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发表于 2009-4-18 17:07:27
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来自 重庆沙坪坝区
本帖最后由 apprent 于 2009-4-18 17:55 编辑
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$ Y* w# `7 p, u9 Z8 }对于——
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3. 取节点或单元的应力6 ]# E* f. Q9 g& W- N; |
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来自《MSC.Fatigue User’s Guide 》的解释:
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Results Location: ! N' V7 b* M6 T1 Q+ ?# Z
This parameter tells MSC.Fatigue whether to expect Nodal stress/strain results or Elemental centroid stress/strain results.
+ c1 ?* M5 P7 _' `$ G6 ]7 PThis dictates whether the user is setting up a global multi-node or global multi-element fatigue analysis.
+ }+ K6 h2 N+ r& T0 q. FSubsequent parameters, results file types, and results displays are dependent on whether nodal or elemental data is being considered. 5 Z- E( w. _. Y$ q3 j% P7 O, F3 B @
If nodal data is being considered, the resulting fatigue lives are reported at the nodes.
* ^7 ?0 }; k0 w( |# XConversely, if elemental data is being considered, the fatigue lives are reported at the element centroids. , ]3 L' _7 M1 U9 _, L& K- h, R
Fatigue cracks invariably occur at free surfaces, and hence when a crack initiation method is used, node points results are usually required. 3 W4 p: x, N$ f. ]
The exception is when a shell model is used, element centroid results may be extrapolated to the top or bottom surface.
?$ {" }) i9 }$ J; gThis is useful when there may be some doubt as to the accuracy of the node point results due to extrapolation and/or nodal averaging practices. ) T2 P, |3 Y3 Z: ~7 r
The spot weld analyzer uses forces and moments from both nodes and elements.
' X& x8 g; u' N0 yThe SEAM-weld analyzer takes stresses from both the top and bottom surface and needs both nodes and elements for this.1 X D5 i" c- ?0 P1 d% q0 R: W7 L
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翻译:
0 }; A6 C) J" q6 L" U/ j这个参数告诉MSC.Fatigue去提取节点的应力应变结果还是单元中心的应力应变结果。+ j( O; g& A- [, |
这决定了用户将要进行全局多节点还是全局多单元的疲劳分析。
+ `: G! @) R8 P7 L( S H! o其后的设置参数——结果文件类型和结果显示,将依赖于上述选择——使用节点还是单元的数据。
3 E/ ]+ f+ D) ~. h3 n) O如果使用节点数据,则疲劳结果将被呈现在节点上。. I* i8 n# v3 {* `3 J/ g) @; i9 X7 i
相反地,如果使用单元数据,则疲劳结果将被呈现在单元中心上。
' N0 d) R% x/ {- h疲劳断裂总是发生在自由表面,因此当进行“裂纹扩展”分析时,通常要求使用节点的数据。
# z" Z/ g9 [1 O+ h6 t& @0 b例外的情况是,当使用壳单元模型时,单元中心的结果可能要被外插到顶层和底层表面。* T* k" I7 V1 H
此时这个选项便发挥作用了,因为外插和/或节点平均操作会给节点结果的精确性带来疑问。7 p# a; ^3 ?! ^; `0 `4 P" `
点焊分析要使用节点和单元两者的力合力矩数据。
$ M- G$ _# C& N, d0 E缝焊分析要使用节点和单元两者的从顶面到底面的应力数据。( @/ V. b1 s6 A5 f8 j2 ]
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Nodal Averaging:
! K j; l4 y. x/ |. MDepending on how the finite element results are defined, nodal averaging of the stresses or strains may take place. - ^- [: x4 u ?2 W: K* I2 K" i- |
If grid point stresses exist and are selected, no averaging will occur. 9 m8 I+ m. Q9 j& s* N! \7 R5 k) N: n
However if the stresses or strains selected for the fatigue analysis are elemental based, such as results at integration points or elemental nodal values such that each element has a different value at the shared grid points, then nodal averaging will occur. ' F. w) B+ s8 E! I- V
This averaging is done on a global basis such that every contributing element surrounding a particular node will be used in the averaging. 2 H$ J* _6 X8 b
The exception to this is if Group is selected in which case only elements in the Current Group will be used in the averaging. " }1 I7 y* i3 R1 C. y
For the SEAM-weld analyzer only the current group can be used and no choice will be given.
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9 ^* n7 N2 e: H' r1 y, `8 ]2 J翻译:
/ G( l6 o4 t0 Z% H$ b4 h9 T根据有限元结果的定义方式(使用节点还是单元数据),应力或应变的节点平均操作可能被采用。
4 ~& Q( M* r5 j9 N5 X如果节点的应力存在并且被选择使用,那么节点平均就不会发生(不需要了)。- P# H0 M! o8 l; K
然而如果基于单元的(比如积分点上值的或各单元在公共节点上有不同值的节点值)应力或应变被选中来进行疲劳分析,那么就需要进行节点平均操作。/ g" k- n7 e& z1 @/ B% A
节点平均操作是全局性的,也就是说,所有环绕着某一节点并对其有贡献的单元(在该节点的值)都将参与平均操作。2 @8 B3 t% }, p1 Y/ X2 R
如果Group被选中,那么只有在当前组中的单元参与平均操作。
& K. l6 a8 a1 q- m& q7 d( [对于缝焊分析,只有当前组选项,因此这个选项变为不可改动。
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) W8 e/ S; v8 @我的模型中使用的单元的结果,节点平均选择了Group,因为我给模型用了“表面覆壳”,表面加了一层0.0001mm厚的壳单元,设为一个单独的组,疲劳计算时只针对这一个组的壳单元。% l/ j. F! G& V- K# M, C; X( q
因为:" H7 u! g6 h* n# I4 o4 ?9 \
1. 疲劳破坏/最低寿命一般都是发生在结构表面;5 H9 ^ Z/ k# H, T6 O4 c
2. 使用壳单元提取出的应力都是结构表层的,而用实体单元的话,表面节点的应力是从表层下的积分点外插上来的,相对而言,使用壳单元提取的应力更精确;
+ W, H$ K6 g; C9 f$ _3. 显然,只计算壳单元一个组,能减少计算量;" y: S2 E1 h; _1 R, B
4. MSC.Fatigue手册中还给出一个理由,由于理论知识的不足,我不是非常明白:The shell element stresses are normally recovered in element coordinate systems which have the advantage of being in the plane of the surface. This makes multiaxial assessment easier.(壳单元的应力坐标系有一个优点:在结构表面的平面上。这个特点使多轴评估更容易。)! d7 S9 j! C1 E
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我想知道:
' M4 H% C6 Q, b; |# g上面用颜色标出来的 global multi-node fatigue analysis or global multi-element fatigue analysis 是什么意思?
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, Z' x* E2 A7 g, C; \resulting fatigue lives are reported at the nodes 跟 fatigue lives are reported at the element centroids 又有什么区别?(这个知道了,见下图——)1 `) W) ~0 A$ Q% {! d9 x& |) N! h: q
5 H/ ~4 I7 N' f8 ?& ]' a, J9 W刚刚做了两个例子试了试,发现基于我的FEM应力结果,选单元和选节点,疲劳计算结果差别不大(如上图),云图分布趋势一致,而且用单元计算的,云图更加圆滑。 |
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