There is many different possible settings to adjust the quality of a mesh in Inventor NASTRAN. In this post, I will present one of the most useful: Suppress Short Features.
You will need to adjust this parameter when a CAD part presents some narrow angle or small feature, like this one:
The problem is that by default, the mesher tries to stick to the topology of the CAD. By meshing this narrow angle, some elements will therefore have a bad aspect ratio. The “Suppress Short Feature/Min Feature Angle” settings control how much the nodes can deviate from the CAD.
You have a solid CAD file of this structure. What do you do: use beam elements, extract mid-surfaces to mesh it with shell elements, or directly mesh the solid 3D CAD file with tetras (the “quick and dirty” approach)? Does the tetra mesh will be able to catch bending of the beams? There is only one way to answer this: try it with different mesh sizes. The study I did for this particular model (linear elastic analysis) told me that using 2 quadratic elements in the thickness results in an error of 5% on the displacement and 4% on the stress (compared to the same structure meshed with shells).
The problem with this approach? The mesh of this structure has a total of 3 millions nodes using the tetra mesh. So, the choice between shell or solid Idealization for this kind of structure is a balance between calculation time and effort needed to transform the solid CAD into a surface model. Good practice with this kind of structure is usually to use structural elements like beams or shells.
If the shell approach is chosen, Inventor NASTRAN has a very good tool to extract the mid-surfaces:
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Topology optimization determines the distribution of material most suitable to a given objective. It is primarily used to produce a fundamental basis for the engineers at the conceptual design stage, or to generate ideas for new alternatives.
In order to express the distribution of materials in topology optimization, density variables of the finite elements created for analysis are used. The element density of 1 represents a part that requires the element, while 0 represents a part that does not require the element. Unlike parametric optimization, the only design variable is the element’s density. As such, the user does not specify separate design variables but composes an optimization problem using only the combinations of objective functions and constraints.
Like any optimization problem, Topology optimization includes the following fundamental elements:
Objective: In this problem, the objective is to minimized the static compliance (a function of element density expressed in the form of global deformation energy):
Where:
f : Load vector u : Global & element displacement vectors K: Global & element stiffness matrices
Design variables: Volume fraction (the n_nodes density values which determine whether material is present (1) or absent (0))
Geometric constraints: the initial unoptimized geometry:
Design evaluator: the linear elasticity solver of MidasNFX, that calculates deformation energy based on specified loads and boundary conditions.
Since we are looking for general guidelines of an optimal design (in practice, we should say “better design”), the next step consists of modifying the original CAD geometry to remove material when it’s not needed:
Topology optimization often leads to complicated organic-like products that cannot be manufactured using traditional processes (which is not necessarily the case here). Additive manufacturing, like 3D printing, is sometimes more adapted for this kind of design. Here, a printed version of the optimized part is produced.
This video summarizes the whole process:
15 August 2020Comments Off on From Design to Prototype using Topology OptimizationBlog Article
