The Paton Welding Journal, 2014, #1, 2-12 pages
DETERMINATION OF THE CAUSES FOR CRACK INITIATION IN STRUCTURAL ELEMENTS OF THE TOWER OF NEW VENTILATION PIPE AT CHERNOBYL NPP
V.M. TOROP1, E.F. GARF1, A.V. YAKIMKIN1 and E.E. GOPKALO2
E.O. Paton Electric Welding Institute, NASU. 11 Bozhenko Str., 03680, Kiev, Ukraine. E-mail: firstname.lastname@example.org
G.S. Pisarenko Institute for Problems of Strength, NASU. 2 Timiryazevskaya Str., 01014, Kiev, Ukraine. E-mail: email@example.com
Analysis of design and service documentation, as well as correspondence of actual loads and service modes of the new ventilation pipe (NVP) to this documentation requirements, has been performed. Results of non-destructive testing of base metal and welded connections of structural elements on the NVP at ChNPP have been analyzed. Cracks in tower nodes of ChNPP NVP are of systematic nature. They formed in the zones of node welded connections of tower three upper tiers, in which the lattice elements are made of pipes of 193.7 x 6 mm cross-section. At present cracks were detected in 17 nodes of slotted gusset of the post and in 17 nodes of brace attachment to straining sill. Templates were cut out, which are crack-containing fragments of gussets, i.e. structural elements of ChNPP NVP, samples were prepared and experimental investigations of the composition, microhardness, mechanical properties and structure of base metal, HAZ and weld metal have been performed. Causes for and mechanisms of cracking in structural elements of the NVP have been determined during performance of materials science investigations in two cut-out templates. Fractographic studies revealed the fatigue nature of surface crack initiation and propagation in gussets. Aerodynamic and strength analyses of the construction and its individual elements have convincingly demonstrated that the cause of cracking are resonance vibrations of braces in the upper three sections of the tower, directed out of face plane and inducing alternating bending stresses in the nodes. Vibration frequencies and forces transferred to the nodes have been determined. Stress fields caused by periodic forces in the braces, confirm the existence of the zone of fracture initiation and development, and maximum stress levels greatly exceed welded connection fatigue limits. A conclusion was made that the tower was designed using out-dated design concepts for nodes, characterized by numerous zones of high stress concentration, low fatigue and corrosion resistance. 12 Ref., 1 Table, 20 Figures.
welded connections, welded nodes, fatigue, resonance, fatigue life, fractography, metallographic investigations, fatigue crack, strength analysis, new ventilation pipe, Chernobyl NPP
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