Fatigue reliability-based assessment of welded joints applying consistent fracture mechanics formulations

Design of welded structures for fatigue limit state is normally carried out by means of either single- or two-sloped SN curves. To properly assess the effect of an inspection and repair strategy of structures degrading due to crack growth, fracture mechanics (FM) models need to be applied to describe crack propagation. To provide a proper tool for making decisions regarding the balance between design criteria and an optimal plan for inspection and repair in view of the inherent uncertainties, reliability methods should be applied. In this paper, alternative SN and FM formulations of fatigue are investigated. They include a crack growth formulation based on bi-linear crack growth law, considering both segments of the crack growth law as correlated and non-correlated in the failure probability calculation. The effect of inspection in the updated reliabilities is illustrated using first and second-order reliability methods as well as Monte Carlo simulation. It is argued that FM formulations need to be calibrated based upon SN-data. This is because the initiation of crack and its initial stages are subjected to uncertainties which are hard to quantify. The calibration of the FM method is specially based upon consideration of the crack initiation time, initial crack size and the crack aspect ratio.