Regarding the negative volume error and constrained release warning
Chamil Dhanasekara
tied node # 84023 on the slave side of surface # 25
type # 2 also belongs to interface # 29
TIED CONSTRAINT IS RELEASED.
list of elements affected follows (at most 20):
Element # 39017 ''
James Kennedy
Dear Chamil,
From this link:
https://www.dynasupport.com/howtos/contact/tied-tied-offset-and-tiebreak-contacts
Constraint-based / Penalty-based Contacts
Tied contacts fall into 2 major categories: constraint-based and penalty-based. Those with
TIEBREAK, OFFSET, or BEAM_OFFSET in the name are penalty-based. All others are
constraint-based. A node, whether it be on the slave or master side, cannot be involved in
more than one constraint-based contact. Also, rigid bodies cannot be included in a constr-
raint-based contact.
The OFFSET option switches the formulation from a constraint type formulation to one that
is penalty based where the force and moment (if applicable) resultants are transferred by dis-
crete spring elements between the slave nodes and master segments.
The BEAM_OFFSET option switches the formulation from a constraint type formulation to
one that is penalty based. Beam-like springs are used to transfer force and moment resultants
between the slave nodes and the master segments. Rigid bodies can be used with this option.
The CONSTRAINED_OFFSET option is a constraint type formulation.
As I read the manual:
http://ftp.lstc.com/anonymous/outgoing/jday/manuals/DRAFT_Vol_I.pdf
The following tied contact type is constraint-based
*CONTACT_TIED_NODES_TO_SURFACE_CONSTRAINED_OFFSET
It is my understanding that for constraint based contact, you cannot have redundant
(multiple/conflicting) definitions. The warnings are indicating that you have tied nodes
on two different interfaces and the tied nodes will thus be released.
Check your input for the conflicting constraints and correct.
Please see the following example (also attached) which uses shared nodes between the
domains:
http://ftp.lstc.com/anonymous/outgoing/ubasu/website/examples/pml_elastic.k
Start with a simple example such as this before moving onto something more complex.
Tied interfaces are used between the building/structure and the soil in the following (much
different than solid material interfaces):
Jayalekshmi, B.R., and Chinmayi, H.K., “Effect of Soil Stiffness on Seismic Response
of Reinforced Concrete Buildings with Shear Walls”, Innovative Infrastructure Solutions,
Vol. 1, Issue 2, pp. 1-18, December, 2016.
https://core.ac.uk/download/pdf/81831283.pdf
https://link.springer.com/article/10.1007/s41062-016-0004-0
I could not able to find any conflicts node sets in tied contacts.
Please check the attached sample file and let me know your inputs or suggestions on "Tied constrained nodes released".
Please find below the link for sample tied contact file.
https://groups.yahoo.com/neo/groups/LS-DYNA/files/dubois_bala/Tied%20contact%20issue.key
Sincerely,
James M. Kennedy
KBS2 Inc.
December 18, 2022
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James Kennedy
Dear Chemil,
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Some messages that indicate an instability has occurred:
out-of-range velocities
negative volume in brick element
termination due to mass increase
Approaches to combating instability of an explicit solution:
First and foremost, use the latest version/revision of LS-DYNA available.
----------------------------------------
See if the following presentations are of some help.
Interior Contact for Foams, Honeycombs and Rubbers to eliminate Negative Volumes
Under large compressive forces, elements belonging to either Foams, Honeycombs and Rubbers
tend to invert causing numerical instabilities. To avoid such difficulties, which is one of the
primary causes of simulation waste (unusable simulation results), a good set of modeling
practices are first necessary as outlined in an earlier post .
As an added protected against “Negative Volume” issues, LS-DYNA allows the definition of
internal contact treatment using *CONTACT_INTERIOR specifically designed for soft materials.
The only argument necessary to define the keyword is a part set consisting of soft materials that
require interior contact treatment. Once the part set is determined, LS-DYNA monitors the smallest
thickness dimension of each solid element in the part set checks if its value is less than Fa*Initial_
Smallest_Thickness_Dimension. If the current smallest thickness dimension falls below the value,
LS-DYNA applies contact forces to separate them much like in classical contact definitions. This
additional force help to keep the opposing surfaces of the element away from each other to avoid
element inversion problem. A graphical representation of the problem is shown below.
https://www.d3view.com/2008/01/contact-interior-for-foams-honeycombs-and-rubbers/
Best Practices for Modeling Recoverable Low Density Foams – By Example
Modeling recoverable foams poses several challenges in crash worthiness as well as in low-to-
medium impact velocity conditions. This is due to its relatively low stiffness when compared
with structural materials which has an indirect effect on its contact-impact interactions with other
materials. To review the best practices when modeling such components, we can consider a simple
example of a rigid steel ball (solid elements) impacting a block of foam (solid elements). To model
the interaction between the rigid block and the foam, a two-way contact such as *AUTOMATIC_
SURFACE_TO_SURFACE is included. The foam is constrained using SPC definitions on the
bottom face. The complete model set up is as shown in Figure 1. The recoverable low density foam
is modeled using *MAT_LOW_DENSITY material model whose inputs include density, elastic-
modulus, and a load curve to define its engineering stress-strain behavior for compression. The
tensile behavior for this model is elastic (uses the compression Young’s modulus, E) and optionally
has a cut-off stress value after which the stress remains constant in tension. Unloading behavior by
default is along the loading curve but optionally we can provide some energy dissipation param-
eters HC and SHAPE which control both the amount of energy dissipation and the shape of the
unloading curve. For simplicity, we will first ignore any hysteresis and assume that the unloading
curve follows the loading curve. As mentioned before, we are going to simulation the impact using
all default parameters and then we incrementally update the modeling parameters by monitoring
the simulations results.
1. Default Parameters
This simulation consists of all default parameters and with this definition, the job terminated
abruptly with messages of ‘Negative Volume’ and ‘Complex sound speed’. The final deformed
shape just prior to termination is as shown in the figure below.
4. Increased Solid Element Thickness in Contact
This simulation works as expected and shows no contact penetration even for large compressive
strains and meets all stability and accuracy criteria. The final deformed shape and the transient
results are shown below.
https://www.d3view.com/2006/10/best-practices-for-modeling-recoverable-low-density-foams-by-example/
----------------------------------------
Sincerely,
James M. Kennedy
KBS2 Inc.
December 18, 2022
From: ls-d...@googlegroups.com [mailto:ls-d...@googlegroups.com]
On Behalf Of Chamil Dhanasekara
Sent: Saturday, December 17, 2022 10:15 PM
To: LS-DYNA2 <ls-d...@googlegroups.com>
Subject: [LS-DYNA2] Regarding the negative volume error and constrained release warning
Dear all,
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