Learning Forum

[Weihan Lin]

Hi all,

I've met problems of using pre-stress modal analysis. The Static Structural simulation turns into "red flash" not "green marker". Although the simulation still run, I don't know if the results are trustable. It's an inflatable body, using rubber material, with 1/16 inch thickness and 10 Pa pressure inside. The boundary conditions are the 7 vertexes of this body. The whole body is around 2 meters long and I'm using 0.03m mesh size.

The error says: An unknown error occurred during solution.  Check the Solver Output on the Solution Information object for possible causes.

The warning says: The solution failed to solve completely at all time points. Restart points are available to continue the analysis. Large deformation effects are active which may have invalidated some of your applied supports such as displacement, cylindrical, frictionless, or compression only.  Refer to Troubleshooting in the Help System for more details.Although the solution failed to solve completely at all time points, partial results at some points have been able to be solved.  Refer to Troubleshooting in the Help System for more details.The unconverged solution (identified as Substep 999999) is output for analysis debug purposes. Results at this time should not be used for any other purpose.

 

 

[Akshay Maniyar]

Hi Weihan,

Can you check the solver output and search for the exact error message? It will give us more information about the issue.

Thanks,

Akshay Maniyar

[Weihan Lin]

 

Hi Akshay,

Thanks for the reply. Here are the error messages I found in the solution information page. Let me know if it helps or no. It seems that my contact regions has some problem, but I don't know why this happened. If I change the rubber thickness from 1/16 inch to 2 inches, there's no error.

 

*** ERROR ***                           CP =       3.891   TIME= 16:56:20
 Element 10417 has excessive thickness change.                          
 *** LOAD STEP     1   SUBSTEP     1  NOT COMPLETED.  CUM ITER =      2
 *** BEGIN BISECTION NUMBER   1    NEW TIME INCREMENT=   7.0000   

 *** NOTE ***                            CP =       3.891   TIME= 16:56:20
 Predictor is ON by default for structural elements with rotational     
 degrees of freedom.  Use the PRED,OFF command to turn the predictor    
 OFF if it adversely affects the convergence.                           

 
 *************************************************
  SUMMARY FOR CONTACT PAIR IDENTIFIED BY REAL CONSTANT SET          28

 *** NOTE ***                            CP =       4.969   TIME= 16:56:21
 Contact pair is inactive.                                              
 
 *************************************************
  SUMMARY FOR CONTACT PAIR IDENTIFIED BY REAL CONSTANT SET          29

 *** NOTE ***                            CP =       4.969   TIME= 16:56:21
 No contact was detected.                                               
 Max.  Pinball distance 6.093701855E-03.                                
 One of the contact searching regions contains at least 8 target        
 elements.                                                              
 *************************************************
 
 *************************************************
  SUMMARY FOR CONTACT PAIR IDENTIFIED BY REAL CONSTANT SET          30

 *** NOTE ***                            CP =       4.969   TIME= 16:56:21
 Contact pair is inactive.                                              
 
 *************************************************
  SUMMARY FOR CONTACT PAIR IDENTIFIED BY REAL CONSTANT SET          31

 *** NOTE ***                            CP =       4.969   TIME= 16:56:21
 No contact was detected.                                               
 Max.  Pinball distance 6.093701855E-03.                                
 One of the contact searching regions contains at least 8 target        
 elements.                                                              
 *************************************************
 
 *************************************************

One of the contact searching regions contains at least 220 target      
 elements.  You may reduce the pinball radius (current value            
 9.972265941E-03) for contact pair identified by real constant set 40   
 to speed up contact searching.                                         

 Range of element maximum matrix coefficients in global coordinates
 Maximum = 6781141.29 at element 16858.                                 
 Minimum = 12160.5806 at element 8110.                                  
     FORCE CONVERGENCE VALUE  =   1.634      CRITERION=  0.8334E-02
     MOMENT CONVERGENCE VALUE =  0.3343E-12  CRITERION=  0.5102E-04

 SPARSE MATRIX DIRECT SOLVER.
  Number of equations =       97485,    Maximum wavefront =     60
  Memory allocated for solver              =   312.443 MB
  Memory required for in-core solution     =   299.944 MB
  Memory required for out-of-core solution =   116.053 MB

 *** NOTE ***                            CP =       5.234   TIME= 16:56:21
 The Sparse Matrix Solver is currently running in the in-core memory    
 mode.  This memory mode uses the most amount of memory in order to     
 avoid using the hard drive as much as possible, which most often       
 results in the fastest solution time.  This mode is recommended if     
 enough physical memory is present to accommodate all of the solver     
 data.                                                                  
 curEqn=  84522  totEqn=  97485 Job CP sec=      5.562
      Factor Done=  68% Factor Wall sec=      0.000 rate=       0.0 GFlops
 curEqn=  97485  totEqn=  97485 Job CP sec=      5.578
      Factor Done= 100% Factor Wall sec=      0.030 rate=     424.3 GFlops
 Sparse solver maximum pivot= 9636186.61 at node 1650 UY.               
 Sparse solver minimum pivot= 1.425333714E-04 at node 7549 ROTY.        
 Sparse solver minimum pivot in absolute value= 1.425333714E-04 at node 
 7549 ROTY.                                                             
     DISP CONVERGENCE VALUE   =   8.242      CRITERION=  0.4205   
    EQUIL ITER   1 COMPLETED.  NEW TRIANG MATRIX.  MAX DOF INC=   22.82   

 

[Sahil Sura]

Hi Weihan Lin,
Since the material, I'm assuming is used in the analysis is a nonlinear material, it might be a bit difficult to converge at first using the default settings, thus it would be a good idea to converge the static structural system in a standalone way to get the required plots and then link it to the modal system.
You can turn on the Large deflection in the static structural system and also consider the Auto time-stepping option and define the adequate amount of timesteps or substeps to help the model get converged.
Also, note that the modal analysis is a subset of the Linear Dynamic Analyses, and as the name suggests the nonlinearities will be invalidated in the same, so please find the following video which helps to solve the Pre-stressed Modal Analysis.
How To Perform Prestressed Modal Analysis — Lesson 2 - ANSYS Innovation Courses 
https://ansyshelp.ansys.com/Views/Secured/corp/v232/en/ans_str/Hlp_G_STR3_OtherModal.html?q=prestressed%20modal 

Hope this helps!

Thanks,

Sahil

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[Weihan Lin]

 

Hi Sahil,

Thanks for you reply. The material I used is linear material as shown in the image below.

I did turn on the large deflection and auto time stepping option. I add more time steps as you mentioned, but it still shows error like this. It seems that my contact regions has some problem, but I don't know why this happened. If I change the rubber thickness from 1/16 inch to 2 inches, there's no error.

*** ERROR ***                           CP =       3.891   TIME= 16:56:20
 Element 10417 has excessive thickness change.                          
 *** LOAD STEP     1   SUBSTEP     1  NOT COMPLETED.  CUM ITER =      2
 *** BEGIN BISECTION NUMBER   1    NEW TIME INCREMENT=   7.0000   

 *** NOTE ***                            CP =       3.891   TIME= 16:56:20
 Predictor is ON by default for structural elements with rotational     
 degrees of freedom.  Use the PRED,OFF command to turn the predictor    
 OFF if it adversely affects the convergence.                           

 
 *************************************************
  SUMMARY FOR CONTACT PAIR IDENTIFIED BY REAL CONSTANT SET          28

 *** NOTE ***                            CP =       4.969   TIME= 16:56:21
 Contact pair is inactive.                                              
 
 *************************************************
  SUMMARY FOR CONTACT PAIR IDENTIFIED BY REAL CONSTANT SET          29

 *** NOTE ***                            CP =       4.969   TIME= 16:56:21
 No contact was detected.                                               
 Max.  Pinball distance 6.093701855E-03.                                
 One of the contact searching regions contains at least 8 target        
 elements.                                                              
 *************************************************
 
 *************************************************
  SUMMARY FOR CONTACT PAIR IDENTIFIED BY REAL CONSTANT SET          30

 *** NOTE ***                            CP =       4.969   TIME= 16:56:21
 Contact pair is inactive.                                              
 
 *************************************************
  SUMMARY FOR CONTACT PAIR IDENTIFIED BY REAL CONSTANT SET          31

 *** NOTE ***                            CP =       4.969   TIME= 16:56:21
 No contact was detected.                                               
 Max.  Pinball distance 6.093701855E-03.                                
 One of the contact searching regions contains at least 8 target        
 elements.                                                              
 *************************************************
 
 *************************************************

One of the contact searching regions contains at least 220 target      
 elements.  You may reduce the pinball radius (current value            
 9.972265941E-03) for contact pair identified by real constant set 40   
 to speed up contact searching.                                         

 Range of element maximum matrix coefficients in global coordinates
 Maximum = 6781141.29 at element 16858.                                 
 Minimum = 12160.5806 at element 8110.                                  
     FORCE CONVERGENCE VALUE  =   1.634      CRITERION=  0.8334E-02
     MOMENT CONVERGENCE VALUE =  0.3343E-12  CRITERION=  0.5102E-04

 SPARSE MATRIX DIRECT SOLVER.
  Number of equations =       97485,    Maximum wavefront =     60
  Memory allocated for solver              =   312.443 MB
  Memory required for in-core solution     =   299.944 MB
  Memory required for out-of-core solution =   116.053 MB

 *** NOTE ***                            CP =       5.234   TIME= 16:56:21
 The Sparse Matrix Solver is currently running in the in-core memory    
 mode.  This memory mode uses the most amount of memory in order to     
 avoid using the hard drive as much as possible, which most often       
 results in the fastest solution time.  This mode is recommended if     
 enough physical memory is present to accommodate all of the solver     
 data.                                                                  
 curEqn=  84522  totEqn=  97485 Job CP sec=      5.562
      Factor Done=  68% Factor Wall sec=      0.000 rate=       0.0 GFlops
 curEqn=  97485  totEqn=  97485 Job CP sec=      5.578
      Factor Done= 100% Factor Wall sec=      0.030 rate=     424.3 GFlops
 Sparse solver maximum pivot= 9636186.61 at node 1650 UY.               
 Sparse solver minimum pivot= 1.425333714E-04 at node 7549 ROTY.        
 Sparse solver minimum pivot in absolute value= 1.425333714E-04 at node 
 7549 ROTY.                                                             
     DISP CONVERGENCE VALUE   =   8.242      CRITERION=  0.4205   
    EQUIL ITER   1 COMPLETED.  NEW TRIANG MATRIX.  MAX DOF INC=   22.82