Microstructure evolution of adiabatic shear band in AZ31 alloy under dynamic compression

Effect of prior heat treatment on the dynamic impact behavior of 4340 steel and formation of adiabatic shear bands

Investigation of the twin-induced adiabatic shear bands evolution in a Mg-Al-Mn alloy under ballistic impact

The evolution of adiabatic shear band in high Co–Ni steel during high strain-rate compression

Ultra-fine-grained (UFG)/Nanocrystalline (NC) materials usually show reduced strain hardening and limited ductility due to formation of adiabatic shear band (ASB) under dynamic loading. In the present study, evolution of ASB in UFG Fe under dynamic shear loading was investigated. The UFG Fe was processed by equal-channel angular pressing (ECAP) via route Bc. After 6 passes, the grain size of UFG Fe reaches ~ 600 nm, as confirmed by means of Electron Back Scatter Diffraction (EBSD). Examination of micro-hardness and grain size of UFG Fe as a function of post-ECAP annealing temperature shows a transition from recovery to recrystallization at 500 0C. The high-strain-rate response of UFG Fe was characterized by hat-shaped specimen set-ups in Hopkinson bar experiments. The characteristics of ASB as a function of shear displacement, such as thickness of shear band and micro-hardness inside the shear band, were examined by SEM and Vickers micro-indentation respectively.

This work studies numerically the development of adiabatic shear banding (ASB) during high strain-rate compression of AISI 1045 steel. Plane strain and cylindrical axisymmetric compressions are simulated in LS-DYNA, considering rectangular and cylindrical steel samples, respectively. Also, a parametric analysis in height-to-base ratio is conducted in order to evaluate the effect of geometry and dimensional ratio of the sample on ASB formation. Doubly structural-thermal-damage coupled finite element models are developed for the numerical simulations, implementing the thermo-viscoplastic Modified Johnson-Cook constitutive relation and damage criterion, while further damage-equivalent stress and strain fields are introduced for the damage coupling. The simulations revealed that plane strain compression promotes more ASB formation, providing lower critical strain for ASB initiation and wider and stronger ASBs compared with axisymmetric compression. Further, X-shaped ASBs initially form during plane strain compression, while as deformation increases, they transform into S-shaped ASBs in contrast to axisymmetric compression, where parabolic ASBs are developed. Also, a lower height-to-base ratio leads to greater ASB propensity, reducing critical strain in axisymmetric compression. Finally, thermal softening is found to precede damage softening and dominate the ASB genesis and its early evolution, while in contrast damage softening drives later ASB evolution and its transition to fracture.

Study of the formation of adiabatic shear bands in steels

Adiabatic shear bands (ASBs) are a critical mechanism for damage initiation under high strain-rate shear impact, whereas the high-current-density-induced shear deformation mechanism of armature and rail materials remains unclear. This study employs a pulsed power source and an electromagnetic repulsion disk device to investigate the shear deformation characteristics of typical armature and rail materials under high strain rates (≥10<sup>4</sup> s<sup>-1</sup> ) coupled with high current densities (≥10<sup>8</sup> A/m<sup>2</sup> ). The results show that the ASB formation energy barrier decreases in the following order: pure copper, oxygen-free copper, CuCrZr alloy, Al<sub>2</sub>O<sub>3</sub> dispersion-strengthened copper alloy, brass, and 7075 aluminum alloy. Therefore, 7075 aluminum alloy is the most prone to ASB formation, followed by brass, while other copper-based rail materials rarely exhibit ASB features. Both 7075 aluminum alloy and brass exhibit a current-induced suppression effect on crack propagation and ASB formation. Electron backscatter diffraction (EBSD) analysis reveals that numerous fine equiaxed grains are present within the shear bands of 7075 aluminum, and the texture within the bands significantly differs from that of the surrounding matrix. With increasing current density, the grain size within the band increases, while the fraction of dynamically recrystallized grains decreases markedly. The formation of ultrafine grains and the texture evolution can be reasonably explained by mechanically assisted rotational dynamic recrystallization. The results indicate that thermal softening alone is insufficient to induce ASB formation; instead, softening caused by rotational dynamic recrystallization is the dominant mechanism. The current-induced temperature rise was calculated, and the yield strength drop under high-strain-rate loading with current was measured, based on which the width of adiabatic shear bands (ASBs) under current was determined. The theoretical predictions show good agreement with experimental results. The results indicate that the temperature rise and softening effect induced by pulsed current lead to an increase in ASB width, which intensifies energy dissipation, suppresses dynamic recrystallization, and inhibits the formation of adiabatic shear bands.

Experimental and finite element simulation study of the adiabatic shear band phenomenon in cold heading process

Effects of process parameters and loading direction on the impact strength of additively manufactured 18%Ni-M350 maraging steel under dynamic impact loading

This paper investigates numerically the effect of damage evolution on adiabatic shear banding (ASB) formation and its transition to fracture during high-speed blanking of 304 stainless steel sheets. A structural-thermal-damage-coupled finite element (FE) analysis is developed in LS-DYNA considering the modified Johnson-Cook thermo-viscoplastic model for both plasticity flow rule and damage law, while further, a temperature-dependent fracture criterion is implemented by introducing a critical temperature. The modeling approach is initially validated against experimental data regarding the fracture profile and ASB width. Next, FE simulations are conducted to examine the effect of strain rate and temperature dependence on damage law, while the effect of damage coupling is also evaluated, aiming to highlight the connection between thermal and damage softening and attribute them a specific role regarding ASB formation and transition to fracture. Also, the influence of dynamic recrystallization (DRX) softening is studied macroscopically, while further, a parametric analysis of the Taylor-Quinney coefficient is conducted to highlight the effect of plastic work-to-internal heat conversion efficiency on ASB formation. The results revealed that the implementation of damage coupling reacts to reduced ASB width and provides an S-shaped fracture profile, while it also decreases the peak force and results in an earlier fracture. Both findings are enhanced when accounting further for DRX softening and a higher value of the Taylor-Quinney coefficient. Finally, the simulations indicated that thermal softening precedes damage softening, showing that the temperature rise is responsible for ASB initiation, while instead, damage evolution drives ASB propagation and fracture.

Effect of tempering conditions on adiabatic shear banding during dynamic compression and ballistic impact tests of ultra-high-strength armor steel

Adiabatic shear banding reflects an unstable and dynamic plastic deformation mechanism occurring at high strain and strain rates which is strongly conjugated to fracture. Current work carries out a numerical study on the initiation and development of adiabatic shear bands (ASBs) in blanking process of AISI 4340 steel sheet. A structural-thermal coupled finite element analysis is developed in LS-DYNA software by implementing a thermo-viscoplastic flow rule for material plasticity and a damage criterion considering dynamic failure. The numerical simulations are focused on capturing ASB genesis through intense shear localization by evincing strain instability. Also, the evolution of ASB mechanism is investigated, aiming to contribute a stage-by-stage propagation and highlight its connection to dynamic failure. Further, the effect of ASB temperature and strain field on fracture is analysed, while the influence of strain/strain rate hardening and thermal softening on strain instability, peak force and the blanked surface is studied. The results revealed an S-shaped ASB due to severe shear localization and significant temperature increase, leading to dynamic recrystallization around punch-die corners and reacting to strain instability and dynamic. Finally, high magnitude thermal softening during ASB development resulted in earlier ASB generation and reduction of the peak blanking force, while further it decreased shear zone expansion and increased fractured length in the blanked surface.

Deciphering the microstructural evolution and adiabatic shearing behavior of the titanium alloy with stress-induced ω phase transformation during dynamic compression

ASB induced phase transformation in high oxygen doped commercial purity Ti

Effect of impurities on adiabatic shear bands (ASB) formation and evolution in U-2.5 wt%Nb alloy under impact