軟鋼の突合せ溶接継手の衝撃疲労き裂進展挙動 Ｉ

Abstract

The impact and non-impact fatigue crack propagation tests were conducted on butt welded mild steel specimens. The impact fatigue tests were carried out using the rotating disk type impact fatigue testing machine. The influence of local constraint in the weld zone which was produced by the residual stress and the difference of microstructure and mechanical properties between heat-affected zone and weld metal on imact fatigue crack growth rate was investigated by means of fracture mechanics and fractography, and the results of impact fatigue tests were compared with the non-impact fatigue test results. The important conclusions were as follows.(1) In non-impact fatigue, it was found that the crack growth rate into the direction perpendicular to the weld bead were almost equal to that in parent metal or weld metal.(2) In impact fatigue, it was found that he crack growth rate in weld metal was almost equal to that in parent metal. But, the crack growth rate into the direction perpendicular to the weld bead showed a drastic decrease in the vicinity of the bond. This drastic decrease of crack growth rate was primarily caused by the residual compressive stresses in heat-affected zone.(3) The dominant fracture appearance in impact fatigue was striation as in non-impact fatigue. It was found that the fatigue crack growth rate for all specimen regions except that which propagated into the direction perpendicular to the weld bead in impact fatigue almost agreed with striation spacing.(4) Comparing impact fatigue test results with non-impact fatigue test results, quite large difference of crack growth rates arises from the difference of striation spacings which should be attributed to the impact load wave and impact loading. Therefore, the impact fatigue crack growth rates, (da/dN)imp, could be expressed as follows, (da/dN)imp=C'(K'max)m'=αC(βKmax)m =αβm(da/dN)non.