1. 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
** Verity is a weld fatigue assessment software developed by Battelle based on Structural Stress & Fracture Mechanics theory proposed by Dr. Dong et al, Battelle

2. Part I: Key Points on SS & Weld Assessments
Zhichao Wang
Sr. Lead Engineer
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)

3. Background

4. Fatigue Assessment – Welded Structures

5. 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 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 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.

6. Fatigue Assessment – Welded Structures

7. Structural Stress & Mesh Sensitivity

8. Is Not Sensitive To FE Mesh?
Why Structure Stress
Is Not Sensitive To FE Mesh?
(a)
(b)

9. Weld Joint Categorization
Master S~N Curve
Weld Joint Categorization
Categorization of Weld Joints (BS 7608)
Weld Joints (IIW)
Most codes divide weld joints into different types

10. 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

11. How Could multiple joint S~N curves be merged into one - Master S~N curve?
Nominal Stress Method (BS)
Hot Spot Stress Method (IIW)
Weld Joints (IIW)
Equivalent SS Method

12. Small Crack Propagation Behavior

13. A Unified Stress Intensity Factor (SIF) Formulation
t1/t=0.1

14. A Unified Stress Intensity Factor

15. A Unified Stress Intensity Factor

16. SIF Magnification Factor Mkn

17. Mkn for 1350 V Notch Specimen
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.

18. Modified Paris Law

19. Master S~N Curve

20. Master S~N Curve Nominal Stress Method Hot Spot Stress Method
Equivalent SS Method
Weld Joints (IIW)

21. 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.

22. 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,
[5]. S.J.Maddox, Review of fatigue assessment procedures…, Int. J. of Fatigue, Vol. 25, 12, 2003,
[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
[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), pp
[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.

23. Sr. Applied Mechanics Engineer Emerson Climate Technology Inc.
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

24. 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.

25. Verity, Fe-Safe Weld analysis
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

26. Ansys Preprocessing- FE Model
Suction Fitting
Shell
Weld Line
Element Type: Solid 186
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.
Linear material properties
(E = 29,000 Ksi, ν = 0.29)
Symmetry BC
On sides
Top & Bottom
fixed
Hex elements along weld line

27. ANSYS Results
Maximum Stress
Equivalent Stress s1
seq

28. 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

29. 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

30. ANSYS Post processing – Weld Life Results
Minimum life at element # 4436, node # 4212

31. Comparison of Verity Predictions with Reliability Test Results
Conservative life estimate!
Predicted Failure Location correlated well with the test failures!!

32. Conclusions
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