Gasket material behavior is highly nonlinear. The full Newton-Raphson solution procedure (the standard Mechanical APDL nonlinear method) is the default method for performing this type of analysis. Other solution procedures for gasket solutions are not recommended.
As with most nonlinear problems, convergence behavior of a gasket joint analysis depends on the problem type. Mechanical APDL provides a comprehensive solution hierarchy; therefore, it is best to use the default solution options unless you are certain about the benefits of any changes.
Some special considerations for solving a gasket problem are as follows:
By default, a zero stress cap is enforced on the gasket. When the element goes into tension, it loses its stiffness and sometimes causes numerical instability.
It is always a good practice to place the lower and upper limit on the time step size (DELTIM or NSUBST). Start with a small time step, then subsequently ramp it up. This practice ensures that all modes and behaviors of interest are accurately included.
When modeling gasket interfaces as sliding contact, it is usually necessary to include adequate gasket transverse shear stiffness. By default, the gasket elements account for a small transverse shear stiffness. You can modify the transverse shear stiffness if needed (TB,GASKET,,,,TSS) command. For better solution stability, use nodal contact detection.
When modeling gasket interfaces via a matching mesh method (that is, with coincident nodes), it is better to exclude transverse shear stiffness to avoid unnecessary in-plane interaction between the gasket and mating components.
Like any other type of nonlinear analysis, Mechanical APDL performs a series of linear approximations with corrections. A convergence failure can indicate a physical instability in the structure, or it can merely be the result of some numerical problem in the finite element model. The program printout gives you continuous feedback on the progress of these approximations and corrections. (The printout either appears directly on your screen, is captured on Jobname.out, or is written to some other file (/OUTPUT).) You can examine some of this same information in POST1, using the PRITER command, or in POST26, using the SOLU and PRVAR commands. Understand the iteration history of your analysis before you accept the results. In particular, do not dismiss any program error or warning statements without fully understanding their meaning. A typical output listing with gasket nonlinearity only is shown in Typical Gasket Solution Output Listing. When other types of nonlinearity such as contact or materials are included, additional information will be printed out.
S O L U T I O N O P T I O N S
PROBLEM DIMENSIONALITY. . . . . . . . . . . . .3D
DEGREES OF FREEDOM. . . . . . UX UY UZ
ANALYSIS TYPE . . . . . . . . . . . . . . . . .STATIC (STEADY-STATE)
PLASTIC MATERIAL PROPERTIES INCLUDED. . . . . .YES
NEWTON-RAPHSON OPTION . . . . . . . . . . . . .PROGRAM CHOSEN
*** NOTE *** CP= 0.000 TIME= 00:00:00
Present time 0 is less than or equal to the previous time.
Time will default to 1.
*** NOTE *** CP= 0.000 TIME= 00:00:00
Nonlinear analysis, NROPT set to the FULL Newton-Raphson
solution procedure for ALL degrees of freedom.
*** NOTE *** CP= 0.000 TIME= 00:00:00
The conditions for direct assembly have been met. No .emat or .erot
files will be produced.
L O A D S T E P O P T I O N S
LOAD STEP NUMBER. . . . . . . . . . . . . . . . 1
TIME AT END OF THE LOAD STEP. . . . . . . . . . 1.0000
AUTOMATIC TIME STEPPING . . . . . . . . . . . . ON
INITIAL NUMBER OF SUBSTEPS . . . . . . . . . 200
MAXIMUM NUMBER OF SUBSTEPS . . . . . . . . . 20000
MINIMUM NUMBER OF SUBSTEPS . . . . . . . . . 20
MAXIMUM NUMBER OF EQUILIBRIUM ITERATIONS. . . . 15
STEP CHANGE BOUNDARY CONDITIONS . . . . . . . . NO
TERMINATE ANALYSIS IF NOT CONVERGED . . . . . .YES (EXIT)
CONVERGENCE CONTROLS. . . . . . . . . . . . . .USE DEFAULTS
COPY INTEGRATION POINT VALUES TO NODE . . . . .YES, FOR ELEMENTS WITH
ACTIVE MAT. NONLINEARITIES
PRINT OUTPUT CONTROLS . . . . . . . . . . . . .NO PRINTOUT
DATABASE OUTPUT CONTROLS
ITEM FREQUENCY COMPONENT
ALL ALL
SVAR ALL
Range of element maximum matrix coefficients in global coordinates
Maximum= 4.326388889E+11 at element 0.
Minimum= 388758681 at element 0.
*** ELEMENT MATRIX FORMULATION TIMES
TYPE NUMBER ENAME TOTAL CP AVE CP
1 2 SOLID185 0.000 0.000000
2 1 INTER195 0.000 0.000000
Time at end of element matrix formulation CP= 0.
ALL CURRENT DATA WRITTEN TO FILE NAME=
FOR POSSIBLE RESUME FROM THIS POINT
FORCE CONVERGENCE VALUE = 0.4200E+07 CRITERION= 0.2143E+05
SPARSE MATRIX DIRECT SOLVER.
Number of equations = 24, Maximum wavefront = 0
Memory available (MB) = 0.0 , Memory required (MB) = 0.0
DISP CONVERGENCE VALUE = 0.1130E-04 CRITERION= 0.2000E-06
EQUIL ITER 1 COMPLETED. NEW TRIANG MATRIX. MAX DOF INC= -0.4000E-05
FORCE CONVERGENCE VALUE = 0.1367E-08 CRITERION= 51.35 <<< CONVERGED
DISP CONVERGENCE VALUE = 0.3257E-20 CRITERION= 0.2000E-06 <<< CONVERGED
EQUIL ITER 2 COMPLETED. NEW TRIANG MATRIX. MAX DOF INC = -0.2234E-20
>>> SOLUTION CONVERGED AFTER EQUILIBRIUM ITERATION 2
*** ELEMENT RESULT CALCULATION TIMES
TYPE NUMBER ENAME TOTAL CP AVE CP
1 2 SOLID185 0.000 0.000000
2 1 INTER195 0.000 0.000000
*** NODAL LOAD CALCULATION TIMES
TYPE NUMBER ENAME TOTAL CP AVE CP
1 2 SOLID185 0.000 0.000000
2 1 INTER195 0.000 0.000000
*** LOAD STEP 1 SUBSTEP 1 COMPLETED. CUM ITER = 2
*** TIME = 0.500000E-02 TIME INC = 0.500000E-02
*** AUTO STEP TIME: NEXT TIME INC = 0.50000E-02 UNCHANGED
FORCE CONVERGENCE VALUE = 0.7716E-09 CRITERION= 100.6
DISP CONVERGENCE VALUE = 0.1951E-20 CRITERION= 0.2000E-06 <<< CONVERGED
EQUIL ITER 1 COMPLETED. NEW TRIANG MATRIX. MAX DOF INC= 0.1642E-20
FORCE CONVERGENCE VALUE = 0.4624E-09 CRITERION= 102.7 <<< CONVERGED
>>> SOLUTION CONVERGED AFTER EQUILIBRIUM ITERATION 1
*** LOAD STEP 1 SUBSTEP 2 COMPLETED. CUM ITER = 3
*** TIME = 0.100000E-01 TIME INC = 0.500000E-02
*** AUTO TIME STEP: NEXT TIME INC = 0.75000E-02 INCREASED (FACTOR = 1.5000)
FORCE CONVERGENCE VALUE = 0.1964E-08 CRITERION= 176.1
DISP CONVERGENCE VALUE = 0.3748E-20 CRITERION= 0.3000E-06 <<< CONVERGED
EQUIL ITER 1 COMPLETED. NEW TRIANG MATRIX. MAX DOF INC= -0.2744E-20
FORCE CONVERGENCE VALUE = 0.8307E-09 CRITERION= 179.7 <<< CONVERGED
>>> SOLUTION CONVERGED AFTER EQUILIBRIUM ITERATION 1
*** LOAD STEP 1 SUBSTEP 3 COMPLETED. CUM ITER = 4
*** TIME = 0.175000E-01 TIME INC = 0.750000E-02
*** AUTO TIME STEP: NEXT TIME INC = 0.11250E-01 INCREASED (FACTOR = 1.5000)
FORCE CONVERGENCE VALUE = 0.3713E-08 CRITERION= 289.4
DISP CONVERGENCE VALUE = 0.7198E-20 CRITERION= 0.4500E-06 <<< CONVERGED
EQUIL ITER 1 COMPLETED. NEW TRIANG MATRIX. MAX DOF INC= -0.4503E-20
FORCE CONVERGENCE VALUE = 0.1468E-08 CRITERION= 295.3 <<< CONVERGED
>>> SOLUTION CONVERGED AFTER EQUILIBRIUM ITERATION 1
*** LOAD STEP 1 SUBSTEP 4 COMPLETED. CUM ITER = 5
*** TIME = 0.287500E-01 TIME INC = 0.112500E-01
*** AUTO TIME STEP: NEXT TIME INC = 0.16875E-01 INCREASED (FACTOR = 1.5000)
FORCE CONVERGENCE VALUE = 0.1833E-07 CRITERION= 459.2
DISP CONVERGENCE VALUE = 0.3753E-19 CRITERION= 0.6750E-06 <<< CONVERGED
EQUIL ITER 1 COMPLETED. NEW TRIANG MATRIX. MAX DOF INC= 0.1800E-19
FORCE CONVERGENCE VALUE = 0.3550E-08 CRITERION= 468.6 <<< CONVERGED
>>> SOLUTION CONVERGED AFTER EQUILIBRIUM ITERATION 1
*** LOAD STEP 1 SUBSTEP 5 COMPLETED. CUM ITER = 6
*** TIME = 0.456250E-01 TIME INC = 0.168750E-01
*** AUTO TIME STEP: NEXT TIME INC = 0.25313E-01 INCREASED (FACTOR = 1.5000)
FORCE CONVERGENCE VALUE = 0.2322E-07 CRITERION= 714.0
DISP CONVERGENCE VALUE = 0.4656E-19 CRITERION= 0.1013E-05 <<< CONVERGED
EQUIL ITER 1 COMPLETED. NEW TRIANG MATRIX. MAX DOF INC= -0.2664E-19
FORCE CONVERGENCE VALUE = 0.6406E-08 CRITERION= 728.5 <<< CONVERGED
>>> SOLUTION CONVERGED AFTER EQUILIBRIUM ITERATION 1
*** LOAD STEP 1 SUBSTEP 6 COMPLETED. CUM ITER = 7
*** TIME = 0.709375E-01 TIME INC = 0.253125E-01
*** AUTO TIME STEP: NEXT TIME INC = 0.37969E-01 INCREASED (FACTOR = 1.5000)