Transition Behaviour of Corrosion Pit to Fatigue Crack

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Project Code: 33593

Objectives

Determine stress/strain fields around corrosion pits of different geometries.
Perform fatigue tests to determine the conditions at which a corrosion pit will develop into a fatigue crack and to compare fatigue performance between pitted and welded specimens.
Develop a fracture mechanics parameter to characterise this transition.

Project Outline

There is a global push towards the use of renewable energy, and the offshore wind industry is growing rapidly. Offshore structures are subject to environmental loading. For example, offshore wind substructures (OWSs) are subjected to severe cyclic loads caused by wind, waves and dynamic structural response. Although most of these structures are protected by coating and/or cathodic protection against corrosion, some parts of the structures may be subject to free corrosion in seawater. The current fatigue design of offshore structures are based on data principally from welds in tubular steel jacket structures for the oil and gas sector. This project will investigate the pit to crack transition in steels for OWSs and mooring chains and the results can be used for other offshore structures.

The project will be composed of three parts: experimental testing, numerical modelling to determine the stress/strain concentration surrounding corrosion pits, and constitutional modelling of the corrosion pit to crack transition. Two types of steels will be investigated: structural steels commonly used in OWSs and high strength steels commonly used for mooring chains. For each type of steel, corrosion pits of various sizes will be introduced artificially. Fatigue testing of pitted plate or strip specimens, some containing a girth weld, will be carried out in both air and free corrosion seawater to establish the critical pit size and pit aspect ratio corresponding to the transition. The test specimens will be sufficiently large to be as closely as possible representative of the actual components. Linear elastic finite element analysis (FEA) will be performed to determine the local stress and stress concentration factor (SCF) for various pit aspect ratio or shapes.

Corrosion pit transition to crack can be influenced by many factors. This project will mainly investigate the following three key factors: (1) effect of corrosion pit geometries on crack initiation; (2) weld profile; (3) effect of small stresses. For (1), specimens with different pit geometries will be tested to determine the threshold stress range for crack initiation. For (2), specimens with different weld profiles, representing that typically seen in OWS, will be tested to determine the conditions at which the fatigue strength of the weld is comparable with that of pitted specimen. For (3), the study will focus on the effect of small stresses on crack initiation from corrosion pits.

Relevant Industry Sectors

Benefits to Industry

The new data and fatigue design guidance generated from the project will allow offshore wind energy companies to design OWSs more efficiently and safely. The results are expected to be included in future revisions of BS 7608 “Guide to Fatigue Design and Assessment of Steel Products”.

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