純チタン板のCup and Cone型破壊過程における剪断変形集中帯について

Abstract

The process of “cup and cone” fracture was examined in commercial pure titanium plate pulled in uniaxial tension. Direct and continuous observation of the fracture process was possible because the fracture process of titanium plate used in this experiment was completed almost in two-dimensional state like plane strain state.The fracture process is summarized as follows: the formation of shear bands which intersect in the center of the necked portion of a specimen occurs prior to the onset of fracture. These macroscopic shear bands are composed of a number of highly localized fine bands of intense shear deformation in layers and along the direction of the macroscopic shear bands.After the formation of these intersecting macroscopic shear bands, the number and size of voids increase extraordinarily in the rhombus region where the shear bands intersected. Voids grow on the spots where the highly localized fine bands of intense shear deformation intersect and coalesce, resulting in the formation of a few small cracks. This is the onset of fracture.Connecting these small cracks, a central crack is formed. It propagates in zig-zag steps producing void sheets within the highly localized fine bands of intense shear deformation. The fracture is completed by separating void sheets having reached the edges of a specimen.The formation of intersecting macroscopic shear bands is a critical stage leading to the onset of fracture and intersecting shear bands also play a predominant role in the fracture process.Comparing the fracture process of the titanium plate with those of the copper round bar specimens in the previous papers, it is concluded that the above results may be applied to the ductile fracture of round bar specimens.