quadratic elements in nx laminates

[mwnuk]

Hello, I did few analysis for laminates in nx with nx nastran, but when I tried to changed element type to CQUAD8 it allways gives me error in f06 file connected to unconstrained mechanism. setting param bailout to -1 make that results are there, but I think it is not a correct solution for that (error is for some reason i think). Does anyone had same problem and could help me?

 

[mwnuk]

As an actual sentence if anyone would read this, there is no problem with flat geometry to calculate witch 2nd order elements laminates. Any ideas?

 

[BlasMolero]

Dear mwnuk,
Surely you have exceeds a threshold defined by the paramater MAXRATIO (default 1.0E7), check the error in the *.F06 file. In the NASTRAN BULK DATA Options (PARAM section) increase the value of MAXRATIO = 1.3E7 and you are done!.
Best regards,
Blas.


~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/

 

[mwnuk]

Thank you very much!


I checked for 1.3e7 but it started works from 1e8. Could you tell me what this parameter do to solver?


regards

 

[BlasMolero]

Hello!,
When performing the solution to a system of linear equations, singularities lead to conditions in which a unique solution is not possible. NX Nastran considers two types of singularities:
• Grid point singularity that is identified by considering the stiffness terms of only one grid point.
• A mechanism type of singularity that requires the consideration of the stiffness terms of more than one grid point.

Singularities cause ill-conditioned matrices that can be detected in several phases of the NX Nastran execution. If PARAM,AUTOSPC,YES is specified (this is the default in the Structured Solution Sequences, except SOLs 106 and 129), the potential singularities are automatically constrained if possible. During decomposition, mechanisms can be detected based on the maximum ratio of the matrix diagonal to the factor diagonal.

All terms whose ratio exceed the value of PARAM,MAXRATIO are printed by NX NASTRAN in the F06 output file. The default for MAXRATIO is 1E07. USER INFORMATIONB MESSAGE (UIM) 4158 prints the statistics for the decomposition that include the number of negative terms on the factor diagonal and the maximum ratio of matrix diagonal to factor diagonal at a specified row number and its corresponding grid point. User Warning Message (UWM) 4698 prints the degrees of freedom that have a factor diagonal ratio greater than the MAXRATIO value or have negative terms on the factor diagonal. Both of these messages are issued by the DECOMP module and are shown below:

*** USER INFORMATION MESSAGE 4158---STATISTICS FOR SYMMETRIC DECOMPOSITION OF DATA BLOCK KLL    FOLLOW
           NUMBER OF NEGATIVE TERMS ON FACTOR DIAGONAL =     1
           MAXIMUM RATIO OF MATRIX DIAGONAL TO FACTOR DIAGONAL = 7.2E+15 AT ROW NUMBER    16
 *** USER WARNING MESSAGE 4698.  STATISTICS FOR DECOMPOSITION OF MATRIX KLL 
    .
 THE FOLLOWING DEGREES OF FREEDOM HAVE FACTOR DIAGONAL RATIOS GREATER THAN 1.00000E+05 OR HAVE
NEGATIVE TERMS ON THE FACTOR DIAGONAL.
      GRID POINT ID       DEGREE OF FREEDOM   MATRIX/FACTOR DIAGONAL RATIO           MATRIX DIAGONAL
           6714                  T1                 -7.19297E+15                       6.02908E+07
 ^^^ DMAP FATAL MESSAGE 9050 (SEKRRS) -  RUN TERMINATED DUE TO EXCESSIVE PIVOT RATIOS
     IN MATRIX KLL. USER PARAMETER BAILOUT MAY BE USED   TO CONTINUE THE RUN.

If, for any row, this ratio is greater than MAXRATIO, the matrix will be considered to be nearly singular (having mechanisms). If any diagonal terms of the factor are negative, the stiffness matrix is considered implausible (nonpositive definite). The ratios greater than MAXRATIO and less than zero and their associated external grid identities will be printed out in the *.F06 foile. The program will then take appropriate action as directed by the parameter BAILOUT.

A negative value for BAILOUT directs the program to continue processing the superelement. Although forcing the program to continue with near-singularities is a useful modeling checkout technique (BUT NOT RECOMMEND AT ALL FOR PRODUCTION MODELS!!), it may lead to solutions of poor quality or fatal messages later in the run. It is recommended that the default values be used for production runs.

The value of "PARAM,BAILOUT,-1" causes the program to continue processing with near singularities and a zero value will cause the program to exit if near singularities are detected.
In SOLs 101 through 200 when PARAM,CHECKOUT,YES is specified, PARAM,MAXRATIO sets the tolerance for detecting multipoint constraint equations with poor linear independence. BAILOUT and MAXRATIO may be specified in the Case Control Section in order to provide different values between superelements but not between different boundary conditions.

Some general causes for singularity can include:

• Degrees of freedom without stiffness because of missing elements.
• A 2-dimensional plate problem with the normal rotation unconstrained.
• A solid model with rotational DOFs at the corners unconstrained.
• Incorrect modeling of offset beams.
• Incorrect multipoint constraints.
• Mechanisms and free bodies, such as sloped plates, beam to plate connections, beam to solid connections, and plate-to-solid connections.
• Low stiffness in rotation.
• A stiff element adjacent to a very flexible element.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/

 

[mwnuk]

Now it's clear

Thank you very much for comprehensive reply.

Regards
m Wnuk