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. NLGEOM,ON), activates large-strain effects supported element types. The large-strain feature is available in most of the solid elements (including all of the large strain elements), and in most shell and beam elements.
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.
In large-strain solutions, all stress-strain input and results are in terms of
true stress and true
(or logarithmic) strain. (In one
dimension, true strain would be expressed as 
). 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 
. To convert from engineering stress to true stress, use
. (This stress conversion is valid only for incompressible
plasticity stress-strain data.)
This feature is available in all beam and most shell elements, as well as in a number of the nonlinear elements. Issue the NLGEOM,ON command to activate large-deflection effects for those elements that are designed for small strain analysis types that support this feature.
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.
Even though stress stiffening theory assumes that an element's rotations and strains are small, in some structural systems (such as in Figure 8.8: Stress-Stiffened Beams (a)), the stiffening stress is only obtainable by performing a large deflection analysis. In other systems (such as in Figure 8.8: Stress-Stiffened Beams (b)), the stiffening stress is obtainable using small deflection, or linear, theory.
To use stress stiffening in the second category of systems, you must issue PSTRES,ON in your first load step.
Large-strain and large-deflection procedures include initial stress effects as a subset of their theory. For most elements, initial stiffness effects are automatically included when large-deformation effects are activated (NLGEOM,ON).