On the Fatigue of Ship-Structures under Wave Loads

Abstract

Background: Fatigue damage is one of the main failure modes in ship structures. This type of damage usually starts from weak point of the structure as welded joints, sites of stress concentrations and cracks, whose propagation can lead to the failure of the ship structures. Cyclic loadings that ships encounter during their service life are one of the main causes that can produce fatigue damage, especially loads due to wave, mainly analyzed in this work. Once the crack is started, also lower stress cycles, which would have negligible effects on intact components, can propagate the crack.

Objective: This paper wants to resume the most used fatigue strength assessment approaches, in order to analyze drawbacks and advantages and to provide the necessary background knowledge for the development of a future and reliable theoretical/numerical models for predicting the fatigue life of ship structures subjected to collision events and different sea states.

Methods: This scientific work will thus consider the main theoretical approaches in time and frequency domain by using energy spectral methods. Collision rules are also assessed to check the hull girder ultimate bending capacity in the damaged state.

Results: Cyclic stresses determination in specific structural details of the hull girder and welded joints is discussed, in order to evaluate the relevant maximum stress range useful for the subsequent fatigue studies, performed by finite element analysis. In a collision scenario, also in minor damages, fatigue cracks may appear and propagate, reducing structural strength and structures fatigue life.

Conclusion: Structural loads determination can be categorized in different methods, but the frequency-domain method is the most extensively used methods in fatigue analysis procedure, because it requires significantly less computational efforts than the time-domain method. Once determined the stress distribution, in most cases fatigue damage is calculated using the Palmgren–Miner cumulative damage rule in combination with the S–N curves of materials, structural details and welded joints. In a scenario where a damaged ship encounters large wave amplitudes, the damage accumulation could lead to low-cycle fatigue.