表面き裂材のCTODについての基礎的検討

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The conversion method to CTOD from measured clip gauge displacement for a through thickness center cracked specimen and a three point bend CTOD specimen has been already established, for the purpose of obtaining the fracture toughness of the material. However the method for a semielliptical surface flaw has not been established yet because of difficulties of considering the three dimensional effect for the COD. In this paper, we assumed that COD for a surface flaw on the cross sectional plane perpendicular to the plate surface, which is the plane with a single edge crack, can be approximately given by the superposition of CODS for the single edge cracked plane under bending loading and for the center cracked plane under tensile loading, of which the center line corresponds to the plate surface. The bending and the tensile load are determined by the measured mouth COD and crack angle at the surface of the cracked body with a surface flaw. Tensile and bending tests for specimens with various sized semi-elliptical surface notches were carried out to confirm the above idea. In the tests, mouth COD and crack angle were measured by using double clip gauge method and simultaneously, the COD inside the surface notch was also measured by using silicon gum casting method. As a result, the converting method to CTOD at the deepest point of a surface flaw is driven from the measured mouth COD and crack angle at the surface which are obtained by double clip gauge method. The testing result of wide plate estst with a semi-elliptical surface fatigue crack on a multi-pass welded joint, of which crack tip located at local brittle zone within HAZ, was also analyzed. It was obtained that the critical CTOD for a surface crack of which the tip locates local brittle zone corresponds to the lowest critical CTOD obtained from the three point bend standard CTOD tests of the joint with a through thickness fatigue pre-crack.
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表面き裂材のCTODについての基礎的検討

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