機械專業(yè)外文翻譯--學習運用proengineer幾何模型建立有限元模型的過程

1、<p>　　畢業(yè)設計(論文)外文資料翻譯</p><p>　　系 部： 機械工程系 </p><p>　　專 業(yè)： 機械工程及自動化 </p><p>　　姓 名： </p><p>　　學 號：

2、 </p><p>　　外文出處： Kistler B L, Technical Report[R]. </p><p>　　SAND-97-8239,1997. </p><p>　　附 件： 1.外文資料翻譯譯文；2.外文原文。 </p><p>　　注：請將該封面與附件裝訂成冊。

3、</p><p>　　附件1：外文資料翻譯譯文</p><p>　　學習運用Pro / ENGINEER</p><p>　　幾何模型建立有限元模型的過程</p><p><b>　　摘 要</b></p><p>　　建立Pro/ENGINEER允許結構一體化模型的方法和生成熱網格和無需重新幾何

4、圖形計算的分析軟件。本學習的目的不是要深入學習Pro/ENGINEER的力學或者生成網格或者分析軟件，而是首次嘗試對將產生有益的分析模型的時間比分析師需要創(chuàng)建一個單獨的模型的時間更短的桑迪亞職員提供建議。該研究評價了運用Pro/ENGINEER建立各種各樣的幾何形狀和對設計師、繪圖員、分析師提供一般建議。</p><p><b>　　致 謝</b></p><p>　

5、　繪圖員Mark Mickelsen和Dennis Fritts直接支持這項研究；設計師Dave Neustel以及有限元分析師Hal Radloff Mike kanouff和Bruce Kistler。 此外，Arlo Ames的見解和運用Pro/ENGINEER 的能力是非常寶貴的。</p><p><b>　　引 言</b></p><p>　　有限元分析系統(tǒng)

6、的執(zhí)行過程或者組成部分一般分為四個步驟：1)定義幾何形狀，2)創(chuàng)建幾何形狀網格，3)應用網格的性能和邊界及載荷條件，和4)執(zhí)行有限元計算，和5) 檢驗分析結果。本研究檢驗前兩步驟之間的關系，本研究的原因是電子設計（幾何圖形）定義的能力在過去十年取得了巨大的增長，像用Pro / ENGINEER這種電腦軟件一樣，[1] 現在能夠經常定義實體幾何。這些電子數據庫不但可以創(chuàng)建傳統(tǒng)的制造目的藍圖，而且還可以對計算機制造工藝和有限元分析傳輸電子

7、信息。可能的話 ，這種電子信息的傳輸可以節(jié)省分析師在1和2 兩個步驟的大量時間。</p><p>　　此外，在過去十年中生成網格代碼也有著顯著的改善。許多不同的代碼，現在有著在一般表面上自動生成殼網的能力。有些人具有（或接近了）將一般網格狀實體要么自動生成四面體或六面體單元的能力。</p><p>　　由于事情正在發(fā)生如此之快的變化以至于分析師和繪圖員在如何最好地運用這些新工具上可能沒有經

8、驗。這項研究以了解一些用當今的電子工具提高創(chuàng)建有限元模型過程和使用電子設計定義作為輸入的分析機制。自桑迪亞已選擇的Pro/ENGINEER作為其設計標準來定義計算機程序，審查Pro/ENGINEER某些細節(jié)。</p><p>　　為了了解詳情和本研究不同階段的重大意義，我們相信，讀者需要對幾個領域有一個基本的了解。這些領域包括簡短的有限元分析過程背景，生成網格能力（包括目前的問題領域）的目前狀況，不同Pro/EN

9、GINEER功能的說明和它們如何運用于有限元分析過程中，和了解這些技術變革可能怎樣影響繪圖員，分析師，以及制造商在設計工作之間的相互關系，鼓舞讀者去深入學習這些章節(jié)的介紹。目的是為了了解主要研究這樣做的意義。</p><p><b>　　背 景</b></p><p>　　在過去對系統(tǒng)或組件進行有限元分析是有難度的，需要給出一個關于結構或熱方面效應的正確估算或者預測，

10、為了進行分析，分析師需要畫出或者以電子文檔的形式建立幾何模型，并提供相關的材料屬性和負載條件。分析師利用這些信息，并對問題進行必要的假設后建立一個有限元模型。這個模型能夠在一定的時間內得出一個近似的求解。因此，分析求解時間的長短是第三個需要考慮的因素。</p><p>　　通常，幾何圖形信息是以圖紙的形式提供給分析師的。分析師需要根據經驗對一些細節(jié)（如螺栓孔，切斷槽等）進行取舍，以便保證分析結果的近似準確度。然后

11、，分析師將初始的幾何圖形以相對簡單的形式重建。對幾何圖形的重建和在此基礎上建立有限元模型的過程將耗費分析師80%的時間和精力。</p><p>　　隨著實體模型設計軟件，如Pro/ENGINEER，和更加強大的計算機編碼以及用于分析計算用的計算機的推廣，我們可以更加方便地對電子實體模型直接進行分析求解。由于結構上的細節(jié)（如螺栓孔，切斷槽等）對計算結果沒什么影響，設計師仍然愿意舍棄它們，盡管在建立Pro/ENGIN

12、EER模型可以將它們考慮進去。</p><p>　　也有些情況下，對于分析師使用實體幾何圖形來建模它可能會無效的一些理由。這有兩個例子，1）薄結構，它可以準確地分析，使用三維殼單元比當用實體單元時更能降低計算成本和模型尺寸大小，和2）軸對稱結構，可充分分析利用二維軸對稱模型代表橫截面。</p><p>　　在這兩種情況下或者重新創(chuàng)建幾何模型或者使用Pro / ENGINEER的實體模型，分

13、析師一定必須提前知道分析什么樣的類型。這就依賴于當前的生成網格和分析技術了。例如，目前的生成網格技術只允許接受使用四面體單元（四環(huán)素）的一般實體幾何圖形的自動生成網格，即使六面體單元（六環(huán)素）通常用更少的單元提供一個更好的方案。因此，如果需要六面體單元的話，該分析師將不得不修改Pro / ENGINEER提供的幾何模型，以適應非自動生成網格。此外，四面體單元往往有問題，甚至超越他們的最低精度。低價四環(huán)素要素往往表現出剪切閉鎖和過度的剛度

14、，而高階四環(huán)素要素中不能使用明確的分析（動態(tài)分析需要非常小的時間間隔）。因此，分析師必須基于分析類型來選擇生成網格類型部分。</p><p>　　另一個考慮是模型的尺寸大小。有的3-D模型可以非常迅速地變的太大以至于無法運行，可能的原因或者是計算時間或者內存容量大小，這兩者都是目前計算機所限制?！靶　?00x100x100單元的3-D網格產生一百萬單元的模型尺寸，而迄今為止傳統(tǒng)有限元模型已低于十萬單元。因此，謹慎

15、的做法是在有可能的情況下，以2-D為模型結構，即使3-D計算方法可能會產生更準確的結果。這又可能需要修改來自Pro / ENGINEER提供的實體幾何模型。</p><p>　　最后一個考慮是，通常繪圖員“創(chuàng)建”一個Pro / ENGINEER模型比分析師“創(chuàng)建” 一個分析模型花費更少的時間和精力。因此，它是合理的（從整體設計到分析過程）以首先集中于可以用Pro / ENGINEER便于分析師的建模來做的事情。盡

16、管這是一個事實，即制圖者幾乎總是由設計師，而不是分析師。因此除非設計師的同意，分析師可能會感到不太愿意對任何模型做出修改。</p><p>　　目前存在的生成網格問題</p><p>　　目前生成網格技術和嚙合過程中有一些已知的障礙。這些障礙包括1）帶有小功能大的幾何模型的嚙合問題，2）復雜的非標準幾何形狀的嚙合問題，3）使用實體幾何模型來創(chuàng)建殼模型的問題，4）連接不同部件的集合建模之間的

17、問題，5）處理公差的問題，和6）如Pro / ENGINEER實體模型代碼轉移生成網格代碼傳輸幾何圖形信息的問題。</p><p>　　傳統(tǒng)的生成網格技術可以自動生成低階網格形狀，具體地說，點、線、四曲面、和六面實體。在2-D中，目前的鋪平技術現在仍然存在著一般性三角形和四邊形單元幾何圖形網格，這些技術相對強勁。在3-D中，目前的技術現已存在的一般四面體（四環(huán)素）單元形狀自動嚙合，但沒有更理想的六面體（六環(huán)素）單

18、元。然而，這些3-D生成網格代碼是不足以讓每個幾何模型網格總是成功，并且他們已越來越難以增加幾何模型的復雜性。具體來說，在大型復雜幾何模型上有許多小特征往往造成生成網格代碼失敗，因為他們無法完成從小單元（約小功能）到大單元和再一次回到（下一個小功能）的過渡 。同樣的問題也可能會發(fā)生在沒有“小”功能部分，但有很多復雜性功能。也就是說，從特征轉換特征將最終失敗，因為生成網格代碼通常在一個起點和“掃描”走向另一個點。一位分析師可能需要分解單一

19、的3-D部分將其分成若干“子部分”以便于部分網格能夠成功。</p><p>　　目前生成網格的另一個領域問題是由3-D幾何模型來創(chuàng)建一個薄殼模型。一個薄殼有限元是沒有厚度的，但假設任何單元有一半的厚度的剛度（即，它是被假定為處于中平面的厚度）。因為他們沒有厚度，薄殼單元零件建模目的是為了與其他部分接觸將現在的幾何間隙隔開。確定這些新的界面往往是困難的。此外，實體模型設計定義代碼（如Pro/ENGINEER）不容易

20、或自動提供這種中平面曲面位置的生成網格代碼擺在首位。因此，分析師可能創(chuàng)建幾何模型可用于薄殼單元模型的決策，也可能創(chuàng)建幾何模型定義殼單元模型的界面。</p><p><b>　　附件2：外文原文</b></p><p>　　A Study of the process of using pro/ENGINEER Geometry models to Create fin

21、ite Element Models</p><p><b>　　Abstract</b></p><p>　　Methods for building pro/ENGINEER models which allowed integration with structural and thermal mesh generation and analyses softw

22、are without recreating geometry were evaluated. This study was not intended to be an in-depth study of the mechanics of pro/ENGINEER or of mesh generation or analysis software, but instead was a first cut attempt to prov

23、ide recommendation for Sandia personnel which would yield useful analytical models in less time than an analyst would require to create a separate mode</p><p>　　Acknowledgments</p><p>　　This stu

24、dy was directly supported by Mark Mickelsen and dennis fritts ,drafters ; Dave neustel and Hal Radloff , designers ;and Mike kanouff and bruce kistler finite element analysts. Also ,Arlo Ames was invaluable for his insig

25、ht into the behavior and capabilities of pro/ENGINEER .</p><p>　　Introduction</p><p>　　The process of performing finite element analysis of systems or components consists generally of four step

26、s :1) geometry definition ,2) mesh creation from the geometry,3) application to the mesh of properties and boundary and load conditions, and 4) performing the finite element calculations ,and 5) examining the result of

27、 the analysis . This study examines the link between the first two steps. The reason for the study is that the past decade has seen a tremendous growth in the capabilities of</p><p>　　In addition , the mesh

28、generation codes have also improved significantly in the last decade . Many different codes now have the capability of automatically generating shell meshes on general surfaces .and some have (or are close to having ) th

29、e ability to mesh general-shaped solids automatically with either tetrahedral or hexahedral elements.</p><p>　　Because things are changing so quickly analysts and drafters may not have experience in how to b

30、est use these new tools .This study was undertaken to understand some of the mechanisms which would enhance the process of creating finite element models using today's electronic tools and using electronic design def

31、inition as input to the analyst. Since sandia has chosen pro/ENGINEER as its standard design definition computer program, pro/ENGINEER was examined in some detail.</p><p>　　In order to understand the detail

32、s and the significance of the different phases of this study, we believe that the reader needs to have a basic understanding of several areas. These areas include a brief background of the finite element process, a curre

33、nt status of mesh generation capabilities(including current problem areas), a description of different pro/ENGINEER capabilities and how they apply to the finite element analysis process, and an understanding of how thes

34、e technological changes mig</p><p>　　Background</p><p>　　The process of performing finite elements analysis of systems or components has in the past been challenging .the analyst could be call o

35、n to give either a very preliminary estimate of a structural or thermal response, or a very detailed prediction of that same response. To perform the evaluation, the analyst was typically given a geometry definition , ei

36、ther in paper or electronic form ,some materials information , and some load information .the analyst took this information and made enough ass</p><p>　　Often, the geometry information was given to the analy

37、st in paper form . The analyst needed to make decisions based on experience to determine how much of the detail (such as bolt holes ,cut-outs, etc.) to include in order to have an acceptable level of accuracy in the anal

38、ysis .then the analyst recreated, in some form , a simplified version of the geometry which had already been created by a drafter, this process , of reconstructing the geometry for the finite element model, and then of c

39、reatin</p><p>　　With the more prevalent use of solid modeler design definition programs, such as pro/ENGINEER [1], and the more powerful codes and computers used by the analyst, it is now more feasible to at

40、tempt an analysis which directly utilizes an electronic solid model definition of the design. however, this is only beneficial if the analyst does not have to recreate or significantly modify the geometry to be compatibl

41、e with the required analysis typically, the analyst would still like to ignore much of th</p><p>　　There are also instances where it may be inefficient for the analyst to work with a solid geometry for some

42、reason . Two examples of this are 1) thin structures, which can be accurately analyzed using 3-dimensional shell elements at a lower computational cost and model size than when using solid elements; and 2) axisymmetric s

43、tructures, which may be adequately analyzed using a 2-dimensional axisymmetric model representing the cross-section.</p><p>　　In either recreating a geometry or using a solid geometry form Pro/ENGINEER, the

44、analyst must know ahead of time what types of analyses are going to be required .This is dependent on the current state of mesh generation and analysis technology . for instance , current mesh generation technology only

45、allows acceptable automatic mesh generation of general solid geometries using tetrahedral (tet) elements , even though hexahedral (hex) elements typically provide a better answer with fewer elements </p><p>

46、　　Another consideration is model size. There-dimensional models can very quickly become too large to run either because of calculation time or memory size, both of which are limitations of the current generation of compu

47、ters. A “small” 3-dimensional mesh of 100x100x100 cells result in a model size of a million elements, while traditional finite element models to date have been less than 100,000 elements. Therefore, it is prudent whereve

48、r possible to model structures as 2-dimensional, even when a 3</p><p>　　A final consideration is that typically it takes much less time and effort for a drafter to “build” a Pro/ENGINEER model than it does f

49、or an analyst to “build” an analysis model. Therefore, it is reasonable (from an overall design-to-analysis process) to focus first on things which can be done in Pro/ENGINEER to facilitate the analyst’s model building.

50、This is despite the fact that the drafter is almost always funded by the designer rather than the analyst, and therefore might feel reluctant to </p><p>　　Problems with Current Mesh Generation</p><

51、;p>　　Current mesh generation technology and the meshing process have some known obstacles. These include 1) problems meshing large geometries with small features, 2) problems meshing complex non-standard geometric sha

52、pes, 3) problems using solid geometries to create shell models, 4) problems modeling the connectivity between different parts of assemblics, 5) problems handling tolerances, and 6) problems transferring geometry informat

53、ion from solid modeler codes such as pro/ENGINEER into mesh generati</p><p>　　Traditional mesh generation technology can automatically mesh low order shapes, specifically, points, lines, four-sided surfaces,

54、 and 6-sided solids. In two dimensions, current paving techniques now exist to also mesh general geometries with three –and four-sided elements. These techniques are relatively robust. In three dimensions, current techni

55、ques now exist for automatically meshing general shapes with tetrahedral (tet) elements, but not the more desirable hexahedral (hex) elements. However,</p><p>　　Another area where current mesh generation has

56、 problems is in creating a shell model from a three-dimensional geometry. A shell finite element has no thickness, but assumes the stiffness of something which has half of the thickness on either side of the element ( th

57、at is, it is assumed to be positioned at the midplane of the thickness). Because they have no thickness, parts modeled with shell elements which are supposed to physically interface with other parts will now be geometric