时间历程曲线

xiaowei331

我点了第一个图鼠标指定的地方list data  出现第二个图  我想问一下   为什么这个time 居然是负的  为什么啊  这个time是什么意思
是指荷载吗  还是指频率啊     （应该是荷载吧 ，指随着荷载的变化） 那为什么刚开始是负的呢

[ 本帖最后由 xiaowei331 于 2008-4-24 20:27 编辑 ]

xinyuguang

怎么都没人回答啊，这个是盲点点吗

Chelsea

你是不是用的弧长法？
弧长法不能应用 TIME 作为参照号，因为 TIME 在弧长分析中不总是单调增加的(即一个 TIME 值可能与多个解相对应)。
你可以采用荷载步和子步数( LSTEP 和 SBSTEP )来作为合适结果的参照，或用数据集号( NSET )。

Chelsea

转载一下别人对这个问题的回复

Arc-length method
1）This method can circumvent global instability when forces are applied. More importantly, it can simulate the negative slope portion of a load-vs.-displacement curve.

2）Avoid using the JCG solver (EQSLV) with the arc-length method. The arc-length procedure can result in a negative definite stiffness matrix (negative pivot), which can cause a solution failure with the solver.

3）The total arc-length load factor (SOLU,,ALLF) can be either positive or negative. Similarly, TIME, which in an arc-length analysis is related to the total arc-length load factor, can also be either positive or negative. Negative values of ALLF or TIME indicate that the arc-length feature is applying load in the reverse direction in order to maintain stability in the structure. Negative ALLF or TIME values are commonly seen in various snap-through analyses.

4）Do not reference results by a TIME value, because TIME in an arc-length analysis is not always monotonically increasing. (A single value of TIME might reference more than one solution.) Additionally, the program cannot correctly interpret negative TIME values (which might be encountered in a snap-through analysis).

5）If TIME becomes negative, define an appropriate variable range (/XRANGE or /YRANGE) before creating any POST26 graphs.

6） Nonlinear Stabilization vs. the Arc-Length Method
You can use nonlinear stabilization for both local and global instability with few limitations related to compatibility with other algorithms and materials. However, nonlinear stabilization cannot detect the negative-slope portion of a load-vs.-displacement curve problem with global instability (if any).

Although the results obtained before the negative slope portion of the problem are always correct, the results for the substeps after the negative-slope portion are also correct if the materials are not deformation-history-dependent. (Consider the results to be questionable if the materials are deformation-history-dependent.)

The arc-length method can detect the negative-slope portion of a load-vs.-displacement curve, but it cannot solve problems with local instability and material softening. Other limitations exist, related mostly to compatibility with certain algorithms and materials.