Material coordinate system
identification number. The x-axis of this coordinate system is projected onto
the element to define the x-axis of the material coordinate system.
Offset from the surface of
grid points to the element reference plane. 6 Overrides the ZOFFS
specified on the PSHELL entry.
Default = 0.0
(Real, Character Input =
TOP/BOTTOM, or
blank)
Comments
Grid points G1 through
G8 must be numbered as shown:Figure 1.
The element coordinate system is a
Cartesian system defined locally for each point (, ).Figure 2. It is based on the following rules:
The plane containing Xelem and Yelem is tangent to
the surface of the element.
Xelem and Yelem are obtained by doubly bisecting
the lines of constant ξ and η.
Xelem increases in the general direction of increasing
and Yelem of .
Figure 3.
The orientation of the material coordinate
system is defined locally at each interior integration point by
THETA, which is the angle from the line of constant η
(essentially the same as the -axis) to the material x-direction (Xmaterial).
If
MCID is used in place of THETA,
the local material x-direction (Xmaterial) is obtained at any
point in the element by projection of the x-axis of the
MCID coordinate system onto the surface of the
element at this point. The local z-direction is aligned with the normal to
the surface and the material y-direction (Ymaterial) is
constructed accordingly to produce right-handed local material system
X-Y-Zmaterial.Figure 4.
Note: Since Xelem changes directions throughout the element
based on element shape, the material coordinate system varies similarly.
Because of this, an orthotropic or anisotropic material will cause the
CQUAD8's stiffness to be biased by both its shape
and grid ordering. Use the CQUAD4 element if a
constant material coordinate system direction is desired with
orthotropic and anisotropic materials.
T1,
T2, T3, and T4 are
optional. If they are not supplied, the element thickness will be set equal to
the value of T on the PSHELL entry. If 0.0
is specified for Ti, then the thickness at that node is zero.
If Ti's are supplied, PID cannot reference
PCOMP or PCOMPP data. If the property
referenced by PID is selected as a region for Size
(parameter) optimization, then any Ti values defined here are
ignored. If you input Ti for elements in the design space for
Topology or Free-Size (parameter) optimization, the run will error out.
It is required that the mid-side grid
points be located within the middle third of the edge; that is the interval
(0.25, 0.75) excluding the quarter points 0.25 and 0.75. If the edge point is
located at the quarter point, the program may fail with an error or the
calculated stresses will be meaningless.
The shell reference plane can be offset from the
plane defined by element nodes by means of ZOFFS. In this case
all other information, such as material matrices or fiber locations for the
calculation of stresses, is given relative to the offset reference plane. Similarly,
shell results, such as shell element forces, are output on the offset reference
plane.
ZOFFS can be input in two
different formats:
Real
A positive or a negative value of ZOFFS is specified
in this format. A positive value of ZOFFS implies
that the reference plane of each shell element is offset a distance of
ZOFFS along the positive z-axis of its element
coordinate system.
Surface
This format allows you to select either "Top" or
"Bottom" option to specify the offset value.
Top
The top surface of the shell element and the plane defined
by the element nodes are coincident.
This makes the effective "Real" ZOFFS
value equal to half of the thickness of the
PSHELL property entry referenced by
this element. (The sign of the ZOFFS
value would depend on the direction of the offset relative
to the positive z-axis of the element coordinate system, as
defined in the Real section).Figure 5. Top option in ZOFFS
Bottom
The bottom surface of the shell element and the plane
defined by the element nodes are coincident.
This makes the effective "Real" ZOFFS
value equal to half of the thickness of the
PSHELL property entry referenced by
this element. (The sign of the ZOFFS
value would depend on the direction of the offset relative
to the positive z-axis of the element coordinate system, as
defined in the Real section).Figure 6. Bottom option in ZOFFS
Note: When ZOFFS is used, both MID1 and
MID2 must be specified on the PSHELL
entry referenced by this element (otherwise, singular matrices would
result).
Offset is applied to all element matrices (stiffness, mass, and
geometric stiffness), and to respective element loads (such as gravity). Hence,
ZOFFS can be used in all types of analysis and optimization.
Automatic offset control is available in composite free-size and sizing (parameter)
optimization where the specified offset values are automatically updated based on
thickness changes.
However, while offset is correctly applied in
geometric stiffness matrix and hence can be used in linear buckling analysis,
caution is advised in interpreting the results. Without offset, a typical simple
structure will bifurcate and loose stability "instantly" at the critical load. With
offset, though, the loss of stability is gradual and asymptotically reaches a limit
load.Figure 7. Therefore, the structure with offset can reach excessive deformation before
the limit load is reached. The above illustrations apply to linear buckling - in a
fully nonlinear limit load simulation, additional instability points may be present
on the load path.
Stresses and strains are output in the
local coordinate system identified by Xelem and Yelem
above.
Size optimization of the property
referenced by PID is not possible if Ti values
are defined here. If the property referenced by PID is selected
as a region for free-size optimization, then any Ti values
defined here are ignored.
These 2nd order shell elements
do not have normal rotational degrees-of-freedom (often referred to as "drilling
stiffness"). No mass is associated with these degrees-of-freedom. If unconstrained,
massless mechanisms may occur. It is therefore advisable to use PARAM,AUTOSPC,YES
when working with these elements.
String based labels allow for easier
visual identification of properties, when being referenced by the elements
cards. For more details, refer to String Label Based Input File in the
Bulk Data Input File.
This card is represented as a quad8
element in HyperMesh.