切欠形状が溶接継手の脆性破壊強度におよぼす影響について

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The authors have investigated in their experiments the effects of residual stresses due to welding upon the initiation of brittle fracture in welded structures from the several points of view.As one of these series of their tests, an experiment was carried out to investigate the effect of notch shape, such as, length of notch and radius of notch root, on brittle fracture strength of welded steel plate, by conducting the notch tensile tests on wide longitudinal welded specimens and base metal specimens.The main results obtained are summarized as follows : 1) Providing the location of the notch root to the un-affected zone of base metal apart from the weld heat affected zone, wide longitudinal welded specimens caused brittle fracture under lower applied stress levels than yield stress at the temperature range below nil ductility transition temperature of the notch root, while the fracture strength of the base metal specimen, having same shape of notch, reached to high stress level as high as yield stress, at the same low temperature range. (cf. Fig. 5 and Fig. 6) The exhaustion of ductility in steel at the root of notch is, of course, one of the reasons why brittle fracture initiates as Mylonas pointed out.Although the exhaustion of ductility in steel may be a necessary condition for initiation of brittle fracture, it might not be a sufficient condition for low stress fracture, 2) In order to investigate the effect of notch length on brittle fracture in residual stress field, wide longitudinal specimens with different length of notch but constant radius of 0.1 mm were tested at the temperature of -30℃. Fracture test on the base metal was also conducted with different length of the notch. As the results seen from Fig. 9,the fracture strength of base metal decreases with the length of notch, while the fracture strength of welded specimens increases with the length of notch except the length below 36 mm. For the case of specimens with notch less than 36 mm the fracture strength increases as notch length decreases. This is mainly due to the higher initiation strength of weld metal. The reason why the fracture strength of welded specimens increases with the length of the notch is due to the reduction in residual stresses while the stress concentration factor at the root of the notch remains constant because of the same sharpness of the notch. This is a strong evidence of the effect of residual stress upon the initiation of brittle fracture.3) Effect of notch radius on the brittle fracture strength of welded joint was clearly confirmed as shown in Fig. 10. The radius of notch was varied from 0.1 mm to 1.0 mm for base metal and welded specimens respectively. If the radius of notch is greater than 0.3 mm, the reduction in fracture strength due to residual stress seems to diminish except extremly low test temperature. Therefore, it is concluded that the effect of residual stresses on brittle fracture can be evaluated only if the sharp notch exists in the residual stress field.
社団法人日本船舶海洋工学会の論文

切欠形状が溶接継手の脆性破壊強度におよぼす影響について

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