warning message using *MAT_PIECEWISE_LINEAR_PLASTICITY
PJOST
Евгений Калентьев
l...@schwer.net
My guess this Warning is caused by your use of the parameter VP=1 Viscoplastic formulation in your MAT024 definition. The Viscoplastic algorithm requires an iterative scheme to return the trial stress to the shear yield surface.
When the effective plastic strain increments are large the iteration scheme can fail to meet the internally set tolerance.
Try reducing the time step size via TSSFAC on *CONTROL_TIMESTEP, perhaps TSSFAC=0.8 --len
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James M. Kennedy
Dear Paul,
Are you using a double precision executable?
------------------------------------
Some notes I found:
Evidently, MAT_024 does not rediscretize the S-S curve when LCSS is a
curve while it does rediscretize the curves when LCSS is a table.
As I understand, LS-DYNA uses the re-discretized curves in the strain
direction [when LCSS is a table]. This means that the original intervals
are retained in the strain-rate direction but not in the strain direction.
------------------------------------
*CONTROL_ACCURACY
Purpose: Define control parameters that can improve the accuracy of the
calculation.
EXACC Explicit accuracy parameter:
EQ.0.0: Off (default)
GT.0.0: On (see Remark 5)
5. EXACC. The EXACC option is developed to improve the numerical accuracy
for an explicit analysis. Currently, nodal coordinates are computed and
stored in double precision in all versions. In most cases, this is suffic-
iently accurate and EXACC is recommended to be off. However, in some cases,
particularly when initial coordinates are large and displacements are small,
EXACC can increase the accuracy of computations. To use this option, the
EXACC parameter should be set to a positive value which is a characteristic
element length for the mesh. For example, if the typical edge length in a
model is 5.0 mm and the length units are millimeters, then set EXACC = 5.0.
------------------------------------
See discussion on pages 22 and 23 in the following:
Question: Plasticity Algorithm Warning Messages
https://www.dynalook.com/fea-newsletters/fea-newsletters-2006/fea-newsletter-april-2006.pdf
------------------------------------
Sincerely,
James M. Kennedy
KBS2 Inc.
April 15, 2021
From: ls-d...@googlegroups.com [mailto:ls-d...@googlegroups.com] On Behalf Of PJOST
Sent: Thursday, April 15, 2021 9:00 AM
To: LS-DYNA2 <ls-d...@googlegroups.com>
Subject: [LS-DYNA2] warning message using *MAT_PIECEWISE_LINEAR_PLASTICITY
Hello all. I work as a CAE engineer for a plastics supplier in Auburn Hills, Michigan.
--
PO
PO
James M. Kennedy
Dear Paul,
Did you try Len’s suggestion of reducing the time step safety factor?
Different element formulations use different characteristic lengths (Le) for determining its stable
time step (please see the following). Some time steps may be more conservative than others.
The following (Table 1) provides time step and run time characteristics of the various tetrahedron
(constant stress hexahedron included as a base comparison) for a simple cantilever beam simulation
(ls971d R5.1.1 - SMP with 2 CPUs) composed of either 360 hexahedron elements (cubic) or 1800
tetrahedron elements (5 equal tetrahedron per cubic hexahedron).
Table 1. Tetrahedron Elements Timing Information | ||||
Formulation (elform) | Nodes/Type (hex-tet) | Characteristic Length (Le) | Time Step (dt) | Normalized Run Time |
1 | 8-node hex | V/Amax | 1.75e-5 | 1.0 |
1 | 4-node tet | hmin | 1.02e-5 | 6.0 |
4 | 4-node tet | 0.850 hmin | 8.66e-6 | 14.5 |
10 | 4-node tet | hmin | 1.02e-5 | 2.0 |
13 | 4-node tet | hmin | 1.02e-5 | 2.3 |
16 | 10-node tet | 0.3889 hmin | 3.96e-6 | 27.0 |
17 | 10-node tet | V/Amax | 3.40e-6 | 48.0 |
Sincerely,
James M. Kennedy
KBS2 INC.
April 16, 2021
PO
added mass = 2.1765E-08
physical mass= 6.0970E-02
ratio = 3.5698E-07
1 t 0.0000E+00 dt 9.00E-08 flush i/o buffers 04/16/21 11:24:24
1 t 0.0000E+00 dt 9.00E-08 write d3plot file 04/16/21 11:24:24
1112 t 9.9990E-05 dt 9.00E-08 write d3plot file 04/16/21 11:24:41
2223 t 1.9998E-04 dt 9.00E-08 write d3plot file 04/16/21 11:24:59
3334 t 2.9997E-04 dt 9.00E-08 write d3plot file 04/16/21 11:25:16
$ dtinit tssfac isdo tslimt dt2ms lctm erode ms1st
0 0.9 0 0 -1E-07 0 1 0
$ dt2msf dt2mslc imscl unused
0 0 0
l...@schwer.net
The time step in the provided snip it is dt 9.00E-08
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PO
l...@schwer.net
Your time step dt 9.00E-08 is determined by the combination of TSSFAC=0.9 and the mass scaling factor DT2MS=1E-7, i.e. (0.9)(1E-7)=9E-8
This FIXED time step causes effective plastic strain increments that are large and the interactive scheme in MAT024 fails to return the trial stress to the yield surface with the resulting warning message you posted -- as I stated originally.
If this explanation is “Greek” to you, you need more self-study about plasticity algorithms.
--len
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PO
l...@schwer.net
Removing the mass scaling MIGHT help.
A lack of knowledge is not an excuse, but a danger to others that might use your results.
To view this discussion on the web visit https://groups.google.com/d/msgid/ls-dyna2/CACs4vk16Us-4nY0LG%3DQ%2B4-usTcUyaDDZ-1u3MsPmvKNGA05UfA%40mail.gmail.com.
PO
James M. Kennedy
Dear Paul,
computational plasticity
Benson, D.J., "Computational Plasticity", Livermore Software Technology Corporation,
Livermore, California, March-April, 2005.
http://www.dynasupport.com/tutorial/computational-plasticity
Keller, C., and Herbrich, U., "Plastic Instability of Rate-Dependent Materials – A Theoretical
Approach in Comparison to FE-Analyses -", 11th European LS-DYNA Users Conference, Salzburg,
Austria, May, 2017.
time integration
https://www.dynasupport.com/tutorial/ls-dyna-users-guide/time-integration/
https://www.dynasupport.com/tutorial/ls-dyna-users-guide/time-step-size/
Bala, S., "Time Integration in LS-DYNA", Livermore Software Technology Corporation,
Livermore, California, April, 2007.
http://blog2.d3view.com/wp-content/uploads/2007/04/timeintegration_v1.pdf
Jensen, M.R., "Introduction to LS-DYNA - Level II (Chapter 11 - Timestep and Mass-
Scaling)", Livermore Software Technology Corporation, Livermore, California, (undated
publication).
(please contact sup...@lstc.com)
conventional mass scaling
Mass-scaling, Bala [2006], is a term that is used for the process of scaling the element’s
mass in explicit simulations to adjust its timestep. The primary motivation is to change
(usually increase) the global compute time step which is limited by the Courant’s stability
criteria.
Conventional mass scaling (CMS): The mass of small or stiff elements is increased to prevent
a very small time step. Thus, artificial inertia forces are added which influence all eigen freq-
uencies including rigid body modes. This means, this additional mass must be used very
carefully so that the resulting non-physical inertia effects do not dominate the global solution.
This is the standard default method that is widely used.
Bala, S., "Overview of Mass-Scaling in LS-DYNA", Livermore Software Technology
Corporation, Livermore, California, October, 2006.
http://blog2.d3view.com/?p=106
Day, J., "Mass Scaling", Livermore Software Technology Corporation, Livermore, California,
November, 2010.
http://www.dynasupport.com/howtos/general/mass-scaling
http://ftp.lstc.com/anonymous/outgoing/jday/faq/mass_scaling
selective mass scaling
Selective mass scaling (SMS): Using selective mass scaling only the high frequencies are affected,
whereas the low frequencies (rigid body modes) are not influenced. Thereby, a lot of artificial
mass can be added to the system without adulterate the global solution. This method is very
effective, if it is applied to limited regions with very small critical time steps. SMS is invoked
with the IMSCL command over a single part or multiple parts
Bala, S., "Overview of Mass-Scaling in LS-DYNA", Livermore Software Technology
Corporation, Livermore, California, October, 2006.
http://blog2.d3view.com/?p=106
Borrvall, T., "Selective Mass Scaling (SMS): Theory and Practice", 2011 Developers' Forum,
Stuttgart, Germany, October, 2011.
https://www.dynamore.de/en/downloads/papers/forum11/entwicklerforum-2011/borrvall.pdf
Tkachuk, A., "Variation Methods for Consistent Singular and Scaled Mass Matrices", Ph.D.
Thesis, Institut für Baustatik und Baudynamik, Universität Stuttgart, Stuttgart, Germany,
December, 2013.
https://www.ibb.uni-stuttgart.de/publikationen/fulltext_new/2013/tkachuk_2013.pdf
Sincerely,
James M. Kennedy
KBS2 Inc.
April 17, 2021
James M. Kennedy
Dear Paul,
Vogler, M., Kolling, S., and Haufe, A., "A Constitutive Model for Polymers with a
Piecewise Linear Yield Surface", Proceedings Applied Mathematics and Mechanics
(PAMM), Vol. 6, Issue 1, pp. 275-276, December, 2006.
http://onlinelibrary.wiley.com/doi/10.1002/pamm.200610118/pdf
Vogler, M., Kolling, S., and Haufe, A., "A Constitutive Model for Plastics with Piecewise
Linear Yield Surface and Damage", 6th German LS-DYNA Forum, Frankenthal, Germany,
October, 2007.
Kolling, S. and Vogler, M.., "A Constitutive Plasticity Model with Piecewise Linear Yield
Surface and Damage", Institut für Mechanik und Materialforschung, Technische Hochschule
Mittelhessen, Giessen, Germany.
https://www.thm.de/me/images/user/IMM-110/Papers/Festschrift_Juckenack_30-51.pdf
Sincerely,
James M. Kennedy
KBS2 Inc.
April 17, 2021
.
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