FEA FAQ's
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What is Finite Element Analysis (FEA)?

FEA is typically used to model or simulate the behavior of complex mechanical parts to external stimuli. It is most useful on complex parts or assemblies that would be very difficult or impossible to solve in a mathematically closed form manually. It is frequently used to minimize building physical prototypes for testing. Eliminating physical testing completely is not a good idea, it places too much faith in the analysis alone.

FEA involves building an accurate 2D or 3D geometry model of the component to be analyzed. The model is broken up into discrete elements with nodes at their corners. Material properties are assigned for all materials used in the part. Boundary conditions are used to model physical connections to the part by setting the appropriate degrees of freedom (DOF) for all boundary nodes. Each node has up to six DOF comprised of three for translation, and three for rotation. Boundary conditions can also be used to model: dynamic, thermal, fluidic, and electrostatic connections. This part of the modeling process is done with a CAD like preprocessor to the FEA software, or geometry data can be imported from your existing CAD or solid modeling software which may be an easier way for people to work. You want to avoid recreating the geometry if at all possible.

Once the geometry, materials, and boundary conditions are set, the next step is for the engineer to decide on the element type(s) and analysis type and then run the FEA solver to solve thousands to millions of simultaneous differential equations to obtain a physical displacement at each node. This strain data is then used to compute stress data at each node. A graphical postprocessor is then used to digest all of this data and display it superimposed over the geometry model of the part with color coded stress (or other analytical parameters) contour lines. Models can also be sliced to display stress contours inside the part.

What qualifications do you need to be an engineering analyst?

You should have academic training in an appropriate field of engineering - at least at the Bachelor's level, extensive experience doing manual engineering analysis calculations, thoroughly understand the FEA process, and finally be able to use your FEA software correctly and efficiently. Finally you need to be able to plan the entire analysis process deciding which questions need to be answered, how you will answer them, what software tools you will use, and what criteria will indicate success or failure of the part or assembly in question.

What are elements and nodes?

Elements are the discrete geometric subdivisions of the model. A 2D FEA model will normally have either triangular and/or quadrilateral elements. A 3D FEA model will normally have either 3D bricks and/or 3D tetrahedra as elements.  Nodes are at the intersection points of the lines that make up an element. A wide variety of element types are available to model different types of structures for different types of analysis.

What are boundary conditions?

Boundary conditions (BC) are the way that a specific node in a FEA model is attached to the ground or some other node in the model. A variety of  BC's are available including: rigid or fixed, elastic spring, thermal, etc. BC's can be specified to be fixed in any or all of the six DOF. BC's define how a part or assembly of parts is attached to the real world.

How do you build a FEA model?

You must create 2D or 3D geometry which matches the real object that you want to analyze to a degree that is commensurate with your analysis objectives. It can be created within the preprocessor of the FEA software or previously created geometry can be imported from a CAD system or a solid modeler. Analyst's rarely need the degree of detail necessary to design the part for actual manufacture. Usually you end up with at least two models: a very detailed one created by the design engineer that deals with all of the parts complexity, and a simpler model used for FEA. If numerous types of analyses are to be performed you may need somewhat different FEA models for each.

What do the terms linear, non-linear, static, and dynamic refer to?

In engineering analysis, whether FEA is used or not, linear refers to an analysis or FEA model where linear behavior of the item being analyzed and its materials are assumed to occur. That is the object being analyzed will not be loaded beyond the elastic limits of its materials. Deflections vary linearly in proportion to the applied load.

Non-linear refers to a model deflection, its materials, and/or boundary conditions which will be subjected to non-linear behavior. Some materials exhibit non-linear  stress-strain behavior and this is referred to as material non-linearity. If large deflections are present they result in geometric non-linearity. Finally if boundary conditions change for different load levels this is referred to as boundary non-linearity.

Static refers to a model which is loaded statically or is loaded dynamically so slowly that it can be assumed to be loaded statically. That is the applied load is fixed in amplitude with respect to time or changes with time so slowly that it can be treated as a static load. This is also referred to as "quasi-static" loading.

Dynamic refers to a model which is loaded dynamically or in a time dependent fashion. Examples would be shock, vibration, and seismic loading where the load amplitudes vary significantly with respect to time. A modal analysis is also a dynamic analysis that determines the resonant frequencies and mode shapes of the structure. Modal analysis models are unloaded.

How much computer hardware do I need?

At least as much as is required for CAD work - see the answer to this question for CAD/CAE FAQ's. FEA, 3D solid modeling, and graphics image processing are three disciplines that require enormous amounts of computing horsepower. Computer system power must increase with model size and analysis complexity. A transient, non-linear, dynamic analysis is much more complex, time consuming, and expensive to create, run, and evaluate than a linear static analysis.

How long does it take to become proficient with FEA software?

In spite of what the sales and marketing people say, for any FEA company, it takes quite a while to become proficient with FEA software - months is very optimistic, years are more typical. With effective training it takes less time, without training it takes much longer. Utilizing FEA software correctly relies heavily on engineering judgment and experience. Practitioners should have a significant amount of academic training and professional experience in: mechanical design, statics, dynamics, materials, and stress analysis or other related analytical techniques. The concept of "push button" analysis where a designer, who is totally inexperienced at stress analysis, can just automatically mesh his/her solid model, run a FEA, and interpret the results correctly is ludicrous. When you get that model meshing and running successfully for the first time, you aren't done, you're just getting started.


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