ADIABATIC SHEAR LOCALIZATION IN AISI 1340 AND 4340 STEELS: THE INFLUENCE OF MICROSTRUCTURE AND GEOMETRY
MOSTAGHIMI GHOMI, HOMA
The mechanical behaviour of AISI 1340 and 4340 steel under high strain-rate loading in compression and in torsion were investigated using direct impact Hopkinson bar and torsion split Hopkinson bars. Both alloys contained 0.40 wt. % C, but different amounts and types of alloying additions. The materials are commonly used in high performance structural applications, where they could be subjected to dynamic shock loading. The objective of this study was to study the effects of microstructure, strain rates and specimen geometry on the occurrence and failure of adiabatic shear bands in these alloys under dynamic shock loading. Cylindrical specimens of the AISI 1340 alloys were heat treated to produce martensitic, dual-phase or pearlitic structure and subjected to impact loading at strain rates ranging between 1000 and 8000 /s. The martensitic test specimens were tempered at 205, 315 and 425 ºC to determine the effects of tempered condition on the adiabatic shear failure of the alloy. The effects of geometry on strain localization and adiabatic shear banding in both alloys were investigated by subjecting cylindrical-, cubical-, and truncated conical-shaped specimens to high velocity impact. The dynamic torsion test involved rapidly twisting of heat-treated thin-walled tubular specimens of the alloys and determining the damage evolution during the high strain torsional loading. Both optical and scanning electron microscopes were used to evaluate the damage evolution in the specimens after high strain rate loading. The types of shear band formed in the alloys depended on the microstructure and strain rate. Deformed bands were formed at low strain rates and there was a minimum strain rate required for formation of transformed band in both alloys. This minimum strain rate was highest in the specimens with pearlitic structure and lowest in the specimen with martensitic structure. The susceptibility of the martensitic specimens to the occurrence of transformed shear band decreased with increasing tempering temperature. Cracks were initiated and propagated along transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by strain-rates, initial microstructure and the specimens̕ geometry. The geometry of the impacted specimen determined the shape of the adiabatic shear band and the topography of the fracture surface of fragmented specimens. Fractographic investigation of fragmented specimens showed ductile shear failure and knobby fracture mode along the transformed band. Investigations of the transformed band using X-ray Photo Emission Electron Microscopy and Near Edge X-ray Absorption Fine structure Spectroscopy showed more nickel and less chromium inside the transformed bands in impacted AISI 4340 steel than in the region outside the shear band.
DegreeMaster of Science (M.Sc.)
SupervisorOdeshi, Akindele G.
CommitteeSzpunar, Jerzy A.; Johnston, James; Wegner, Leon D.
Copyright DateSeptember 2011
Dynamic shock loading
AISI 1340 steel
AISI 4340 steel
Adiabatic shear bands