求高手：ABAQUS开启和不开启几何非线性Nlgeom在计算时什么公式不一样？

吃草的鸟

ABAQUS在显式分析时，默认开启几何非线性，隐式分析则需要自主选择，开启和不开启几何非线性Nlgeom在理论上主要体现在哪的公式不一样？非线性应变，还是其他的地方？

gdczzz

这个问题我也想知道

吴聊SP

有兴趣的话，可以看看我的帖子，呵呵
http://forum.simwe.com/thread-1075510-1-1.html

UVAShawn

变形梯度F在没打开非线性的时候始终是单位张量，打开了以后才有值进去吧……？
比较明显的是，大变形和小变形情况下应变的计算公式变了。

sunnygu

http://ansys.net/?mycat=tnt_sheldon6
ANSYS

Stress-stiffening, large deflection, large strain, consistent tangent matrix
          Q: Can you explain the differences between large strain, large deflection, stress-stiffening, and consistent tangent matrix?

A: Below is an explanation for each term.

    Large strain (or finite strain) - The shape change of the elements need to be taken into account (i.e., strains are finite). Rigid-body effects (e.g., large rotation) are also taken into account. An example is metal yielding. Note that ANSYS uses the term "large strain", though I prefer the term "finite strain." Finite strain implies that the strains are not infinitesimal, but a finite amount. (Sometimes users think that "large strain" is a lot of strain, but really, it's any case where there is a finite amount, not an excessively large amount, of strain.)

    Large deflection (or large rotation) - The strains are assumed to be small, but rigid-body effects (e.g., large rotation) are taken into account. An example is a long, slender fishing rod; when it bends due to the fish, each segment of the rod may not strain, but the total deformation may be large. Basically, this allows the actual strain to be "weeded out" from the displacements; it separates displacements due to rigid-body motion and those associated with the small strains.

    Stress stiffening (only) - Strains and rigid-body motions are assumed to be small, and stiffening (or softening) of the structure due to the stress state is takening into account. Stress stiffening is needed for structures which are thin in one (or two) dimensions (e.g., bending stiffness of beams and shells small compared with in-plane/transverse stiffness) since a coupling between the two occurs (examples - membrane of a drum or a guitar string). It is also needed when doing other types of analyses like buckling or modal where the stress state affects the response of the system. In ANSYS, stress stiffening terms are constant, so be aware of this assumption (since in prestress modal or eigenvalue buckling, analyses are linear too). Stress stiffening may also be known as geometric stiffness matrix, differential stiffness matrix, stability coeff matrix, initial stress stiffness matrix, incremental stiff matrix, etc. Note that the commands SSTIF and PSTRES essentially do the same thing, but are used in different situations (PSTRES is used to request that a stress-stiffening matrix be written for use in a future eigenvalue buckling or pre-stressed modal analysis). For the 18x family of elements, stress-stiffening is not available independent of large deflection. For other elements, the decision to include stress-stiffening with large deflection is generally based on ease of convergence, since large deflection and stress-stiffening are redundant.

    Consistent Tangent Stiffness Matrix (CTS Matrix, for short) - Matrix used in nonlinear problems which is comprised of the "main tangent stiffness matrix" (the "regular" stiffness matrix we think of, especially in linear analyses), the "initial displacement matrix" (accounts for shape changes in elements), the "initial stress matrix" (the stress stiffening matrix; stiffness due to stress state), and the "initial load matrix" (stiffness associated with change in follower force loads during deformation -- pressure load stiffness for elements 154/181/188/189). The 181/188/189 elements always use a fully consistent tangent stiffness matrix. The rest of the 18x family of elements does include a stress stiffening matrix, but not necessarily a fully CTS matrix.

This should clarify the differences between stress stiffening and large deformation (or finite strain). Basically, you can think of them in terms of a hierarchy:

    finite strain (or large strain)
    large deformation (or large rotation)
    stress stiffening (there's also spin softening, which is not discussed here)
    regular linear analysis

So (1) encompasses (2)-(4), (2) encompasses (3)-(4), etc. This may be a bit of an oversimplification, but it may help to think of it in these terms. When SSTIF is not ON with NLGEOM,ON, it just leads to slower convergence; but that effect is included in the overall response/results (again, thin beams and shells may be exceptions).

Note that, if you use NLGEOM,ON, SSTIF,ON will automatically be activated (at least for v5.5 and above). The defaults should be left alone (i.e., you do not need to manually activate SSTIF,ON). SSTIF,ON helps aid convergence, especially for thin beams and shells. Although it has no effect for the 18x elements, as noted above, it's just good practice to leave the defaults (i.e, default is SSTIF,ON when NLGEOM is ON).

吃草的鸟

sunnygu 发表于 2013-4-24 04:40
http://ansys.net/?mycat=tnt_sheldon6
ANSYS

谢谢版主！

侠梦瘪瘪

学习了。。。。。。