ANSYS中如何解决非线性不收敛的问题？
2015-11-01 09:00阅读：
ANSYS中如何解决非线性不收敛的问题？
一、问题描述
3D
1/2对称竖直水泥土加劲抗拔桩，带三块分散板的钢绞线埋于水泥土桩体内，钢绞线分自由段和锚固段。建立了钢绞线杆轴，没有建立分散板，假定其刚度无穷大，其作用是通过与相邻的杆轴节点耦合实现的。没有考虑初始地应力，同时作用了竖向重力和上拔位移。
当材料全部为弹性时，计算很容易收敛，但将土体材料改为弹塑性材料DP时，连第一步都不能收敛，弹性材料和弹塑性材料的收敛历史如下图所示。
如何解决非线性不收敛的问题？
弹性材料
弹塑性材料
二、基本理论
1、8.11.2. Overcoming Convergence Problems
This section provides information to help you fix convergence
problems in a nonlinear analysis. The following topics are
available:
Overview of Convergence Problems
Performing Nonlinear Diagnostics
Tracking Convergence Graphically
Automatic Time Stepping
Line Search
Nonlinear Stabilization
Arc-Length Method
Artificially Inhibit Divergence in Your Model's Response
Use the Rezoning Feature
Dispense with Extra Element Shapes
Using Element Birth and Death Wisely
Read Your Output
Graph the Load and Response History
2、
Overview of Convergence Problems
Some examples may be initially open contact
surfaces causing rigid body motion, large load increments causing
nonconvergence, material instabilities, or large deformations
causing mesh distortion that result in element shape
errors.
CHECK,
MCHECK,
and CNCHECK
commands help you
verify if there are any obvious problems with the model before you
start the solution.
When you analyze models
with large
deformations, some portions of the initial mesh can become highly
distorted. Highly distorted elements can take on unacceptable
shapes, providing inaccurate results. This can cause your nonlinear
solution to stop. When this happens, use the ESCHECK command to perform shape checking of
deformed elements in the postprocessor
(based on the current set of results in database). This
deformed-shape checker helps you to identify the portions of your
model that require different meshing, thereby allowing them to
retain acceptable shapes. Using ESCHECK
at different time
points helps you to identify the load conditions that cause mesh
deterioration.
A convergence failure can also indicate
a physical instability in the
structure, or it can merely be the result of some numerical problem in the finite element
model.
3、Performing Nonlinear
Diagnostics
The nonlinear diagnostics
tool NLDIAG
can help you find
problems in your model when an analysis does not
converge.
Identify
Regions of High Residual Forces
Issue the NLDIAG,NRRE
command to write the Newton-Raphson residuals from equilibrium
iterations to a file
(Jobname.nrxxx).
You can then contour plot the residual
forces via the PLNSOL,NRRE
command, which helps to identify regions of high residual
forces.
Such a
capability can be useful when you experience convergence
difficulties in the middle of a load step, where the model has a
large number of contact surfaces and other nonlinearities. You can
restart the analysis and issue an
NLDIAG,NRRE
command to write out the residuals. By tracking the way the residuals change over several
equilibrium iterations you can identify a portion of your
model where large residuals persist. You can then focus on the
nonlinearities in that area (for example, a contact pair's
properties) instead of having to deal with the entire
model.
Process the
Tracked Results Issue the NLDPOST
command
to process the .ndxxx
nonlinear
diagnostics files. The command creates
components of elements that violate a certain criterion for a
particular equilibrium iteration (or
iterations).
Monitor the Diagnostics Results in Real Time
The NLHIST
command allows you
to monitor results of interest in real time during solution. Before starting the
solution, you can request
nodal data
such as displacements or reaction forces at specific nodes. You can
also request element nodal data such
as stresses and strains at specific elements to be graphed. Pair-based contact data are also
available. The result data are written to a
file named Jobname.nlh.
For example, a reaction
force-deflection curve could indicate when possible buckling
behavior occurs. Nodal results and contact results are monitored at
every converged substep while element nodal data are written as
specified via the OUTRES
setting.
4、Automatic
Time Stepping（静态的塑性问题可以不选此项）
This can be important in the following
situations:
- Problems that have only localized dynamic behavior
(for e
