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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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
• 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
• 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):
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
(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
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)
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
• 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
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