Implementation of a nominal stress approach for the fatigue assessment of aluminium naval ships

Abstract

Fatigue analysis is an important part of the structural design of weight-optimised naval ships, because they are often constructed from aluminium alloys or high tensile steel. Due to the low fatigue strength of these metals, the welded joints are vulnerable to cracking. Furthermore, naval ships can be required to operate in demanding environments, and remain in service longer than assumed during design. Thus, ship designers and in-service managers require efficient fatigue evaluation approaches. One industry-accepted and relatively quick method, documented in design codes, to assess the fatigue strength of welded joints is the nominal stress or S-N curve approach. However, there are uncertainties associated with using a design code; if all of the conditions for its applicability are not met, the analyst must reliably interpret the code. Further, different specifications for the use of S-N curves leads to different fatigue life estimates. In this paper, a refinement of the nominal stress approach for joints, typical of aluminium welded ship details, is proposed. The goal is to inject rigour and practicality into the fatigue analysis of naval ships. The refinement process leverages both in-service maintenance information and long-term strain measurements of a 56 m aluminium patrol boat. The sensitivity of the predicted fatigue life of welded details to the choice of S-N curve, available from a structural design code, and stress parameter extraction is investigated. Finally, recommendations for future work are provided.