17. Crack-Arrest Properties of 9% Nickel Steel and its Weldment

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An investigation was carried out to establish the crack-arrest concept which is directly related to the safety assessment of LNG storage tank and to determine the crack arrest properties of 9% nickel steel and its weldment. Firstly, a new crack-arrest model was proposed which delineated the initiation, propagation and arrest of a brittle crack in the vicinity of the tip of a pre-existing surface flaw. Further, the SNDT (Surface-Notched Double Tension) test designed to faithfully simulate the run/arrest behaviour of a brittle crack on this model was developed and performed on 9% nickel steels and their weldments. In those tests, it was revealed that a brittle crack could be arrested at the tip of a surface notch simulating a pre-existing flaw, and a significant plastic deformation surrounding the front of the arrested brittle-crack was observed. Those observations indicate that the factor governing crack-arrest is a ductile fracture of macroscopic expanse occurring in the plate surface layer in relaxed restraint conditions. The SNDT test results were analyzed dynamically by a specially designed three-dimensional finite element method. As a result, it was demonstrated that the possibility of restricting potentially extensive propagation of a brittle crack in an infinite plate in constant temperature and stress conditions could not be assessed by the Ka-concept and that the crack arrest properties of material were in essence ascribable to its dynamic plastic deformability. The SNDT tests performed in the present study provided suggestive evidence that the direct quenching and tempering treatment is effective in improving the crack-arrest properties of 9% nickel steel.
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17. Crack-Arrest Properties of 9% Nickel Steel and its Weldment

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