The failure of the internally threaded centre spool of large valves is investigated. Brittle fracture occurred at the root of the internally threaded area. Several visual tests were performed. Visual investigation shows that the failure occurs on the last thread engaged with the bolt. Primary finite element method analysis shows that the last thread root has the highest stress concentration and opening mode of fracture mechanics. Redesign optimization and a parametric study are carried out. Reduction of the stress concentration is achieved and so the fracture is prevented.
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