1、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 A system of units 单位制度 22.Mass Scaling 质量缩放 33.Long run times 长分析时间 84.Quasi-static 准静态 105.Instability 计算不稳定 .136.Negative Volume 负体积 .167.Energy balance 能量平衡 198.Hou
2、rglass control 沙漏控制 269.Damping 阻尼 .3110.ASCII output for MPP via binout .3611.Contact Overview 接触概述 4012.Contact Soft 1 接触 Soft=14413.LS-DYNA 中夹层板 (sandwich)的模拟 .4614. 怎样进行二次开发 .491.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 第一个时间步之后的”added mass” glstat 和mat
3、sum 中的”added mass”。2. 当 DT2MS 为负值且*mat_spotweld 卡 DT0 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 高速冲击 type 2 或者 3。* 对泡沫用四面体(tetrahedral)单元来建模，使用类型 10 体单元。* 增加 DAMP 参数(foam model 57) 到最大的推荐值 0.5。* 对包含泡沫的接
4、触，用*contact 选项卡 B 来关掉 shooting node logic。* 使用*contact_interior 卡用 part set 来定义需要用 contact_interior 来处理的 parts，在 set_part 卡 1 的第 5 项 DA4 来定义 contact_interior 类型。缺省类型是 1，推荐用于单一的压缩。在版本 970 里，类型 1 的体单元可以设置 type=2，这样可以处理压缩和减切混合的模式。* 如果用 mat_126，尝试 ELFORM=0* 尝试用 EFG 方程(*section_solid_EFG) 。因为这个方程非常费时，所以只
5、用在变形严重的地方，而且只用于六面体单元。See also: instabilityEnglish Version:Negative Volumes in Foams (or other soft materials)In materials that undergo extremely large deformations, such as soft foams, an element may become so distorted that the volume of the element is calculated as negative. This may occur without
6、 the material reaching a failure criterion. There is an inherent limit to how much deformation a Lagrangian mesh can accommodate without some sort of mesh smoothing or remeshing taking place. A negative volume calculation in LS-DYNA will cause the calculation to terminate unless ERODE in *control_ti
7、mestep is set to 1 and DTMIN in *control_termination is set to any nonzero value in which case the offending element is deleted and the calculation continues (in most cases). Even with ERODE and DTMIN set as described, a negative volume may cause an error termination (see erode/negvol.k). Some appro
8、aches that can help to overcome negative volumes include the following.- Simply stiffen up the material stress-strain curve at large strains. This approach can be quite effective.- Sometimes tailoring the initial mesh to accommodate a particular deformation field will prevent formation of negative v
9、olumes. Again, negative volumes are generally only an issue for very severe deformation problems and typically occur only in soft materials like foam.- Reduce the time step scale factor. The default of 0.9 may not be sufficient to prevent numerical instabilities.- Avoid fully-integrated solids (form
10、ulations 2 and 3) which tend to be less stable in situations involving large deformation or distortion. (The fully integrated element is less robust than a 1-point element when deformation is large because a negative Jacobian can occur at one of the integration points while the element as a whole ma
11、intains a positive volume. The calculation with fully integrated element will therefore terminate with a negative Jacobian much sooner than will a 1-point element. (lpb)- Use the default element formulation (1 point solid) with type 4 or 5 hourglass control (will stiffen response). Preferred hourgla
12、ss formulations for foams are: type 6 with coef. = 1.0 if low velocity impact types 2 or 3 if high velcocity impact- Model the foam with tetrahedral elements using solid element formulation 10 (see pdf/dubois-foam-tets.pdf).- Increase the DAMP parameter (foam model 57) to the maximum recommended val
13、ue of 0.5.- Use optional card B of *contact to turn shooting node logic off for contacts involving foam.- Use *contact_interior. A part set defines the parts to be treated by contact_interior. Attribute 4 (DA4 = 5th field of Card 1) of the part set defines the TYPE of contact_interior used. The defa
14、ult TYPE is 1 which is recommended for uniform compression. In version 970, solid formulation 1 elements can be assigned TYPE=2 which treats combined modes of shear and compression.- If mat_126 is used, try ELFORM = 0.- Try EFG formulation (*section_solid_EFG). Use only where deformations are severe
15、 as this formulation is very expensive. Use only with hex elements.*See also: instablity.tips7.Energy balance 能量平衡GLSTAT(参见 *database_glstat)文件中报告的总能量是下面几种能量的和：内能 internal energy动能 kinetic energy接触(滑移 )能 contact(sliding) energy沙漏能 houglass energy系统阻尼能 system damping energy刚性墙能量 rigidwall energyGLSTA
16、T 中报告的弹簧阻尼能”Spring and damper energy”是离散单元(discrete elements)、安全带单元(seatbelt elements)内能及和铰链刚度相关的内能(*constrained_joint_stiffness)之和。而内能”Internal Energy”包含弹簧阻尼能”Spring and damper energy”和所有其它单元的内能。 因此弹簧阻尼能”Spring and damper energy”是内能”Internal energy”的子集。由 SMP 5434a 版输出到 glstat 文件中的铰链内能 ”joint intern
17、al energy”跟*constrained_joing_stiffness 不相关。它似乎与*constrained_joint_revolute(_spherical,etc)的罚值刚度相关连。这是 SMP 5434a 之前版本都存在的缺失的能量项，对 MPP 5434a 也一样。这种现象在用拉格朗日乘子(Lagrange Multiplier)方程时不会出现。与*constrained_joint_stiffness 相关的能量出现在 jntforc 文件中，也包含在glstat 文件中的弹簧和阻尼能和内能中。回想弹簧阻尼能”spring and damper energy”，不管是从
18、铰链刚度还是从离散单元而来，总是包含在内能里面。在 MATSUM 文件中能量值是按一个 part 一个 part 的输出的(参见*database_matsum)。沙漏能 Hourglass energy 仅当在卡片 *control_energy 中设置 HGEN 项为 2 时才计算和输出。同样，刚性墙能和阻尼能仅当上面的卡片中 RWEN 和 RYLEN分别设置为 2 时才会计算和输出。刚性阻尼能集中到内能里面。质量阻尼能以单独的行”system damping energy”出现。由于壳的体积粘性(bulk viscosity)而产生的能量耗散(energy dissipated)在版本
19、970.4748 之前是不计算的。在后续子版本中，设置 TYPE=-2 来在能量平衡中包含它。最理想的情况下能量平衡：总能量 total energy 初始总能量 外力功 external work换句话说，如果能量比率 energy ratio(指的是 glstat 中的 total energy/initial energy，实际上是 total energy/(initial energy + external work) 等于 1.0。注意，质量缩放而增加质量可能会导致能量比率增加。注意在 LSprepost 的 HistoryGlobal energies 中不包含删掉的单元(ero
20、ded elements)的能量贡献，然而 GLSTAT 文件中的能量包含了它们。注意它们的贡献可以通过 ASCIIglstat 中的”Eroded Kinetic Energy”先试着用一个小的值，比如 0.01。如果在*control_energy 中设置 RYLEN=2，因为刚性阻尼而能会计算且包含在内能中。正的接触能：当在接触定义中考虑了摩擦时将得到正的接触能。摩擦将导致正的接触能。如果没有设置接触阻尼和接触摩擦系数，你将会看到净接触能为零或者一个很小的值(净接触能从边和主边能量和)。 所说的小是根据判断在没有接触摩擦系数时，接触能为峰值内能的 10%内可以被认为是可接受的。负的接触能
21、：突然增加的负接触能可能是由于未检测到的初始穿透造成的。在定义初始几何时考虑壳的厚度偏置通常是最有效的减小负接触能的步骤。查阅LS-DYNA 理论手册的 23.8.3 其二，在大变形应用时更不稳定(更容易出现负体积);其三，类型 2 体单元当单元形状比较差时在一些应用中会趋向于剪切锁死(shear-lock)，因而表现得过于刚硬。三角形壳和四面体单元没有沙漏模式，但缺点是在许多应用中被认为过于刚硬。减小沙漏的一个好的方法是细化网格，但这当然并不总是现实的。加载方式会影响沙漏程度。施加压力载荷优于在单点上加载，因为后者更容易激起沙漏模式。为了评估沙漏能，在*control_energy 卡片中设
22、置 HGEN2，而且用*database_glstat 和*database_matsum 卡分别输出系统和每一个部件的沙漏能。这一点是要确认非物理的沙漏能相对于每一个 part 的峰值内能要小(经验上来说Mischourglass energy。对于流体部件，缺省的沙漏系数通常是不合适的(太高)。因此对于流体，沙漏系数通常要缩小一到两个数量级。对流体用基于粘性的沙漏控制。缺省的沙漏方程(type 1)对流体通常是可以的。对于结构部件一般来说基于刚性的沙漏控制(type 4,5)比粘性沙漏控制更有效。通常，当使用刚性沙漏控制时，习惯于减小沙漏系数到 0.030.05 的范围，这样最小化非物理的
23、硬化响应同时又有效抑制沙漏模式。对于高速冲击，即使对于固体结构部件，推荐采用基于粘性的沙漏控制(type 1,2,3)。粘性沙漏控制仅仅是抑制沙漏模式的进一步发展，刚性沙漏控制将使单元朝未变形的方向变形。类型 8 沙漏控制仅用于单元类型 16 的壳。这种沙漏类型激活了 16 号壳的翘曲刚度，因此单元的翘曲不会使解退化。如果使用沙漏控制 8，16 号壳单元可以用于解被称为扭曲梁(Twisted Beam)问题。对于单元类型 1 的体和减缩积分 2D 体(shell types 13 & 15)类型 6 沙漏控制调用了一种假设应变协同转动方程。使用沙漏控制类型 6 和系数 1.0，一个弹性部件在厚
24、度方向仅仅需要划分一层类型 1 的体单元就可以获得正确的弯曲刚度。在隐式计算里面，对于类型 1 的体单元应该总是使用类型 6 的沙漏控制(实际上，在 V970 里面这是自动设置的)。(More on type 6 HG control from Lee Bindeman)类型 6 的沙漏控制与类型 4,5 不在于它用了一个假设应变场和材料属性来估算出假设应力场。这个应力在单元封闭域内进行积分得到沙漏力，因此单元表现的像一个有同样假设应变场的全积分单元。这种假设应变场设计成用来阻止纯弯曲中不真实的剪切变形和近似不可压材料中的体积锁死。类型 4 和 5 的沙漏控制基于单元体积，波速和密度像在 LS
25、-DYNA 理论手册中方程 3.21 那样来计算沙漏刚度。沙漏类型 6 主要的改进是应力场在单元域内积分。这使得当使用大的长细比或者歪斜形状的体单元时沙漏控制非常鲁棒。类型 4 和 5 的沙漏控制对大长细比和歪斜形状单元反应变不好，它趋向于对某些沙漏模式反应的过于刚硬而对其它模式反应得过弱。沙漏控制类型 6 另一个理论上的优点是对在厚度方向只有一个单元的梁可以在弹性弯曲问题中得到准确的解。要做到这一点，设置沙漏刚度参数为 1.0。同样，对弹性材料方形截面杆的扭曲问题，当沙漏系数设为 1.0 时可以用很少的单元来解。然而，对于非线性材料，用粗糙的网格得到好的结果是不可能的，因为应力场不是像沙漏类
26、型 6 假设的那样线性变化的。在梁厚度方向上如果没有更多积分点的话，没有办法捕获应力场的非线性状态。对于选择沙漏控制，下面几个问题要考虑。对于单元有大的长细比或者明显歪斜( 不管是初始还是变形过程中)，推荐采用类型 6 的沙漏控制。类型 6 的沙漏控制通常对软的材料更好，像泡沫或蜂窝材料在计算中会有非常明显的变形。在材料不是特别软或者单元有合理的形状且网格不是太粗糙时，类型 4,5和 6 沙漏控制似乎都能得到同样的结果。这种情况推荐用类型 4 的沙漏控制，因为它比其它的更快。类型 6 的沙漏控制在 LS-DYNA Users Manual 中参考的 Belytschko 和Bindeman 的
27、论文中有更详细的描述。English Version:Hourglass (HG) modes are nonphysical, zero-energy modes of deformation that produce zero strain and no stress. Hourglass modes occur only in under-intetgrated (single integration point) solid, shell, and thick shell elements. LS-DYNA has various algorithms for inhibiting h
28、ourglass modes. The default algorithm (type 1), while the cheapest, is generally not the most effective algorithm.A way to entirely eliminate hourglass concerns is to switch to element formulations with fully-integrated or selectively reduced (S/R) integration. There can be a downside to this approa
29、ch. For example, Type 2 solids are much more expensive than the single point default solid. Secondly, they are much more unstable in large deformation applications (negative volumes much more likely). Third, type 2 solids have some tendency to shear-lock and thus behave too stiffly in applications w
30、here the element shape is poor.Triangular shells and tetrahedral solid elements do not have hourglassing modes but have drawbacks with regard to overly stiff behavior in many applications.A good way to reduce hourglassing is to refine your mesh but, of course, that isnt always practical.The method o
31、f loading can affect the degree of hourglassing. A pressure loading is preferred over loading individual nodes as the latter approach is more likely to excite hourglassing modes.To evaluate hourglass energy, set HGEN to 2 in *control_energy and use *database_glstat and *database_matsum to report the
32、 HG energy for the system and for each part, respectively. The point is to confirm that the nonphysical HG energyis small relative to peak internal energy for each part ( Misc hourglass energy. For fluid parts, the default HG coefficient is generally inappropriate (too high). Thus for fluids, the ho
33、urglass coefficient should generally be scaled back one or two orders of magnitude. Use only viscosity-based HG control for fluids. The default HG formulation (type 1) is generally ok for fluids.Stiffness-based HG control (types 4,5) is generally more effective than viscous HG control for structural
34、 parts. Usually, when stiffness-based HG control is invoked, I like to reduce the HG coefficient, usually in the range of .03 to .05, soas to minimize nonphysical stiffening of the response and at the same time effectively inhibiting hourglass modes. For high velocity impacts, viscosity-based HG con
35、trol (types 1,2,3) is recommended even for solid/structural parts. “Viscous HG control only stops the HG mode from developing further. Stiffness HG control will push the element back toward its undeformed configuration. CAUTION: Stiffness hourglass control tends to be overly stiff!” (Paul Dubois)Typ
36、e 8 HG control applies only to shell formulation 16. This HG type activates warping stiffness in type 16 shells so that warping of the element does not degrade the solution. Type 16 shells will solve the so-called Twisted Beam problem correctly if HG type 8 is invoked. Type 6 HG control invokes an a
37、ssumed-strain co-rotational formulation for type 1 solid elements and under-integrated 2D solids (shell types 13 and 15). With the HG type set to 6 and the hourglass coefficient set to 1.0, an elastic part need only be modeled with a single type 1 solid through its thickness to achieve the exact ben
38、ding stiffness. Type 6 HG control should always be used for type 1 solids in implicit simulations (in fact, this is done automatically in v. 970). (More on type 6 HG control from Lee Bindeman) “Type 6 hourglass control for solid elements differs from types 4 and 5 because is uses an assumed strain field and material properties to evaluate an assumed stress field. This stress is integrated in
