2008 International ANSYS Conference · 2008 International ANSYS Conference Fatigue Assessment of Weld Joints Using ANSYS, ... (For crack propagation life) – The parameter widely

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2008 International ANSYS Conference

Fatigue Assessment of Weld Joints Using ANSYS, BSS & FE-Safe **

Zhichao Wang , Manager Of Analytical ServicesAditya Sakhalkar, Engineering SpecialistEmerson Climate Technology** BSS, Battelle Structural Stress Method Proposed by Dr. Dong and

His Colleagues at Battelle


Fatigue Assessment of Weld Joints Using ANSYS, BSS (Verity) & FE-Safe **

• Background• Battelle Structural Stress (BSS) Method & Mesh

Sensitivity• Master S~N Curve?• Example Application• References


BackgroundFatigue Assessment – Unwelded Structures


BackgroundFatigue Assessment – Welded Structures

• It’s been commonly recognized that the fatigue life of the polished specimen is dominated by fatigue crack initiation, whereas that of welded structures is dominated by small crack propagation from some pre-existing discontinuity.



• 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



• Hotspot Stress Approach (structural stress, geometric stress, BS, IIW, CEN,DNV)– This procedure uses hot-spot stress range as a

parameter– The S–N curves are obtained from tests of actual

welded joints based on the hot-spot stress range rather than the nominal stress range



• 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 S~N curve

– Thus a single S~N 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


• 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):

• Verity Equivalent Structural Stress Method (Battelle)– Equivalent Structural Stress + Single Master S~N Curve

nda C ( K) (1)dN

= Δ

f

0

a

na

da C N (2)( K)

=Δ∫



(b) Comparison of SCF predicted by various modelingprocedures and extrapolation based HSS at the weld toe [12].

(c) Structural Stress Using Verity Method

H o t S p o t N o m i n a

N o m i n a l

l

F E

( H u m a n F a c t o r )

~0 ∞

σ

σ

< σ <σ

xσ

X~0.4 t

H o t S p o tσF Eσ

N o m i n a lσ

(a) Normal Stress at the sharp corner

(Structural Stress)tσ

xσ sp

A

A

P

Y

X

0.4 t

A

A

t

Battelle Structural Stress (BSS) Method & Mesh Sensitivity


Calculation of Structure Stress3-D Solid Element:i. Calculate nodal forceii. 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 satisfiedThus: m/=m

Q/=Q AndN/=N

tm

N (σ )=

A

tb

m (σ )=

W

( ) ( ),/ /t t t t

m m b bσ = σ σ = σ

The equations in the calculation ofstructural stress are the same. Hence, thestructural stress is mesh independent

NP1

P2

Q

m

P1 P2

P1 P2N/

Q/

m/

P1P2

t

X

Y

Fx

P1P2

Fy

Neutral axistσ t

mσ tbσ

)′t(σ )′tm(σ )′t

b(σ

NP1

P2

Q

mN

P1P2

Q

m

P1 P2P1 P2

P1 P2N/

Q/

m/ P1 P2N/

Q/

m/

P1P2

t

X

Y P1P2

P1P2

t

X

Y

Fx

P1P2

Fy

Neutral axis

Fx

P1P2

FyFx

P1P2

Fy

Neutral axistσ t

mσ tbσtσ t

mσ tbσ

)′t(σ )′tm(σ )′t

b(σ)′t(σ )′tm(σ )′t

b(σ

Why BSS Is Not Sensitive To FE Mesh?

(c) Structural Stress Using Verity Method(c) Structural Stress Using Verity Method


Most codes divide weld joints into different typesCategorization of Weld Joints (BS 7608)

Weld Joints (IIW)

Master S~N CurveWeld Joint Categorization


Example Design S~N curves for weldedjoints: (a) Steel weld joint S~N curves(BS 7608); (b) Weld joint type and S~Ncurves (IIW recommendations); (c)Aluminum weld joint S~N curves (IIWrecommendations)

(a)

(b)

(c)

Multiple S~N curves provide flexibilityfor the selection of life curves andincrease difficulty to select the properone due to the variation of actual joints

Master S~N CurveMultiple S~N Curves


Nominal Stress Method (BS) Hot Spot Stress Method (IIW)

BSS MethodWeld Joints (IIW)

Master S~N Curve


(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

xσl 0

( a )

A

A

tl

( c )

t1

t

1

2

3

1σ

2σ

3σA

A R1

R2

′xσ

t1

t

1

2

3

A

A Δσ

tσ

A

A

tσt1

t

12

3

A

A

Δσ

( d )( e ) ( f )

/ tx= -σΔ σσ/ t

x =σ σ + Δσ

Master S~N Curve?A Unified Stress Intensity Factor


t ( )( ) (0 l t)Δσ + < <σsn KK = KNotch 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 Δσ (notch effect)

xt /

s( ) (p , (0 l t) )( )Δσ ≈ = σσ < <s s sK K K

tl

( a ) ( b )A

A

tσt1

t

12

3

A

A

Δσ

( c )

t

sp

l( d )

′xσt1t

3

A

Al

/s x

t p = f ︵Δσ ︶= σf ︵ ,σ ︶( e )



ts(p( ) (0 l t) )σ + < <n sK = KK

Notch Effect Structural Stress, Far field stress

tl

( d ) ( e )A

A

tσ

( f )

t

sp

l

Notch Effect Notch Effect

Far Field SIF

Far Field SIF

Stress intensity solutionsusing published weightfunction results: (a) remotetension; (b) remotebending.



a/t a/t

ts(p( ) (0 l t) )σ + < <n sK = KK

Introduction of notch induced SIF magnification factor Mkn :t

ts(p )( ) 1.0 (0 l t)

( )σ +

≥ < <σkn

sn

n

KM =K

K

t( ) (0 l t)σ < <kn nK = M K

Comparisons of stress intensity magnification factor Mkn at 135° sharp Vnotch for various specimen geometries and loading conditions:(a) Edge crack solutions; (b) Elliptical crack solutions for a/c = 0.4

SIF Magnification Factor Mkn


mn

n mkn n

Paris Law :da C K (a)dNIntroduce tda CM KdN

heUnified SIF :

(b)= Δ

= Δ

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

Modified Paris Law


−

=

=

= ⋅

=

→

Δ

−

Δ

−

=

=

∫

-msΔσ

f

m1

n m

2

nkn

kn n

a a

n ma 0

m b

n

m

kn

Using the Unified Paris Law,

daN (b)CM K

Introduction of

S~Ncurve can be obtained

1N t I(r) (c)C

d(a/ t)Where,

generallized SIF ra

da C

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 1m 2m m m

2 m 12

1 1m

m m

m

Then

C t I(r ) N (e)

Introduce Equivalent Structrual Stress Range,

t I(r )C N (f )

−

− =

=

= →

s

sss Δ S

Δ σ

Δ σΔ S

Master S~N Curve


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 validationand comparison

Reliability Testing

CalibrationQuantify loading

relative to the test for FEA

Example ApplicationANSYS, Verity, Fe-Safe: Weld Analysis Procedure


Suction FittingShell

Weld Line

Element Type: Solid 186

Hex elements along weld line Linear material properties (E = 29,000 Ksi, ν = 0.29)

Symmetry BCOn sides

Top & Bottomfixed

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.


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


Fe-Safe Analysis – Weld life predictions using Master S~N Curve

• Fully reverse loading defined

• Fe-Safe calculates weld fatigue life based on the Battelle’s Master SN curve

• The weld life (log N) data at each node is written in the ANSYS .rst format


Minimum life at element # 4436, node # 4212

ANSYS Post processing – Weld Life Results


Conservative life estimate!Predicted Failure Location correlated well with the test failures!!

Comparison of Verity Predictions with Reliability Test Results


Conclusions

• 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

• ANSYS could be used for the fatigue assessment of welded joints in conjunction with Verity & Fe-Safe

• Verity Generally provides conservative estimate of the weld life• Good correlation with the predicted and test failure locations

• Note: – We believe that the weld joints have to be stress concentration

dominant to achieve consistent results with test data


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. 865–876.[11]. Dong, P., Hong, J. K, Osage, D., and Prager, M., ‘‘Assessment of ASME’s FSRF Rules for Pipe and Vessel

Welds Using A New Structural Stress Method,’’ Welding In the World, Vol. 47, No. 1/2, 2003, pp. 31–43.[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 FPSO’s and Ships

Based on Round-Robin FE Analysis,’’ ISOPE Proceedings, Stavanger, Norway, June.

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2008 International ANSYS Conference · 2008 International ANSYS Conference Fatigue Assessment of Weld Joints Using ANSYS, ... (For crack propagation life) – The parameter widely