Mechanical APDL Structural Analysis Guide

The following input illustrat es a typical (temperature-independent) nonlinear axial behavior:

sectype,1,genb,plastic! Elasto-plastic response

bsax ,0.0008,200! Axial strain 0.0008, Force of 200

bsax ,0.001 ,240! Axial strain 0.001 , Force of 240

bsax ,0.0014,300! Axial Strain 0.0014, Force of 300

The range of strain values must cover the anticipated maximum deformation of the structure. For all
nonlinear beam section specifications, the stiffness disappears if the structural response is beyond the
maximum strain value of the section data. You can define a maximum of 20 generalized stress-strain
points at each temperature value, and the successive slopes must be smaller than the first slope. Iso-
tropic hardening is assumed for the material response.

For a plastic nonlinear beam section subtype (SECTYPE,,GENB,PLASTIC), you must define all section
components with two or more stress-strain points. However, you can define a linear behavior for any
section component by specifying a larger maximum strain value. Following is an example where axial
behavior is nonlinear, but bending response is linear:

sectype,1,genb,plastic! Elasto-plastic response

bsax,0.0008,200

bsax,0.001 ,240

bsax,0.0014,300

! bending

bsm1,0.1,10000

bsm1,1,100000

For an elastic nonlinear beam section subtype (SECTYPE,,GENB,ELASTIC), a single stress-strain point is
adequate for defining linear behavior.

15.4.2. Considerations for Using Nonlinear General Beam Sections

You can define nonlinear general beam sections only when using element BEAM188 or BEAM189. When
using nonlinear beam section data, the following conditions apply:

• The section data defined by each command listed in Table 15.2: Commands for Specifying Nonlinear
  General Beam Section Data (p. 444) is associated with the section most recently defined via the SECTYPE
  command.


• Beam stresses are not available for output; however, the stress resultants are available as ETABLE quant-
  ities.


• Section offsetting (SECOFFSET) is not available.


• General beam sections do not support linear perturbation.


• Only the temperature of the beam axis is relevant.

Beam section rotary inertia is calculated internally based on axial and bending stiffnesses provided.

If beam section rotary inertia of a more general form is available, use MASS21 elements (rather than

the BSMD command) to define beam section mass and rotary inertia at the beam nodes.

15.5. Using Preintegrated Composite Beam Sections

The behavior of beam elements is governed by the beam-section stiffness matrix and the section mass
matrix. A generalized nonlinear beam section (SECTYPE,,GENB) allows only diagonal terms in the cross-
section stiffness matrix defining the relationship between generalized stress and generalized strains; it
is therefore suitable only for beams with homogeneous and isotropic materials where the coupling
between different generalized strains is small. For composite beams made of dissimilar constituent

Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information

Using Preintegrated Composite Beam Sections