Mechanical APDL Structural Analysis Guide

Figure 7.1: Buckling Curves

F

u

Snap-through

buckling

(a) (b)

Bifurcationpoint

Limitload(from

nonlinearbuckling)

F

u

(a) Nonlinear load-deflection curve, (b) Linear (Eigenvalue) buckling curve

7.2. Commands Used in a Buckling Analysis

You use the same set of commands to build a model and perform a buckling analysis that you use to
do any other type of finite element analysis. Likewise, you choose similar options from the graphical
user interface (GUI) to build and solve models no matter what type of analysis you are doing.

Sample Buckling Analysis (GUI Method) (p. 186) and Sample Buckling Analysis (Command or Batch
Method) (p. 191) show you how to perform an example eigenvalue buckling analysis via the GUI or via
commands, respectively. For detailed, alphabetized descriptions of the ANSYS commands, see the
Command Reference.

7.3. Performing a Nonlinear Buckling Analysis

A nonlinear buckling analysis is a static analysis with large deflection active (NLGEOM,ON), extended
to a point where the structure reaches its limit load or maximum load. Other nonlinearities such as
plasticity may be included in the analysis. The procedure for a static analysis is described in Structural
Static Analysis (p. 9), and nonlinearities are described in Nonlinear Structural Analysis (p. 193).

7.3.1. Applying Load Increments

The basic approach in a nonlinear buckling analysis is to constantly increment the applied loads until
the solution begins to diverge. Be sure to use a sufficiently fine load increment as your loads approach
the expected critical buckling load. If the load increment is too coarse, the buckling load predicted may
not be accurate. Turn on bisection and automatic time stepping (AUTOTS,ON) to help avoid this
problem.

7.3.2. Automatic Time Stepping

With automatic time stepping on, the program automatically seeks out the buckling load. If automatic
time stepping is ON in a static analysis having ramped loading and the solution does not converge at
a given load, the program bisects the load step increment and attempts a new solution at a smaller
load. In a buckling analysis, each such convergence failure is typically accompanied by a "negative pivot"
message indicating that the attempted load equals or exceeds the buckling load. You can usually ignore
these messages if the program successfully obtains a converged solution at the next, reduced load. The
program normally converges to the limiting load as the process of bisection and resolution continues

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Buckling Analysis