LS-DYNA常见问题汇总10

LS-DYNA常见问题汇总1.0资料来源：网络和自己的总结yuminhust2005Copyright of original English version owned by relative author. Chinese version owned by /Kevin目录1.Consistent system of units 单位制度 (2)2.Mass Scaling 质量缩放 (4)3.Long run times 长分析时间 (9)4.Quasi-static 准静态 (11)5.Instability 计算不稳定 (14)6.Negative Volume 负体积 (17)7.Energy balance 能量平衡 (20)8.Hourglass control 沙漏控制 (27)9.Damping 阻尼 (32)10.ASCII output for MPP via binout (37)11.Contact Overview 接触概述 (41)12.Contact Soft 1 接触Soft=1 (45)13.LS-DYNA中夹层板(sandwich)的模拟 (47)14. 怎样进行二次开发 (50)1.Consistent system of units 单位制度相信做仿真分析的人第一个需要明确的就是一致单位系统(Consistent Units)。

计算机只认识0&1、只懂得玩数字，它才不管你用的数字的物理意义。

而工程师自己负责单位制的统一，否则计算出来的结果没有意义，不幸的是大多数老师在教有限元数值计算时似乎没有提到这一点。

见下面LS-DYNA FAQ中的定义：Definition of a consistent system of units (required for LS-DYNA):1 force unit = 1 mass unit * 1 acceleration unit1 力单位＝1 质量单位× 1 加速度单位1 acceleration unit = 1 length unit / (1 time unit)^21 加速度单位= 1 长度单位/1 时间单位的平方The following table provides examples of consistent systems of units.As points of reference, the mass density and Young‘s Modulus of steel are provided in each system of units. ―GRA VITY‖ is gravitational acceleration.2.Mass Scaling 质量缩放质量缩放指的是通过增加非物理的质量到结构上从而获得大的显式时间步的技术。

在一个动态分析中，任何时候增加非物理的质量来增大时间步将会影响计算结果（因为F=ma）。

有时候这种影响不明显，在这种情况下增加非物理的质量是无可非议的。

比如额外的质量只增加到不是关键区域的很少的小单元上或者准静态的分析（速度很小，动能相对峰值内能非常小）。

总的来说，是由分析者来判断质量缩放的影响。

你可能有必要做另一个减小或消除了质量缩放的分析来估计质量增加对结果的灵敏度。

你可以通过人工有选择的增加一个部件的材料密度来实现质量缩放。

这种手动质量缩放的方法是独立于通过设置*Control_timestep卡DT2MS项来实现的自动质量缩放。

当DT2MS设置为一个负值时，质量只是增加到时间步小于TSSFAC*|DT2MS|的单元上。

通过增加这些单元的质量，它们的时间达到TSSFAC*|DT2MS|。

有无数种TSSFAC和DT2MS的组合可以得到同样的乘积，因而有相同的时间步，但是对于每一种组合增加的质量将是不一样的。

一般的趋势是TSSFAC越小，增加的质量越多。

作为回报，当TSSFAC减小时计算稳定性增加（就像在没有做质量缩放的求解中一样）。

如果TSSFAC缺省的值0.9会导致稳定性问题，可以试试0.8或者0.7。

如果你减小TSSFAC，你可以相应增加|DT2MS|，这样还是可以保证时间步乘积不变。

为了确定什么时候和位置质量自动增加了，可以输出GLSTAT和MATSUM 文件。

这些文件允许你绘出完整的模型或者单独部件所增加的质量对时间的曲线。

为了得到由壳单元组成的部件增加的质量云图，将*database_extent_binary 卡的STSSZ项设置为3。

这样你可以用ls-prepost绘出每个单元的质量增加量的云图，具体方法是通过选择Fcomp>Misc>time step size。

在*control_timestep中设置DT2MS正值和负值的不同之处如下：负值：初始时间步将不会小于TSSFAC*-DT2MS。

质量只是增加到时间步小于TSSFAC*|DT2MS|的单元上。

当质量缩放可接受时，推荐用这种方法。

用这种方法时质量增量是有限的。

过多的增加质量会导致计算任务终止。

正值：初始时间将不会小于DT2MS。

单元质量会增加或者减小以保证每一个单元的时间步都一样。

这种方法尽管不会因为过多增加质量而导致计算终止，但更难以作出合理的解释。

*control_timestep卡中的参数MS1ST控制是否只是在初始化时增加一次质量（MS1ST=1）还是任何需要维持由DT2MS所指定的时间步时都增加质量（MS1ST=0)。

你可以通过在*control_termination卡片中设置参数ENDMAS来控制当质量增加到初始质量一定比率时终止计算（只对自动质量缩放有效）－－－－－－－－－－－－－－－－－－－－－－－－－－可变形点焊梁的质量缩放*mat_spotweld卡的质量缩放参数DT只影响点焊单元。

如果*control_timestep 卡中没有指定质量缩放(DT2MS=0)，而且时间由可变形点焊控制，可以用参数DT来在初始化时增加惯量到点焊单元上来提高时间步达到DT指定的值。

当DT 不为0时，增加到可变形点焊梁元上的质量会输出到d3hsp文件里。

MATSUM 中动量和动能不受增加到可变形点焊上的质量的影响。

GSLTAT中DOES和总的KE受增加的质量的影响。

考虑三种调用可变形点焊的质量缩放的情况：1.当DT2MS为负值*mat_spotweld卡DT＝0时，尽管在d3hsp文件中可变形点焊质量增量百分比不真实。

下面几个值是正确的：d3hsp中‖added spotweld mass‖;第一个时间步之后的‖added mass‖& ―percentage increase‖;glstat和matsum中的‖added mass‖。

2. 当DT2MS为负值且*mat_spotweld卡DT≠0时，可变形点焊质量增加不会包含在d3hsp、glstat、matsum文件中的‖added mass‖里。

这非常容易令人误解。

用户必须检查d3hsp文件的‖added spotweld mass‖。

建议不要同时使用两种质量缩放标准，推荐使用第一种方法（即负的DT2MS&DT=0）。

3. 如果DT2MS＝0且DT≠0，初始时间步将不考虑增加点焊的质量，但是之后每一个周期时间步都会增加10%，直到时间步达到正确的值（考虑点焊质量增加）。

glstat & matsum不包含‖added mass‖的行。

注意质量增加会引起能量比率增长。

English Version：Mass-scaling refers to a technique whereby nonphysical mass is added to a structure in order to achieve a larger explicit timestep.Anytime you add nonphysical mass to increase the timestep in a dynamic analysis, you affect the results (think of F = ma). Sometimes the effect is insignificant and in those cases adding nonphysical mass is justifiable. Examples of such cases may include the addition of mass to just a few small elements in a noncritical area or quasi-static simulations where the velocity is low and the kinetic energy is very small relative to the peak internal energy. In the end, it‘s up to the judgement of the analyst to gage the affect of mass scaling. You may have to reduce or eliminate mass scaling in a second run to gage the sensitivity of the results to the amount of mass added.One can employ mass scaling in a selective manner by artificially increasing material density of the parts you want to mass-scale. This manual form of mass scaling is done independently of the automatic mass scaling invoked with DT2MS in *control_timestep.When DT2MS is input as a negative value, mass is added only to those elements whose timestep would otherwise be less than TSSFAC * |DT2MS|. By adding mass to these elements, their timestep becomes equal to TSSFAC * |DT2MS|. An infinite number of combinations of TSSF and DT2MS will give the same product and thus the same timestep but the added mass will be different for each of those combinations. The trend is that the smaller the TSSF, the greater the added mass. In return, stability may improve as TSSF is reduced (just as in non-mass-scaled solutions). If stability is a problem with the default TSSF of 0.9, try 0.8 or 0.7. If you reduce TSSF, you can increase |DT2MS| proportionally so that the product/timestep is unchanged.To determine where and when mass is automatically added, write GLSTAT and MATSUM files. These files will allow you to plot added mass vs. time for the complete model and for individual parts, respectively. To produce fringe plots of added mass in parts comprised of shell elements (DT2MS negative), set STSSZ=3 in *database_extent_binary. You can then fringe the added mass (per element) using LS-POST by choosing Fcomp > Misc > time step size. (Here, the label ―time stepsize‖ is really the element added mass.)The difference between using a positive or negative number for DT2MS in *control_timestep is as follows:Negative: Initial time step will not be less than TSSF * -DT2MS. Mass is added to only those elements whose timestep would otherwise be less than TSSF*abs(DT2MS). When mass scaling is appropriate, I recommend this method. The amount of mass that can be added using this method is limited. ‗Excessive‘ added mass will cause the job to terminate.Positive: Initial time step will not be less than DT2MS. Mass is added OR TAKEN AWAY from elements so that the timestep of every element is the same. This method is harder to rationalize although it is not subject to termination from ‗excessive‘ added mass.The parameter MS1ST in *control_timestep controls whether mass is added only once during initialization (MS1ST=1) or anytime as necessary to maintain the desired timestep specified via DT2MS (MS1ST=0).You can use ENDMAS in *control_termination to stop the calculation after a certain amount of mass has been added (active for automatic mass scaling only)._____________________________________________________________________ Mass-scaling of deformable spotweld beams:The mass-scaling parameter in *mat_spotweld (DT) affects only the spotwelds. If no mass-scaling is invoked in *control_timestep (DT2MS=0.) AND the timestep is controlled by the deformable spotwelds, DT can be used to add inertia to the spotwelds during intialization in order to increase the timestep to a value of DT. When DT is nonzero, mass added to spotweld beams is reported to d3hsp. MATSUM momentum and KE does NOT factor in added mass to def. spotwelds. GLSTAT DOES factor in added mass to total KE (spotweld.beam.type9.mscale.initvel.k) Consider 3 cases of invoking mass-scaling in a model with deformable spotwelds:1.Although ―percentage mass increase‖ under ―Deformable Spotwelds:‖ in d3hsp is bogus when DT2MS is neg. and DT in *mat_spotweld = 0, the following arecorrect:―added spotweld mass‖ in d3hsp―added mass‖ and ―percentage increase‖ in d3hsp AFTER the first time step―added mass‖ in glstat and matsum2. Added spotweld mass controlled by DT in *mat_spotweld is NOT INCLUDED in ―added mass‖ given in d3hsp, glstat, or matsum when DT2MS is neg. and DT in *mat_spotweld is nonzero. This can be quite misleading. User must check for ―added spotweld mass‖in d3hsp. Recommended: Do not invoke both mass-scaling criteria. Neg. DT2MS with DT=0 (case 1 above) is preferred.3. If DT is nonzero and DT2MS=0, the initial timestep will NOT consider added spotweld mass but the time step will increase by 10% each cycle until the correct timestep (considering added spotweld mass) is achieved. Glstat and matsum contain no ―added mass‖ line item.The above can be illustrated using/j5000a_2/jday/test/weld/spotweld.beam.type9.mscale.k._____________________________________________________________________ Note that added mass may cause the energy ratio to rise.(See /j5000a_2/jday/test/erode/taylor.mat3.noerode.mscale.k)3.Long run times 长分析时间当用显式时间积分时，对于仿真非常小的部件而分析时间又要相当长时没有好的方法。