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Negative Pivot Ratios and Matrix Diagonals

Negative Pivot Ratios and Matrix Diagonals

Negative Pivot Ratios and Matrix Diagonals

(OP)
Good Morning,

I have a model in Femap being solved with NX Nastran. It is a combination of 10-noded tets, shells, and bars, however there is not any sharing of nodes between the solid and non-solid elements. I am getting fatal pivot ratio errors and the error messages and ratios are below. I have checked the nodes and the elements they are attached to. All but one of these nodes (930272) are mid-side nodes of the 10-noded tets. Several of the elements that these nodes are connected to have very high aspect ratios and are very flat and long, and are attached to much larger elements that are not nearly as skewed. I have run the analysis with PARAM,BAILOUT,-1, and PARAM,AUTOSPC,NO, and it runs to completion without showing any excessive deflections or mechanisms, indicating to me that there are no constraint issues. I have read that negative pivot ratios indicate a constraint problem or that the stiffness around that node is too small compared to the rest of the structure. Is there any steps I can take, other than re-meshing the poor-quality elements? All the elements have the same material properties as the elements surrounding the nodes in question.

I am not concerned about the stress results in the area of these nodes, either (the stress has been benign in this location in previous analyses).

Can anyone tell me what negative matrix diagonals can indicate?

Am I correct in interpreting the very small pivot ratios as locations that are very soft compared to other locations in the model?

Thank you very much,

Ryan Bolin
Laboratory for Atmospheric and Space Physics (LASP)
University of Colorado at Boulder


*** USER WARNING MESSAGE 4698 (DCMPD)
STATISTICS FOR DECOMPOSITION OF MATRIX KLL .
THE FOLLOWING DEGREES OF FREEDOM HAVE FACTOR DIAGONAL RATIOS GREATER THAN
1.00000E+10 OR HAVE NEGATIVE TERMS ON THE FACTOR DIAGONAL.
USER INFORMATION:
THIS MESSAGE MAY BE IGNORED IF NO GRID POINT IDS OR HIGH RATIO MESSAGES APPEAR IN THE TABLE ON THE NEXT PAGE.
User information:
During decomposition, the degrees of freedom listed have pivot
ratios that are that are greater than maxratio or are negative.
Verify that the degrees of freedom are not part of a mechanism
and that elements do not have excessive stiffness. In superelement
analysis this condition causes run termination. PARAM,BAILOUT may
be used to continue the run. See the NX NASTRAN Numerical Methods
User's Guide.
1 THERMAL LOAD TEST MARCH 8, 2016 NX NASTRAN 5/ 5/15 PAGE 27107

0 SUBCASE 1
0
GRID POINT ID DEGREE OF FREEDOM MATRIX/FACTOR DIAGONAL RATIO MATRIX DIAGONAL

694931 T1 1.00000E+00 -8.98965E+05
694931 T2 1.14357E+00 -3.56215E+06
694931 T3 2.36942E+00 -2.50657E+06
698722 T1 -1.67846E+01 -2.63450E+07
698722 T2 -2.64036E+01 -3.21134E+07
698722 T3 -9.55444E+01 -1.31610E+08
698725 T1 7.66168E-01 -3.57518E+07
698725 T2 7.85739E-01 -3.54738E+07
698725 T3 8.10252E-01 -1.34659E+08
699979 T1 -2.64877E+01 -3.65597E+07
699979 T2 -2.88857E+01 -3.45196E+07
699979 T3 -1.16893E+02 -1.39241E+08
702688 T1 -4.37348E+00 -3.64078E+07
702688 T2 -4.11375E+00 -3.44845E+07
702688 T3 -4.98574E+00 -1.33350E+08
930272 T2 -5.47892E+01 2.30117E+07
956797 T3 -3.23780E+01 2.79006E+06
956804 T2 6.13333E-01 -1.28178E+05
956804 T3 -4.15746E-01 -2.03479E+05
962333 T1 1.63257E+00 -1.36044E+06
962333 T2 9.64163E-01 -4.46875E+05
962333 T3 9.01502E-01 -1.48995E+06
962334 T1 -2.37824E+00 -7.24582E+05
962334 T2 -4.51640E-01 -1.04717E+05
962334 T3 -4.49636E+00 -8.38459E+05
962457 T1 -1.29889E+11 -1.83486E+17
962457 T2 -1.27100E+11 -1.83486E+17
962457 T3 -2.71684E+11 -7.23151E+17
962458 T1 -6.90744E+13 -1.05939E+20
962458 T2 -4.78409E+13 -1.05939E+20
962458 T3 -2.49147E+14 -4.17524E+20
962459 T1 -5.79693E+13 -2.15820E+19
962459 T2 -3.00535E+13 -2.15820E+19
962459 T3 -1.90947E+14 -8.50584E+19
962479 T1 -1.91882E+13 -6.93706E+19
962479 T2 -2.29297E+13 -6.93706E+19
962479 T3 -4.45868E+13 -2.73402E+20
962500 T1 -1.73038E+14 -9.95338E+19
962500 T2 -2.19124E+14 -9.95338E+19
962500 T3 -1.99524E+14 -3.92280E+20
962501 T1 1.00000E+00 -5.61502E+20
962501 T2 1.00000E+00 -5.61502E+20
962501 T3 1.00000E+00 -2.21298E+21
962502 T1 -2.91487E+14 -1.28808E+20
962502 T2 -2.42761E+14 -1.28808E+20
962502 T3 -3.04789E+15 -5.07657E+20
962509 T1 1.89992E+00 -8.71186E+19
962509 T2 1.89992E+00 -8.71186E+19
962509 T3 1.89992E+00 -3.43350E+20
962510 T1 -2.88057E+13 -1.24534E+19
1 THERMAL LOAD TEST MARCH 8, 2016 NX NASTRAN 5/ 5/15 PAGE 27108

0 SUBCASE 1
0
GRID POINT ID DEGREE OF FREEDOM MATRIX/FACTOR DIAGONAL RATIO MATRIX DIAGONAL

962510 T2 -2.86427E+13 -1.24534E+19
962510 T3 -3.69654E+13 -4.90810E+19
962511 T1 1.00000E+00 -3.07143E+17
962511 T2 1.00000E+00 -3.07143E+17
962511 T3 1.00000E+00 -1.21050E+18
962515 T1 -7.70845E+13 -6.65257E+19
962515 T2 -6.30383E+13 -6.65257E+19
962515 T3 -9.19426E+13 -2.62190E+20
962516 T1 -1.06089E+14 -4.49541E+19
962516 T2 -7.62982E+13 -4.49541E+19
962516 T3 1.46974E+15 -1.77172E+20
966490 T2 -5.48640E-02 1.46698E+06
975308 T2 -2.33829E+00 2.96598E+06
983154 T1 1.87088E+01 -5.94240E+05
983154 T2 -1.19128E-02 -1.89759E+04
983154 T3 -1.05154E+00 -8.73878E+05
984867 T1 -5.97553E+13 -9.95338E+19
984867 T2 -8.88183E+13 -9.95338E+19
984867 T3 -2.00626E+14 -3.92280E+20
984868 T1 -3.44780E+13 -2.42327E+19
984868 T2 -3.96562E+13 -2.42327E+19
984868 T3 -1.47613E+14 -9.55055E+19
984870 T1 -1.55974E+13 -1.42537E+20
984870 T2 -1.65014E+13 -1.42537E+20
984870 T3 -1.83551E+13 -5.61765E+20
984884 T1 -4.44525E+14 -5.71612E+20
984884 T2 -4.77468E+14 -5.71612E+20
984884 T3 -4.42168E+15 -2.25282E+21
984885 T1 1.96415E+00 -1.41320E+20
984885 T2 1.96415E+00 -1.41320E+20
984885 T3 1.96415E+00 -5.56969E+20
984887 T1 -1.67219E+14 -1.40099E+20
984887 T2 -1.79420E+14 -1.40099E+20
984887 T3 -9.46730E+14 -5.52155E+20
984888 T1 1.00000E+00 -7.89434E+19
984888 T2 1.00000E+00 -7.89434E+19
984888 T3 1.00000E+00 -3.11130E+20
985027 T1 -1.04706E+00 -8.74507E+05
985027 T2 -4.74036E-01 -2.53954E+05
985027 T3 -2.34762E+00 -1.28921E+06
^^^ USER FATAL MESSAGE 9137 (SEKRRS)
^^^ RUN TERMINATED DUE TO EXCESSIVE PIVOT RATIOS IN MATRIX KLL.
^^^ USER ACTION: CONSTRAIN MECHANISMS WITH SPCI OR SUPORTI ENTRIES OR SPECIFY PARAM,BAILOUT,-1 TO
^^^ CONTINUE THE RUN WITH MECHANISMS.

RE: Negative Pivot Ratios and Matrix Diagonals

To my best understanding a very large pivot ratio at a grid means the following: the connections to this particular grid have very different stiffness values. The bad thing about this is that it can introduce significant numerical round-off errors (hence the error thrown by Nastran).

From your description it sounds indeed like the badly shaped elements could be the underlying problem.

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