強度ミスマッチ継手の脆性破壊強度に対するき裂先端の応力上昇および塑性ひずみ集中の影響

Abstract

In order to investigate the influence of strength mismatching on brittle fracture strength for large heat-input welded joints, a series of tension tests was conducted using surface notched specimens. Furthermore, notched round-bar tension tests were carried out for simulated bond material (homogeneous material) which was base metal heat-treated on the same condition as the weld bond. Thereby, cleavage fracture strength of the bond material without the influence of strength mismatching was obtained. The point of fracture initiation was identified by SEM observation and, the critical maximum principal stress at the point (σ1c) was calculated by FE analysis for each specimen. σ1c values of surface notched specimens were found to be distributed in the range of 900-1,400N/mm². This result indicates that the strength mismatching does not significantly affect σ1c. In addition, distribution of σ1c for the notched round-bar specimens is in good agreement with that for surface notched specimens. This indicates that the cleavage fracture of the weld bond is a dominant fracture on the welded joints. Peak values of maximum principal stress σ1ᵖ and equivalent plastic strain epᵖ increase with increase in the strength mismatching ratio defined as the ratio of yield strength of weld metal to that of HAZ (σYWM/σYHAZ). The increase in σ1ᵖ causes the decrease in fracture strength of the weld joints. ep concentration at notch tip of HAZ side is due to the lower yield strength of HAZ. Except for small-scale and large-scale yielding conditions, the influence of ep distribution caused by strength mismatching on fracture strength also needs to be considered.