1. 實驗力學研究室 1 CAD Modeling for FEA

2. 實驗力學研究室 2 Four typical scenarios seen in product design: CAD models prepared by the design group for eventual FEA. CAD models prepared without consideration of the analyst’s needs. CAD models unsuitable for use in analysis due to the amount of rework required. Analytical geometry developed by or for the analyst for the sole purpose of FEA.

3. 實驗力學研究室 3 Design versus Analytical Model The importance of mixed element models, thoughtful boundary conditions, and informed results interpretation are just as frequently ignored or downplayed. Ignoring the latter tends to obscure the fact that the mesh is only one part, and often a small one, of overall solution quality, and that in many cases, the design model and the analysis model may be, should be, or must be quite different.

4. 實驗力學研究室 4 The conditions of using the design model as the analysis model are 1.Design models are built in 3D solid or surfaces that fully enclose volumes. 2.The part can and should be meshed with tetrahedrons, or is simple enough to provide the foundation for solid mapped brick meshing or mid-plane surface extraction for building shell models. 3.The CAD model exists at the time the analysis is to performed.

5. 實驗力學研究室 5 Building CAD Models for Eventual FEA Use Geometry providers as well as downstream users should be aware of the inconsistencies and issues which cause problems later. Solid Chunky Parts

6. 實驗力學研究室 6 There are several areas of modeling which can improve the transfer of data between applications that must be known by the geometry providers. 1.Clean geometry 2.Fragile “parent-child” or dependency relations

7. 實驗力學研究室 7 Building Clean Geometry (avoid short edges and silver surfaces) 1.The geometric features must not prevent the mesh from being created and must also contain surfaces of consistent size and shape ratios to prevent forcing high aspect ratio elements and/or transitions between element edges that may compromise accuracy. 2.Simplification or manipulation of feature in an attempt to clean up the geometry should not reduce the structural integrity of the part.

8. 實驗力學研究室 8 Limiting the size of small edges to no les than one-third of the expected nominal element size is good practice.

9. 實驗力學研究室 9 Sliver Surfaces Sliver surfaces are faces on a part with a high aspect ratio.

10. 實驗力學研究室 10

11. 實驗力學研究室 11 Voids in Solids

12. 實驗力學研究室 12 High Order Surfaces and Edges

13. 實驗力學研究室 13 Corrupt Geometric Definition Standards such as ACIS, Parasolids, VDA, and STEP have greatly reduced the problems typically associated with model conversion. IGES and DXF will continue to cause headaches, primarily because of the disparity between standard interpretations and level implementation.

14. 實驗力學研究室 14 The impact on downstream geometry quality can be minimized by following the steps listed below. 1.Try to use the same CAD system for all components in a design. 2.When the above is not possible, translate geometry through kernel based tools such as ACIS or Parasolids. Using standards based (i.e., IGES, DXF, or VDA) translations may lead to problems. 3.Visually and systematically inspect the quality of imported geometry before it is incorporated into the product database. Do not assume it is clean.

15. 實驗力學研究室 15 4.When possible, avoid modifications of imported geometry in a second CAD system. Recreating the part in a native system may be preferable if modifications cannot be made in the original system. 5.Use the original geometry for analysis when available. If the native CAD geometry cannot be used directly, use a translation directly from the original model. Minimize the iterations of translation to reduce the error which can be introduced by these manipulations.

16. 實驗力學研究室 16 Assigning Properties

17. 實驗力學研究室 17 Material Properties Types of Materials Isotropic materials have properties that are independent of geometric orientation. On the other hand, anisotropic material properties always require definition of material orientation. Stiffness Properties Young’s modulus, Poisson’s ratio, and shear modulus.

18. 實驗力學研究室 18 Other Properties 1.A thermal expansion analysis requires a coefficient of thermal expansion, conductivity and specific heat values 2.Orthotropic studies request values for all structural and thermal quantities in each of three orthogonal directions. 3.Modal analyses make use of a mass density, and dynamic studies allow for the input of a material damping coefficient as well. Units It is extremely important that your units are consistent with those of the rest of the model.

19. 實驗力學研究室 19 Nonlinear Material Properties Nonlinear material models can be input in one of several manners. 1.A bilinear model simply requires a plastic modulus and a transition stress to identify when to switch the element stiffness definition from the elastic Young’s modulus to its plastic counterpart. 2.A multilinear model requires the input of stress-strain data pairs to essentially communicate the stress-strain curve from material suppliers or testing to the FE model.

20. 實驗力學研究室 20 General Element Properties Beams Cross-sectional area, A. Principal area moments of inertia, I yy and I zz. Stress recovery points, C y and C z. These points define the distances along the principal y and z directions and away from the neutral axis (principal x-axis ) at which you want bending stresses to be calculated. Torsional stiffness factor, K.

21. 實驗力學研究室 21 Advanced Beam Properties Neutral offset and shear center offset allow you to physically attach the beam element in a convenient location, but force the model to interpret the beam as if it were in a different spot.

22. 實驗力學研究室 22 Shells 1.The mathematical section property of a shell need only specify thickness. 2.For the examination of shell results, you can select the orientation of their normal vector to tell the system which side is “up” and which is “down”. Solids Solids do not require any mathematical definition because they are completely defined by the geometry.

23. 實驗力學研究室 23 Special Element Properties Mass Elements In a static analysis, this definition may simply consist of a mass value. If a mass, its rotation is used to simulate a nonmodeled component of a size comparable to or larger than of the modeled structure, its moments of inertia are likely to have a significant effect on the overall dynamic response of the system.

24. 實驗力學研究室 24

25. 實驗力學研究室 25 Springs Whether a spring is used between two element nodes or to connect a single node to ground, there are basically two types of stiffness properties that must be considered: extensional and torsional. Although both types are used to limit motion, extensional stiffness opposes linear translation while torsional stiffness fights back angular rotation. Point-to-point springs simply require a spring constant, either torsional or axial. These stiffness quantities are always input in terms of coordinate directions. Hence, to define them, a spring orientation must be known or declared.

26. 實驗力學研究室 26 Dampers Damper elements are line elements that, in their most basic use, represent dashpot-like devices in a system. The units of a damper element are force/velocity (or force-time/length). As with any new element type, you should experiment with them in test models. Contact Elements The property definition of contact elements depends on the technique(s) your code allows for creating contact pairs. If surface- to-surface or curve-to-curve contact is allowed, you may not have properties to enter because the contact behavior is derived from the material properties of the elements on each side of the contact pair.

27. 實驗力學研究室 27 Gap Elements The two basic properties of a gap element are its compressive stiffness and initial gap. The compressive stiffness must simulate the local stiffness of the elements in the contact pair. The gap element provides a means to specify interaction between two otherwise independent nodes. Therefore, the stiffness property is a spring constant that allows the gap to resist one node from crossing the spatial location of the other. You can estimate the spring constant of local elements using the equation,. The point at which the gap begins to resist this displacement is defined by the initial gap.

28. 實驗力學研究室 28 Slide Line Elements Slide line elements require a compressive stiffness property just like gap elements. However, slide lines cannot support tension.