Effective notch stress based CA-VA fatigue resistance similarity of welded joints in steel maritime structures. Damage accumulation modelling advances

Fatigue is typically a governing limit state for steel (maritime) structures operating in variable amplitude loading and response conditions. Arc-welded joints connecting the structural members are often identified as the weakest links and accurate lifetime estimates for fatigue design are essential. However, a generalised constant amplitude resistance formulation incorporating the mid- and high-cycle fatigue strength and mechanism contributions, as well as a generalised damage accumulation model taking the variable amplitude induced resistance decrease into account, is not available. In particular one providing explicitly defined damage reliability and confidence levels for design. Adopting the average effective notch stress as fatigue strength parameter, a random fatigue limit resistance model, as well as a non-linear damage accumulation model including the response history induced fatigue limit degradation, one vector of most likely constant and variable amplitude fatigue parameters is obtained, applicable for a frequency domain based fatigue assessment of all arc-welded joints. Analysing fatigue resistance data containing a broad range of spectral distributions, constant and variable amplitude fatigue resistance similarity is obtained, as reflected in a mean damage estimate of one. The generalised random fatigue limit resistance formulation and generalised non-linear damage accumulation model provides the most accurate lifetime estimates in comparison to state-of-the-art guidelines and codes. Incorporating global mean stress provides more accurate fatigue lifetime estimates. Consistent reliability and confidence levels for constant and variable amplitude fatigue assessment for design suggests an allowable damage of 0.2. For guidelines (e.g. IIW) and codes (e.g. DNV) a similar value illustrates over conservatism from fatigue resistance perspective.