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Terminology issues in the Finite Element Analysis
The Finite Element Analysis (FEA), or Finite Element Method as
mathematicians call it, is one of
many numerical techniques of solving partial differential
equations that describe, among others,
the structural and thermal problems presented in this book. FEA
has seen rapid development
during the last few decades and it has displaced other numerical
techniques into niche applications
assuming a dominant position in the market of engineering
analysis tools. Still, FEA is a relatively
new engineering tool that has evolved from being an exclusive
tool for highly trained analysts, to
the present day where it has become an everyday tool of design
engineers. Deeply rooted inmathematics and developed, often
independently by competitive commercial firms, FEA shows
discrepancies in development of terminology, which has not yet
been unified across the industry.
Users of different FEA programs may use different terminology
for the similar problems or use the
same term describing different things. Constraints, restraints,
supports and fixtures may all mean
the same for some people while others will understand them
differently. Many FEA users will
argue that loads and boundary conditions are different entities;
while other will say that loads are
just one type of boundary condition because they are applied to
the boundary of a model (loads
external to the model are in fact boundary conditions, volume
loads are not). Make sure you
understand what is meant by each term you use and do not be
afraid to ask what exactly does it
mean that element "locks" or what is "A nonconforming hexahedral
element" when you hear such
a term. Many of those terms come from legacy sources and have
long lost their relevance in
modern programs such as SolidWorks Simulation.
While volumes could and in fact should be written about FEA
terminology, here we will only
review terminology issues that apply to names of analysis types
used by SolidWorks Simulation.
As you know, the following studies are available: Static,
Frequency, Buckling, Thermal, Drop
test, Fatigue, Nonlinear, Linear Dynamic and Pressure Vessel
Design . Don't take each name
literally as a short description of the analysis capabilities of
each study. Instead treat them just as
labels, here is why:
Static
This can be linear static analysis or nonlinear static analysis
however nonlinear analysis is limited
to large displacements and/or contact. In nonlinear analysis
conducted under a Static study, the
user has no control over the load time history which must be
linear ("ramping-up" the load at auniform pace). Nonlinear material
is not available.
Frequency
A common name for this type of analysis is modal analysis as you
find in every textbook on
vibration analysis. Modal analysis finds natural frequencies and
the associated shapes of vibration.
A combination of frequency and shape is called a mode of
vibration. Modal analysis does not find
displacements, strains and stresses.
Buckling
This is linear buckling analysis which finds buckling load
factors and the associated bucklingshapes. The name "Eigenvalue
based buckling analysis" is sometimes used. Linear buckling
analysis does not say how far a structure will buckle or if it
will survive buckling. To solve these
questions, you must use a nonlinear buckling analysis which is
available in Simulation under
Nonlinear analysis.
Thermal
Thermal analysis can be executed as Steady State thermal
analysis or Transient Thermal
analysis and is utilized to find temperatures, temperature
gradients and heat flux. Notice that
thermal stresses are not calculated in thermal analysis; they
are calculated in Static orNonlinear
analysis using the temperature results from Thermal
analysis.
Drop Test
This is a specialized type of analysis intended for analysis of
collision between two bodies. This is
dynamic analysis based on the direct integration method, which
is stable but very time consuming.
Fatigue
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Fatigue analysis used results of Static analysis to calculate
fatigue life under cyclic loads.
Nonlinear
Nonlinear analysis will do everything that Static analysis can
do and much more but at a higher
computational cost. All types of nonlinear behaviors can be
analyzed including nonlinear buckling
and nonlinear materials. Simulation features an extensive
library of nonlinear materials available
in a Nonlinear study. Beware of common misconception that the
only reason why Nonlinear
analysis may be required is nonlinear materials. In this book we
have presented many examples
where other types of nonlinear behavior were present.
Additionally, Nonlinear analysis can beexecuted as static or
dynamic. And so it is more general than Linear Dynamic
analysis.
Linear Dynamic
This should be really called Linear Vibration analysis. Remember
that FEA is a tool of structural
analysis and as such, deals with elastic bodies. Any motion of
elastic bodies can only take a form
of vibration about the position of equilibrium. Linear Dynamic
(Vibration) analysis is based on the
Modal Superposition method and this makes it very numerically
efficient, but less general than
Nonlinear Dynamic (Vibration) analysis. Linear Dynamic analysis
has four sub-categories in
Simulation: Modal Time History, Harmonic, Random Vibration
Analysis and ResponseSpectrum Analysis.
Modal Time History
Vibration analysis textbooks call this Time Response analysis
(the term Dynamic Time is also
used). This analysis is intended for problems where load is an
explicit function of time.
Harmonic
Vibration analysis textbooks often call this Frequency response
(the terms Steady State
Harmonic analysis and Dynamic Frequency analysis are also used).
This analysis is intended
for problems where load is a function of frequency which in turn
is a function of time. It is
assumed that frequency changes very slowly (if at all), hence
the alternative name: Steady
State Harmonic analysis.
Random Vibration Analysis
Here, loads are given as a Power Spectral Density (PSD) of
displacements, velocities or
accelerations. Results such as RMS and PSD displacements,
velocities and accelerations are
calculated only in probabilistic terms.
Response Spectrum Analysis
This analysis is intended for excitation loads of longer
duration that are non-stationary and
therefore, cannot be presented as PSD. Instead, the excitation
is presented as a Response
Spectrum which is useful to analyze events such as
earthquakes.
Pressure Vessel Design
This analysis offers a convenient way of superposing results of
different Static studies as required
in the analysis of pressure vessels for compliance with safety
codes. Notice that a Pressure Vessel
Design study can be used to analyze superposed results of
anything, not just pressure vessels.
