PSI - Issue 57

Arne Fjeldstad et al. / Procedia Structural Integrity 57 (2024) 692–700 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The stress intensity factors plates under membrane and bending loading are normally based on the Newman Raju equations (1981, 1983). The geometry functions or stress increase due to weld notches are based on the equations presented by Bowness and Lee (2002). Calibrated fracture mechanics models to S-N curves in DNV-RP-C203 are presented in DNV-RP-C210.

t

φ

c

B

θ

T

Plane of symmetry

a

A

L

Fig. 3. Semi-elliptic crack at weld toe. Excerpt form Lotsberg et al. (2016)

4. Probabilistic analysis

4.1. Failure probability in service life

Fatigue damage is accumulated over the service life for structures subjected to dynamic loading and it is practical to relate this damage to an accumulated failure probability. The calculated failure probability is the probability that the structure fails in the time period prior to the time . The accumulated failure probability can be derived from a limit state function defined as: ( ) = ∆ − ( ) (1) where ∆ is a function describing the uncertainty in the Palmgren-Miner damage accumulation and ( ) is accumulated fatigue damage at time from Equation (1) based on S-N. The following limit state function can be formulated based on fracture mechanics ( ) = − ( ) (2) where c is crack depth at failure and ( ) is crack size at time as derived by integration of the Paris Erdogan equation. Failure is defined when ( ) < 0 . 4.2. Uncertainties in fatigue life calculation

Calculated fatigue damages are associated with several uncertainties: • Environmental description • Calculation of loads • Analysis of load effect at the hot spot from structural analysis • Scatter in S-N data • Uncertainty related to the Palmgren-Miner rule.