Abstract
The mechanical heterogeneity in welds influences, remarkably the results of fracture toughness tests, But such an influence has not yet been clarified sufficiently.
In the present paper, as the first step to establish the rational method to evaluate the fracture toughness of welds with mechanical heterogeneity, the 3 point bending COD test on 9%Ni steel welded joint made of austenitic electrode was investigated. FEM analysis and experiments on deformation behavior of bending specimen with notch located at heat affected zone made clear that the mechanical heterogeneity in welds causes the unsymmetrical spread of plastic deformed region from notch tip, resulting in the unsymmetrical crack opening behavior, that is, the opening displacement of the soft weld metal side of crack becomes larger comparative to that of base metal side with the increase of bending stress, and this behavior is expressed accurately by using the two different rotational factors which are not same to that in case of homogeneous material.
These results imply that the ordinarily adopted 3 point bending COD test method is not adequate as the fracture toughness testing method for the 9%Ni steel welded joint.
Instead of ordinary one, the new method taking into account the effect of mechanical heterogeneity on the crack opening behavior was proposed as follows:
δ= [r1 (W-a)/{a+r1 (W-a)+z}⋅ a/(l+a)+r2 (W-a)/ {a+r2 (W-a)+z}⋅1/(l+a)] Vg
where r1=0.6, r2=0.1 a=V1/V2 Vg: crack mouth opening displacement a: crack length W: specimen width Z: distance of clip gage location from test piece surface
