8 finite element modelling
TRANSCRIPT
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FE-MODELING
Bertil Jonsson
Expert weld strength
Nov 2011
2
Content General
Program overview
Boundary conditions
Accuracy
Non linearity
Sub modeling
3D-models
Some notes about software
Summary
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General
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Method used determines often the tool
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Program overview
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Some software tools
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Boundary conditions
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A HAULER AT WORK
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Boggi beam
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GEOMETRY REAR FRAME
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GEOMETRY BOGGI BEAM
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Example bearing in boggi
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Example bearing in boggi
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Project LOST example of HSS Light Optimized STructures
Duration 2006-2009
Budget appr 18 mSek
End-conference 24-25 march 2010 in Sweden
13 Work-packages, 3 of them are
New weld class system
Analysis course on local based methods
Demonstrator boggi beam
Education &
seminars planned
Continuation
project: WIQ
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LOST Workpackage no 10GOAL : decrease weight by at least 20%
Demonstrator boggi beam
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Old BB compare to the new
Old New
t = 15 / 8 mm t = 12 / 6 mm
183 kg 143 kg (-22%)
Mtrl grade: 350 Mpa Usage of high strength steel (HSS)
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Max static load material grade
Yield 350 Mpa at bearing
Yield 460 Mpa in flanges
Yield 600 Mpa in webs
Control of buckling
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Fatigue analysis using local based methods
Equivalent load from test track
Main principal stress range + FAT 225 Mpa
Fatigue life > 1000 hours on test track
allowed stress level 370 Mpa
Radius = 1 mm
Notch method
Loads on global model Transferring displacements
local sub models and study
the stresses
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Test set up
Rig setup Spectrum load (range pair)
Range Pai r
0
100
200
300
400
500
600
10 10 0 10 00 10 00 0 100 00 0
Antal ranger
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Test of boggi no 1, failure after 120 h
Expected > 1000 h
Failure after 120 hours Lack of fusion
(demand penetration i = 2 mm)
However :
Chocking !!! this could not be the only reason
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Multi axial stresses !
Normally This case is in shear loading
max Principal Stress range use von Mises (diff signs !) & FAT 200
PS = 516 Mpa vM = 745 Mpa
N = 420 hours N = 100 h [agrees well with test 120 !]
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Test of boggi no 3 failure after 920 h
Fully penetrated on root side
Now failure from toside !
Same cause: Multiaxial stress state with different signs
in principal stresses
PS = 241 Mpa vM = 393 Mpa
N = 4100 hours N = 660 h (appr in agree with test 920 h)
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Multi axial proportional cases
- effective notch method -
Use Mises stress range & FAT 200 Mpa
in cases when Mises > Max principal stress range
Comparison of life calculation in notch method
( multi axial cases with diff. signs in principal stresses, proportional loads )
10
100
1000
10000
10 100 1000 10000
Tested life (hours BII)
Calculatedlife(hoursBII)
Mises
Princ stress
Equality
Non c
onser
vative
Conse
rvative
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Cost estimations
Lowered weight & production cost
win-win situation for customer & producer
COST ESTIMATION (% of total) OF NEW BOGGI BEAM
-10
-8
-6
-4
-2
0
2
4
6
slit r emo ved mtrl+cut plate a way fixture
improved
extra weld weld
prearation
TIG
treatment
Totally
Percentage
(%)
-8%
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Demonstrator Boggi Beam results:
Lowered weight 22%
Using HSS & improved weld quality
Life target almost reached
Lower production cost 8 %
Lessons learned:
Multiaxial problem found and verified
Use Mises when > principal stresses
A high production quality is needed !
Process variation & control
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Accuracy & non linearities
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A-stay
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Assembly of an A-stay
with studied welds
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FE-models
SimplifiedSimplified linearlinear
elasticelastic modelmodel
ComplexComplex modelmodel
includingincluding screwsscrews,,
axleaxle casingcasing,,
contactcontact elementselements
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Global model
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Sub model
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Example of the NOTCH-method
Rear frame of a loader
(without a weld prepared plate)
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GEOMETRY OF REAR FRAME
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Section through the sub model
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Result symetric load
Global model w/o notch
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Zoomed picture
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Submodell incl notch
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Submodell incl notch
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Submodell incl notch
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Submodell incl notch
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Sectio through the sub model
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Zoom around the weld / notch
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Result w/o deformation
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Result w deformation
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Max principal stress in the notch
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Zoom, max stress = 1932 MPa
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Hand calculation on white board !
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Brottmekanisk ansats
Global modell
Submodell 1
Submodell 2
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Fracture mechanicsFracture mechanics
> @)4()4(21
21122
CBCBI vvvvl
GK
S
N
mKCdN
da)(* '
)/( WafaK SV''
'
fa
a
mda
KCN
0
1
ParisParis lawlaw DDisplacementisplacementcorrelation techniquecorrelation technique
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Automatic crack propagation in
3D
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Tvrsnitt genom submodellen
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XCRACK
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Submodel 2Section through weld with crack
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Submodel 2 swept to 3D
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Submod 2 merged into submod 1
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Result in section
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Zoom at the root crack
Opening of the crack determinesthe stress intensity
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Stress intensity along the sub model
Bakrams-spricka
SPG-INT lngs axelgenomfringen
0,0
2,0
4,0
6,0
8,0
10,0
12,0
14,0
0 200 400 600 800 1000
s (mm)
dK[(MPa_
rot(m)
]
Chan
TH=2
TH=5
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3 sub models are analysed:
Spricklngd Spg-intensitet Lutning
a=8 mm K = 12 MPam (16-12)/(12-8) = 1
a=12 K = 16 (40-16)/(16-12) = 6
a=16 K = 40
Approx. bi-linjr fkn : K = 1*a+4 resp 6*a-56
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Stress-intensity as fcn of crack growth
Antag linjr fkn : K = a+4 resp 6a-56
Spnnings-intensitet sfa sprickvg
0
5
10
15
20
25
30
35
40
45
5 7 9 11 13 15 17
Sprickstorlek (mm)
Spg.int(Mparot(m))
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Hand calculation on white board !
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Comparison of calculated lifes
Konservativ berkning kan vara fr skert
Notchmetod stmmer bra med brottmekanik
Livslngd med olika metoder
0
50000
100000
150000
200000
250000
300000
350000
400000
450000
1 rt linje 2 rta linjer krkt linje notch-metoden
Livslngd(cykler)
N
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More about accuracy
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Use the right stiffness in the
FE-model...
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Arm to excevator
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B/BRG
Initial root
crack
4mm~6mm
Field Crack Section View
CRACK PATTEN
Section view
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LEFM Analysis of arm
[Case A]
16t
2mm
2mm
2.8mm
5.6mm
4mm
Sub1
Sub2
Sub3
Sub4
Model Cases
Digging Force Side Force
Load Cases
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Definition of Crack Direction in Sub1 Model
- Principal Max Stress Direction Check
Crack Growth Direction
Principal Max Drection
2mm
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L
Sub2 Model Analysis Result
- Sub2 Model
2.8mm
Principal Max Directio
Crack Growth
Direction
Keff = 28.75
Digging Force
Keff = 1.57
Side Force
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LEFM Analysis of arm
2mm
2.8mm
5.6mm
4mm
Sub1
Sub2
Sub3
Sub4
Model Cases
Digging Force Side Force
Load Cases
[Case B]
18t
2mm
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Arm FE Model ( Case B )
[ Analysis Result ] [ Coupled Model ]
[ Sub Model ]
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Conclusion
Bench Test Loading
Failure Life=99677(Cycle)
Failure Life=79095(Cycle)
Initial Crack length : 2mm
Failure Crack length : 25mm
(Median 50%)
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Cold lap
Spatter induced cold lap
Compared with
line cold lap
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What is the difference between a line
cold lap and a spatter induced cold lap
?
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Geometry spatter induced cold lap
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Cold lap 2D vs 3D
Question: what is the expected increase in life for a spatter induced
cold lap compared to a line cold lap ?
Spatter induced: has a/c=1 at start, this requires a 3D-model &
analysis.
Line cold lap: has a/c=0 at start, can be calculated in a 2D-
model.
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Studied case
Korsprovstav:
t = 12 mm
= 100 MPa
a = 5 mm = 70 grader
R = 0
i = 2 mm
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2D analysis in FRANC2D
of
a line cold lap
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2D-modellen (a/c=0)
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3D analysis in FRANC3D
of
a spatter induced cold lap
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Initial flaw: cold lap of crack size a=0.2 [mm] and a/c=1
(fringe values are crack opening displacement in [mm])
(deformation magnified 5)
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Crack trajectory at step 3(fringe values are crack opening displacement in [mm])
(deformation magnified 5)
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Crack trajectory at step 8(fringe values are crack opening displacement in [mm])
(deformation magnified 50)
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Crack trajectory at step 13(fringe values are crack opening displacement in [mm])(deformation magnified 5)
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Resultat 3D
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Resultat 3D
a/c goes to 0 relatively slow Is appr 0,1-0,2 at full life (N=2,7E6 cycles)
N is calculated along the edge with K=K(c) (not area) and
with extrapolated.
Mixed mode in the beginning, there after mode I KI+KII at bottom gives a great kink-angle KI+KIII at edge gives a small kink-angle
Crack grows deepest at bottom (comp kink) Similar conditions as in the 2D-analysis
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Result in lifeLife for cold lap = 0,2 mm - comparison 2D (a/c=0) with 3D (a/c=1)
( crusiform joint, t12, a5, 70 deg, R0, 100 MPa )
0
1
2
3
4
5
6
0,0E+00 5,0E+05 1,0E+06 1,5E+06 2,0E+06 2,5E+06 3,0E+06
LIFE (cycles)
Crac
kgrowth(mm)
3D (a/c=1)
2D (a/c=0)
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Comparison line CL vs spatter CL
Life N cold lap from spatter = 2,7E6
Life N lilne cold lap = 1,0E6
Thus is N appr 3 times bigger
Edge crack has K(a/c=1) 0,63*K(a/c=0)
Theoretical longer N is then (0,63) -3 4
Fits well with 3 above
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Some notes on softwares
Ansys Classic fracture mechanics 2D
Ansys WorkBench notch method 3D
Afgrow fracture mechanics 2D
(load plane)
Franc2D fracture mechanics 2D
(load//plane)
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Ansys Classic
fracture mechanics, 2D
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Ansys Classic
fracture mechanics, 2D
Result:
KI, KII, KIII for a point
Repeat of growth points
Integration gives life
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Ansys WorkBench (+DesignModeler)
notchmetoden 3D
Result:
Stresses in the notch
SN-curve gives life
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A new layout is seen in WB.12
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Afgrow
fracture mechanics in 2D
Result
Cycles = life
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Afgrow, result
Result
Cycles = life
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Franc2D
Automatic crack growth in 2D
Result
KI, KII, KIII as fcn
of crack path.
(Integration of
Paris law
gives life)
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Summary
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Some short conclusions
Daily work: Use notch-method
Failure analysis: Use linear fracture mechanics
Free of charge ! 2D: Agfrow (?) and Franc2D
Be careful with: Contacts, stiffnes
Do not forget: Root side of welds
Useful technique: Sub-modeling