bodies
• Joint-Types:
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*CONSTRAINED_NODAL_RIGID_BODY
• A nodal rigid body consists of several nodes that form a
Rigid Body.
Nodal
Rigid
Body
• Important: Nodal rigid bodies cannot have nodes of
another Rigid Body
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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*CONSTRAINED_RIGID_BODIES
•
One Rigid Body is united with another one. The resulting body has
the ID of the master body.
COG 1
COG 2
Rigid body 1Rigid body 2
Mass = 20kgMass = 20kg
*CONSTRAINED_RIGID_BODIES
Master = RB 1
Slave = RB 2
Rigid body 1
Mass = 40kg
COG
1
 M.S. Ramaiah School of Advanced Studies, Bengaluru
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*CONSTRAINED_RIVET
*CONSTRAINED_SPOTWELD
• Connection of two nodes with a rivet or weld
• Failure of weld points can be defined
 M.S. Ramaiah School of Advanced Studies, Bengaluru
69
PEMP
AME2510
*DEFINE_CURVE
•
•
•
Definition of a xy curve
Pairs of points are defined and get interpolated linearly.
Example: Stress-strain curve of a material
 M.S. Ramaiah School of Advanced Studies, Bengaluru
70
PEMP
AME2510
*INITIAL_VELOCITY
*INITIAL_VELOCITY_NODE
*INITIAL_VELOCITY_GENERATION
•
Definition of initial velocities of node sets, nodes, or parts
respectively.
*LOAD_BODY_OPTION
•
•
Definition of base accelerations to the whole system
(LOAD_BODY_X, LOAD_BODY_Y,
LOAD_BODY_Z).
Mostly used for gravity loads
 M.S. Ramaiah School of Advanced Studies, Bengaluru
71