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本帖最后由 wdhd 于 2016-5-11 09:38 编辑
nastran中的平面应力单元是2D-Membrane shelll单元是平面应力和弯曲单元的组合，可以设置两者的组合
Can you give me a brief description of the difference in DOFs supported for the 2D shell, bending panel, and 2-D solid elements types? How do these property types relate to the MSC/NASTRAN property entries?
1) 2D SHELL
The 2D shell is the QUAD4/8 with stiffness in membrane and bending with transverse shear flexibility. There are 5 degrees of freedom per node: 3 translations and 2 rotations. The element does not have in-plane rotational stiffness unless PARAM,K6ROT is set greater than 0.0. The amount of in-plane rotational stiffness is proportional to the value of K6ROT. If your plate is in the XY plane and k6rot=0.0, then there will be stiffness in DOF 12345 (1=x, 2=y, 3=z, 4=rx, 5=ry, 6=rz).
The PSHELL/QUAD bulk data for the 2D shell looks like this:
$ Elements and Element Properties for region : 2d-shell
$pshell pid mid1 thick mid2 mid3
PSHELL 1 1 .1 1 1
CQUAD4 1 1 1 2 8 7
MAT1 1 1.+7 0.3
GRID 1 0. 0. 0.
GRID 2 .2 0. 0.
GRID 7 0. 1. 0.
GRID 8 .2 1. 0.
The Grid Point Singularity Table in the .F06 file will show that DOF 6 has no stiffness:
G R I D P O I N T S I N G U L A R I T Y T A B L E
POINT TYPE FAILED STIFFNESS OLD USET NEW USET
ID DIRECTION RATIO EXCLUSIVE UNION EXCLUSIVE UNION
1 G 6 0.00E+00 B F SB S *
2 G 6 0.00E+00 B F SB S *
7 G 6 0.00E+00 B F SB S *
8 G 6 0.00E+00 B F SB S *
(The asterisk at the end of each line means that DOF was internally constrained to avoid singularities)
2) 2D BENDING PANEL
The 2D bending panel is a subset of the 2D shell. It only has bending stiffness (MID2 on PSHELL). It does not have membrane stiffness or transverse shear flexibility.
The PSHELL/QUAD for the bending panel looks like this:
$ Elements and Element Properties for region : 2d-bending-panel
PSHELL 2 .1 1
CQUAD4 2 2 22 33 99 88
MAT1 1 1.+7 .3
GRID 22 .2 0. 0.
GRID 33 .4 0. 0.
GRID 88 .2 1. 0.
GRID 99 .4 1. 0.
If the bending panel is in the XY plane as in the above input, it will have transverse stiffness in the Z direction (DOF3), and rotational stiffness about the x and y axes (DOF 45). The grid point singularity output in the .F06 file shows that DOF 126 are constrained to avoid singularities.
G R I D P O I N T S I N G U L A R I T Y T A B L E
POINT TYPE FAILED STIFFNESS OLD USET NEW USET
ID DIRECTION RATIO EXCLUSIVE UNION EXCLUSIVE UNION
22 G 1 0.00E+00 B F SB S *
22 G 2 0.00E+00 B F SB S *
22 G 6 0.00E+00 B F SB S *
33 G 1 0.00E+00 B F SB S *
33 G 2 0.00E+00 B F SB S *
33 G 6 0.00E+00 B F SB S *
88 G 1 0.00E+00 B F SB S *
88 G 2 0.00E+00 B F SB S *
88 G 6 0.00E+00 B F SB S *
99 G 1 0.00E+00 B F SB S *
99 G 2 0.00E+00 B F SB S *
99 G 6 0.00E+00 B F SB S *
3) 2D solid Element
The 2D solid element in PATRAN writes out either a plane strain QUAD/PSHELL or an axisymmetric CTRIAX6 depending on the option selected in the properties menu. For the rest of this discussion, my comments pertain only to the plane strain option. You can look up the definition of plane strain in an elasticity text book, but basically, plane strain dictates that the strain in the z direction (normal to the element face) is zero all the way through the thickness. This type of analysis is useful for thick solids loaded on edge. For example, consider the "dam" below loaded on edge. We are looking at the cross-section and assume is very long (thick) such that Z strain is essentially zero.. Therefore we make the plane strain assumption and mesh with 2D solids. Patran does not prompt you for a thickness, and will insert a thickness of 1.0 on the PSHELL so that all edges loads are input in units of force per unit depth. Note that all loads for the 2D solid should be inplane edge loads.
-->_____
-->/ \
-->/ \ Y
-->/ \ ^
-->/ \ |
-->/ \ |
-->/_______________\ |----> X
The PSHELL/QUAD for the 2D solid looks like this: (the -1 in the MID2 field is the flag that tells MSC/NASTRAN to use a plane strain formulation.)
$ Elements and Element Properties for region : 2d-solid
PSHELL 3 1 1. -1
CQUAD4 3 3 333 444 101 999
MAT1 1 1.+7 .3
GRID 333 .4 0. 0.
GRID 444 .6 0. 0.
GRID 999 .4 1. 0.
GRID 101 .6 1. 0.
If the 2D solid is in the XY plane as shown above, it will have stiffness in the X and Y directions only. (There will be RZ stiffness if K6ROT >0.0)Examination
of the grid point singularity output below shows that DOF 3456 are constrained to avoid singularities.
G R I D P O I N T S I N G U L A R I T Y T A B L E
POINT TYPE FAILED STIFFNESS OLD USET NEW USET
ID DIRECTION RATIO EXCLUSIVE UNION EXCLUSIVE UNION
101 G 3 0.00E+00 B F SB S *
101 G 4 0.00E+00 B F SB S *
101 G 5 0.00E+00 B F SB S *
101 G 6 0.00E+00 B F SB S *
333 G 3 0.00E+00 B F SB S *
333 G 4 0.00E+00 B F SB S *
333 G 5 0.00E+00 B F SB S *
333 G 6 0.00E+00 B F SB S *
444 G 3 0.00E+00 B F SB S *
444 G 4 0.00E+00 B F SB S *
444 G 5 0.00E+00 B F SB S *
444 G 6 0.00E+00 B F SB S *
999 G 3 0.00E+00 B F SB S *
999 G 4 0.00E+00 B F SB S *
999 G 5 0.00E+00 B F SB S *
999 G 6 0.00E+00 B F SB S *
4) 2D-Membrane
The 2D membrane is similar to the 2D-solid except that Plane Stress instead of plane strain assumtions are made. (This is a BIG difference.) Patran writes
out a plane stress PSHELL/QUAD combinations with only membrane material properties on the PSHELL. This is suitable for thin flat surfaces with inplane
loads only. Transverse loads should only be applied to membrane elements if the element is supported on the edges by elements that can carry transverse loads (e.g. bars/beams). The PSHELL/QUAD data for the 2D-membrane looks like this:
$ Elements and Element Properties for region : 2d-membrane
PSHELL 4 1 .1
CQUAD4 4 4 4444 555 111 1011
MAT1 1 1.+7 .3
GRID 4444 .6 0. 0.
GRID 555 .8 0. 0.
GRID 1011 .6 1. 0.
GRID 111 .8 1. 0.
If the element is in the XY plane as shown above, it will only have stiffness in the X and Y directions.(There will be RZ stiffness if K6ROT >0.0) The grid point singularity output for this case shows that DOF 3456 are constrained to avoid singularities.
G R I D P O I N T S I N G U L A R I T Y T A B L E
POINT TYPE FAILED STIFFNESS OLD USET NEW USET
ID DIRECTION RATIO EXCLUSIVE UNION EXCLUSIVE UNION
111 G 3 0.00E+00 B F SB S *
111 G 4 0.00E+00 B F SB S *
111 G 5 0.00E+00 B F SB S *
111 G 6 0.00E+00 B F SB S *
555 G 3 0.00E+00 B F SB S *
555 G 4 0.00E+00 B F SB S *
555 G 5 0.00E+00 B F SB S *
555 G 6 0.00E+00 B F SB S *
1011 G 3 0.00E+00 B F SB S *
1011 G 4 0.00E+00 B F SB S *
1011 G 5 0.00E+00 B F SB S *
1011 G 6 0.00E+00 B F SB S *
4444 G 3 0.00E+00 B F SB S *
4444 G 4 0.00E+00 B F SB S *
4444 G 5 0.00E+00 B F SB S *
4444 G 6 0.00E+00 B F
SB S * |
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发表于 2006-11-23 21:59
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