屈服准则、失效准则、硬化准则、速率
0)为什么讨论这些基本问题
有限元技术发展到今天，其算法基本上已经成熟。对从事有限元软件开发的⼈员⽽⾔，主要的⼯作就是根据新材料的发展不断补充各种材料模型，不断完善材料库，同时也不断完善单元库；⽽对有限元使⽤⼈员来说，主要的⼯作就是建⽴⼏何模型，选择合适的材料及单元，设置求解参数。选择单元及设置求解参数主要牵涉到有限元基本算法，通过集中的学习可以较快的掌握；⽽材料模型种类繁杂，有时候并不容易选择，有必要群策群⼒，共同学习。
我先抛砖引⽟，真诚希望⼤家能把这些⼯作做起来。为容易理解计，尽量避免使⽤特别专业的词汇。
(1)屈服
对许多延展性较好的材料（如⼤多数⾦属）⽽⾔，其弹性和⾮弹性⾏为⼀般⽤屈服强度（yield strength)这个标量来区分、界定。在ANSYS⾥，屈服点（yield point)和⽐例极限(proportional limit)被假定为是⼀致的。应⼒分量的组合千变万化，不可能对每个应⼒状态都指定屈服强度，屈服准则的作⽤就是将林林总总的多向应⼒转化为单向应⼒（屈服强度⼀般通过单向拉伸试验来测定，因为这个实验最简单。），然后将转化后的等效应⼒和屈服强度进⾏⽐较。在ANSYS⾥，主要有von Mis 和 Hilll（可以通过TB,HILL指定Hill Potential)两类准则。当然⼀些塑性模型有⾃⼰特殊的屈服准则，如Drucker-Prager 。
失效指材料失去承载能⼒或者不能满⾜规定的使⽤要求（如过⼤的变形等），对脆性材料，失效⼀般表现为断裂，对延展性材料，失效的表现形式可以是最后的断裂，或者是产⽣永久变形，或过⼤的变形等等。ANSYS6.0以后，FC系列命令可以⽤来为所有的单元指定失效准则，如最⼤主应⼒，最⼤主应变，蔡-吴准则等等，和TB,FAIL命令有些类似。对复合材料单元（如SHELL91/99，SOLID46/191)也可以⽤TB,FAIL指定失效；对混凝⼟（SOLID65)，可以使⽤TB,CONCR指定裂纹的产⽣条件。
对各向同性材料，可以指定最⼤主应⼒，最⼤主应变，最⼤剪应⼒，最⼤Mis应⼒为失效应⼒。对各项异性材料或正交各项异性材料，通常采⽤最⼤应⼒或最⼤应变准则。应⽤FC命令指定最⼤拉伸应⼒和最⼤压应⼒，如果不指定后者，其缺省值与前者相同。剪应⼒或剪应变也应通过FC输⼊。蔡-吴准则也是通过FC指定。对拉压强度不同的材料（如铸铁），可以通过TB,HILL或TB,ANISO指定失效条件。FC对应的菜单路径是Main Menu > General Postproc > Failure Criteria。
必须指出的是，在ANSYS⾥，失效单元并不从计算中删除，因此，FC命令位于后处理中，它只是被⽤来从设计者的⾓度来衡量已建⽴的模型。所以，最⼤应⼒（应变）可能可能⼤于失效条件中指定的应⼒（应变）。
-- 作者：paradiboy
实用版-- 发布时间：2004-8-7 17:14:44
--
为了将失效的单元去除，⼀般要应⽤单元的‘⽣死技术。当然在ls-dyna⾥就很简单了。
下⾯的例⼦是杀死总应变超过许⽤值的单元：
/SOLU !进⼊求解器
qq浏览器广告...
... !标准的求解过程
SOLVE
FINISH
!
/POST1 !进⼊POST1
SET,...
张辽文远
ETABLE,STRAIN,EPTO,EQV !将总应变存⼊ETABLE
ESEL,S,ETAB,STRAIN,0.20 !选择所有总应变⼤于或等于0.20的单元
FINISH
!
/SOLU !重新进⼊求解器
ANTYPE,,REST
EKILL,ALL !杀死选择（超过允许值）的单元
ESEL,ALL !读⼊所有单元
目光所及皆是你...
... !继续求解
体重秤怎么看请参阅ETABLE,ESEL,ANTYPE和EKILL命令得到更详细的解释。
等向强化：弹性区域等向扩⼤（如右上图所⽰）
随动强化：弹性区域等⼤
宝鸡小吃Isotropic hardening can be ud for large-strain analys of metals (> 5-10% true strain).
Isotropic hardening is not meant for cyclic loading applications becau it does not account for the Bauschinger effect. Moreover, applicability of isotropic hardening for non-proportional loading is
left up to the ur, although, generally speaking, it is meant for proportional loading only.2
On the other hand, kinematic hardening is usually meant for non-proportional, cyclic loading
瑟庄妮皮肤since the Bauschinger effect is approximated with this model. However, it is generally meant for
small-strain applications.
Combined hardening (and Chaboche nonlinear kinematic hardening), though not discusd in
detail in this memo, can be utilized to model complex, large-strain cyclic behavior such as cyclic
hardening/softening and rachetting/shakedown.
-- 作者：paradiboy
-- 发布时间：2004-8-7 17:17:32
--
The bilinear kinematic hardening model (BKIN) usually cannot reprent large-strain effects well becau of the constant tangent modulus. The true stress-strain slope of most metals usually changes as the strains increa, but the bilinear model fails to account for this due to its simple
大米发糕配方
reprentation. This means that the yield surface can translate forever in principal stress space, even allowing for the unrealistic possibility of passing through the origin.
There are two multilinear kinematic hardening models available in ANSYS, namely MKIN and KINH. Both models u the sublayer model, which can be thought of as a weighted respon of multiple elasto-perfectly-plastic ‘layers.’ A simplified view of this is that, as a layer ‘yields,’ it becomes perfectly plastic, so it provides no stiffness respon; this allows for the modeling of a piecewi linear curve. The author recommends using KINH over MKIN due to the following reasons:
" KINH allows up to 20 points per stress-strain curve, whereas MKIN only allows up to 5 points.
" For KINH, input is done via TBPT commands, which is more consistent with other piecewilinear models such as MISO and MELAS, but MKIN relies on TBDATA input.
" KINH allows up to 40 temperature-dependent curves, whereas MKIN allows only 5 temperature-dependent curves. Furthermore, in the ca of temperature-dependent curves,MKIN requires each curve to have the same strain values, whereas KINH does not.KINH is the same as MKIN with TBOPT=2, or u of Rice’s model for temperature-dependency.
As a result, KINH behaves the same as MKIN (TBOPT=2), so, due to the reasons mentioned above, the ur should consider using KINH.
