Occurrence of Dynamic Shear Bands in AISI 4340 Steel under Impact Loads

In this study, occurrence of adiabatic shear bands in AISI 4340 steel under high velocity impact loads is investigated using finite element analysis and experimental tests. The cylindrical steel specimen subjected to impact load was divided into different sections separated by nodes using finite element method in ABAQUS environment with boundary conditions specified. The material properties were assumed to be lower at the section where the adiabatic shear bands are expected to initialize. The finite element model was used to determine the maximum flow stress, the strain hardening, the thermal softening, and the critical strain for the formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for formation of transformed band in the alloy. The experimental results also show that 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. The simulation results obtained were compared with experimental results obtained for the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results was obtained.

Measurement of the Deformation of Aluminum Alloys under High Strain Rates Using High Speed Digital Cameras

Aluminum alloys exhibit an attractive combination of mechanical and physical properties such as high stiffness and low density, which favors their utilization in many structural applications. Thus, increasing the structural applications of aluminum alloy is the driving force for the need to adequately understand its deformation and failure mechanisms under various types of dynamic loading conditions. In this study, full field plastic deformation of AA6061-T6 aluminum alloy at high strain-rates under compressive and torsion loads are measured using split Hopkinson compression, torsion Kolsky bars, and a high speed digital image correlation system. The stress-strain curves obtained using the high speed digital cameras are compared with results obtained from the elastic waves in the compression and torsion bars. A post deformation analysis of the specimen also shows strain localization along narrow adiabatic shear bands in the AA6061-T6 alloy.

Microstructure and Quasi-Static Mechanical Behavior of Cryoforged AA2519 Alloy

In this study, AA2519 alloy was initially processed by multi axial forging (MAF) at room and cryogenic temperatures. Subsequently, the microstructure and the mechanical behavior of the processed samples under quasi-static loading were investigated to determine the influence of cryogenic forging on alloys’ subgrains dimensions, grain boundaries interactions, strength, ductility and toughness. In addition, the failure mechanisms at the tensile rupture surfaces were characterized using scanning electron micro-scope (SEM). The results show significant improvements in the strength, ductility and toughness of the alloy as a result of the cryogenic MAF process. The formation of nanoscale crystallite microstructure, heavily deformed grains with high density of grain boundaries and second phase breakage to finer particles were characterized as the main reasons for the increase in the mechanical properties of the cryogenic forged samples. The cryogenic processing of the alloy resulted in the formation of an ultrafine grained material with tensile strength and toughness that are ~41% and ~80% higher respectively after 2 cycles MAF when compared with the materials processed at ambient temperature. The fractography analysis on the tested materials shows a substantial ductility improvement in the cryoforged (CF) samples when compared to the room temperature forged (RTF) samples which is in alignment with their stress-strain profiles. However, extended forging at higher cycles than 2 cycles led only to increase in strength at the expense of ductility for both the CF and RTF samples.

Fatigue Responses of Three AA 2000 Series Aluminum Alloys

In this paper, smooth specimens of three aluminum alloys: AA 2219-T8, AA 2519-T8 and AA 2624-T351, were subjected to the same level of uniaxial (tension/compression) fatigue loading to compare their fatigue responses. Fractographic investigations of the failed specimens after fatigue loading was also conducted using a scanning electron microscope. The fatigue test results showed considerable differences in the fatigue lives of the three investigated alloys with AA 2219-T8 having the shortest fatigue life and AA 2624-T351 the longest fatigue life. The fractographic analysis showed that coalescence of micropores, microvoids, particles cleavage and microcracks are the predominant features in the fracture surface of AA 2219-T8. The fracture surface features of AA 2519-T8 revealed higher resistance to fatigue cracks nucleation and growth when compared to AA 2219-T8. The features depicted mainly partly ductile and partly brittle fracture. The AA 2624-T351 fracture surface features revealed noteworthy ductile failure mechanism. The results suggest a strong correlation between the surface fractographic features and the fatigue lives of the alloys. It is also observed that in addition to the yield strengths and ultimate tensile strengths, the total strain energy densities (SED) may provide a reasonable indication of the relative fatigue performance of the three alloys. AA 2219-T8 had the lowest SED and the lowest fatigue life, while AA 2624-T351 had the highest SED and the highest fatigue life. Thus, AA 2624-T351 would be the most suitable materials for components subjected to fatigue loading.

Shear Band Formation in AISI 4340 Steel Under Dynamic Impact Loads:Modeling and Experiment

In this study, the occurrence of the adiabatic shear bands in AISI 4340 steel under high velocity impact loading was investigated using finite element analysis and experimental tests. The cylindrical specimen subjected to the impact load was divided into different regions separated by nodes using finite element method in ABAQUS environment with boundary conditions specified. The material properties were assumed to be lower in the region where the probability of strain localization is high based on prior experimental results in order to initialize the formation of the adiabatic shear bands. The finite element model was used to determine the maximum flow stress, the strain hardening, the thermal softening, and the time to reach the critical strain for the formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for the formation of the transformed band in the alloy and the cracks were initiated and propagated along the transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by the strain-rates and the initial material microstructure. The simulation results obtained were compared with the experimental results obtained from the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results was obtained.