Fatigue_Assessment_of_Weld_Joints_Using_ANSYS_Verity_and_FESafe.pdf
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Transcript of Fatigue_Assessment_of_Weld_Joints_Using_ANSYS_Verity_and_FESafe.pdf
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Fatigue Assessment of **
Weld Joints Using ANSYS,
Verity & FE-Safe
Zhichao Wang
Aditya Sakhalkar
5/8/2007
** Verity is a weld fatigue assessment software
developed by Battelle based on Structural
Stress & Fracture Mechanics theory proposed
by Dr. Dong et al, Battelle
Part I: Key Points on Equivalent SS Method
Part II: Example Application
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Part I: Key Points on SS & Weld Assessments
Zhichao Wang
Sr. Lead Engineer
Emerson Climate Technology Inc.
Background
Why The Battelles Structure Stress Is Not Sensitive To FE Mesh Size?
How Could Multiple Joint S~N Curves Be Reduced to A Single S~N Curve, the Master S~N Curve?
Example Application (Part ii, Aditya)
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Background
Fatigue Assessment Unwelded Structures
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Fatigue Assessment Welded Structures
Its been commonly recognized that the fatigue life of the polishedspecimen is dominated by fatigue crack initiation, whereas that of
welded structures is dominated by small crack propagation from some
pre-existing discontinuity.
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(i). Nominal Stress Method (BS,IIW)
The nominal stress range is used to develop the S~N curves using
samples with actual weld joint geometry. The life curves refer to
particular weld details, there is no need for the user to attempt to quantify
the local stress concentration effect of the weld detail itself.
(ii). Hotspot Stress Approach (structural stress, geometric stress,
BS, IIW, CEN,DNV) This procedure uses hot-spot stress range as a parameter. The SN curves are obtained from tests of actual welded joints based on the hot-
spot stress range rather than the nominal stress range.
(iii). Local Notch Stress Method (ASME, BS, IIW). The notch stress approach attempts to include all sources of stress
concentration in the stress used with the design SN curve. Thus a single SN curve may be sufficient for a given type of material. The problem is that the local geometry of the toe or root of a weld is highly variable. It
may be hard to achieve consistent results.
Fatigue Assessment Welded Structures
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Fatigue Assessment Welded Structures
(iv). The Fracture Mechanics Approach (For crack propagation life)
The parameter widely used is SIF, K. The fatigue resistance is represented by
fatigue crack growth rate da/dN. Many crack propagation laws are available. The
simplest one is Paris Law in which the crack growth law approximates to a linear
relationship:
For a flaw size starts from a0 to a critical fatigue crack size af, the remaining
fatigue life N under stress range S is obtained by integrating Eq (1):
(v) Verity Equivalent Structural Stress Method (Battelle)
Equivalent Structural Stress + Single Master S~N Curve
nda C ( K) (1)dN
f
0
a
na
daC N (2)
( K)
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Structural Stress & Mesh Sensitivity
(b) Comparison of SCF predicted by various modeling
procedures and extrapolation based HSS at the weld toe [12].
(b) Comparison of SCF predicted by various modeling
procedures and extrapolation based HSS at the weld toe [12].
(c) Structural Stress Using Verity Method(c) Structural Stress Using Verity Method
Hot Spot Nomina
Nominal
l
FE
(Human Factor)
~
0
x
X~0.4 t
Hot Spot
FE
Nominal
(a) Normal Stress at the sharp corner
(Structural Stress)t
x sp
A
A
P
Y
X
0.4 t
A
A
t
Hot Spot Nomina
Nominal
l
FE
(Human Factor)
~
0
x
X~0.4 t
Hot Spot
FE
Nominal
(a) Normal Stress at the sharp corner
x
X~0.4 t
Hot Spot
FE
Nominal
(a) Normal Stress at the sharp corner
(Structural Stress)t
x sp
A
A
(Structural Stress)t
x sp
(Structural Stress)t
x sp
A
A
P
Y
X
0.4 t
A
A
tP
Y
X
0.4 t
A
A
A
A
t
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Why Structure Stress Is Not Sensitive To FE Mesh?
(a)
(b)
(c) Structural Stress Using Verity Method(c) Structural Stress Using Verity Method
N
P1P2
Q
m
P1 P2
P1 P2N/
Q/
m/
P1P2
t
X
Y
Fx
P1P2
Fy
Neutral axis
t tmtb
)t( )tm( )
tb(
N
P1P2
Q
mN
P1P2
Q
m
P1 P2P1 P2
P1 P2N/
Q/
m/ P1 P2N/
Q/
m/
P1P2
t
X
YP1
P2
P1P2
t
X
Y
Fx
P1P2
Fy
Neutral axis
Fx
P1P2
Fy
Fx
P1P2
Fy
Neutral axis
t tmtb
t tmtb
)t( )tm( )
tb()t( )
tm( )
tb(
Calculation of Structure Stress3-D Solid Element:
i. Calculate nodal force
ii. Transformation of nodal force to neutral
axis to obtain resultant forces (N, m).
iii.Calculation of structural stress
Shell Element:
i. Calculate nodal force ( N/, m/, Q/ )
ii. Calculation of structural stressBecause equilibrium has to be satisfied
Thus: m/=m
Q/=Q And
N/=N
tm
N ()=
A
tb
m ()=
W
,/ /
t t t t
m m b b = =
The equations in the calculation of
structural stress are the same. Hence, the
structural stress is mesh independent
Calculation of Structure Stress3-D Solid Element:
i. Calculate nodal force
ii. Transformation of nodal force to neutral
axis to obtain resultant forces (N, m).
iii.Calculation of structural stress
Shell Element:
i. Calculate nodal force ( N/, m/, Q/ )
ii. Calculation of structural stressBecause equilibrium has to be satisfied
Thus: m/=m
Q/=Q And
N/=N
tm
N ()=
A
tb
m ()=
W
,/ /
t t t t
m m b b = =
The equations in the calculation of
structural stress are the same. Hence, the
structural stress is mesh independent
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Master S~N Curve
Most codes divide weld joints into different types
Categorization of Weld Joints (BS 7608) Weld Joints (IIW)
Weld Joint Categorization
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Multiple S~N Curves Due to Weld Joint Categorization
Example Design S~N curves for
welded joints: (a) Steel weld joint S~N
curves (BS 7608); (b) Weld joint type
and S~N curves (IIW
recommendations); (c) Aluminum weld
joint S~N curves (IIW
recommendations) (a)
(b)
(c)
Multiple S~N curves provide flexibility
for the selection of life curves and
increase difficulty to select the proper
one due to the variation of actual
joints
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How Could multiple joint S~N curves be
merged into one - Master S~N curve?
Nominal Stress Method (BS) Hot Spot Stress Method (IIW)
Equivalent SS Method Weld Joints (IIW)
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Small Crack Propagation Behavior
The data does not fall into one curve for the same specimen;
Multiple da/dN curves obtained for the same or different
specimens, which tells that the Paris Law does not hold for
small crack growth. This is called anomalous crack growth
behavior.
(g) K-correlated small
crack vs. large crack
growth, 7075-T6 AI [13]
The curves show that
small crack growth faster
than corresponding large
cracks at the same value
of the driving force K.
Specimens for notch induced short crack growth test. (a)
Compact tension (CT) with keyhole by Ramulu; (b) single edged
notched (SEN) by Shin and Smith; (c) double edged notched
(DEN) by Shin and Smith; (d) center notched (CB) by Tanaka and
Nakai; (e) for CT specimen; (f) for three different
specimens
daK
dN
daK
dN
Specimens for notch induced short crack growth test. (a)
Compact tension (CT) with keyhole by Ramulu; (b) single edged
notched (SEN) by Shin and Smith; (c) double edged notched
(DEN) by Shin and Smith; (d) center notched (CB) by Tanaka and
Nakai; (e) for CT specimen; (f) for three different
specimens
daK
dN
daK
dN
Different factors may attribute to short crack anomalous growth such as the effect of crack
closure, micro structure interaction and that of notch details.
A unified SIF formulation for both short and long crack and a two stage crack propagation model
were proposed by Dr. Dong & his coworkers, Battelle, to use Equivalent Structural Stress as a
parameter for weldments fatigue life assessment.
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A Unified Stress Intensity Factor (SIF) Formulation
(a) Weld geometry with a hypothetical crack l ; (b) Actual normal stress distribution; (c)
Simplification; (d) Decomposition; (e) Equilibrium-equivalent structural stress or far-field
stress; (f) Self-equilibrating stress (notch stress) with respect to a reference depth t1.
( b )
t
A
A
xl 0
( b )
t
A
A
xl 0
( a )
A
A
t
l
( a )
A
A
t
l
( c )
t1t
1
2
3
1
2
3A
AR1
R2
x
( c )
t1t
1
2
3
1
2
3
1
2
3
1
2
3A
AR1
R2
x
t1t
1
2
3
A
A
t
A
A
t
t1t
1
2
3
A
A
( d )( e ) ( f )
t1t
1
2
3
A
A
t
t1t
1
2
3
A
A
t
A
A
t
A
A
t
A
A
t
t1t
1
2
3
A
A
t1t
1
2
3
A
A
( d )( e ) ( f )
/ t
x= - / t
x = / t
x= - / t
x =
t1/t=0.1
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A Unified Stress Intensity Factor
t ( )( ) (0 l t) sn KK = K
Notch Stress
Structural Stress, Far field stress
Notch Stress
Structural Stress, Far field stress
The drive force for crack to start and grow is the crack tip stress, introduce crack
surface traction ps called self equilibrating surface traction due to (notch effect)
x
t /
s( ) (p , (0 l t) )( ) s s sK K K
t
l
t
l
( a ) ( b )A
A
t
A
A
t
A
A
t
t1t
1
2
3
A
A
t1t
1
2
3
A
A
( c )
t
sp
l
( d )
t
sp
l
tt
sp
l
( d )
x
t1t
3
A
Al
/
s x
t p = f( )= f( , )( e )
x
t1t
3
A
Al
/
s x
t p = f( )= f( , )( e )
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A Unified Stress Intensity Factor
t
s(p( ) (0 l t) ) n sK = KK
Notch Effect
Structural Stress, Far field stress
t
s(p( ) (0 l t) ) n sK = KK
Notch Effect
Structural Stress, Far field stress
Notch Effect
Structural Stress, Far field stress
t
l
( d ) ( e )A
A
t
( f )
t
sp
lt
l
( d )
t
l
( d ) ( e )A
A
t
( e )A
A
t
A
A
t
A
A
t
( f )
t
sp
l
( f )
t
sp
l
tt
sp
l
Notch Effect Notch Effect
Far Field SIF
Far Field SIF
Notch Effect Notch Effect
Far Field SIF
Far Field SIF
Stress intensity solutions
using published weight
function results: (a) remote
tension; (b) remote
bending.
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SIF Magnification Factor Mkn
t
s(p( ) (0 l t) ) n sK = KK
Introduction of notch induced SIF magnification factor Mkn :
t
t
s(p )( ) 1.0 (0 l t)( )
kn
sn
n
KM =
K
K
t( ) (0 l t) kn nK = M K
a/t a/ta/t a/t
Comparisons of stress intensity magnification factor Mkn at 135 sharp V notch for various
specimen geometries and loading conditions: (a) Edge crack solutions; (b) Elliptical crack
solutions for a/c = 0.4
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Mkn for 1350 V Notch Specimen
a/t a/t
Conclusions:
Mkn approaches unity as crack size a/t approaches 0.1 i.e., a/t=0.1 (short crack correction factor) Thus a/t=0.1 can be taken as a characteristic
parameter beyond which the notch effect is negligible
The difference between edge crack and elliptical crack solutions are not significant.
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Modified Paris Law
m
n
n m
kn n
Paris Law :
daC K (a)
dN
Introduce t
daCM K
dN
heUnified SIF :
(b)
Figures showed the
significant improvement
for the application of
Paris Law and its
application
More important the
crack starts from zero
that covers crack
initiation life
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Master S~N Curve
-m
s
f
m1
n m
2
n
kn
kn n
a a
n ma 0
m b
n
m
kn
Using the Unified Paris Law,
daN (b)
CM KIntroduction of
S ~ N curve can be obtained
1N t I(r) (c)
Cd(a / t)
Where,
generallized SIF ra
daC
I(r)M {f (a
M K (a)dN
nge
/ t) r [f (a / t) f
, leads
(a
to,
/ t)]
a / t 1
ma / t 0(d)
}
1 2 m 1 1
m 2m m m
2 m 1
2
1 1
m
m m
m
Then
C t I(r) N (e)
Introduce Equivalent Structrual Stress Range,
t I(r)
C N (f )
s
sss S
S
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Master S~N Curve
Nominal Stress Method Hot Spot Stress Method
Equivalent SS Method Weld Joints (IIW)
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Summary
The Structural Stress Method developed at Battelle is mesh insensitive that removes the uncertainty in the calculation of structural stress for weld joint fatigue assessment
The nature of weld joint fatigue is considered through the introduction of Equivalent Structural Stress based on fracture mechanics, which enable most fatigue curves of weld joints merged into a narrow band, the Master S~N curve. Thus one S~N curve can be used for majority of weld joints
Note: I believe that the weld joints have to be stress concentration dominant to achieve consistent results with test data.
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References
[1]. BS PD 5500: , BSI Standards, London, 2000.
[2]. European Standard for Unfired Pressure Vessels, EN 13445: 2002, BS EN 13445:2002, BSI, London, 2002
[3]. ASME Boiler and Pressure Vessel Code, Section VIII, Rules for construction of pressure vessels, Division 2-
Alternative rules, ASME, 2003.
[4]. Carl E. Jaske, FSRF for WPVP, Journal of Pressure Vessel Technology, AUGUST 2000, Vol. 122, 297-304
[5]. S.J.Maddox, Review of fatigue assessment procedures, Int. J. of Fatigue, Vol. 25, 12, 2003, 1359-1378.
[6]. Maddox S J: 'Fatigue aspects of pressure vessel design, Spence J and Tooth A S, E & F N Spon, London, 1994.
[7]. Harrison J D and Maddox S J: 'A critical examination of rules for the design of pressure vessels subject to fatigue
loading' in Proc. 4th Int. Conf. on 'Pressure Vessel Technology', Mech E, London, 1980.
[8]. Taylor N (Ed): 'Current practices for design against fatigue in pressure equipment', EPERC Bulletin No.6, European
Commission, NL-1755ZG, Petten, The Netherlands, 2001.
[9]. Dong, P., 2005, A Robust Structural Stress Method for Fatigue Analysis of Offshore/Marine Structures, Journal of Offshore Mechanics and Arctic Engineering , Vol. 127, pp. 68-74.
[10]. Dong, P., 2001, A Structural Stress Definition and Numerical Implementation for Fatigue Evaluation of Welded Joints, Int. J. Fatigue, 23/10, pp. 865876.
[11]. Dong, P., Hong, J. K, Osage, D., and Prager, M., Assessment of ASMEs FSRF Rules for Pipe and Vessel Welds Using A New Structural Stress Method, Welding In the World, Vol. 47, No. 1/2, 2003, pp. 3143.
[12] Dong, P., Hong, J. K., Osage, D., Prager, M., 2002, Master S-N Curve Method for Fatigue Evaluation of Welded Components, WRC Bulletin, No. 474, August.
[13]. Lankford, J. , Fatigue of Eng Mater and Structures 5 (1982), pp233-248
[14]. R. Craig McClung, et al, Behavior of Small Fatigue Cracks, ASME, Vol. 19, Fatigue & Fracture, P153
[15]. Fricke W., 2001, Recommended Hot-Spot Analysis Procedure for Structural Details of FPSOs and Ships Based on Round-Robin FE Analysis, ISOPE Proceedings, Stavanger, Norway, June.
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Part II: Example Application
Aditya Sakhalkar
Sr. Applied Mechanics Engineer
Emerson Climate Technology Inc.
Background ANSYS, Verity, Fe-Safe Weld Assessment Procedure ANSYS Preprocessing Verity Analysis Fe-Safe Analysis ANSYS Results Comparison of Analysis Results with the Test Results Conclusions
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Resistance welded suction fitting
Three failures in the suction fitting weld during reliability testing
Crack initiated at the weld toe (9 o clock) and propagated through the shell.
Failure due to reverse bending fatigue.
Background
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ANSYS, Verity, Fe-Safe Weld Analysis Procedure
Verity, Fe-Safe Weld analysis
Ansys Preprocessing
Ansys Post processing
- Meshing per Verity requirements
- Linear material properties
- Apply load
- Solution
Verity Analysis (Eq. Structural Stress
calculations along the weld line)
- Fe-Safe analysis (Weld life calculations
using Master S-N curve)
- Import .rst file
- Post processing
Verity result validation
and comparison
Reliability Testing
Calibration
Quantify loading
relative to the test
for FEA
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Suction Fitting
Shell
Weld Line
Element Type: Solid 186
Ansys Preprocessing- FE Model
Mesh Requirements
Shell Elements: 3D mid-surface element with no through-thickness dimension
Solid Element: Rectangular faces required along the through-thickness cut from the weld line
Regular mesh along the through thickness cut from weld line
Recommended elements: Hexahedral (brick) and Pentahedral (wedge)
Tetrahedral elements can be used. Requires special handling.
Hex elements along weld line Linear material properties
(E = 29,000 Ksi, = 0.29)
Symmetry BC
On sides
Top & Bottom
fixed
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ANSYS Results
eq1
Maximum Stress Equivalent Stress
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Verity Procedures
Structural Stresses calculated along the weld line at each node
Fe-Safe builds a connectivity table and maps the stresses to elemental stresses.
These stresses are inserted into the
stress matrix to be used for fatigue
evaluation
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Fe-Safe Analysis Weld life predictions using Master SN Curve
Fully reverse loading defined
Fe-Safe calculates weld fatigue life
based on the Battelles Master SN curve
The weld life (log N) data at each
node is written in the ANSYS .rst
format
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ANSYS Post processing Weld Life Results
Minimum life at element # 4436, node # 4212
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Comparison of Verity Predictions
with Reliability Test Results
Conservative life estimate!
Predicted Failure Location correlated well with the test failures!!
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ANSYS could be used for the fatigue assessment of welded joints in conjunction with Verity & Fe-Safe
Verity provides conservative estimate of the weld life
Good correlation with the predicted and test failure locations
Verity & Fe-Safe provides a excellent weld design tool in conjunction with ANSYS
Conclusions