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Application of the Notch Stress Intensity and Crack Propagation Approaches to weld toe and root fatigue

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Abstract

Several fatigue failures are initiating from the root of non-penetrating welds, which are widely applied in several industrial sectors. The structural optimization and reduction of weld material increase the danger of root cracking. The fatigue assessment during design is mainly based on the nominal, structural hot-spot or notch stress approach. These approaches are only partly applicable to weld roots, give rough estimates such as the nominal stress approach or over-conservative life estimation such as occasionally the notch stress approach when keyhole notches are modelled. An alternative assessment is possible with the crack propagation approach assuming the non-welded root gap and a short crack at the weld toe as initial cracks. A new approach is the notch stress intensity approach (N-SIF approach) which is able to assess the fatigue life of V-shaped notches at weld toes as well as crack-like notches at weld roots using the strain energy density around the notch. Both approaches are applied to different joints where toe and root failures are probable. The results are compared with a previous analysis using the notch stress approach. Fatigue test results are also available for a comparison which allows to draw conclusions with respect to the approaches for an appropriate fatigue strength assessment.

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Abbreviations

a:

weld throat thickness

a:

initial crack length

ac :

critical crack length

C:

material constant

da/dN:

crack propagation rate

E:

modulus of elasticity

E′:

modified modulus of elasticity

e1, e2 :

parameters for dependency from α

Kc :

critical stress intensity factor

K1, K2 :

stress intensity factor for Mode I and Mode II loading

ℓ:

length

λ1, λ2 :

eigenvalues of the stress field for K1 and K2 modes

m:

slope exponent of S-N curve

m:

material constant (in Paris equation)

N:

number of load cycles

Ps :

probability of survival

R:

stress ratio (lower stress/upper stress)

ReH :

yield stress

R0 :

control radius

R*:

enlarged control radius

rref :

reference radius ref. to notch stress

t:

plate thickness

Tσ :

scatter index ref. to fatigue strength for Ps= 10 and 90%

W:

elastic strain energy density

α:

half opening angle of V-notch

Δa:

crack increment

ΔK:

range of stress intensity factor

ΔKth :

threshold range of stress intensity factor

Δσc :

characteristic fatigue strength

Δσeq :

equivalent stress range

Δσn :

nominal stress range

ΔσT :

range of T-stress (parallel to slit)

ΔW:

range of elastic strain energy density

ΔWT :

range of elastic strain energy density due to T-stress

v:

Poisson’s number

σ:

stress

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Authors and Affiliations

  1. Institute of Ship Structural Design and Analysis, Hamburg University of Technology (TUHH), Hamburg, Germany

    C. Fischer,  O. Feltz &  W. Fricke

  2. Department of Management and Engineering, University of Padova, Vicenza, Italy

    P. Lazzarin

Authors
  1. C. Fischer
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  2. O. Feltz
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  3. W. Fricke
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  4. P. Lazzarin
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Correspondence to C. Fischer.

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Fischer, C., Feltz, O., Fricke, W. et al. Application of the Notch Stress Intensity and Crack Propagation Approaches to weld toe and root fatigue. Weld World 55, 30–39 (2011). https://doi.org/10.1007/BF03321305

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