Developing a successful and effective Finite Element Analysis (FEA) model can result in a frustrating experience for design engineers. The model needs to be simple and easy to replicate, while still complex enough to provide valid test results. This creates a problem where models are often either too simplified and approximated to provide accurate analysis, or the model is too complicated for easy processing.
An important step in improving an FEA model is model simplification. However, the model must be simplified in the right way in order to provide accurate analysis.
Generating model geometry is one of the most difficult aspects of a Finite Element Analysis (FEA). A common mistake among beginner FEA users is assuming that a computer aided design (CAD) model created as part of the product design process can be plugged directly into an FEA study. Designer CAD models typically include tremendous detail and it may take hours or days of processing to incorporate into an FEA.
CAD models help designers visualize the product, generate instructive drawings for machinists, and help manufacturing engineers or packaging engineers determine how the product will safely be shipped to the customer, and accomplish a wide variety of other activities. For some of these activities, the tiniest detail could be important, and for that reason designer CAD models can be very intricate.
However, much of this detail is often unnecessary in FEA. Worse, the inclusion of unnecessary detail in an FEA model can result in poor quality meshes, inefficient simulations run times, and inaccurate results. For an analyst, understanding when and how to simplify a model that was passed to them by a designer is a key skill for effective FEA simulation.
Removing Unnecessary Object Features
Perhaps the most common details that can be immediately removed from most CAD models are the fillets and rounds. Truly square edges rarely exist in the real-world--edges are typically rounded and CAD models will often include this rounding on many, if not all, of the geometric bodies. However, square edges are much easier for meshing in the FEA world, and most small fillets/rounds will not affect global displacement calculations. CAD tools will typically have a feature to aid in the removal of fillets/rounds, like the fill command in ANSYS Spaceclaim. Appropriately using these features can rapidly decrease model complexity with little user effort.
Incorporating Effective Geometries and Constraints
Another common simplification is removing insignificant bodies or replacing them with effective geometries or constraints. For example, most mechanical assemblies include fasteners, like bolts and rivets. Sometimes it may be necessary to include the geometry of a bolt in the model; however, in many cases, bolt geometries can be replaced with greatly simplified 3D geometries, 1D beam elements, or even removed entirely and approximated with rigid contact constraints or fixed boundary conditions.
For example, if mechanical shock is being simulated on a 12” by 12” PCBA, very small components like 0201 resistors will have no effect on the global stiffness of the model and can be removed entirely. Larger components, like a 16 PIN SOIC may need to be modeled, but the solder can be replaced and approximated with a rigid contact between the leads and the board. ANSYS Sherlock is a tool that can aid in generating an FEA-ready model from information available at the design stage of a PCBA by taking in the ECAD information meant for PCBA manufacturing and automating the creation of a simplified, meshed, FEA-ready PCBA model.
How to improve an fea model: proper mesh generation