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, material properties, 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. The design
engineer has to deal with all of the parts needed to manufacture a real product,
down to every single screw and lock washer. The engineering analyst needs a
simplified, but structurally significant model for FEA. Turning a design model
into an analysis model can be quite time consuming.
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, and usually a precursor to doing
any other type of dynamic analysis.
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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