一些表示計(jì)算不穩(wěn)定的消息如：
“out-of-range velocities” 速度超出范圍
“negative volume in brick element” 體單元負(fù)體積
“termination due to mass increase” 因質(zhì)量增加而終止

用來(lái)克服顯式求解中的不穩(wěn)定的方法如下：

首先(也是最重要的)是使用可獲得的最新的LS-DYNA版本。最新的執(zhí)行塊可以從ftp://user@ftp.lstc.com上下載(注：前提是你有訪問權(quán)限)。聯(lián)系LSTC獲得user帳號(hào)的密碼。最新的BETA版執(zhí)行塊可以在ftp://ftp.lstc.com/outgoing/ls971上找到(不需要密碼，但lstc公司對(duì)ftp訪問有IP限制)。

其次是增加d3plot的輸出頻率到可以顯示出不穩(wěn)定的出現(xiàn)過程。這可以提供導(dǎo)致不穩(wěn)定性發(fā)生的線索。

其它的不些解決數(shù)值不穩(wěn)定性的技巧：

* 試著用雙精度LS-DYNA版本運(yùn)行一次

* 試著減小時(shí)間步(timestep)縮放系數(shù)(即使使用了質(zhì)量縮放mass-scaling)

* 單元類型和/或沙漏(hourglass)控制。對(duì)出現(xiàn)不穩(wěn)定的減縮體和殼單元，試著用沙漏控制type 4 和沙漏系數(shù)0.05

。或者試著用類型16的殼單元，沙漏控制type 8。如果殼響應(yīng)主要是彈性，設(shè)置BWC=1 和 PROJ=1 (僅對(duì)B-T殼)。 避免使用type=2體單元。對(duì)體單元部件，在厚度方向最少用兩個(gè)體單元。

* 接觸。設(shè)置接觸的bucket sorts之間周期數(shù)為0，這樣會(huì)使用缺省的分類間隔。如果參與接觸的兩個(gè)部件的相對(duì)速度異常的大，可能需要減小bucket sort的間隔(比如減小到5,2甚至1)。

如果仿真過程中有明顯的接觸穿透出現(xiàn)，轉(zhuǎn)換到使用*contact_automatic_surface_to_surface或者*contact_automatic_single_surface，并設(shè)置SOFT=1。 確保幾何考慮了殼單元的厚度。如果殼非常薄，比如小于1mm，放大或者設(shè)置接觸厚度到一個(gè)更加合理的值。

* 避免冗余的接觸定義，也就是說(shuō)不要對(duì)同樣的兩個(gè)部件定義多于一個(gè)的接觸對(duì)。

* 查找出現(xiàn)不穩(wěn)定的部件的材料定義中的錯(cuò)誤(比如誤輸入，不一致的單位系統(tǒng)等)

* 關(guān)掉所有的*damping

這些技巧是一些通用的方法，可能并不適合于所有的情況。

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.