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1、 Fundamental FEA Concepts and Applications A Guidebook for the Use and Applicability of Workbench Simulation Tools from ANSYS, Inc. FUNDAMENTAL FEA CONCEPTS PAGE 1 OF 46 Forward Demystifying FEA 2 In Perspective . 2 What is FEA? 3 Pre-Processing . 3 ANSYS Workbench Meshing 6 Solving. 7 ANSYS Workben
2、ch Solving 7 Post-Processing (Interpretation Of Results). 7 ANSYS Workbench Post-Processing. 8 Types of Engineering Analysis 8 Stress and Strain The Basics . 9 Users of FEA.10 Advanced10 Intermediate.10 Fundamental11 In What Areas Can FEA Help?.11 Over-Designed Products.11 Under-Designed Products11
3、Miscellaneous Product Performance Issues .12 Engineering Assumptions for ANSYS Workbench12 General Assumptions and Limitations.14 Assumptions and Limitations Continued.15 FEA Terms and Definitions16 FUNDAMENTAL FEA CONCEPTS PAGE 2 OF 46 Forward Demystifying FEA There are many high-quality FEA Fundam
4、entals books available, “Building Better Products With Finite Element Analysis” by Vince Adams and Abraham Askenazi is one such highly recommended book (available from A). The main purpose of this primer is to provide the reader with enough basic understanding of FEA fundamentals to understand how A
5、NSYS Workbench Simulation works with a bias toward how to apply this tool in order to reap the most reward from its implementation. A definition of terms, analysis types, assumptions and limitations will be covered and, where meaningful, additional explanation as to “Why is this important?” and “How
6、 and where would it be used?” will be provided. In addition, a section will cover when to apply ANSYS Workbench Simulation and when to look to more advanced tools such as ANSYS Professional or other ANSYS configurations that are offer more detailed analysis capabilities but also require more trainin
7、g and expertise. In Perspective FEA, for this discussion, is simply a tool to better understand how a design will perform under certain conditions. All analysis are approximations accounting for the most significant variables among many. What is almost as useful as obtaining an “exact” answer with r
8、egard to “reality,” which might take a highly trained specialist a great deal of time performing what is often called a “detailed” analysis, is the trending and behavioral information that can be used to help predict and improve ones design (a “basic” or “fundamental” analysis). This approach substa
9、ntially improves the overall engineering process. A very simple example is: how and where does this design bend? Noting how and where parts move can be often be just as helpful as knowing if it moves 1 inch or 1.0625, especially early in the design. Accuracy is important and ANSYS is known for quali
10、ty, accurate products. Within its scope of functionality, ANSYS Workbench Simulation is perfectly capable of performing detailed analysis on par with traditional FEA methods in a fraction of the time. For any analysis, detailed or fundamental, it is vital to keep in mind the nature of approximations
11、 study the results and test the final design. Proper use of ANSYS Workbench Simulation will greatly reduce the number of physical tests required while allowing the designer/engineer/analyst to experiment on a wider variety of design options and improve the end product. Product Terminology ANSYS Work
12、bench in and of itself is not a product, rather it is a product development platform and user GUI built for analysis needs with the objective of providing elegant next generation functionality and intelligent automation to the engineering community. Workbench can be thought of as a “kernel” from whi
13、ch applications are built in a way similar to how solid modeling kernels such as ACIS or Parasolid are used. ANSYS Workbench Simulation is the primary application module used in the Workbench environment and it is available in several product configurations. DesignSpace V6.0 was the first product bu
14、ilt and delivered using Workbench technology in September of 2001. Today, the Workbench Simulation module can invoke any product license available from ANSYS DesignSpace through ANSYS Multiphysics; the higher the license level the more analysis capability exposed. The term “Workbench Simulation” is
15、used throughout this document and will typically reference the product DesignSpace but may also reference functionality only available when using an ANSYS Professional or higher-level license. While other ANSYS Workbench product modules are available (DesignXplorer, DesignModeler, FE Modeler), this
16、document will focus only on the Simulation module, regardless of license level used. FUNDAMENTAL FEA CONCEPTS PAGE 3 OF 46 What is FEA? Finite Element Analysis is a mathematical representation of a physical system comprising a part/assembly (model), material properties, and applicable boundary condi
17、tions collectively referred to as pre-processing, the solution of that mathematical representation solving, and the study of results of that solution post-processing. Simple shapes and simple problems can be, and often are, done by hand. Most real world parts and assemblies are far too complex to do
18、 accurately, let alone quickly, without use of a computer and appropriate analysis software. Pre-Processing To do this, FEA software typically uses a CAD representation of the physical model and breaks it down into small pieces called finite “elements” (think of a 3-D puzzle). This process is called
19、 “meshing.” The higher the quality of the mesh (collection of elements), the better the mathematical representation of the physical model. The primary purpose of an element is to connect nodes with predictable mathematical equations based on stiffness between nodes; the type of element used often de
20、pends upon the problem to be solved. The behavior of each element, by itself, is very well understood. By combining the behaviors of each element using simultaneous equations, one can predict the behavior of shapes that would otherwise not be understood using basic “closed form” calculations found i
21、n typical engineering handbooks. A block (element) with well defined thermal, mechanical, and modal behaviors. Example of a simple part whose structural behavior would be difficult to predict using equations by hand. The same part broken into small blocks (meshed into elements) each with well-define
22、d behaviors capable of being summed (solved) and easily interpreted (post-processed). FUNDAMENTAL FEA CONCEPTS PAGE 4 OF 46 There are many different types and classes of elements, most created for specialized purposes (cable, piping, beams, truss structures, e-mag, etc.). A one-dimensional element r
23、epresents line shapes, such as beams or springs. A 2D element, also known as a quadrilateral element, will represent triangles and squares. 3D elements represent solid shapes and are usually in 2 basic shapes: brick (hexahedrons or “hex”) and pyramids (tetrahedrons or “tets”). Examples of applicatio
24、ns for specialized elements would be: scaffolding consisting of connecting 1D line elements. Car bodies and other stamped or formed sheet metal parts are typically very thin relative to their overall size and are usually best represented by 2D shell/plate elements. Many thin shapes can, and are, mes
25、hed with 3D solid elements but at the cost of increased processing time and sometimes a loss in accuracy because of the special formulation of 2D shell elements. The tradeoff is that, in order to mesh with 2D shell elements, there is often significant modification and preparation required to the CAD
26、 geometry in order to obtain a meshable surface model, or models in the case of an assembly. In other words, the pre-processing requirement increases substantially. The guidelines are: can it be meshed and solved in 3D solids (sometimes the resultant model is simply too large)? If yes and the user i
27、s not looking for ultimate accuracy but only trending and behavioral information, then 3D solid meshing is often appropriate due to the human time savings. 3D elements are ideal for thick and chunky parts and assemblies such as engine blocks, machine components, etc. General purpose, modern mechanic
28、al FEA programs typically use a select set of elements chosen for their versatility, robustness, and their overall contribution to product ease of use. Workbench Simulation uses several primary element types and will default to high-order (10 node quadratic) tetrahedral (H) elements (SOLID 187 in AN
29、SYS-Speak) for solid model geometries if they are not sweepable, in which case high-order (20 node) brick elements (SOLID 186) are employed. On closed surface models, “quad-dominate” (4 node) shell elements (SHELL 181), are used providing both accuracy and efficiency while being suitable for the rob
30、ust automatic meshing algorithms used in Workbench Simulation. And, for part-to-part interaction within assemblies, high-end surface-to- surface contact elements (CONTACT 170/174) are used. For mixed beam/shell models and for spot-weld features, beam elements are employed (BEAM 188). ANSYS Workbench
31、 Simulation applies these various element types automatically. FUNDAMENTAL FEA CONCEPTS PAGE 5 OF 46 Common Element Types Used in ANSYS Workbench Simulation 4-Noded Shell 10-Noded Tetrahedral 20-Noded Hexahedral Other Common Element Types Each element is comprised of 2 or more “nodes” which help def
32、ine its shape as well as to convey physical reactions from one element to the next. The “finite” in FEA comes from the fact that there are a known number of elements in a finite element model. The solver adds up the individual behaviors of each element to predict the behavior of the entire physical
33、system. FUNDAMENTAL FEA CONCEPTS PAGE 6 OF 46 Other aspects of the pre-processing phase involve identifying material properties and environmental conditions the design will be subject to. These conditions include various forms of physical forces (loads, pressures, moments, etc.), thermal loads and c
34、onditions (temperature, conductivity, convection, etc.), and constraints (fixed, pinned, frictionless/symmetrical, etc.). Some Workbench Simulation pre-processing fundamentals: ANSYS Workbench Simulation Meshing ANSYS Workbench Simulation provides 2 forms of automated meshing: Fully automatic and Ma
35、nually Directed Automatic. Both forms employ a fault-tolerant philosophy meaning that, if a problem occurs, at least 12 attempts of automatic trouble-shooting are made before the mesher fails and tags the area of difficulty with a label. Manually directed means that the user may specify meshing over
36、rides on specific areas of a part (edge(s), face(s) or the baseline mesh density on entire parts that differ from other parts within the assembly, either for accuracy or efficiency purposes. These changes remain associative. Workbench Simulation Material Data Structural and thermal material data are
37、 defined, modified, and used in Workbench Simulation for structural and thermal analyses. Material properties include Youngs modulus, Poissons ratio, density, coefficient of thermal expansion, and thermal conductivity. The latter quantity, conductivity, can be temperature-dependent. Workbench Simula
38、tion Convection Data Temperature-dependent film coefficients are defined, modified, and used in Workbench Simulation for thermal analysis. Many categories of film coefficients can be devised to take into account laminar, turbulent, forced, and natural convection conditions, as well as various geomet
39、ric configurations Workbench Simulation Fatigue Material Data Materials in the Workbench Simulation material library may include a fatigue stress-life curve populated with data from engineering handbooks. Fatigue data has been pre-populated for the Structural Steel and Aluminum Alloy material data f
40、iles from the MIL-SPEC handbook (MIL-HDBK- 5H) (http:/analyst.gsfc.nasa.gov/FEMCI/links.html). Since material data is crucial to accurate fatigue results, Workbench Simulation readily allows the input of this information to new or existing materials by hand or through load history files. The Fatigue
41、 Tool will then use the information in the stress-life curves for each material in the model when calculating life, damage, safety factors, etc. Workbench Simulation Pre-Processing Extensions Many additional pre-processing capabilities can be realized by using the ANSYS Preprocessing Command Builder
42、 in which the user has greatly expanded capabilities to modify the analysis. APDL commands can also be added to the command stream to extend capabilities even further. The ANSYS Preprocessing Command Builder is enabled in Workbench Simulation when using an ANSYS Professional or higher-level license.
43、 More sophisticated analysis types such as fluid flow, Micro Electro-Mechanical Systems (MEMS), and transient (time dependent) have many additional environmental and material property considerations, which require additional expertise to properly employ and are beyond the scope of discussion here. F
44、UNDAMENTAL FEA CONCEPTS PAGE 7 OF 46 ANSYS Workbench Solving ANSYS Workbench employs 3 of the ANSYS solvers and automatically chooses the most appropriate or efficient solver for the job at hand. In addition to linear/static, ANSYS Workbench performs Coupled analysis types (thermal-stress, stress-mo
45、dal, thermal-stress- modal) as well as some limited non-linear analysis types (thermal with temperature-dependent material properties and convection, geometric/contact with contact supporting lift-off). All solver settings and iteration propagations from one solve step to the next are performed auto
46、matically. Post-Processing (Interpretation Of Results) The output of a solver is generally a very substantial quantity of raw data. This quantity of raw data would normally be difficult and tedious to interpret without the data sorting and graphical representation referred to as post-processing. Pos
47、t-processing is used to create graphical displays that show the distribution of stresses, strains, deformations, temperatures, and other aspects of the model. Interpretation of these post-processed results is the key to identifying areas of potential concern (weak areas in a model), areas of materia
48、l waste (areas of the model bearing little or no load), or valuable information on other model performance characteristics (thermal, modal) that otherwise would not be known until a physical model were built and tested prototype. The post-processing phase of FEA is where the most critical thinking m
49、ust take place, where the user looks at the results (the numbers vs. color contours, movements, etc.), and compares results with what might be expected. It cannot be stressed enough that it is up to the user to determine if the results make sense, to be able to explain the results based upon engineering “common sense.” If the results are other than expected, one must search until an explanation can be found before the results can be fully trusted. ANSYS Workbench Post-Processing A select set of results is avail