Turnbull, A; Crocker, L (2020) Finite element calculation of stress intensity factor for cracks developing from corrosion pit. NPL Report. National Physical Laboratory. MAT 95

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Abstract

Finite element analysis (FEA) has been undertaken to calculate the stress intensity factor (K) of cracks emerging from a corrosion pit in a martensitic stainless steel cylinder stressed remotely. The corrosion pit was in the form of a truncated spheroid with depth equal to the full mouth opening. This geometry reflects the configuration found in laboratory testing and in service in low chloride solution, albeit the pit is considered ideally smooth. The pit depth was fixed at 150 µm. A semi-circular crack was adopted with depths ranging from 25 µm to 250 µm. Computations were performed assuming purely elastic condition in most cases but the impact of plasticity around the pit was evaluated for selected crack depths.
A key observation of this investigation is that cracks on either side of the pit that are small compared to the pit depth act independently with K values unaffected by the presence of the other crack. However, as the cracks on either side of the pit grow to a depth approaching the pit depth the K value is mutually increased. For cracks with depth close to that of the pit depth the stress intensity factor is increased due to the presence of the pit such that the expected increase of K with increasing crack size does not immediately materialise due to the decreasing stress gradient away from the pit.
Elastic plastic analysis in discreet cases confirms these trends but highlights the need to avoid simple comparison of K values that may not be applicable because of plasticity around the pit.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	MAT 95
Subjects:	Advanced Materials > Corrosion
Divisions:	Electromagnetic & Electrochemical Technologies
Publisher:	National Physical Laboratory
Identification number/DOI:	10.47120/npl.MAT95
Last Modified:	08 Jan 2021 16:00
URI:	http://eprintspublications.npl.co.uk/id/eprint/9007

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