Deformation and failure mechanism in AISI 4340 steel under ballistic impact

Deformation and failure mechanism in quench-hardened AISI 4340 steel under ballistic impact is investigated. The influence of microstructure on damage evolution is also evaluated. Strain localization and shear failure along adiabatic shear bands are the dominant deformation and failure mechanisms. The time and critical strain for the commencement of strain localization is influenced by strain rate and microstructure. The microstructure of the steel sample also influenced the type of adiabatic shear bands formed during impact. Failure mechanism involves nucleation of micro-voids, which clusters to form bigger pores. Extremely fine micro-cracks are initiated adjacent to the pores and in shear flow direction along the shear bands. These micro-cracks become interconnected and grow to macro-cracks, which cause fracture of some of the investigated cylindrical steel samples under impact. The susceptibility of the adiabatic shear bands to cracking increases with decreasing tempering temperature of the steel.