- What
Wh t Is
I Needed
N d d To
T Run
R A Finite
F it Element
Fi
El
tA
Analysis?
l i ?-
Wh t Is
What
I Needed
N d d To
To Run
R A Finite
Fi it
Element Analysis?
?
Thiss cchapter
apte includes
c udes material
ate a from
o tthe
e boo
book “Practical
act ca Fin ite
te Element
e e t Analysis”.
a ys s Itt a
also
so has
as bee
been reviewed
e e ed a
and
d has
as
additional material included by Matthias Goelke and Jan Grassmannsdorf
smannsdorf.
Basic Information Needed To Run A Finite Element Analysis
n a high
g level summary,
y, the “working”
g steps
p involved in a finite element
e
analysis
y mayy be categorized
g
as:
•
Modeling (pre
(pre-processing)
processing)
•
Solution
•
Vi li ti off solution
Visualization
l ti results
lt (post-processing)
( t
i )
This image depicts the three elementary working steps involved in a FEM analysis.
analysis Some details about the individual steps are
summarized below.
below
2
2.
M d li g / Pre-Processing
Modeling
P P
i g
CAD Data
D t
Most commonly,
y, an FEM simulation p
process starts with the impor
p rt of the component’s
p
((or p
part’s)) CAD g
geometryy ((e.g.
g CATIA,, STEP,,
UG IGES
UG,
IGES, solidThinking,
solidThinking etc.) into the pre
pre-processor
processor i.e. HyperMessh
In many cases,
cases the imported geometry is not ready for meshing.
meshing Qu
uite often the geometry needs a cleanup first due to
•
“b k ” surfaces
“broken”
f
•
surfaces which are not stitched together
g
•
redundant (multiple) surfaces
•
surfaces which are too small to be meshed in a reassonable way later on
1
- What
Wh t Is
I Needed
N d d To
T Run
R A Finite
F it Element
Fi
El
tA
Analysis?
l i ?-
Another issue related to g
geometryy is depicted
p
in the following
g ima
age:
g
In the image on the left, the imported geometryy is shown. Note the
t lateral offset of the green edges. Here, the surface edges (in
green) do not meet at a single point i.e. there is a very small latteral offset of the surface edges. As meshing is carried out with
respect to the surfaces,
surfaces this small offset will be automatically take
en into account during meshing,
meshing which,
which unfortunately will result in
very poor quality
lit elements.
l
t The
Th image
i
i the
in
th middle
iddl depicts
d i t the
th meshed
m h d “initial”
“i iti l” geometry.
t Note
N t how
h the
th mesh
h is
i locally
l ll distorted.
di t t d
The updated
p
((cleaned)) and meshed g
geometryy is shown on the righ
ght.
Here,
H
r the
th surface
rf
edges
dg (in
(i green)
gr
) do
d nott meett in
i a single
i gl point,
i t i.i.e. there
th r is
i a very
r smallll llateral
t r l offset
ff t off the
th surface
rf
edges.
dg
A
As
meshing
g is carried out with respect
p
to the surfaces, this small offsset will be automaticallyy taken into account during
g meshing,
g
which, unfortunately will result in very poor quality elements.
Once these
O
h
“h dl ” are mastered,
“hurdles”
d one needs
d to ask
k whether
h h alllll the
h CAD information
i f
i is
i really
ll needed.
d d What
Wh about
b
li l fillets
little
fill
and
a
d rounds,
ou ds, ttinyy holes
o es o
or e
even
e co
company
pa y logos
ogos which
c ca
can o
often
te be found
ou d in C
CAD data
data? Do
o tthey
ey really
ea y co
contribute
t bute to tthe
eo
overall
ea
performance of the component?
Meshing
g
Once the geometry is in an appropriate state,
state a mesh is create
ed to approximate the geometry.
geometry Either a beam mesh (1D),
(1D) shell
mesh
h (2D) or a solid
lid mesh
h (3D) will
ill be
b created.
t d This
Thi meshing
hi step
t p is
i crucial
i l to
t the
th finite
fi it element
l
t analysis
l i as the
th quality
lit off the
th mesh
h
directlyy reflects on the q
qualityy of the results g
generated. At the same time,, the number of elements ((number of nodes)) affects the
computation time.
time That is the reason why in certain cases a 2D and 1D mesh is preferred over 3D mesh.
mesh For example,
example in sheet
metals a 2D approximation
appro imation of the structure
str ct re uses
ses much
m ch less elem
ments and thus
th s reduces
red ces the CPU time (which
( hich is the time while
hile you
o
are waiting
g for yyour results).
)
See the p
picture above for structures that are typically
yp
y meshed with
w 1D,, 2D and 3D elements. Which element type
yp would yyou
choose for which part?
2
- What
Wh t Is
I Needed
N d d To
T Run
R A Finite
F it Element
Fi
El
tA
Analysis?
l i ?-
Despite
p the fact that meshing
g is ((at least optionally)
p
y) a highly
g y auto
omated p
process, mesh q
quality,
y its connectivityy ((i.e. compatibility),
p
y)
and element normals needs to be checked. If necessary, these element
e
“issues”
issues may need to be improved by updating (altering)
the underlying geometry or by editing single elements.
elements
Material And Property
p y Information
After meshing is completed,
completed material (e.g.
(e g Young’s Modulus) and
d property information (e.g.
(e g thickness values) are assigned to the
elements.
Loads Constraints And Solver Information
Loads,
Various loads and constraints are added to the model to repr
present the loading
g conditions that the p
part(s)
( ) are subjected
j
to.
Different load cases can be defined to represent different loading
g conditions on the same model.
model Solver information is also added
to tell the solver what kind of analysis is being run,
run which results to
t export,
export etc.
etc
To determine your relevant loads, your engineering skills are nee
eded. Think of all kinds of load situations that can occur on your
structure and decide whether you want to use them in your sim
mulation or not.
not To determine the load from a static or dynamic
event,
t a Multibody
M ltib d Simulation
Si l ti (MBD) might
ight be
b helpful.
h l f l
The FEM model (consisting of nodes,
nodes elements,
elements material propertties,
ties loads and constraints) is then exported from within the pre
preprocessor HyperMesh.
HyperMesh The exported FEM model,
model typically called solver input deck,
deck is an ASCII file based on the specific syntax of
the FEM solver chosen for
f the analysis (e.g. RADIOSS
OSS or OptiStru
O S uct). A section out off an OptiStruct
O S
solver deck is depicted in the
figure below.
As you will see, the bulk of information stored in the analysis file is related to the definition of nodes (or grids). Each single node is
defined by its nodal number (ID) and its xx-,, yy- and z coordinates. Each element is then defined by its element number (ID) and its
nodes (IDs are referenced).
referenced) This completes the pre-processing
pre processing ph
hase.
hase
3
- What
Wh t Is
I Needed
N d d To
T Run
R A Finite
F it Element
Fi
El
tA
Analysis?
l i ?-
3.
Solution
During the solution phase off a simple linear static analysis or an eigenfrequency
e
f
study, there is not much for
f you to do. The default
f
settings of the Finite Element program do handle these classes of
o problems pretty well. Practice will show you that if the solution
process is aborted by an “error”
error , it is due to mistakes you have ma
ade during the model building phase.
phase Just to mention a few typical
errors:
•
Element quality (http://altair
(http://altair-2
2.wistia.com/medias/rm
wistia com/medias/rm
mretoumym)
•
Invalid material properties
•
Material property not assigned to the elements
•
I ffi i tl constrained
Insufficiently
t i d model
d l (the
(th model
d l shows
h
a rigid
ri id body
b d motion
ti due
d to
t external
t
l loads)
l d )
Some of these model issues are discussed in a free video seriess available on the Academic Training Center (http://training.
(http://training
altairuniversity.com/e-learning/by-altair-_2/hypermesh-related/fr
l i i
i
/ l
i /b l i 2/h
h l d/free-hyperworks-starter-kit-video-series/)
h
k
ki id
i /)
44
4.4
Vi
Visualization
li ti / Post-Processing
P tP
i g
Once the solution has ended successfully,
successfully post-processing (in HyperView
H
for contour plots and HyperGraph for 2D/3D plots) of
th simulation
the
i l ti results
lt is
i done
d
next.
t Stresses,
St
strains,
t i and
d deform
d f mations
ti
are plotted
l tt d and
d examined
i d to
t see how
h the
th partt responded
d d
to the various loading
g conditions. Based on the results,, modificattions mayy be made to the p
part and a new analysis
y mayy be run to
examine how the modifications affected the part.
part
This eventually completes the FEM process.
process
Remarks
•
Practice will show, that in many projects, the above depicted
d process must be re
re-entered
entered again, because simulation results
indicate that the part is not performing as requested.
requested
It is
i quite
it obvious
b i
th t going
that
i back
b k to
t CAD (to
(t apply
l changes)
h
) and
a d working
ki through
th
h the
th entire
ti FEM process becomes
b
t di
tedious.
A very efficient (and exciting) technology to speed up this process is
i called Morphing.
Morphing Employing morphing allows the CAE engineer
to modify the geometry of the FEM model,
model e.g.
e g change radii,
radii thickn
ness of ribs,
ribs shape of hard corners,
corners etc.
etc Quite often the morphed
4
- What
Wh t Is
I Needed
N d d To
T Run
R A Finite
F it Element
Fi
El
tA
Analysis?
l i ?-
FEM model can be exported
p
instantaneouslyy ((without anyy reme
eshing)
g) allowing
g the CAE engineer
g
to re-run the analysis
y of the
modified part on the fly.
An example of morphing a given finite element model is depicted below (http://altair-2.wistia.com/medias/dp9q29f3jn)
(http://altair 2 wistia com/medias/dp9q29f3jn)
A nice introduction in morphing is given in the video below (http:///altair-2.wistia.com/medias/xycj10c4y2).
/altair 2.wistia.com/medias/xycj10c4y2).
Note:
The individual
Th
i di id l working
ki steps
t
off the
th FEM process are nott only
l sub
b t d to
bj
bjected
t many “user”
“
” errors e.g. typo
t
while
hil defining
d fi i material
t i l or
loads. A lot of attention must also be p
paid to the chosen modelin
ng
g assumptions
p
((for instance,, simplification
p
of g
geometry,
y, chosen
element type and size,
size etc.).
etc ) Even though the FEM solver may detect
d
some of the most striking errors,
errors the likelihood that your
res lts have
results
ha e bypassed
b passed “errors” is high.
high
The following chapters aim at creating awareness about FEM challenges and pitfalls.
5