Papers by Keyword: Adiabatic Shear Band (ASB)

Abstract: In this paper, the shock phase transformation of β phase in Ti-5Al-5Mo-5V-3Cr-0.5Fe (Ti-5553) was investigated. Split Hopkinson Pressure Bar (SHPB) and light gas gun were employed to investigate the dynamic properties under high strain rates from 1000s^-1 to 3500s^-1. Microstructure characterization was carried out by optical microscopy (OM), scanning electronic microscopy (SEM) and transmission electron microscope (TEM). The experimental results demonstrate that the Ti-5553 alloy with β phase exhibit no obvious strain rate hardening effect with the high strain rate from 1000s^-1 to 3000s^-1. However, compared with the quasi-static compression test results (10^-3s^-1), this alloy shows an evident strain rate hardening effect, with the yield strength significantly improved. Second time loading indicates light gas gun dynamic tensile loading and then SHPB dynamic compression loading in Ti-5553 alloy with β phase. The results show that the shock-induced β to αʺ martensite phase transformation dramatically influences the postshock mechanical properties of these alloys. The yield strength of this alloy decreased after the shock wave effect of light gas gun, its ductility increasing. Higher shock pressures yielded an increased dislocation density and a gradual increase in the yield strength. Adiabatic shear band (ASB) exists in second time loading Ti-5553 alloy under 10³s^-1 strain rate. SHPB loaded the alloy: The results show that the Ti5553 alloy with β phase is adiabatic shear failure in high strain rate (3000s^-1).

Abstract: Cubic-shaped specimens and hat-shaped specimens were used to investigate ASBs formed in Mg-12Gd-3Y-0.5Zr magnesium alloy under different initial strain rate impact. It shows that no adiabatic shear bands (ASBs) is observed in micro-structure of cubic-shaped specimens by scanning electron microscope (SEM) , but obvious ASBs can be observed in hat-shaped specimens. Johnson-Cook model based on thermoviscoplasticity constitutive relation was used to simulate internal stress distribution and fracture mode, and it indicated that the result of failure analysis from specimens under high velocity impact tests was the same as that obtained by computer simulation.

Abstract: Adiabatic shear bands are microstructural features that appear when metals, and some non-metals are subjected to impact loading at strain rates in excess of 10³ s^-1 and large strains. The formation of these bands is generally attributed to several competing mechanisms, among them is an initial strain hardening followed by adiabatic thermal softening that may lead to crack initiation within the bands. The authors have developed a model for formation of adiabatic shear bands in metallic materials as they are formed during testing using a torsional Hopkinson Bar. The model relies on a one dimensional analysis which predicts accurately the two steps of forming adiabatic shear bands in terms of strain hardening followed by thermal softening. In this current research, the model is extended to a two-dimensional analysis which would be suitable for application in either a two bar compression Split Hopkinson Bar or in a direct impact compression system developed by the author (Nabil Bassim) to produce high strain rates and large strains. The algorithm relies on applying the concept of dynamic recrystallization in order to determine the onset or initiation of the adiabatic shear bands.

Abstract: The adiabatic shear bands (ASB) of the thick-walled cylinder have been studied by many researchers in the recent years. The onset and evolutions of the multiple shear failure of metal cylinder under explosive loadings are affected by many factors such as the characteristics of the impulsive loadings, the dynamic behavior of the materials, etc. In this work, a tube compression driven by electro-magnetic forces is introduced, which enables to carry out the experiments of the spontaneous evolution of multiple adiabatic shear bands in metal tube. The FEM simulation was conducted to investigate the evolution process of strain localization with coupled thermo-mechanical analysis. The FEM results show that ASB initiates when the stress drops rapidly and strain growth and not when it reaches the maximum shear stress. Once the shear band is formed, elastic unloading occurs beside the shear band. The different behaviors of the damage introduced in the strain softening model affect the initial nucleation strain and the distribution of ASBs. With the increase of material damage softening, the initial strain of shear band decreases and the number of shear bands increases.

Abstract: A set of 18 armour steel plates were stacked on top of each other and subjected to shape charges that went through the plates and created a hole that decreased in diameter as it went through consecutive plates. Afterwards, the plates were examined and the hardness near the hole and away from the hole was taken to determine the effect of the passing of the shaped charge through the plates. Also, specimens from the walls of the holes were taken to determine changes in the microstructure due to the shock wave and the resulting excessive heating from the shape charge. It was observed that the shock wave produced significant changes in the microstructure resulting in the appearance adiabatic shear bands (ASBs). These ASBs persisted in holes in plates placed further down the stack (up to 8^th in the stack). More complex microstructural mechanisms are thought to take place as opposed to erosion from the flow of the molten metal through the holes in the plates.

Abstract: In order to develop new Al alloy, the quasi-static and dynamic compression deformation behavior of an advanced Al-Mg alloy, which contained Zn and rare earth, were investigated. Deformed microstructures in the shear bands of this alloy were characterized by optical microscope. Results show that the compressive strengths increase with strain and strain rates being increased, and microstructures of deformed Al-Mg alloy along compression directions are sensitive to strain rates. The distorted deformation twins near the deformed band can be formed under low strain rate of 5.6×10^-3 s^-1, while multiple shear bands slipping can be induced by dynamic deformation under high strain rates from 1.9×10³ s^-1 to 5×10³ s^-1. Therefore, compressive strength and fracture strain of dynamic deformation are higher than that of the quasi-static deformation, which is attributed to the strain hardening and grain refining in the multiple shear bands.

Abstract: In this paper, a novel model combining the microstructure prediction model and a modified constitutive model of the Johnson-Cook (JC) model was developed and embedded into FEM software via the user subroutine. The chip formation and microstructure evolution in high speed cutting of Ti-6Al-4V alloy were simulated. The results indicated that dynamic recrystallization mainly happened in adiabatic shear bands (ASBs), where the grain size had a big decline. Then FEM simulations were carried out to investigate the effect of cutting velocity, uncut chip thickness, and the rake angle on the ASBs width of the serrated chips. It can be concluded that the width of ASB increases with the increasing of cutting depth and cutting velocity, and decreases with the increasing of rake angle of the tool.

Abstract: In order to solve the problem of rivet head tending to crack in riveting, the effect of die forms on TA1 rivet deformation and microstructure was investigated by experimental method from macro and micro aspects in electromagnetic riveting. The results show that rivet material axial and radial flow can be controlled using different die forms, and thus the distribution of adiabatic shear band and grains in two sides of that in formed rivet head can be changed. With flat head die, severe axial and radial flows result in the nonuniform deformation and crack is prone to produced in rivet head. The adiabatic shear band of rivet head is obvious and grains in both sides of that deform severely. Material radial flows of rivet head are well constrained by spherical and 400 head die. The deformations are relatively uniform and the adiabatic shear bands are not obvious. It is an effective way to avoid crack formation and improve riveting quality using different die forms.

Abstract: In order to research the formation and spread of adiabatic shear bands in Ti-6Al-4V targets, LS-DYNA code is used to simulate the ballistic impact process. The projectile used in the impact test is a flat-headed steel cylinder with diameter of 7.62mm and length of 39mm. The results of simulated and impact test are in good agreement. Multiple adiabatic shear bands form in Ti-6Al-4V targets under high-speed ballistic impact. Adiabatic shear bands were found to extend parallel with a certain distance. The formation and distribution of adiabatic shear bands was related to the breaking-off of projectiles, which was caused by the distribution of maximum shear stress in Ti-6Al-4V targets and projectiles.

Abstract: Four different heat treatment routes which were called step quenching were taken and accordingly different morphology dual-phase samples have been obtained. It is found that Maximum shear stress increases with martensite volume increases. They have the following relationship: Maximum shear stress = 1.14(MVF)2 – 90.7(MVF) + 24266 MPa. Adiabatic shear bands and cracks led by them are important signs during the dynamic torsional tests