Abstract
Deformation behavior and ductile crack initiation in HAZ softening joints by fine grained steel are studied in this paper. Not only deformation but also fracture of heterogeneous material are affected by plastic constraint due to strength mismatch, and are different from those of homogeneous material. Large-deformation finite-element analysis is done in a tensile round-bar specimen with base metal, weld metal and HAZ softening zone, and ductile crack initiation is discussed. Necking initially generates in HAZ for all HAZ softening joints, and HAZ necking usually continues before fracture. There are some cases that the location of necking after proceeding deformation finally shifts from HAZ to weld metal or base metal when the joint is fabricated by high-strength steel with high plastic constraint condition. However, the locations of final necking and fracture initiation do not always correspond. This is because the location of ductile crack initiation is determined from the relation between stress triaxiality and equivalent plastic strain, and they are independent of necking deformation. It is found that the effects of mechanical and geometrical mismatch on tensile strength of HAZ softening joint depend on the behavior of deformation and fracture.
