|
|
经典图书
Following is a list of commonly encountered errors during the execution of
nonlinear gap/contact problems.
1. During the first step, the program stops with the error message: “Stop, the
diagonal term in equation ...., node ...., direction .. is ....." (a zero or negative).
This error usually indicates that the whole model, or portion of it, is externally
statically unstable due to improper constraints. It should always be remembered
that the gap elements do not alter the stiffness. If a portion of the model is
supported only by gap elements, then that portion can be stabilized through the
use of soft trusses (see verification problems NS17 and NS18).
2. The program runs successfully, but the postprocessing module shows that the
one node gap elements go beyond the contact surface which should have
stopped them.
Make sure that you are looking at the structure's iso-scale displaced plot (both
the dimensions of the structure and the deformations have the same scale in the
plot). The default setting of the postprocessing programs show the deformedshape with an exaggerated deformation scale. Therefore, try to use a scale factor
of 1.0 in plotting the deformation (see the DEFPLOT (Results > PLOT >
Deformed Shape) command in COSMOSM Command Reference Manual).
If the gap elements still exceed the contact surface after issuing the above
commands, then the conclusion is that the gap elements are not properly closed.
The following two possibilities then should be considered:
a. The orientation of the gap surfaces might be wrong. The gap elements are
allowed to remain on the positive side of the surface.
b. The original displacement is too large that the displaced location of the gap
elements cannot be compared correctly with respect to the contact surfaces.
This case usually occurs when one of the bodies is an unconstrained structure
supported only by soft trusses. To investigate this possibility, the analyst
should calculate the displacements of the soft trusses under the loads. If the
resultant displacements are excessive and the gap elements are pushed far
beyond the contact surface, it is likely that the gap iterations also will not
converge. This situation can be overcome by decreasing the applied load step
through modifying the “time” curve, or by using stiffer trusses to support the
unattached portion of the model.
3. The program stops with the error message: “Stop, wrong definition for the
contact surfaces.”
Check the target surface connectivity and direction. Make sure that each target
contact surface is represented by continuous sub-surfaces.
4. In nonlinear dynamic problems, the program converges but the structure
behaves erratically after the gaps close.
This condition often occurs due to the assumed perfect rigidity of the closed
two-node gaps. To avoid that situation, some flexibility for the contact can be
introduced through the third real constant of the two-node gap elements.
5. The program completes one or more steps with some gap elements closed, but
finally stops with one of the following error messages:.
a. “Stop, the diagonal term in equation..., node ..., direction ... is zero or
negative."
b. “Stop, convergence not achieved for gap elements.”
c. “*** ERROR: Convergence is not achieved in 200 iterations” or
“Convergence is not achieved in 100 contact iterations.”
These errors basically imply difficulties in problem convergence due to:
– System stiffness has deteriorated and become singular or close to singular
due to other nonlinearities (geometric or material).
– The load increment is too large.
In either case, reducing the load increment is most likely to solve the problem.
However, in case I, if the stiffness has extremely deteriorated, a solution
continuation may not be possible.
✍ If friction forces are present, the analysis is nonconservative (dependent on the
load application sequence). Therefore, the loads must be applied in increments
which resemble the actual load history.
✍ Contact problems which involve large-deflection analysis are likely to require
mesh refinement in the regions where contact is expected. |
|
发表于 2007-10-17 14:46:59
QQ好友和群
QQ空间
腾讯微博
腾讯朋友
收藏
转播
教程|习题|模型|技巧
点赞
拍砖
鲁公网安备 37028502190335号