How to Make Valid FEA Assumptions

Finite Element Analysis is being popular tool to evaluate the product performance without a prototype having built.  Though it is being used as a qualitative analytical tool for majority of applications, there are products wherein it is required that a model calibration is done to ensure the quantitative parameters predicted matches with the actual results.

While it is important to include the geometrical details when conducting an FEA Analysis, it is more important to have knowledge that which details should be excluded.  This is required since FEA models are based upon assumptions and the role of FEA engineer is to make valid assumptions. We will be discussing about some of the examples for simplifying the problem.

(1)   Use of Linearity vs Non linearity:

If the problem is more or less a linear, make sure to conduct linear elastic analysis first.  In real physics, almost all problems are nonlinear, however linearity is an approximation method.  There are different types of non-linearity such as geometric(shape based), material non linearity and contact non linearity.  If the geometry is not much slender, you can assume it to be geometrically linear. However, for slender structures having loading at the end of the span, one should chose nonlinear geometry option. Common examples are bending of the fishing rods, deformation of off shore structures (Very long span), deformation of bridges etc. 

For general application materials such as metals (Fe based), one should go with linear material option.  However, if the intention is to carry out an explicit (crash) analysis, one must use nonlinear materials.  For gaskets, rubbers and seals, one has to use nonlinear materials.

Contact non linearity is generally applicable to the frictional, sliding or no separation kind of contacts in an assembly level analysis.  One should ensure that if the parts are assembled with locked degrees of freedom as in case of bolted or glued, MPC based contact algorithm should be used.

(2) Use of beams and shells:

Use of elements such as beams and shells can reduce the model size by significant amount while one can still have the intended result sets out of the FEA analysis.  Beams are defined as a line with a cross section and material properties assigned to them.  Examples of using beams while modeling structures is: FEA models for building pillars and beams, FEA models for machine structures etc.  One variant of beams is pipe element which is used in carrying out strain analysis for piping systems.  Usually these systems are very long (in the ranges of km) and hence usage of line based elements such as pipes and beams helps to solve such problems very quickly.  Shells are popularly used in automotive and aerospace industry to model the panels however nowadays they're being used in the pressure vessel industry as well.  One can easily change the thickness parameters for shells and assess the designs quickly.  The biggest advantage of using shells is one can model composite lay up using shell elements.  This is really helpful with increasing popularity of composites in various industries.

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