Grid point identification
numbers of connection points.
No default (Integers > 0, all
unique)
Theta
Material orientation angle in
degrees.
Default = 0.0
(Real)
MCID
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.
= 0
Specifies the basic coordinate system.
MCID must be an integer ≥ 0
Offset from the plane
defined by element grid points to the shell reference plane. 7 Overrides the ZOFFS
specified on the PSHELL entry.
Default = 0.0
(Real, Character Input =
TOP/BOTTOM, or
blank)
Ti
Thickness of the element
at the grid points. Overrides the thickness specified on the
PSHELL entry. The values of
Ti specified here will be directly used in
the solution.
PARAM,SHELLTI,NO can be used to
switch to using the average value of T1,
T2, and T3 as the
shell thickness.
The x-axis of the element coordinate system
is aligned with side 1-2 of the shell element. Figure 1. Element Coordinate System Figure 2. Forces in CTRIA3 Element Figure 3. Moments in CTRIA3 Element
For H3D and OUTPUT2 output formats,
stresses and strains are always output in the elemental system.
For HM, PUNCH, and OPTI output formats,
stresses and strains are output by default in the material coordinate system. PARAM,OMID can be set to
NO to output results in the elemental system. For elements with
blank Theta/MCID, the material coordinate
system is aligned with elemental coordinate system. For elements with
THETA, the material x-axis is rotated from side
G1-G2 by angle THETA.
For elements with assigned MCID, the material system is
constructed by projecting the MCID onto the plane of the
element. Figure 4. Orientation when Theta (real value) is Entered in 8th Field Figure 5. Orientation when MCID (integer value) is Entered in 8th Field
If any of the Ti fields
are blank, the thickness specified on the PSHELL data will be
used for that node's thickness. If 0.0 is specified for Ti, the
thickness at that node is zero.
If the property referenced by
PID is selected as a region for free-size or size
optimization, then any Ti values defined here are ignored. If you
input Ti for elements in the design space for Topology or
Free-Size optimization, the run will error out.
If Ti is present, the
PID cannot reference PCOMP or PCOMPP data.
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 6. 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 7. 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.
Note: For first order shell elements (CQUAD4 and
CTRIA3), the offset operation does not correct for
secondary effects, such as change of shell area when offset is applied on curved
surfaces. Therefore, the value of ZOFFS should be kept within
a reasonable percentage (10% - 15%) of the local radius of curvature.
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 8.
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.
PHFSHL properties are
only valid with an @HYPERFORM statement in the
first line of the input file.
The CTRIA3 element
utilizes 3 integration points at the standard Gauss point locations.
