R.L.G. Slange
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Fatigue is often a governing limit state for steel (maritime) structures operating in variable amplitude loading conditions. Arc-welded joints connecting the structural members are typically the weakest links, and accurate lifetime estimates are essential for structural design. However, a resistance formulation and damage accumulation model incorporating the mid- and high-cycle fatigue strength and mechanism contributions is not available. In particular, one containing explicitly defined damage reliability and confidence levels. A generalized random fatigue limit resistance formulation and a generalized non-linear damage accumulation model, including the variable amplitude loading induced fatigue limit degradation, are proposed to improve the lifetime estimates applicable for fatigue assessment in the frequency domain. The most likely fatigue resistance parameters are obtained by analysing fatigue resistance data from literature containing a broad range of loading distributions. Adopting the average effective notch stress as a fatigue strength parameter, including global mean stress effects, constant and variable amplitude fatigue resistance similarity is obtained, as reflected in a mean damage estimate of one. The fatigue lifetime estimate accuracy significantly improved in comparison to state-of-the-art guidelines and codes (e.g. IIW, DNV, BS). Consistent reliability and confidence levels for constant and variable amplitude fatigue assessment for design suggest an allowable damage of 0.2. A similar value for guidelines and codes illustrates over-conservatism from a fatigue resistance perspective.
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Fatigue is often a governing limit state for steel (maritime) structures operating in variable amplitude loading conditions. Arc-welded joints connecting the structural members are typically the weakest links, and accurate lifetime estimates are essential for structural design. However, a resistance formulation and damage accumulation model incorporating the mid- and high-cycle fatigue strength and mechanism contributions is not available. In particular, one containing explicitly defined damage reliability and confidence levels. A generalized random fatigue limit resistance formulation and a generalized non-linear damage accumulation model, including the variable amplitude loading induced fatigue limit degradation, are proposed to improve the lifetime estimates applicable for fatigue assessment in the frequency domain. The most likely fatigue resistance parameters are obtained by analysing fatigue resistance data from literature containing a broad range of loading distributions. Adopting the average effective notch stress as a fatigue strength parameter, including global mean stress effects, constant and variable amplitude fatigue resistance similarity is obtained, as reflected in a mean damage estimate of one. The fatigue lifetime estimate accuracy significantly improved in comparison to state-of-the-art guidelines and codes (e.g. IIW, DNV, BS). Consistent reliability and confidence levels for constant and variable amplitude fatigue assessment for design suggest an allowable damage of 0.2. A similar value for guidelines and codes illustrates over-conservatism from a fatigue resistance perspective.
Increasing demand for the extended lifetime of structures stresses the need for reliable high-cycle fatigue lifetime predictions for variable amplitude-loaded structures. Design standards (including Eurocode 3, AASHTO and British Standard 7608) provide a fatigue design model based on constant amplitude S-N curves and a linear damage accumulation model. There are minor differences in detail categories between standards, but larger differences between the extensions of the S-N curves for variable amplitude loading. It is known that the load spectrum influences this extension due to load interaction and sequence effects. The validity of the predicted fatigue life by available design standards is checked using a compilation of variable amplitude fatigue test data for steel arc-welded joints from the literature with different loading spectra. The results indicate that the standards are generally conservative in the mid- but not in the high-cycle regime, suggesting that resistance non-linearities should be explicitly addressed.
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Increasing demand for the extended lifetime of structures stresses the need for reliable high-cycle fatigue lifetime predictions for variable amplitude-loaded structures. Design standards (including Eurocode 3, AASHTO and British Standard 7608) provide a fatigue design model based on constant amplitude S-N curves and a linear damage accumulation model. There are minor differences in detail categories between standards, but larger differences between the extensions of the S-N curves for variable amplitude loading. It is known that the load spectrum influences this extension due to load interaction and sequence effects. The validity of the predicted fatigue life by available design standards is checked using a compilation of variable amplitude fatigue test data for steel arc-welded joints from the literature with different loading spectra. The results indicate that the standards are generally conservative in the mid- but not in the high-cycle regime, suggesting that resistance non-linearities should be explicitly addressed.