Folks,

In Mechanical, there is an option for "rigid" or "deformable" behavior when building joints.  (in APDL, this is "rigid surface constraint" and "force-distributed constraint)
What thought process does an analyst use to choose one or the other?  I know for a model with thermal loads, you have to choose deformable so that the model can move naturally with heat.  In the absence of any temperature loads, what thought process would one use to decide?

Matt Ridzon, PE, MSME
Sr. Engineering Analyst

Email    matt@prime-engineer.commailto:matt@prime-engineer.com
Mail      266 Main St, Burlington, VT 05401
Web      www.prime-engineer.comhttp://www.prime-engineer.com/
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PRIME ENGINEERING LLC

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Dear Mr. Ridzon,
I almost always use the rigid formulation.
MNF-based flex bodies badly work in an MBS environment when generated with the deformable formulation at the interface nodes.
The joint's kinematics is better represented with the rigid formulation when simulating mechanisms inside Ansys.
I use the deformable formulation for remote entities only to apply loads not related to kinematic constraints.

Best regards
Riccardo Testi

Development and Strategies
2 Wheeler Engines Technical Centre
Piaggio & C. S.p.A
Viale Rinaldo Piaggio, 25
56025 Pontedera (Pisa) - ITALY
Phone:  +39 0587 272850
Fax:        +39 0587 272010
Mobile: +39 339 7241918
E-mail:    riccardo.testi@piaggio.com

-----Original Message-----
From: Matthew Ridzon, PE via Xansys xansys-temp@list.xansys.org
Sent: Wednesday, February 26, 2025 9:15 PM
To: xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com
Subject: [Xansys] Choosing Rigid or Deformable Behavior

CAUTION:This email originated from outside the Piaggio Group. Do not click links or open attachments unless you recognize the sender and know the content is safe.

Folks,

In Mechanical, there is an option for "rigid" or "deformable" behavior when building joints.  (in APDL, this is "rigid surface constraint" and "force-distributed constraint) What thought process does an analyst use to choose one or the other?  I know for a model with thermal loads, you have to choose deformable so that the model can move naturally with heat.  In the absence of any temperature loads, what thought process would one use to decide?

Matt Ridzon, PE, MSME
Sr. Engineering Analyst

Email    matt@prime-engineer.commailto:matt@prime-engineer.com
Mail      266 Main St, Burlington, VT 05401
Web      https://urlsand.esvalabs.com/?u=http%3A%2F%2Fwww.prime-engineer.com&e=6e97a7e3&h=87561189&f=y&p=y < https://urlsand.esvalabs.com/?u=http%3A%2F%2Fwww.prime-engineer.com%2F&e=6e97a7e3&h=63971262&f=y&p=y >
[A blue hexagon with white letters  Description automatically generated] PRIME ENGINEERING LLC

This message (including any attachments) may contain confidential, proprietary, privileged and/or private information. The information is intended to be for the use of the individual or entity designated above. If you are not the intended recipient of this message, please notify the sender immediately, and delete the message and any attachments. Any disclosure, reproduction, distribution or other use of this message or any attachments by an individual or entity other than the intended recipient is prohibited.

Matt,

For kinematic analyses that your question seems to imply, Mr Testi's answer makes a lot of sense.  For structural/dynamic analyses, one helpful paradigm is to ask if the behavior of the 'many grids' (or, 'many nodes') is best modeled as infinitely stiff with respect to each other and the 'single grid' - or - if it is better to represent the effect of the 'single grid' as distributed onto the 'many grids' without additional stiffness (like the mass of a small subassembly being added to the component 'many grids' without adding extra stiffness).

Both these constructs are MPC's with a one-to-many design.  In the 'rigid' formulation, the 'one-grid' is the independent grid and the 'many-grids' are dependent.  The rigid MPC design is set to force the dependent grids to have zero relative displacement with respect to the 'one grid'.  In the 'flexible' formulation, the 'one-grid' is dependent on the displacement of all of the 'many grids'.  The effect is the displacement of the 'one grid' is the average of the 'many'.  I've described the resulting behavior - the behind-the-scenes math is actually much more complicated because each of the grids has 6 degrees-of-freedom so the coupling of all the DOF of all the grids is quite profound.

Beware of connecting MPC's together in a chain without elements between the different MPC's (sometimes tempting to do in APDL, hard to do in Workbench).  Also beware of having multiple MPC's assigned to the same surface in Workbench (profoundly easy to do).  I've seen both mistakes made, not crash the solver, and not appear to be obviously wrong until the engineer starts probing more deeply.

Alan Baker
Principal Engineer
Engine Systems & Component Analysis
Honeywell | Aerospace
alan.baker2@honeywell.com

-----Original Message-----
From: Matthew Ridzon, PE via Xansys xansys-temp@list.xansys.org
Sent: Wednesday, February 26, 2025 1:15 PM
To: xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com
Subject: [External] [Xansys] Choosing Rigid or Deformable Behavior

Folks,

In Mechanical, there is an option for "rigid" or "deformable" behavior when building joints.  (in APDL, this is "rigid surface constraint" and "force-distributed constraint) What thought process does an analyst use to choose one or the other?  I know for a model with thermal loads, you have to choose deformable so that the model can move naturally with heat.  In the absence of any temperature loads, what thought process would one use to decide?

Matt Ridzon, PE, MSME
Sr. Engineering Analyst

Email    matt@prime-engineer.commailto:matt@prime-engineer.com
Mail      266 Main St, Burlington, VT 05401

Thanks Riccardo!  Thanks Alan!

I guess I should have clarified...my question pertains to joints that are NOT being used for kinematics.  They are fixed joints acting like "bonded contact."  However, if using joints for kinematics, I can understand why "rigid" is preferred.

I built a small test model and found odd stress contours at the periphery of the joint scopings when using rigid.  In other words, the scoped locations were behaving rigidly but the neighboring elements were flexible, which created an unusual stress pattern that was incorrect.

"Also beware of having multiple MPC's assigned to the same surface in Workbench (profoundly easy to do).  I've seen both mistakes made, not crash the solver, and not appear to be obviously wrong until the engineer starts probing more deeply."
I agree with you.  I have run into this problem.  Sometimes it works, sometimes it doesn't.  I will usually click on Solution Information and click the Geometry tab under the workspace to see all of the spider webs to confirm they were all built correctly.  I don't know if that checking process is foolproof, but it seems to have worked for me so far.  And in the few times I found a missing spider web, I rebuilt the joint connections to create some separation.

-Matt

-----Original Message-----
From: Baker, Alan (E&PS) via Xansys xansys-temp@list.xansys.org
Sent: Thursday, February 27, 2025 7:40 AM
To: XANSYS Mailing List Home xansys-temp@list.xansys.org
Cc: Baker, Alan (E&PS) Alan.Baker2@Honeywell.com
Subject: [Xansys] Re: [External] Choosing Rigid or Deformable Behavior

Matt,

For kinematic analyses that your question seems to imply, Mr Testi's answer makes a lot of sense.  For structural/dynamic analyses, one helpful paradigm is to ask if the behavior of the 'many grids' (or, 'many nodes') is best modeled as infinitely stiff with respect to each other and the 'single grid' - or - if it is better to represent the effect of the 'single grid' as distributed onto the 'many grids' without additional stiffness (like the mass of a small subassembly being added to the component 'many grids' without adding extra stiffness).

Both these constructs are MPC's with a one-to-many design.  In the 'rigid' formulation, the 'one-grid' is the independent grid and the 'many-grids' are dependent.  The rigid MPC design is set to force the dependent grids to have zero relative displacement with respect to the 'one grid'.  In the 'flexible' formulation, the 'one-grid' is dependent on the displacement of all of the 'many grids'.  The effect is the displacement of the 'one grid' is the average of the 'many'.  I've described the resulting behavior - the behind-the-scenes math is actually much more complicated because each of the grids has 6 degrees-of-freedom so the coupling of all the DOF of all the grids is quite profound.

Beware of connecting MPC's together in a chain without elements between the different MPC's (sometimes tempting to do in APDL, hard to do in Workbench).  Also beware of having multiple MPC's assigned to the same surface in Workbench (profoundly easy to do).  I've seen both mistakes made, not crash the solver, and not appear to be obviously wrong until the engineer starts probing more deeply.

Alan Baker
Principal Engineer
Engine Systems & Component Analysis
Honeywell | Aerospace
alan.baker2@honeywell.com

-----Original Message-----
From: Matthew Ridzon, PE via Xansys xansys-temp@list.xansys.org
Sent: Wednesday, February 26, 2025 1:15 PM
To: xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com
Subject: [External] [Xansys] Choosing Rigid or Deformable Behavior

Folks,

In Mechanical, there is an option for "rigid" or "deformable" behavior when building joints.  (in APDL, this is "rigid surface constraint" and "force-distributed constraint) What thought process does an analyst use to choose one or the other?  I know for a model with thermal loads, you have to choose deformable so that the model can move naturally with heat.  In the absence of any temperature loads, what thought process would one use to decide?

Matt Ridzon, PE, MSME
Sr. Engineering Analyst

Email    matt@prime-engineer.commailto:matt@prime-engineer.com
Mail      266 Main St, Burlington, VT 05401

Xansys mailing list -- xansys-temp@list.xansys.org To unsubscribe send an email to xansys-temp-leave@list.xansys.org If you are receiving too many emails from XANSYS please consider changing account settings to Digest mode which will send a single email per day.

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Dear Mr. Ridzon,
looking at the stress field near regions involved in constraint equations (joint are converted into constraint equations before the solver starts working) is always a risky business. When using joints, I suggest keeping in mind Saint-Venant's principle.

Best regards
Riccardo Testi

Development and Strategies
2 Wheeler Engines Technical Centre
Piaggio & C. S.p.A
Viale Rinaldo Piaggio, 25
56025 Pontedera (Pisa) - ITALY
Phone:  +39 0587 272850
Fax:        +39 0587 272010
Mobile: +39 339 7241918
E-mail:    riccardo.testi@piaggio.com

-----Original Message-----
From: Matthew Ridzon, PE via Xansys xansys-temp@list.xansys.org
Sent: Thursday, February 27, 2025 2:37 PM
To: XANSYS Mailing List Home xansys-temp@list.xansys.org
Cc: Baker, Alan (E&PS) Alan.Baker2@Honeywell.com; Matthew Ridzon, PE Matt@prime-engineer.com
Subject: [Xansys] Re: [External] Choosing Rigid or Deformable Behavior

CAUTION:This email originated from outside the Piaggio Group. Do not click links or open attachments unless you recognize the sender and know the content is safe.

Thanks Riccardo!  Thanks Alan!

I guess I should have clarified...my question pertains to joints that are NOT being used for kinematics.  They are fixed joints acting like "bonded contact."  However, if using joints for kinematics, I can understand why "rigid" is preferred.

I built a small test model and found odd stress contours at the periphery of the joint scopings when using rigid.  In other words, the scoped locations were behaving rigidly but the neighboring elements were flexible, which created an unusual stress pattern that was incorrect.

"Also beware of having multiple MPC's assigned to the same surface in Workbench (profoundly easy to do).  I've seen both mistakes made, not crash the solver, and not appear to be obviously wrong until the engineer starts probing more deeply."
I agree with you.  I have run into this problem.  Sometimes it works, sometimes it doesn't.  I will usually click on Solution Information and click the Geometry tab under the workspace to see all of the spider webs to confirm they were all built correctly.  I don't know if that checking process is foolproof, but it seems to have worked for me so far.  And in the few times I found a missing spider web, I rebuilt the joint connections to create some separation.

-Matt

-----Original Message-----
From: Baker, Alan (E&PS) via Xansys xansys-temp@list.xansys.org
Sent: Thursday, February 27, 2025 7:40 AM
To: XANSYS Mailing List Home xansys-temp@list.xansys.org
Cc: Baker, Alan (E&PS) Alan.Baker2@Honeywell.com
Subject: [Xansys] Re: [External] Choosing Rigid or Deformable Behavior

Matt,

For kinematic analyses that your question seems to imply, Mr Testi's answer makes a lot of sense.  For structural/dynamic analyses, one helpful paradigm is to ask if the behavior of the 'many grids' (or, 'many nodes') is best modeled as infinitely stiff with respect to each other and the 'single grid' - or - if it is better to represent the effect of the 'single grid' as distributed onto the 'many grids' without additional stiffness (like the mass of a small subassembly being added to the component 'many grids' without adding extra stiffness).

Both these constructs are MPC's with a one-to-many design.  In the 'rigid' formulation, the 'one-grid' is the independent grid and the 'many-grids' are dependent.  The rigid MPC design is set to force the dependent grids to have zero relative displacement with respect to the 'one grid'.  In the 'flexible' formulation, the 'one-grid' is dependent on the displacement of all of the 'many grids'.  The effect is the displacement of the 'one grid' is the average of the 'many'.  I've described the resulting behavior - the behind-the-scenes math is actually much more complicated because each of the grids has 6 degrees-of-freedom so the coupling of all the DOF of all the grids is quite profound.

Beware of connecting MPC's together in a chain without elements between the different MPC's (sometimes tempting to do in APDL, hard to do in Workbench).  Also beware of having multiple MPC's assigned to the same surface in Workbench (profoundly easy to do).  I've seen both mistakes made, not crash the solver, and not appear to be obviously wrong until the engineer starts probing more deeply.

Alan Baker
Principal Engineer
Engine Systems & Component Analysis
Honeywell | Aerospace
alan.baker2@honeywell.com

-----Original Message-----
From: Matthew Ridzon, PE via Xansys xansys-temp@list.xansys.org
Sent: Wednesday, February 26, 2025 1:15 PM
To: xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com
Subject: [External] [Xansys] Choosing Rigid or Deformable Behavior

Folks,

In Mechanical, there is an option for "rigid" or "deformable" behavior when building joints.  (in APDL, this is "rigid surface constraint" and "force-distributed constraint) What thought process does an analyst use to choose one or the other?  I know for a model with thermal loads, you have to choose deformable so that the model can move naturally with heat.  In the absence of any temperature loads, what thought process would one use to decide?

Matt Ridzon, PE, MSME
Sr. Engineering Analyst

Email    matt@prime-engineer.commailto:matt@prime-engineer.com
Mail      266 Main St, Burlington, VT 05401

Xansys mailing list -- xansys-temp@list.xansys.org To unsubscribe send an email to xansys-temp-leave@list.xansys.org If you are receiving too many emails from XANSYS please consider changing account settings to Digest mode which will send a single email per day.

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