8.2.5. Nonlinear Transient Analyses
The procedure for analyzing nonlinear transient behavior is similar to that used for nonlinear static be-
havior: you apply the load in incremental steps, and the program performs equilibrium iterations at
each step. The main difference between the static and transient procedures is that time-integration effects
can be activated in the transient analysis. Thus, "time" always represents actual chronology in a transient
analysis. The automatic time stepping and bisection feature is also applicable for transient analyses.
8.3. Using Geometric Nonlinearities
Small deflection and small strain analyses assume that displacements are small enough that the resulting
stiffness changes are insignificant.
In contrast,large strain analyses account for the stiffness changes that result from changes in an element's
shape and orientation. By issuing NLGEOM,ON (GUI path Main Menu> Solution> Analysis Type> Sol'n
Control ( : Basic Tab) or Main Menu> Solution> Unabridged Menu> Analysis Type> Analysis Op-
tions), you activate large strain effects in those element types that support this feature. The large strain
feature is available in most of the solid elements (including all of the large strain elements), as well as
in most of the shell and beam elements. Large strain effects are not available in the ANSYS Professional
program; however, large deflection effects (NLGEOM command) are supported for shell and beam
elements in ANSYS Professional, if indicat ed as such in the Element Reference.
The large strain procedure places no theoretical limit on the total rotation or strain experienced by an
element. Certain element types are subject to practical limitations on total strain, as described below.
The procedure requires that strain increments be restricted to maintain accuracy, however, so the total
load should be broken into smaller steps.
8.3.1. Stress-Strain
In large strain solutions, all stress-strain input and results are in terms of true stress and true (or logar-
ithmic) strain. (In one dimension, true strain would be expressed as ε = ln ( ℓ/ ℓ 0 ). For small-strain regions
of response, true strain and engineering strain are essentially identical.) To convert strain from small
(engineering) strain to logarithmic strain, use εln = ln (1 + εeng). To convert from engineering stress to
true stress, use σtrue = σeng (1 + εeng). ( This stress conversion is valid only for incompressible plasticity
stress-strain data.)
8.3.1.1. Large Deflections with Small Strain
This feature is available in all beam and most shell elements, as well as in a number of the nonlinear
elements. Issue NLGEOM,ON (Main Menu> Solution> Analysis Type> Sol'n Control ( : Basic Tab) or
Main Menu> Solution> Unabridged Menu> Analysis Type> Analysis Options) to activate large de-
flection effects for those elements that are designed for small strain analysis types that support this
feature.
8.3.2. Stress Stiffening
The out-of-plane stiffness of a structure can be significantly affected by the state of in-plane stress in
that structure. This coupling between in-plane stress and transverse stiffness, known as stress stiffening,
is most pronounced in thin, highly stressed structures, such as cables or membranes. A drumhead,
which gains lateral stiffness as it is tightened, would be a common example of a stress-stiffened structure.
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