Learning Forum

[helen.durand]

Hello,

What is the difference between a single frame and multiframe restart in ANSYS transient structural?

Using inserted command objects, I want to change the material type (MPCHG) and use element birth and death (EKILL and EALIVE) between each loadstep. Do I want to use single frame or multiframe restarts?

I found some resources on single and multiframe restarts, but it's still not clear to me which to use:
https://simutechgroup.com/single-frame-restarts-in-mechanical/
https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v222/en/ans_bas/Hlp_G_BAS3_12.html

Thank you!

[Sean Harvey]

Hello Helen,

You can change material properties between load steps by using the ‘MPCHG’ command with certain limitations such as material properties cannot be changed from linear to nonlinear, or from one nonlinear option to another. So for this, you don't need restart.

Ekill and ealive also do not need restarts and they are natively exposed inside Mechanical if you are using Mechanical that is.

Single frame restart is the original method of restarting in APDL. In fact if you are in MAPDL, you solve a load step, then add some loads, and solve again, you are using a single frame restart. There are all the files such as esav,full,db, etc. needed to do this.  You may have seen input files for mapdl that have multiple solve commands. That doc you shared is a good resource that if you are using mechanical, it will naturally set the model up for multiframe restarts.  With multiframe, we can have multiple points, but there are some limitations as listed in that help doc.

So, in your case, you can use single frame restart and just put in mpchg in a command object, and use ekill and alive natively. The code provided to delete the rdb and turn of restarts should not be necessary if you go to Analysis Settings> Restart Controls> Generate Restart Points to No.

MPCHG is valid in /solu, so you don't need to even leave /solu to go to /prep7

Keep in mind Mechanical deletes the single frame restart files (and other files) by default after the solve, and you can change this under Analysis Settings > Analysis Data Management> Delete Unneeded files to No

Good luck!

Sean

 

[helen.durand]

Thank you for the detailed reply! This is extremely helpful. I just have a couple of follow up questions:

What do you mean by using ekill and ealive "natively" in transient structural?

Is there a way to change between two nonlinear materials? For example, is it possible to change some elements from one nonlinear material to another nonlinear material at the end of each load step using command objects?

[Sean Harvey]

Hi Helen,

See the screenshot below.  It is in the Mechanical UI as an option as shown.

I answered on the other thread but I can repeat it here for the benefit of others.

MPCHG command allows you to change from one nonlinear material to another, but it does not support if there are different options.

per the help doc on MPCHG  "Changes the material number of the specified element. Between load steps in SOLUTION, material properties cannot be changed from linear to nonlinear, or from one nonlinear option to another."

So you can't switch the type of plasticity model, etc.  If you do, you will get an error like this.

 *** FATAL ***                           CP =     191.328   TIME= 14:33:48
 Change a nonlinear material model, 2, to another nonlinear material    
 model, 1, is not allowed for element type 186.                         

 

Regards,

Sean