Fatigue_Assessment_of_Weld_Joints_Using_ANSYS_Verity_and_FESafe.pdf

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.


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(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


x

X~0.4 t

Hot Spot

FE

Nominal



x sp

A

A


x sp


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


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


t

s(p( ) (0 l t) ) n sK = KK

Notch Effect


Notch Effect


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

Fatigue_Assessment_of_Weld_Joints_Using_ANSYS_Verity_and_FESafe.pdf

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