外文翻譯--學習運用ProENGINEER幾何模型建立有限元模型的過程.doc

畢業(yè)設計(論文)外文資料翻譯系 部： 機械工程系 專 業(yè)： 機械工程及自動化 姓 名： 學 號： (用外文寫)外文出處： Kistler B L, Technical ReportR. SAND-97-8239,1997. 附 件： 1.外文資料翻譯譯文；2.外文原文。 指導教師評語： 外文資料內容與課題相關，譯文能正確地表達原文的意文，語言表述基本符合漢語的習慣，語句較通順，層次清晰。 簽名： 年 月 日注：請將該封面與附件裝訂成冊。附件1：外文資料翻譯譯文學習運用Pro / ENGINEER幾何模型建立有限元模型的過程摘 要建立Pro/ENGINEER允許結構一體化模型的方法和生成熱網(wǎng)格和無需重新幾何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(wǎng)格或者分析軟件，而是首次嘗試對將產(chǎn)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。致 謝繪圖員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。引 言有限元分析系統(tǒng)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(wǎng)格，3)應用網(wǎng)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(xiàn)在能夠經(jīng)常定義實體幾何。這些電子數(shù)據(jù)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。此外，在過去十年中生成網(wǎng)格代碼也有著顯著的改善。許多不同的代碼，現(xiàn)在有著在一般表面上自動生成殼網(wǎng)的能力。有些人具有（或接近了）將一般網(wǎng)格狀實體要么自動生成四面體或六面體單元的能力。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(jīng)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)。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(wǎng)格能力（包括目前的問題領域）的目前狀況，不同Pro/ENGINEER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。背 景在過去對系統(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，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。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(jù)經(jīng)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，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。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/ENGINEER模型可以將它們考慮進去。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。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(wǎng)格和分析技術了。例如，目前的生成網(wǎng)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(wǎng)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(wǎng)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(xiàn)出剪切閉鎖和過度的剛度，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(wǎng)格類型部分。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(wǎng)格產(chǎn)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(chǎn)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。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。目前存在的生成網(wǎng)格問題目前生成網(wǎng)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(wǎng)格代碼傳輸幾何圖形信息的問題。傳統(tǒng)的生成網(wǎng)格技術可以自動生成低階網(wǎng)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(xiàn)在仍然存在著一般性三角形和四邊形單元幾何圖形網(wǎng)格，這些技術相對強勁。在3-D中，目前的技術現(xiàn)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）單元。然而，這些3-D生成網(wǎng)格代碼是不足以讓每個幾何模型網(wǎng)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(wǎng)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(wǎng)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的3-D部分將其分成若干“子部分”以便于部分網(wǎng)格能夠成功。目前生成網(wǎng)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(xiàn)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）不容易或自動提供這種中平面曲面位置的生成網(wǎng)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。附件2：外文原文A Study of the process of using pro/ENGINEER Geometry models to Create finite Element ModelsAbstractMethods for building pro/ENGINEER models which allowed integration with structural and thermal mesh generation and analyses software 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 provide recommendation for Sandia personnel which would yield useful analytical models in less time than an analyst would require to create a separate model. The study evaluated a wide variety of geometries built in pro/ENGINEER and provide general recommendations for designers, drafters, and analysts.AcknowledgmentsThis study 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 insight into the behavior and capabilities of pro/ENGINEER .IntroductionThe process of performing finite element analysis of systems or components consists generally of four steps :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 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 electronic design (geometry) definition, with such computer software as pro/ENGINEER . 1 now being able to routinely define solid geometries. These electronic databases can create traditional blueprints for manufacturing purposes, but can also transfer information electronically to computerized manufacturing processes and to finite element analysts. Potentially, this electronic transfer of information can save the analyst a significant amount of time in both steps 1 and 2.In addition , the mesh 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 ) the ability to mesh general-shaped solids automatically with either tetrahedral or hexahedral elements.Because things are changing so quickly analysts and drafters may not have experience in how to best 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 todays electronic tools and using electronic design definition 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.In order to understand the details 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 current 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 these technological changes might affect the interrelationship between the work the designer, the drafter, the analyst, and the manufacturer ,the reader is encouraged to thoroughly study these introductory sections .in order to understand the significance of things that were done in the main study. BackgroundThe process of performing finite elements analysis of systems or components has in the past been challenging .the analyst could be call on 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 , either in paper or electronic form ,some materials information , and some load information .the analyst took this information and made enough assumptions about the problem to allow a finite element modal to be built which would result in an acceptable answer within the available amount of time. Thus, a limited time to perform an analysis was a third constraint.Often, the geometry information was given to the analyst 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 analysis .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 creating the finite element model , took up to 80% of the analysts time and efforts. 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 attempt 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 compatible with the required analysis typically, the analyst would still like to ignore much of the detail (such as bolt holes ,cut-outs, ect) because that detail does not contribute to the accuracy of the solution ,even though that detail may be integrated into the pro/ENGINEER model。There are also instances where it may be inefficient for the analyst to work with a solid geometry for some 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 structures, which may be adequately analyzed using a 2-dimensional axisymmetric model representing the cross-section.In either recreating a geometry or using a solid geometry form Pro/ENGINEER, the 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 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 . Thus , if hex elements are required ,the analyst will have to modify the geometry provided from Pro/ENGINEER, to accommodate the non-automatic mesh generation. In addition, tet elements tend to have problems even beyond their lower accuracy. Low order tet elements tend to exhibit shear locking and excessive stiffness, while higher order tet elements cannot be used in explicit analyses (dynamic analyses requiring very small time steps). So the analyst must choose the type of mesh generation based partly on the type of analysis.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 computers. 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 wherever possible to model structures as 2-dimensional, even when a 3-dimensional calculation may yield more accurate results. This again may require modification of solid geometry provided from Pro/ENGINEER.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 for 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 analysts model building. 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 do any model modification for the analyst unless agreed to by the designer.Problems with Current Mesh GenerationCurrent 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 shapes, 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 information from solid modeler codes such as pro/ENGINEER into mesh generation codes.Traditional mesh generation technology can automatically mesh low order shapes, specifically, points, lines, four-sided surfaces, 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 techniques now exist for automatically meshing general shapes with tetrahedral (tet) elements, but not the more desirable hexahedral (hex) elements. However, there three-dimensional mesh generation codes are not robust enough to always successfully mesh every geometry, and they have increasing difficulty with increased geometry complexity. Specifically, having many small features in a large complex geometry often causes mesh generation codes to fail because they cannot complete the t