讨论单元类型对模态分析的影响
讨论单元类型对模态分析的影响计算一块薄钢板的振动模态及本征频率，这块平板的支撑方式如下：在板的四个角上各钻一个孔，
底边两个孔用螺丝和螺帽将其固定到两根竖直的刚性支柱上，因此简化建模时底边两个两孔周围一定距离的环形区域的自由度固定。但采用不同的单元类型时(shell63,plane42,solid45)，得到的结果相差很大，这究竟是怎么回事，我是百思不得其解，不知谁能帮忙看看问题出在哪里，不甚感激！！
其命令流和计算结果如下：
/PREP7
!**有限元单元类型**
ET,1,PLANE42 !
!若采用SHELL63,改为ET,1,SHELL63
R,1,0.002 ! 单元厚度为2mm
!**材料模型**
MP,DENS,1,7740 ! ASTM A240 Stainless Steel
MP,EX,1,200E9
MP,NUXY,1,0.32
!**几何模型**
BLC4,0,0,0.2,0.1 ! 200*100*2mm
CYL4,0.0275,0.0275,,,0.0035,360
CYL4,0.0275,0.0725,,,0.0035,360
CYL4,0.1725,0.0275,,,0.0035,360
CYL4,0.1725,0.0725,,,0.0035,360
ASEL,S,AREA,,1,5
ASBA,1,ALL
!**分网**
K,10001,0.0275,0.0275
K,10003,0.1725,0.0275
CIRCLE,10001,0.007,,,360,4 !生成螺母固定区域
CIRCLE,10003,0.007,,,360,4
L,5,21 !生关键点生成线
L,6,22
L,7,23
L,8,24
L,13,25
L,14,26
L,15,27
L,16,28
LESIZE,1,,,50
LESIZE,3,,,50
LESIZE,2,,,25
LESIZE,4,,,25
LSEL,S,LINE,,5,20
LESIZE,ALL,,,2
LSEL,S,LINE,,21,28
LESIZE,ALL,,,2
LSEL,S,LINE,,29,36
LESIZE,ALL,,,1
ALLSEL,ALL
TYPE,1
MAT,1
AMESH,6
ALLSEL,ALL
!**施加约束**
ASEL,S,AREA,,6
WPOFFS,0.0275,0.0275 ! 平移工作平面到约束圆面中心
CSWPLA,11,1 ! 在圆心处建一个局部柱坐标系
NSEL,S,LOC,X,0.0030,0.008
WPOFFS,0.145,0.0
CSWPLA,11,1
NSEL,A,LOC,X,0.0030,0.008
WPSTYL,DEFA
WPSTYL,,,,,,,,0 ! 不显示工作平面
D,ALL,UX,0
D,ALL,UY,0
FINISH!退出前处理模块
ALLSEL,ALL
!**模态求解**!
/SOLU
ANTYPE,MODAL ! Mode-Frequency analysis
MODOPT,LANB,10! Mode extraction method
MXPAND,10 ! Expanding modes
SOLVE !开始求解
FINISH !退出后模块
/POST1!进入后处理模块POST1
SET, 1, 1 !读入第一阶频率和振型
PLDISP,2 ! 在图形窗口显示结构变形
ANMODE,10,0.05!用10帧每隔0.05秒钟的动画显示振型
FINISH
我计算得到的结果
采用PLANE42时，为
SET TIME/FREQ LOAD STEP SUBSTEPCUMULATIVE
17381.6 1 1 1
27839.2 1 2 2
313093. 1 3 3
413826. 1 4 4
517919. 1 5 5
619230. 1 6 6
721562. 1 7 7
823368. 1 8 8
924671. 1 9 9
1025069. 1 10 10
而采用SHELL63单元时，结果
*****INDEX OF DATA SETS ON RESULTS FILE*****
SET TIME/FREQ LOAD STEP SUBSTEPCUMULATIVE
10.0000 1 1 1
2 0.41341E-03 1 2 2
3 0.90614E-03 1 3 3
4263.27 1 4 4
5323.38 1 5 5
6710.08 1 6 6
7728.62 1 7 7
81076.3 1 8 8
91234.6 1 9 9
101267.8 1 10 10
此外，我采用了solid45单元来计算，结果也相差很大，
SET TIME/FREQ LOAD STEP SUBSTEPCUMULATIVE
1263.88 1 1 1
2436.13 1 2 2
3521.20 1 3 3
4697.52 1 4 4
51021.0 1 5 5
61414.6 1 6 6
71542.7 1 7 7
81721.7 1 8 8
91981.6 1 9 9
102251.0 1 10 10
这究竟是怎么回事，我是百思不得其解，不知谁能帮忙看看问题出在哪里，不甚感激！！ PLANE42肯定不对，SHELL63应该是正确的，你用solid95再算一下。另外SHELL63的前3阶是刚体运动，还是没有加约束。
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