Fatigue Assessment of ** Weld Joints Using ANSYS, Verity & FE-Safe Zhichao Wang Aditya Sakhalkar 5/8/2007
Part I: Key Points on Equivalent SS Method Part II: Example Application Application ** Verity is a weld fatigue assessment software developed by Battelle based on Structural Stress & Fracture Mechanics theory proposed by Dr. Dr. Dong et al, Battelle
Part I: Key Points on SS & Weld We ld Assessments Zhichao Wang Sr.. Lead Engineer Sr Emerson Climate Technology Inc.
Background Why The Battelle’s 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)
Part I: Key Points on SS & Weld We ld Assessments Zhichao Wang Sr.. Lead Engineer Sr Emerson Climate Technology Inc.
Background Why The Battelle’s 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)
Background
Fatigue Assessment – Unwelded Structures
Fatigue 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. ’
Fatigue Assessment – Welded Structures (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 S –N curves are obtained from tests of actual welded joints based on the hotspot 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 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.
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:
da dN
C ( K)
n
(1)
For a flaw size starts from a 0 to a critical fatigue crack size a f , the remaining fatigue life N under stress range S is obtained by integrating Eq (1):
a f
a0
da (K)
n
C N
(2)
(v) Verity Equivalent Structural Stress Method (Battelle) Equivalent Structural Stress
+ Single Master S~N Curve
Structural Stress & Mesh Sensitivity A
A
(Structural Stress)
Y
0.4 t
A
X A
t 0
x
(b) Comparison of SCF predicted by various modeling procedures and extrapolation based HSS at the weld toe [12].
FE
Hot Spot
P
No min al
~
No min al
(Human Factor)
FE
Hot Spot
Nomin al
X ~0.4 t (a) Normal Stress at the sharp corner
(c) Structural Stress Using Verity Method
Master S~N Curve Weld Joint Categorization
Categorization of Weld Joints (BS 76 08)
Weld Joints (IIW)
Most codes divide weld joints into different types
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)
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
(c)
How Could multiple joint S~N curves be merged into one - Master S~N curve?
Nominal Stress Method (BS)
Weld Joints (IIW)
Hot Spot Stress Method (IIW)
Equivalent SS Method
Small Crack Propagation Behavior
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 da da Nakai; (e) dN K for CT specimen; (f) dN K for three different specimens 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.
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.
A Unified Stress Intensity Factor (SIF) Formulation
A
A 1
A
R1
t1
t
t
2
t
3
A
(a)
(b)
A
A R2
(c)
t1 /t=0.1 1
A
A
1
t1 t
t1
2
(d)
2
t
3
A
3
A
A (e)
(f)
A
(a) Weld geometry with a hypothetical crack ; (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 t 1.
A Unified Stress Intensity Factor
A
1
t1
t
A
2
t
3
(a)
A
(b)
(t )
()
A (c)
(0 l t)
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 (e)
(d)
A
t
t
t1
3
A
()
(ps )
t
( ,
/ x
)
(0 l t )
(notch effect)
A Unified Stress Intensity Factor A
t
t A
(d)
(t )
(ps )
(e)
(f)
(0 l t ) Notch Effect Structural Stress, Far field stress
Notch Effect
Notch Effect
Far Field SIF Far Field SIF
Stress intensity solutions using published weight function results: (a) remote tension; (b) remote bending.
SIF Magnification Factor Mkn (t )
(ps )
(0 l t )
Introduction of notch induced SIF magnification factor Mkn : (t )
(ps ) t
( )
(t )
1.0
(0 l t)
(0 l t)
a/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
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.
Modified Paris Law Paris Law : da
CK m n
CMnkn K nm
(a) dN Introduce theUnified SIF : da dN
(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
Master S~N Curve Using the Unified Paris Law, da CMnkn K mn S ~ N curve can be obtained (a) dN a a f da N (b) a 0 CMn K m kn n Introduction of generallized SIF range, leads to, m 1 1 2 N t I(r) (c) C a / t 1 d(a/t) Where, I(r) a / t 0 Mn {f (a / t) r [f (a / t) f (a /t)] } m kn m m b
Then
1 m
C t
2 m 2m
1 m
I(r) N
1 m
(e)
Introduce Equivalent Structrual Stress Range,
t
2 m 2m
I(r)
1 m
1 m
1 m
C N
(f )
(d)
Master S~N Curve
Nominal Stress Method
Weld Joints (IIW)
Hot Spot Stress Method
Equivalent SS Method
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.
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.
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
Background • 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.
ANSYS, Verity, Fe-Safe Weld Analysis Procedure Ansys Preprocessing - Meshing per Verity requirements - Linear material properties
Calibration Quantify loading relative to the test for FEA
- Apply load - Solution
Verity, Fe-Safe Weld analysis Verity Analysis (Eq. Structural Stress calculations along the weld line) - Fe-Safe analysis (Weld life calculations using Master S-N curve)
Reliability Testing Ansys Post processing - Import .rst file - Post processing
Verity result validation and comparison
Ansys Preprocessing- FE Model
Mesh Requirements • Shell Elements: • 3D mid-surface element with no throughthickness dimension • Solid Element: • Rectangular faces required along the throughthickness 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.
Suction Fitting
Shell
Weld Line
Element Type: Solid 186
Top & Bottom fixed Symmetry BC On sides
Hex elements along weld line
Linear material properties (E = 29,000 Ksi, ν = 0.29)
ANSYS Results
Maximum Stress
Equivalent Stress
1
eq
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 SN 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
ANSYS Post processing – Weld Life Results
Minimum life at element # 4436, node # 4212
Comparison of Verity Predictions with Reliability Test Results
Conservative life estimate! Predicted Failure Location correlated well with the test failures!!
