Optimal life-cycle mitigation of fatigue failure risk for structural systems

Fatigue failure risk can be mitigated both by increasing the design fatigue capacity of the structural components and by conducting more frequent inspection and maintenance actions. The optimal combination of these two types of safety measure is structure dependent. It depends, among others, on the relative cost of the safety measures, the consequences of failure, the level of redundancy, the number of deteriorating components and the statistical dependence among components. In this article, a generic system representation is used to parametrise deteriorating structures according to these system characteristics. Based on this system representation, we investigate patterns of optimal life-cycle fatigue mitigation and provide recommendations for fatigue design. Results show that it can be cost-efficient to achieve system-level safety requirements with high component reliabilities at design and less frequent inspections. Furthermore, we show that the minimum requirements for fatigue design that are typically prescribed in design standards to avoid the need for inspections are not enough unless sufficient redundancy is ensured.