Page 48 - Using ANSYS for Finite Element Analysis Dynamic, Probabilistic, Design and Heat Transfer Analysis

composite materials • 35
• If you don’t want the failure stress or strain to be checked in a par-
ticular direction, specify a large number in that direction (as shown
in the previous example).

User-written failure criteria may be specified via user subroutines
USRFC1 through USRFC6. These subroutines should be linked with the
ANSYS program beforehand; see the ANSYS Advanced Analysis Tech-
niques Guide for a brief description of user-programmable features.

2.2.4 ADDiTionAL MoDeLing AnD PoSTPRoCeSSing
gUiDeLineS

Some additional guidelines for modeling and postprocessing of composite
elements are presented in the following list.

1. Composites exhibit several types of coupling effects, such as cou-
pling between bending and twisting, coupling between extension
and bending, and so on. This is due to stacking of layers of differing
material properties. As a result, if the layer stacking sequence is not
symmetric, you may not be able to use model symmetry even if the
geometry and loading are symmetric, because the displacements
and stresses may not be symmetric.
2. Interlaminar shear stresses are usually important at the free edges of
a model. For relatively accurate interlaminar shear stresses at these
locations, the element size at the boundaries of the model should
be approximately equal to the total laminate thickness. For shells,
increasing the number of layers per actual material layer does not
necessarily improve the accuracy of interlaminar shear stresses.
With elements SOLID46, SOLID95, and SOLID191, however,
stacking elements in the thickness direction should result in more
accurate interlaminar stresses through the thickness. Interlaminar
transverse shear stresses in shell elements are based on the assump-
tion that no shear is carried at the top and bottom surfaces of the
element. These interlaminar shear stresses are only computed in the
interior and are not valid along the shell element boundaries. Use of
shell-to-solid submodeling is recommended to accurately compute
all of the free-edge interlaminar stresses.
3. Since a large amount of input data is required for composites, you
should verify the data before proceeding with the solution. Several
commands are available for this purpose:
• ELIST (Utility Menu> List> Elements) lists the nodes and
attributes of all selected elements.