Question about defining proper joint constraints.

Evan Yi

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Mar 31, 2022, 10:54:13 AM3/31/22

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Hello everyone,

Recently, I have a model that needs to simulate Joint ( Spherical and Revolute). I built a simple model to understand how joints are defined and work. This scenario contains two balls hung by strings, both of which are modeled with RIGID materials as shown below. And I applied gravity by *LOAD_BODY_Z and initial velocity by *INITIAL_VELOCITY_NODE so that the right ball is supposed to collide with the left one. However, it is turned out that both of them just fall under gravity, the spherical joints are not properly defined though no error reported. The two spherical joints look just do not exist at all. However, when I go to check the kinematic energy of parts, it tells me that the KE is zero for the left ball.

I also studied the example about joints on dynaexamples.com, and cannot recognize the essence of defining joints yet. Could anyone help me with this problem? The keyword file is attached. Thanks a lot in advance.

test.k

l...@schwer.net

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Mar 31, 2022, 12:27:22 PM3/31/22

to Evan Yi, LS-DYNA2

The beam (B7) that suspends the rigid ball need to have a “dog leg” that is coincident with the horizontal beam (B1372) and the nodes of the “dog leg” need to be coincident with the horizontal beam, i.e. A=B and C=D and then the joint definition has

N1 N2 N3 N4

A B C D --len

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Evan Yi

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Mar 31, 2022, 1:15:40 PM3/31/22

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Dear Len,

Thanks for your reply, this figure↓ shows how the two revolute joints are defined. It looks the same as you indicate but the result is as same as before.

l...@schwer.net

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Apr 1, 2022, 1:48:37 PM4/1/22

to Evan Yi, LS-DYNA2

Evan –

I spent quite a bit of time looking at your model and here are a few things I think are wrong:

1\ Your rigid body labeled “Boundary Beam” has two revolute joints – I do not believe a rigid body make have more than one rigid body joint.

2\ Since you merged the rigid “Ball” to their corresponding “String” each now represents ONE rigid body. The center of gravity (rotation) of that rigid body does not align with the center of rotation of the revolute joint.

3\ Do not use nodal constraints on rigid bodies. Rigid body constraints are applied via the *MAT_RIGID keyword.

I suggest you break your “Boundary Beam” into two rigid bodies and similarly break each “String” into two rigid bodies, i.e. one for the revolute joint and the other attached to the ball. Insert a singe elastic element to connected these two rigid bodied at the end of the string nearest the revolute joint.

--len

From: ls-d...@googlegroups.com <ls-d...@googlegroups.com> On Behalf Of Evan Yi
Sent: Thursday, March 31, 2022 10:16 AM
To: LS-DYNA2 <ls-d...@googlegroups.com>
Subject: Re: [LS-DYNA2] Question about defining proper joint constraints.

Dear Len,

Thanks for your reply, this figure↓ shows how the two revolute joints are defined. It looks the same as you indicate but the result is as same as before.

在2022年4月1日星期五 UTC+8 00:27:22<L...@Schwer.net> 写道：

The beam (B7) that suspends the rigid ball need to have a “dog leg” that is coincident with the horizontal beam (B1372) and the nodes of the “dog leg” need to be coincident with the horizontal beam, i.e. A=B and C=D and then the joint definition has

N1 N2 N3 N4
A B C D --len

From: ls-d...@googlegroups.com <ls-d...@googlegroups.com> On Behalf Of Evan Yi
Sent: Thursday, March 31, 2022 7:54 AM
To: LS-DYNA2 <ls-d...@googlegroups.com>
Subject: [LS-DYNA2] Question about defining proper joint constraints.

Hello everyone,
Recently, I have a model that needs to simulate Joint ( Spherical and Revolute). I built a simple model to understand how joints are defined and work. This scenario contains two balls hung by strings, both of which are modeled with RIGID materials as shown below. And I applied gravity by *LOAD_BODY_Z and initial velocity by *INITIAL_VELOCITY_NODE so that the right ball is supposed to collide with the left one. However, it is turned out that both of them just fall under gravity, the spherical joints are not properly defined though no error reported. The two spherical joints look just do not exist at all. However, when I go to check the kinematic energy of parts, it tells me that the KE is zero for the left ball.
I also studied the example about joints on dynaexamples.com, and cannot recognize the essence of defining joints yet. Could anyone help me with this problem? The keyword file is attached. Thanks a lot in advance.

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Evan Yi

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Apr 9, 2022, 5:07:41 AM4/9/22

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Dear Len,

Thank you. It looks like I have some misunderstandings on Constrained_Joints before. After receiving your second reply, I spent several days digging this keyword. So, now I realized that one rigid body cannot be connected to more than one joint, and a joint can only be permissible on rigid body nodes.
I created a simple model and compared it with the example provided by Mr. James and on the dynasupport website, to make sure it has identical structures with the example keyword file except for the unit system and coordinates of nodes, which should not be influence to joints as I can understand. However, the animation of the keyword file I created still show an unexpected result, the rotating plate flies away ignoring the existence of a revolute joint. Could you please help me review this keyword file? Thanks in advance.

try.k

l...@schwer.net

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Apr 9, 2022, 1:10:40 PM4/9/22

to Evan Yi, LS-DYNA2

I think you do not understand a key aspect of rigid bodies and how they interact with constrained joints.

Rigid bodies move (translate and rotate) about their mass centers – see circles below. Although you have defined a revolute joint at the edge of the two shells, the rigid bodies attempt to rotate about their mass centers.

You could define two rigid beams that form the revolute joint and then merge those beam nodes with deformable shell elements; if you make the shells rigid the resulting beam/shell rigid bodies will be back to your original model with mass centers off the axis of the revolute joint.

--len

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James M. Kennedy

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Apr 9, 2022, 1:43:12 PM4/9/22

to Evan Yi, LS-DYNA2

Dear Evan,

Your conversion leaves a bit to be desired.

It appears that you are trying to use gram, millimeter, millisec, Newton, megaPa.

The example I shared used ton, millimeter. Sec, Newton, megaPa.

Please check your conversion and missing input.

*KEYWORD

*TITLE

$# title

LS-DYNA keyword deck by LS-PrePost

*CONTROL_TERMINATION

$# endtim endcyc dtmin endeng endmas nosol

200.0 0 0.0 0.01.000000E8 0

*CONTROL_TIMESTEP

$# dtinit tssfac isdo tslimt dt2ms lctm erode ms1st

0.01 0.9 0 0.0 0.0 0 0 0

$# dt2msf dt2mslc imscl unused unused rmscl unused ihdo

0.0 0 0 0.0 0

*DATABASE_GLSTAT

$# dt binary lcur ioopt

0.1 0 0 1

*DATABASE_JNTFORC

$# dt binary lcur ioopt

0.1 0 0 1

*DATABASE_NODOUT

$# dt binary lcur ioopt option1 option2

0.1 0 0 1 0.0 0

*DATABASE_RBDOUT

$# dt binary lcur ioopt

0.1 0 0 1

*DATABASE_BINARY_D3PLOT

$# dt lcdt beam npltc psetid

5.00000E-4 0 0 0 0

$# ioopt rate cutoff window type pset

0 0.0 0.0 0.0 0 0

*DATABASE_HISTORY_NODE

$# id1 id2 id3 id4 id5 id6 id7 id8

6 8 0 0 0 0 0 0

*LOAD_NODE_POINT

$# nid dof lcid sf cid m1 m2 m3

6 3 1 1.0 1 0 0 0

5 3 1 1.0 1 0 0 0

*PART

$# title

rotating

$# pid secid mid eosid hgid grav adpopt tmid

1 1 1 0 0 0 0 0

*SECTION_SHELL_TITLE

shell

$# secid elform shrf nip propt qr/irid icomp setyp

1 2 1.0 3 1.0 0 0 1

$# t1 t2 t3 t4 nloc marea idof edgset

2.54 2.54 2.54 2.54 0.0 0.0 0.0 0

*MAT_RIGID_TITLE

rigid free

$# mid ro e pr n couple m alias

1 0.00783 200000.0 0.3 0.0 0.0 0.0

$# cmo con1 con2

0.0 0 0

$#lco or a1 a2 a3 v1 v2 v3

0.0 0.0 0.0 0.0 0.0 0.0

*PART

$# title

fixed

$# pid secid mid eosid hgid grav adpopt tmid

2 1 2 0 0 0 0 0

*MAT_RIGID_TITLE

rigid fixed

$# mid ro e pr n couple m alias

2 0.00783 200000.0 0.3 0.0 0.0 0.0

$# cmo con1 con2

1.0 7 7

$#lco or a1 a2 a3 v1 v2 v3

0.0 0.0 0.0 0.0 0.0 0.0

*DEFINE_COORDINATE_NODES

$# cid n1 n2 n3 flag dir

1 5 7 6 0X

*DEFINE_CURVE_TITLE

load

$# lcid sidr sfa sfo offa offo dattyp lcint

1 0 1.0 1.0 0.0 0.0 0 0

$# a1 o1

0.0 100.0

9999.0 100.0

*DEFINE_CURVE_TITLE

timestep

$# lcid sidr sfa sfo offa offo dattyp lcint

2 0 1.0 1.0 0.0 0.0 0 0

$# a1 o1

0.0 4.1500002226e-06

9999.0 4.1500002226e-06

*CONSTRAINED_JOINT_REVOLUTE_ID

$# jid title

1

$# n1 n2 n3 n4 n5 n6 rps damp

1 7 2 8 0 0 1.0 1.0

*ELEMENT_SHELL

$# eid pid n1 n2 n3 n4 n5 n6 n7 n8

1 2 3 4 2 1 0 0 0 0

2 1 7 8 6 5 0 0 0 0

*NODE

$# nid x y z tc rc

1 25.0 25.0 0.0 0 0

2 25.0 50.0 0.0 0 0

3 50.0 25.0 0.0 0 0

4 50.0 50.0 0.0 0 0

5 0.0 25.0 0.0 0 0

6 0.0 50.0 0.0 0 0

7 25.0 25.0 0.0 0 0

8 25.0 50.0 0.0 0 0

*END

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ LS-DYNA(970) DECK WRITTEN BY : eta/FEMB-PC version 28.0

$ ENGINEER :

$ PROJECT :

$ UNITS : MM, TON, SEC, N

$ TIME : 03:09:16 PM

$ DATE : Thursday, July 08, 2004

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*KEYWORD

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*TITLE

hinged shell w/ stop angle

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ CONTROL CARD $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*CONTROL_TERMINATION

$ ENDTIM ENDCYC DTMIN ENDENG ENDMAS

0.020

*CONTROL_TIMESTEP

$ DTINIT TSSFAC ISDO TSLIMT DT2MS LCTM ERODE MS1ST

5

$ DT2MSF

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ DATABASE CONTROL FOR ASCII $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*DATABASE_GLSTAT

$ DT BINARY

0.00010

*DATABASE_JNTFORC

$ DT BINARY

0.00010

*DATABASE_NODOUT

$ DT BINARY

0.00010

*DATABASE_RBDOUT

$ DT BINARY

0.00010

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ DATABASE CONTROL FOR BINARY $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*DATABASE_BINARY_D3PLOT

$ DT/CYCL LCDT BEAM NPLTC

0.00050

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ DATABASE HISTORY CARDS $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*DATABASE_HISTORY_NODE

$^

$ NID1 NID2 NID3 NID4 NID5 NID6 NID7 NID8

3 4

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ PART CARDS $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*PART

$HEADING

ROTATING

$ PID SECID MID EOSID HGID GRAV ADPOPT TMID

1 1 1 0

*PART

$HEADING

FIXED

$ PID SECID MID EOSID HGID GRAV ADPOPT TMID

2 1 2 0

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ SECTION CARDS $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*SECTION_SHELL

$ SECID ELFORM SHRF NIP PROPT QR/IRID ICOMP SETYP

1 2 3

$ T1 T2 T3 T4 NLOC MAREA

2.54 2.54 2.54 2.54

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ MATERIAL CARDS $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*MAT_RIGID

$^MAT0001

$ MID RO E PR N COUPLE M ALIAS

17.4808E-09 206842.7 0.30

$ CMO CON1 CON2

$LCO_OR_A1 A2 A3 V1 V2 V3

*MAT_RIGID

$^MAT0002

$ MID RO E PR N COUPLE M ALIAS

27.4808E-09 206842.7 0.30

$ CMO CON1 CON2

1.0 7.0 7

$LCO_OR_A1 A2 A3 V1 V2 V3

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ CONSTRAINED JOINT CARDS $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*CONSTRAINED_JOINT_STIFFNESS_GENERALIZED

$^

$ JSID PIDA PIDB CIDA CIDB JID

1 1 2 5 5

$ LCIDPH LCIDT LCIDPS DLCIDPH DLCIDT DLCIDPS

$ ESPH FMPH EST FMT ESPS FMPS

350.94758

$ NSAPH PSAPH NSAT PSAT NSAPS PSAPS

50.0

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ LOAD NODE CARDS $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*LOAD_NODE_POINT

$^

$ NODE DOF LCID SF CID M1 M2 M3

3 3 1 -1.0

*LOAD_NODE_POINT

$^

$ NODE DOF LCID SF CID M1 M2 M3

4 3 1 -1.0

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ COORDINATE SYSTEM CARDS $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*DEFINE_COORDINATE_NODES

$^

$ CID N1 N2 N3 FLAG

5 1 2 3

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ LOAD CURVE CARDS $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*DEFINE_CURVE

$^

$ LCID SIDR SFA SFO OFFA OFFO DATTYP

5

$ A1 O1

0.0 0.00000415

1.0 0.00000415

*DEFINE_CURVE

$^

$ LCID SIDR SFA SFO OFFA OFFO DATTYP

1

$ A1 O1

0.0 1.0

1.0 1.0

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ NODE INFORMATION $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*NODE

$ NID X Y Z TC RC

1 0.0 0.0 0.0 0.0 0.0

2 25.4 0.0 0.0 0.0 0.0

3 0.0 25.4 0.0 0.0 0.0

4 25.4 25.4 0.0 0.0 0.0

7 0.0 -25.4 0.0 0.0 0.0

8 25.4 -25.4 0.0 0.0 0.0

9 0.0 0.0 0.0 0.0 0.0

10 25.4 0.0 0.0 0.0 0.0

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ ELEMENTS INFORMATION $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ SHELL ELEMENTS $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*ELEMENT_SHELL

$ EID PID NID1 NID2 NID3 NID4

1 1 1 3 4 2

3 2 7 9 10 8

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

$ $

$ JOINT REVOLUTE CARDS $

$ $

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*CONSTRAINED_JOINT_REVOLUTE

$ N1 N2 N3 N4 N5 N6 RPS DAMP

1 9 2 10 0 0 1.0

$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8

*END

Sincerely,

James M. Kennedy

KBS2 Inc.

April 9, 2022

To view this discussion on the web visit https://groups.google.com/d/msgid/ls-dyna2/2da6e654-c968-4a77-b882-e82c69ff7634n%40googlegroups.com.

Evan Yi

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Apr 9, 2022, 10:44:02 PM4/9/22

to LS-DYNA2

Dear Len,

Thank you again.

Do you mean the constrained_extra_nodes keyword? I did not reach that far yet since this is a simple model following the manual example. In that example, only rigid bodies are involved and the example run fine. But your suggestions are still enlightening for me in the future. After all, practical works always contain interactions between deformable bodies.

I have found out the problem bothering me is the timestep control. I recalled the theory about defining proper timestep. Before this, my models did not encounter timestep issues so I thought the software will give a proper timestep size if I leave it aside. However, in this example, it seems that the timestep should be prudently defined since elements are all rigid, so the default timestep value leads to an unrealistic result. If I define the Load curve ID(LCTM) in *CONTROL_TIMESTEP as 4.15e-3 milisec (4.15e-6 second in the original example), which was missed because of my carelessness, the revolute joint can work without changing any other keyword. So, I am believed that timestep size is the reason.

Anyway, I would like to express my appreciation for your time and effort spent on my question, your suggestions would help me and any other young or part-time engineers understanding how joints work in LS-DYNA and what needs to be paid attention to.

Sincerely,

Evan

Evan Yi

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Apr 9, 2022, 10:53:03 PM4/9/22

to LS-DYNA2

Dear James,

Thank you so much. I realized that the problem is caused by the improper timestep size. I missed the LCTM in *CONTROL_TIMESTEP because of my carelessness. After I fix this negligence and make the timestep reasonable according to my unit system, the model gives a realistic result.

It is a good lesson for me because this problem bothered me for several days. I truly appreciate your timely help.

Best Regards

Evan

James M. Kennedy

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Apr 10, 2022, 12:47:19 PM4/10/22

to Evan Yi, LS-DYNA2

Dear Evan,

Did you make your units consistent?

Did you try adding *CONSTRAINED_JOINT_STIFFNESS_GENERALIZED?

Sincerely,

James M. Kennedy

KBS2 Inc.

April 10, 2022

To view this discussion on the web visit https://groups.google.com/d/msgid/ls-dyna2/a123ec32-ff92-43c1-bac4-df67f2bfbfc1n%40googlegroups.com.

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