Evolution Of Adiabatic Shear Bands Research Articles

A structural-thermal coupled modeling approach on the formation of adiabatic shear bands in steel sheet blanking process

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.

Two-dimensional evolution of temperature and deformation fields during dynamic shear banding: In-situ experiments and modeling

Adiabatic shear band (ASB) is a significant failure mechanism observed in metals and alloys under impact loading. Though ASB formation has been widely assumed to be a one-dimensional thermo-mechanically-coupled instability problem, it is crucial to recognize that adiabatic shear banding is essentially a two-dimensional propagating event in space. However, it is challenging to perform in-situ characterization of temperature-deformation fields during ASB formation due to the extremely small spatial and temporal scales involved. To obtain the two-dimensional features of ASB evolution, a newly developed plane-array infrared imaging system and microspeckle-based digital image correlation (DIC) technique are synchronized with the Kolsky bar system. By incorporating interrupted tests, “quasi-synchronous” characterization of temperature-deformation-microstructure evolution during ASB formation in hat-shaped specimens of Ti–6Al–4V is achieved. A phase-field model incorporating energy-based shear banding criteria and independently calibrated model parameters is established to simulate the dynamic shear failure process, which is demonstrated to be able to well reproduce experimentally observed temperature and deformation evolution. Based on experimental characterization and simulation results, the two-dimensional features and thermo-mechanical aspects of ASB formation are presented. Energy dissipation of shear banding is estimated based on the measured temperature field, demonstrating good agreement with the calibrated values in the phase-field model. The “propagation” and “percolation” modes along the band are analyzed, which can be predicted by the introduction of a shear band process zone. The influences of thermal and microstructural softening on shear failure are also clarified through a comprehensive analysis of temperature and microstructure evolution.

High-strain-rate deformation: Stress-induced phase transformation and nanostructures in a titanium alloy

Microstructural evolution in adiabatic shear bands of a Ti-10V-2Fe-3Al (wt.%) alloy subjected to high strain-rate dynamic loading was investigated in detail via high-resolution transmission electron microscopy (HRTEM), focusing on the underlying plastic deformation mechanisms. The alloy after solution treatment consisted of a coarse body-centered cubic β single-phase microstructure, which exhibited double-yield and stress-collapse characteristics. The former was a result of stress-induced martensite transformation accompanied by the formation of nanotwins in the martensite, and the latter was caused by thermal softening stemming from dynamic recovery. Substantial grain refinement with three orders of magnitude grain size reduction occurred in the center of adiabatic shear bands due to severe plastic deformation along with dynamic recovery. Multiple deformation mechanisms in the adiabatic shear bands were identified, including the formation of face-centered orthorhombic α″ martensite, {111}α″ type I nanotwins, and hexagonal close-packed α phase, where distinctive nano-twinned structures and phase transformation from α″ to α with an orientation relationship of {1¯11}α″//{1¯101}α were observed. The transition area of the adiabatic shear bands contained mainly high-density dislocation regions and plate-like structures with relatively fewer nanoscale grains, where the transition from nanokinks to nanotwins along with phase transformation was also revealed, corroborating that kink bands served as nanotwin nucleation sites. The mechanisms of the microstructural evolution in relation to the mechanical response during the high strain-rate dynamic loading were discussed.

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

The formation and evolution of twin-induced adiabatic shear bands (ASBs) in a Mg-Al-Mn alloy under ballistic impact were investigated by analyzing the microstructures at three representative locations with different distances from the crater rim. Results show that localized shear deformation occurred at the twin boundaries, resulting in the formation of ASBs. The ASBs thicken at the expense of the deformation twins with increasing plastic deformation, forming ASBs which are mainly composed of subgrains, ultra-fine recrystallized grains and Mg17Al12 precipitates. The appearance of the twin-induced ASBs is mainly attributed to the twin-twin intersections caused by high strain rate deformation. The twin-twin intersections under high strain rate deformation lead to a different twinning activity in which the subsequent plastic deformation mainly occurs by shear deformation along the twin boundaries rather than thickening the twins. These results and analysis can shed new light on understanding the nucleation and evolution of ASBs in Mg alloys under dynamic loadings.

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

The microstructural and damage evolution of 18%Ni M350 maraging steel fabricated by laser-based directed energy deposition (DED) process under dynamic impact loading are investigated in this study. The influence of additive manufacturing (AM) processing parameters, including laser power, powder feed rate, and energy area density (EAD) are discussed. The mechanical testing of the additively manufactured alloy was done using the split Hopkinson pressure bar (SHPB) system. The dynamic impact test was conducted using strain rates ranging from 100 to 4000 s−1. The test was conducted on cylindrical specimens (7 mm diameter and 8 mm long) with axes parallel to the build direction (Z-axis) and perpendicular (X- and Y-axis) to the build direction to evaluate the effects of loading directions on mechanical responses and microstructural evolution in the alloy under the impact loading conditions. The test results revealed that the materials processed using the lowest EAD parameters exhibit the greatest impact strength under dynamic impact loading for all directions. Specimens impacted along the build direction (Z-axis) exhibit a continuous drop in peak flow stress once the strain rates exceeded 2300 s−1 indicating the greatest susceptibility to thermal softening at high strain rates. Strain localization leading to the formation of adiabatic shear bands (ASB) was observed in some specimens. Cracks developed along the ASBs, while some AM-process-related defects, such as the lack-of-fusion, interacted with adiabatic shear bands, promoting cracking inside the shear bands. Despite possessing the highest impact strength, the materials produced by the lowest EAD parameters are the most susceptible to the development of adiabatic shear bands leading to cracking and fracture.

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

The formation of adiabatic shear band (ASB) and its damage behaviour in AZ31 alloy under high strain rate compression (2000 s−1) were investigated in this study by using a split Hopkinson pressure bar. Microstructure of the ASB was characterised by electron back-scattering diffraction and transmission electron microscopy, and the adiabatic shear damage behaviour was analysed through finite element simulation by LS-DYNA. The results show that the ASB and the surrounding microstructure form a gradient microstructure distribution, and the formation of ultra-fine grains in the ASB is due to rotational dynamic recrystallisation. The combination of work hardening and grain refinement in ASB leads to a high microhardness. Micro-voids grow in the ASB and eventually form macro-cracks, leading to material failure.

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

The microstructural evolution and formation mechanism of adiabatic shear band (ASB) in M54 steel were investigated using split Hopkinson pressure bar (SHPB). Specially, the strain response to ASB was analyzed systematically by strain controlling dynamic compression tests at high strain-rates. The results demonstrated that the ASBs started to form at 23% strain and shearing failure strain was increased to 28% at strain-rate of 4000 s−1. The ASBs in the M54 steel was consist of a transformed adiabatic shear bands (T-ASB) and deformed adiabatic shear bands (D-ASB). The evolution of the ASBs was revealed and the generation of fine equiaxed nano-grains within ASBs was attributed to the rotational dynamic recrystallization mechanism. The twinning process was actually the reverse transformation of ω→bcc and the driving force was the lattice instability of martensite matrix caused by carbon redistribution.

Evolution of local deformation field inside adiabatic shear band of 1018 steel studied using digital image correlation with micro-speckles

Adiabatic shear band (ASB) is widely observed in metals and alloys under impact loading as a significant failure mechanism. Exhibiting severe localization over a short period of time, ASB imposes great challenges in characterizing the evolution of the local deformation field. Most existing theoretical models employed the assumption of laminar-flow-like shear deformation inside the band, while non-uniform “hot spots” have also been observed along a propagating shear band. In this study, the evolution of the local deformation field inside the ASB was investigated based on the digital image correlation (DIC) technique with micro-speckles. Destructive micro-speckles were created using laser etching within the shearing region to resolve localized deformation. Based on experimental results of 1018 steel specimen under impact loading, ASB evolution along the shearing plane was divided into two distinct stages: in the first stage, the deformation localized almost uniformly along the band, which is consistent with the laminar flow assumption; whereas in the second stage, the deformation was highly non-uniform and exhibited distributed strain-localizations, which could be closely associated with hot-spot formation and should be taken into account especially during the post-localization stage. Our study revealed the existence of “hot spots” based on a direct measurement of the deformation field, and shed more light on the understanding of the entire evolution process of ASB formation.

ASB induced phase transformation in high oxygen doped commercial purity Ti

Given that commercial purity titanium (CP-Ti) doped with a high oxygen concentration has recently been reported with high mechanical performance, the present work investigated the failure behavior and its mechanisms of the material under uniaxial impact loading. Besides the strong hardening effect of oxygen, it was found that increasing oxygen content led to increased propensity for adiabatic shear failure in CP-Ti. The texture or grain orientation was also found to have profound influence on the formation of adiabatic shear band (ASB). Microstructural examinations on the postmortem high oxygen CP-Ti suggested that uniform and equi-axed nano-grains were produced within the ASB. The orientations of these nano-grains were analyzed using the precession electron diffraction (PED) technique. It provides direct evidence of phase transformation occurring in ASB. Then we demonstrate that the nano-grains in ASB formed from parent grains by phase transformation, indicating that the α→β→α phase transformation process takes part in ASB evolution and is an underlying mechanism of grain refinement. As such, this result is a supplement for traditional well-accepted dynamic recrystallization mechanism of ASB evolution.

Temperature increases and thermoplastic microstructural evolution in adiabatic shear bands in a high-strength and high-toughness 10 wt.% Ni steel

A 10 wt.% nickel-steel has been developed for high pressures and low-temperature applications, due to its high strength, excellent toughness, and low ductile-to-brittle transition temperature (DBTT). Under dynamic loading conditions this steel is, however, prone to shear localization that manifests as adiabatic shear bands (ASBs). The temperature increases and thermoplastic microstructural evolution in the ASB are studied in detail, from the microscopic length scale to the atomic-scale employing correlative electron-backscatter diffraction (EBSD), transmission electron microscopy (TEM), and atom-probe tomography (APT). From a calculation of the temperature increase under adiabatic conditions, based on the conversion of plastic-work to heat generation, the microstructural transitions in the ASB are discussed specifically for: (i) a b.c.c.-f.c.c. phase-transformation and elemental partitioning of alloying elements; (ii) a thermodynamic model for the compositional change of the V(Nb)-rich carbonitride precipitates during a temperature increase; (iii) grain-refinement and rotation by dynamic/mechanical recrystallization processes. Solute segregation at subgrain boundaries, measured using the Gibbsian interfacial excess methodology, reveals how solute segregation contributes to the instability of localized shear-deformation by promoting the depinning of solute elements and the migration of a grain boundary. Finally, a kinetic model for grain refinement/rotation within an ASB is described by the dynamic recrystallization behavior with: (i) sub-grain formation; (ii) rotation/refinement by deformation; and (iii) grain growth by subgrain coalescence with further rotation and a temperature increase. The temperature increase under dynamic deformation in an ASB promotes grain boundary migration and subgrain coalescence to create a large degree of equiaxed grains with a low density of imperfections.

Complete precipitate dissolution during adiabatic shear localisation in a Ni-based superalloy

ABSTRACT Whereas microstructure evolution in adiabatic shear bands have been thoroughly studied, reports on the stability of hardening precipitates during shear localisation are scarce. We report an atomic scale investigation of solute distribution in adiabatic shear bands in a precipitation strengthened Ni–Fe-based superalloy, showing that the hardening particles are completely dissolved. Temperature estimations indicate that peak temperatures in the shear band above the solvus limits of the precipitates are not unrealistic, and thus diffusion-assisted transformations during the severe plastic deformation cannot be ruled out.

Role of plastic deformation mechanisms during the microstructural evolution and intermetallics formation in dissimilar friction stir weld

During dissimilar Friction Stir Welding (FSW) of titanium (Ti) to aluminum (Al), particles of titanium are inhomogeneously distributed in the aluminum matrix, in the weld nugget. The Al/Ti interfaces are the potential zones for intermetallic formation. The inhomogeneous distribution of Ti particles and intermetallic particles can have a negative impact on the mechanical properties of the weld. The objective of the present investigation is to highlight the mechanism of such inhomogeneity in the distribution and formation of intermetallics within the weld. In the present investigation, FSW of Al to Ti was carried out in butt-weld configuration and microstructure across the weld was characterized using Scanning Electron Microscopy (SEM) and X-ray Tomography (XCT). Results from the weld nugget showed three regions of Ti particle distribution, variation in their fragmentation and chemical composition across the Al/Ti interfaces when investigated from top to bottom of the weld. Such variations were correlated with conventional strain rate and temperature variation in the weld nugget. Detailed analysis of microstructure in the weld nugget illustrated that both titanium and aluminum undergo dynamic recrystallization. However, the mechanisms leading to such microstructural evolution were different. Recrystallization in titanium occurred due to low-temperature deformation and evolution of adiabatic shear bands whereas dynamic recrystallization in aluminum was attributed to its high stacking fault energy. The present results and mechanism can be useful in further development of the weld nugget in dissimilar FSW and development of composite by friction stir processing with homogeneous microstructure and superior mechanical properties.

A thermo-elastic-plastic phase-field model for simulating the evolution and transition of adiabatic shear band. Part I. Theory and model calibration

Adiabatic shear band (ASB) is an important failure mode of solid materials, especially for metal materials under high strain rate loading. In this study, a thermo-elastic-plastic phase-field model, which considers both damage softening and thermal softening, is established to simulate the formation of multiple ASBs and the transition from ASB to the fracture. How to select and calibrate the material parameters in the phase-field model is seldom clearly discussed in the current phase-field model when dealing with ASB. In this paper, the damage parameters in the phase-field method are calibrated using data from pure shear specimens, taking into account both the overall response of the structure and the local response in the ASB. As an application, the calibrated model is used to numerically study the evolution of the ASB of the hat specimen and the process of its transition to the fracture. The simulation results successfully explain the typical phenomena such as transient “hot spots” and double softening in the experiment. The numerical model we developed provides a reliable quantitative description for the evolution and width calculation of ASBs and lays a foundation for the further study of ASB.

A thermo-elastic-plastic phase-field model for simulating the evolution and transition of adiabatic shear band. Part II. Dynamic collapse of thick-walled cylinder

In Part I, a thermo-elastic-plastic phase-field model is established for describing the adiabatic shear band (ASB) in metal materials. In this Part II, the developed model is used to simulate the classical thick-walled cylinder (TWC) experiment to investigate the self-organizing behavior of multiple ASBs. For the first time, the formation process of self-organized ASBs in the TWC experiment is reproduced by the phase-field method and the underlying physical mechanism is analyzed in detail. The simulation results show that the number and spacing of ASBs are related to loading rate and material properties. A higher loading rate leads to more intensive ASBs. For typical engineering materials such as 304L stainless steel (Ss304L) and titanium alloy (Ti6Al4V), the contribution of thermal softening to the formation of ASBs is far less than that of damage softening. However, thermal softening is very important to induce initial ASBs. In addition, we also find that defects, especially large ones, play a dominant role in the initiation and evolution of ASBs, leading to complex patterns of ASBs in the TWC experiments.

Temperature Increases and Thermoplastic Microstructural Evolution in Adiabatic Shear-Bands in a High-Strength and High-Toughness 10 wt.% Ni Steel

A 10 wt.% nickel-steel has been developed for high pressures and low-temperature applications, due to its high strength, excellent toughness, and low ductile-to-brittle transition temperature (DBTT). Under dynamic loading conditions this steel is, however, prone to shear localization that manifests as adiabatic shear bands (ASBs). The temperature increases and thermoplastic microstructural evolution in the ASB are studied in detail, from the macroscopic length scale to the atomic-scale employing correlative electron-backscatter diffraction (EBSD), transmission electron microscopy (TEM), and atom-probe tomography (APT). From a calculation of the temperature increase under adiabatic conditions, based on the conversion of plastic-work to heat generation, the microstructural transitions in the ASB are discussed specifically for: (i) a b.c.c.-f.c.c. phase-transformation and their elemental partitioning; (ii) the thermodynamic model for the compositional change of the V(Nb)-rich carbonitride precipitates during a temperature increase; and (iii) grain-refinement and rotation by dynamic/mechanical recrystallization processes. Solute segregation at subgrain boundaries, measured using the Gibbsian interfacial excess methodology, reveals how solute segregation contributes to the instability of localized shear-deformation by promoting the depinning of solute elements and the migration of a grain boundary. Finally, a kinetic model for grain refinement/rotation within an ASB is described by the dynamic recrystallization behavior with: (i) subgrain formation; (ii) rotation/refinement by deformation; and (iii) grain growth by subgrain coalescence with further rotation and a temperature increase. The temperature increase under dynamic deformation in an ASB promotes grain boundary migration and subgrain coalescence to create a large degree of equiaxed grains with a low density of imperfections.

Size Effect on Onset and Subsequent Evolution of Adiabatic Shear Band: Theoretical and Numerical Analysis

The adiabatic shear instability of ductile materials has attracted more and more attentions of researchers and groups, who have been sparing no effort in further understanding of the underlying mechanism since the first experimental depiction of adiabatic shear instability by Zener and Hollomon. As for the adiabatic shear instability, many factors account for its occurrence, including heat conduction, inertia effect, microstructure effect and so on. However, lots of experimental evidence has shown that metal materials display a strong size effect when the characteristic length scale is in the order of microns. The size effect has also been observed in the analysis of shear band in the ductile materials because the order of the bandwidth stays within the microscale range. However, a comprehensive understanding of the whole process of adiabatic shear banding (ASB), including the early onset and the subsequent evolution, is still lacking. In this work, a gradient plasticity model based on the Taylor-based nonlocal theory feasible for the linear perturbation analysis and convenient for numerical calculation is proposed to investigate the strain gradient on the onset of ASB and the coupling effect of heat conduction, inertia effect and strain gradient at the early stage, as well as on the subsequent evolution process at later stages. As for the onset of ASB, the linear perturbation method is used to consider the effect on the initial formation of ASB. After the investigation of the onset of ASB, the characteristic line method is applied to describe the subsequent nonlinear evolution process of ASB. Three stages of ASB evolution are clearly depicted during the evolution process, and the significance of size effect on the ASB nonlinear evolution process of ASB at different stages is analyzed. With the help of linear perturbation analysis and characteristic line method, a comprehensive description of the role of strain gradient in the ASB from the early onset to the end of the evolution is provided.

Superior mechanical properties and deformation mechanisms of heterogeneous laminates under dynamic shear loading

High-strain rate response of low C steel/304 stainless steel (SS) laminates was characterized by hat-shaped specimen using Hopkinson-bar technique at a strain rate of about 7 × 104 s−1. Better dynamic shear properties were observed in the laminates, compared to the plain low C steel plate and the plain 304 SS plate. The laminates were found to postpone the nucleation of adiabatic shear band (ASB) in the hard zone and to delay the propagation of ASB from the hard zone to the soft zone. The conventional maximum stress criterion on ASB nucleation was found not valid any more in the laminates. The hardness difference between the hard zone and the soft zone in the laminates was found to have great influence on the patterns of ASB evolution. Nanotwins were formed in the 304 SS and grain refinement was observed in the martensite low C steel for strain hardening under dynamic shear loading. The mechanical incompatibility across the interfaces was observed to result in strain gradient and geometrically necessary dislocations at the interfaces under dynamic shear loading, contributing to extra strain hardening. The extra hardening was also found to be triggered at the propagation tip of ASB, which helps for achieving better dynamic ductility in the laminates.

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

Local stress concentration sites and/or microstructural inhomogeneities result in the nucleation and initiation of adiabatic shear bands (ASB). This paper reports the influence of impurities for the initiation and evolution of ASBs in U-2.5 wt%Nb alloy. Both cylindrical specimen and hat-shaped specimen were used to investigate the formation and evolution mechanism of ASBs in U-2.5 wt%Nb alloy using a split Hopkinson pressure bar apparatus. It was observed that brittle inclusions such as carbides in all of the specimens break up to form microcracks at very small strain during impact, then the microcracks coalesce and grow with increasing strain, which revealed that the brittle impurities in specimen not only precedes adiabatic shear failure, but also likely to be a dominant micromechanical factor in the very generation of the band. Adiabatic shear fracture occurs in the top end of hat-shaped specimen, it also appears that the microstructural inhomogeneities play more important role in occurrence of ASBs than the geometrical inhomogeneities. Additionally, it was demonstrated that the ductile inclusion of niobium was susceptible to resist the propagation of shear bands.

Effect of strain rate and temperature on dynamic mechanical behavior and microstructure evolution of ultra-high strength aluminum alloy

The dynamic mechanical response of Al-Zn-Mg-Cu alloy with a high content of Zinc was studied using split Hopkinson pressure bar (SHPB) at strain rate of 1778 s−1–6516 s−1 and temperature of 25–400 °C. The microstructure evolution and fracture characteristics of this alloy were revealed from these studies. The obtained results show that the strain rate sensitivity is slightly positive below 3136 s−1, but becomes negative at higher strain rates because of the development of adiabatic shear bands and cracks. From 25 to 400 °C, the flow stress decreases and there is an obvious decline above 200 °C due to occurrence of athermal softening. The dislocation microbands and geometrically necessary dislocation contribute to grain fragmentation. The fracture develops due to a combination of ductile and shear failure. This study in general provides a significant understanding on the relationship between microstructure evolution and mechanical behavior of high strength aluminum alloy under dynamic loading.

Dynamic Mechanical Responses of Ultrafine-Grained IF Steel Over a Wide Range of Temperatures

The ultrafine-grained interstitial free (UFG-IF) steel was fabricated using the equal channel angular pressing (ECAP) method. Transmission electronic microscope (TEM) observations showed that the grains were refined to sub-micrometer scale and the average grain size was several hundred nanometers (nm). Uniaxial compression tests were conducted over a wide range of strain rates (up to 6000/s) and temperatures (from −196[Formula: see text]C to 300[Formula: see text]C) to understand the dependence of mechanical behaviors on the strain rate and temperature. It is found that the UFG-IF steel showed a weak strain rate sensitivity and displayed a strong temperature sensitivity only within a very narrow temperature range. Microstructural observations on the specimens deformed dynamically at temperatures below −20[Formula: see text]C via scanning electronic microscopy (SEM) and TEM indicated the occurrence of adiabatic shear localization. In order to explain the reason why adiabatic shear localization formed at the low temperatures and high strain rates, the susceptibility factor to adiabatic shear band (ASB) was estimated. It is proposed that more heat generated during the adiabatic plastic deformation process due to the enhanced flow stress of the UFG-IF steel at lower temperatures may play a dominant role in the formation of ASB. Meanwhile, the experimental results also provide strong evidence that for the UFG-IF steel the dynamic recrystallization which results from the severe plastic deformation and high temperature rise only occurs at the later stage of ASB evolution. With regard to the formation of ASB in the UFG IF steel, the localized plastic flow at a lower stress level indicates that localized deformation happens firstly and the temperature rise in the localized shearing zone leads to the final plastic instability.