Effects of strain and strain-rate on the formation of the shear band in metals

An experimental study on the formation of shear bands was carried out in several metals including pure titanium (α-Ti), 304 stainless steel and aluminium (Al-7075-T6) through the compression of cylinder specimens in a Split-Hopkinson-Bar system. The shear band initiation and the distribution within the specimen were examined under different plastic strains and strain rates. The shear band forms along the maximum shear stress plane. Combining the total plastic strain and the angle between the band and the centrally symmetrical axis with the stress-strain curve, the critical strain (e c ) and stress (σ c ) for the formation of the band can be evaluated. It was found that the multiple developing shear bands were stopped in front of the grain boundary in one grain of Al-7075-T6. The present experimental results agree with Armstrong's dislocation pile-up model for the initiation for shear banding. Based on dislocation dynamics, a new constitutive equation is proposed for description of the formation of shear bands. The formation of the band is caused mainly by the strain softening from the sharp increase of mobile dislocations while the thermal softening enhances the development of the shearing process.