Fatigue Assessment of Weld Joints Using ANSYS, Verity

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 MethodPart
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 Battelles Structure Stress Is Not
  Sensitive To FE Mesh Size?
• How Could Multiple Joint SN Curves Be Reduced to
  A Single SN Curve, the Master SN 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 SN 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.
6
Fatigue Assessment Welded Structures
7
Structural Stress Mesh Sensitivity
8
Why Structure Stress Is Not Sensitive To FE Mesh?
(a)
(b)
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Master SN Curve
Weld Joint Categorization
Most codes divide weld joints into different types
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Multiple SN Curves Due to Weld Joint
Categorization
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How Could multiple joint SN curves be merged
into one - Master SN curve?
Equivalent SS Method
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Small Crack Propagation Behavior
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A Unified Stress Intensity Factor (SIF)
Formulation
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A Unified Stress Intensity Factor
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A Unified Stress Intensity Factor
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SIF Magnification Factor Mkn
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Mkn for 1350 V Notch Specimen

• Conclusions
• Mkn approaches unity as crack size a/t approaches
  0.1 i.e., a/t0.1 (short crack correction
  factor) Thus a/t0.1 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
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Master SN Curve
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Master SN Curve
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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 SN curve. Thus one SN
  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 134452002, 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.
23
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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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.

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ANSYS, Verity, Fe-Safe Weld Analysis Procedure
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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.

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ANSYS Results
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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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