Abstract
Abstract A number of perforated-plate fuel oil strainers, called “witch hat” filters for their peculiar shape, were found fractured. Detailed metallurgical failure examinations revealed deflagration within the filter body, i. e. a mild explosion, as the cause of fracture by tensile overload. No evidence of fatigue was found on the fracture surfaces. The microstructure of the subject failed “witch hat” filters was unremarkable, except for the presence of slip lines immediately adjacent to the fracture surfaces. This is consistent with and evidence for plastic deformation in the area of the fracture. While the metallurgical or physical cause of failure was determined to be tensile overload, where the ligaments between the filter holes fractured, the root cause of failure was found to be abuse in engine service, that caused deflagration of fuel oil vapours within the filter body.
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