LS-DYNA（FAQ）中英文版-计算不稳定（Instability）

一些表示计算不稳定的消息如：
“out-of-range velocities” 速度超出范围
“negative volume in brick element” 体单元负体积
“termination due to mass increase” 因质量增加而终止

用来克服显式求解中的不稳定的方法如下：

首先(也是最重要的)是使用可获得的最新的LS-DYNA版本。最新的执行块可以从ftp://user@ftp.lstc.com上下载(注：前提是你有访问权限)。联系LSTC获得user帐号的密码。最新的BETA版执行块可以在ftp://ftp.lstc.com/outgoing/ls971上找到(不需要密码，但lstc公司对ftp访问有IP限制)。

其次是增加d3plot的输出频率到可以显示出不稳定的出现过程。这可以提供导致不稳定性发生的线索。

其它的不些解决数值不稳定性的技巧：

* 试着用双精度LS-DYNA版本运行一次

* 试着减小时间步(timestep)缩放系数(即使使用了质量缩放mass-scaling)

* 单元类型和/或沙漏(hourglass)控制。对出现不稳定的减缩体和壳单元，试着用沙漏控制type 4 和沙漏系数0.05

。或者试着用类型16的壳单元，沙漏控制type 8。如果壳响应主要是弹性，设置BWC=1 和 PROJ=1 (仅对B-T壳)。 避免使用type=2体单元。对体单元部件，在厚度方向最少用两个体单元。

* 接触。设置接触的bucket sorts之间周期数为0，这样会使用缺省的分类间隔。如果参与接触的两个部件的相对速度异常的大，可能需要减小bucket sort的间隔(比如减小到5,2甚至1)。

如果仿真过程中有明显的接触穿透出现，转换到使用*contact_automatic_surface_to_surface或者*contact_automatic_single_surface，并设置SOFT=1。 确保几何考虑了壳单元的厚度。如果壳非常薄，比如小于1mm，放大或者设置接触厚度到一个更加合理的值。

* 避免冗余的接触定义，也就是说不要对同样的两个部件定义多于一个的接触对。

* 查找出现不稳定的部件的材料定义中的错误(比如误输入，不一致的单位系统等)

* 关掉所有的*damping

这些技巧是一些通用的方法，可能并不适合于所有的情况。

See also: negative_volume_in_brick_element.tips，shooting-nodes

English Version:

Some messages that indicate an instability has occurred:

“out-of-range velocities”

“negative volume in brick element”

“termination due to mass increase”

Approaches to combating instability of an explicit solution:

First and foremost, use the latest version/revision of LS-DYNA available.The latest production executables can be downloaded from ftp://user@ftp.lstc.com.Contact LSTC for the password to this “user” ftp account.More recent BETA executables are found inftp://ftp.lstc.com/outgoing/ls971(no password required).

The next step is to write plot states frequently enough to see the evolution of the instability.   This should offer clues into what’s initiating the instability.

Some other general tips toward resolving numerical instabilities:

*   Try running a double precision LS-DYNA executable.

*   Timestep.   Try reducing the timestep scale factor (even if mass-scaling is invoked).

*   Element formulation and/or hourglass control.   For underintegrated solids or shells that go unstable, try hourglass type 4 with a hourglass coefficient of 0.05.    Or, try shell formulation 16 with hourglass type 8.   If response of shells is primarily elastic, set BWC=1 and PROJ=1 (B-T shells only).Avoid type 2 solids.   Use at least two solid elements thru the thickness of any solid part.

*   Contact.   Set number of cycles between bucket sorts to zero so that the default sort interval will be used.   If the relative velocity between two parts in contact is exceptionally high, it may be necessary to reduce the bucket sort interval (for instance to 5, 2, or even 1).

If visible contact penetrations develop during the simulation, switch to *contact_automatic_surface_to_surface or contact_automatic_single_surface with SOFT set to 1.   Make sure geometry takes into account thickness of shells.   If shells are VERY thin, e.g., less than 1 mm, scale up or set the contact thickness to a more reasonable value.

Avoid redundant contact definitions, that is, don’t treat contact between the same two parts using more than one contact definition.

*   Look for mistakes (typos, inconsistent units, etc.) in material input of parts that go unstable.

*   Turn off all *damping.

These tips are of a general nature and may not be appropriate in all situations.See also:   negative_volume_in_brick_element.tips, shooting-nodes.