On the Formation of Shear Bands in the Ductile Fracture Process of Pure Titanium Plate

Abstract

The present study deals with the concentration of plastic deformation into necked region in the fracture process of titanium plate and the condition for the formation of intersecting macroscopic shear bands.
As reported in a previous paper, the intersecting macroscopic shear bands formed prior to the onset of fracture play an important role in the “cup and cone” fracture process of titanium plate and the formation of these shear bands is the critical stage of the fracture process.
Observations on the surfaces of specimens deformed after electrolytical polishing and measurements of deformation at several stages in the deformation process to the formation of these shear bands have made clear the following points. The boundary of the plastically deformed region exists along the direction of zero extensional strain and moves toward the central part of neck with the development of necking. The formation of intersecting macroscopic shear bands is the localization of plastic deformation (plastic instability) within zones along direction of zero extensional strain that intersect in the central part of neck.
The true stress in the smallest section of the neck in titanium plate was obtained by applying Bridgman’s correction method for stress in the necked portion. A maximum in true stress was found at the formation of intersecting macroscopic shear bands, and this corresponds to the condition (maximum in true stress) for the localization of flow within a plastic zone along the direction of zero extensional strain that was obtained by Chakrabarti et al.
Therefore, it is concluded that the formation of shear bands in the fracture process of pure titanium plate with a large amount of necking is thought to be the plastic instability along the direction of zero extensional strain that was confirmed by Chakrabarti et al. as a second basic mode of strain localization leading to fracture initiation.