Initiation Of Adiabatic Shear Bands Research Articles

Effect of stress state on adiabatic shear banding in Al2024-T351

Cubic uniaxial compression and angled cubic biaxial shear-compression specimens are a preferable choice in the context of developing empirical data for models such as the Generalized Incremental Stress State Dependent Damage Model (GISSMO), with a strain-rate dependent extension. The angled cubic specimens introduce a multiaxial stress state of combined shear and compression. Al2024-T351 specimens have been impacted using a direct impact Hopkinson bar coupled with Digital Image Correlation (2D-DIC). 2D-DIC has been used to track the full-field strain evolution of all specimens and obtain the fracture strains for fractured specimens. The shear strain evolution related to failure along the shear planes can be acquired using 2D-DIC, and a quantitative measurement of fracture initiation is acquired from the DIC data. It is also highlighted that the transverse displacement contours can offer qualitative information about the effects of friction on proportional loading during uniaxial compression. Interrupted tests using stop rings are conducted for angled cubic specimens to identify the critical strains for the initiation of adiabatic shear bands (ASBs), which can be defined as the GISSMO instability strains. ASBs are confirmed using the optical microscope, and it is found that for a constant impact momentum, the specimens with a greater shear to compressive stress state exhibit a higher tendency to form ASBs at lower major axial strains. It is also found that ASBs are observed using the optical microscope with no loss of flow stress, and that angled specimens exhibit a lower flow stress than uniaxial compression specimens at higher strain rates. Lastly, pre-impact and non-fractured post-impact Vickers microhardness tests have been conducted, and it is found that for all specimens, the material is on average about 20% harder after impact in regions away from the ASB, and further that regardless of stress-state, the ASB is consistently about 30% harder than the pre-impact microstructure.

The Effects of Microstructure on the Dynamic Mechanical Response and Adiabatic Shearing Behaviors of a Near-α Ti-6Al-3Nb-2Zr-1Mo Alloy

The formation and evolution of adiabatic shear behaviors, as well as the corresponding mechanical properties of a near-Ti-6Al-3Nb-2Zr-1Mo (Ti-6321) alloy during dynamic compression process, were systematically investigated by the split Hopkinson pressure bar (SHPB) compression tests in this paper. Ti-6321 samples containing three types of microstructures, i.e., equiaxed microstructure, duplex microstructure and Widmanstätten microstructure, were prepared to investigate the relationship between microstructures and dynamic mechanical behaviors under different strain rates in a range from 1000 s-1 to 3000 s-1. It was found by the dynamic strain-stress relation that the Ti-6321 alloys containing equiaxed microstructure, duplex microstructure and Widmanstätten microstructure all exhibited a strong strain-hardening effect. The samples containing equiaxed microstructure exhibited a larger flow stress than samples containing duplex microstructure and Widmanstätten microstructure. The adiabatic shearing behaviors in Ti-6321 alloy are significantly influenced by different types of microstructures. The formation of adiabatic shearing bands occurs in equiaxed microstructure when the strain rate is increased to 2000 s-1. The adiabatic shear bands are formed in duplex microstructure when the strain rate reaches 3000 s-1. However, the initiation of adiabatic shear bands is found in Widmanstätten microstructure under the strain rate of 1000 s-1. The Widmanstätten microstructure shows a larger sensitivity to adiabatic shearing than the equiaxed microstructure and duplex microstructure samples.

Effect of rotary swaging on microstructure evolution and adiabatic shear sensitivity of 90W–7Ni–3Fe alloy under dynamic loading

Tungsten heavy alloys (WHAs) are widely used in the preparation of kinetic penetrators. However, the penetrating performance of alloys, measured by the “self-sharpening effect”, is often limited by the low sensitivity of the adiabatic shear band (ASB). To address this issue, the 90W–7Ni–3Fe rods with deformation degrees of 20% and 40% were prepared by rotary swaging (RS) at high strain rates (4000 s−1 and 6000 s−1, respectively). The deformation and ASB behavior of RS-treated alloys were compared to those of undeformed samples. According to the results, RS pretreatment significantly increased the dynamic penetration strength and ASB sensitivity of the alloys. Besides, the characterization of the recrystallized microstructure within ASB and the calculation of the adiabatic temperature rise indicated that microstructure softening could be the primary cause for the initiation of ASB, which was consistent with the rotational dynamic recrystallization (RDR) mechanism. Moreover, the higher strain energy of the 40% RS deformation sample with high densities of dislocations and twins promoted the subgrain rotation and RDR initiation, providing a recrystallization prerequisite for the propagation and proliferation of ASB. The recrystallized twins formed after the completion of the RDR in the 40% RS deformation sample in order to adapt to the reloading impeded the movement of dislocations, which became a stress concentration site promoting microdamage more likely to nucleate and expand, making the ASB more prone to fracture. Therefore, this study is of great significance for the development of self-sharpening WHA kinetic penetrators.

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

In this study, roles of strain-hardening rate on susceptibility of adiabatic shear band (ASB) formation and subsequent cracking were investigated in two ultra-high-strength armor steel plates heat-treated differently. The quenched and tempered steel contained ~2% of retained austenite in the tempered martensitic matrix, while the quenched and austempered steel contained ~4% retained austenite in the bainitic matrix partly with the tempered martensite. The actual ballistic impact test results revealed the lower sensitivity of ASB formation in the austempered steel than in the tempered steel, which corresponded well to the higher critical strain for ASB formation in the dynamic compressive test using a laboratory-scale split Hopkinson pressure bar (SHPB). The austempered steel caused the higher internal stress among various constituents, and all the retained austenite transformed into martensite during the deformation, thereby leading to transformation-induced plasticity (TRIP) effect. The higher strain-hardening rate induced by these higher internal stress and TRIP effect increased resistance to ASB formation, which was confirmed by a calculation of ASB susceptibility. Thus, the austempered steel was much less susceptible to the ASB formation during the ultra-high-speed deformation. Consequently, the increased resistance to ASB formation retarded the initiation and propagation of ASBs and cracks.

Microstructural characteristics and formation mechanism of adiabatic shear bands in Al–Zn–Mg–Cu alloy under dynamic shear loading

The microstructural characteristics and formation mechanism of adiabatic shear bands in ultra-high strength aluminum alloy are studied under dynamic shear loading. It is found that thermal softening alone is insufficient to trigger the initiation of adiabatic shear bands (ASBs). Dynamic recrystallization induced softening is attributed to be the main cause of ASB initiation, with temperature rise playing a secondary role by accelerating the deformation localization process. It is believed that abrupt temperature rise takes place after ASB initiation, and the temperature rise prior to ASB formation is relatively small. Recrystallized ultrafine grains with a strong texture of {112} <110> are formed in ASBs, based on a progressive subgrain misorientation recrystallization mechanism. It is proposed that the translational–rotational motion of the dynamically recrystallized nanograins in the form of vortexes at the mesoscale can account for the microstructural softening, thus causing shear instability, eventually leading to fracture by the combination of ductile and brittle failure mechanisms.

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.

Dynamic failure of titanium: Temperature rise and adiabatic shear band formation

One of the most important issues related to dynamic shear localization is the correlation among the stress collapse, temperature elevation and adiabatic shear band (ASB) formation. In this work, the adiabatic shear failure process of pure titanium was investigated by dynamic shear-compression tests synchronically combined with high-speed photography and infrared temperature measurement. The time sequence of important events such as stress collapse, ASB initiation, temperature rise and crack formation was recorded. The key characteristics of ASB, such as width, critical strain, temperature, propagation speed and cooling rate were systematically studied. The maximum propagation velocity of ASB is found in this work to be about 1900 m/s, about 0.6Cs (Cs is the shear wave speed). The maximum temperature within ASB is in the range of 350–650 °C, while the material close to ASB is also heated. The cooling rate of ASB is on the order of 106 °C/s, indicating that it needs a few hundreds of microseconds for the ASB to cool down to the ambient temperature. One important observation is that the apparent temperature rise occurs after ASB initiation, which indicates that it might not be the causation but the consequences of ASB. Further efforts are called for confirmation of this notion because of its significance.

Formation of adiabatic shear band within Ti–6Al–4V: Effects of stress state

Shear failure is frequently accompanied with the formation of an adiabatic shear band (ASB) under dynamic loading condition. The results obtained by a previous study (Guo et al., 2019) indicate that temperature increase does not play a primary role in the formation of ASB. Moreover, the shear strain to ASB initiation is closely related to the stress state. In this paper, a hybrid experimental–numerical method is developed to evaluate quantitatively the effect of stress stare on the initiation of ASB within Ti–6Al–4V. A variety of geometries of specimens, including the modified shear-compression, shear-tension, and thin-walled tubular specimen are designed to create different stress states. Combined with high-speed photography, split Hopkinson bar systems are utilized to measure the mechanical response of these specimens. Critical shear strains of ASB initiation are acquired based on the high- speed photos. The stress state for each test condition is obtained by numerical simulation. A phenomenological model of ASB initiation considering stress triaxiality and Lode parameter is proposed. Compared to the experimental results, the proposed model can predict the initiation of ASB with good accuracy under the test condition of this work.

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.

Experimental and numerical studies on the expanding fracture behavior of an explosively driven 1045 steel cylinder

The expanding fracture of an explosively driven 1045 steel cylinder was studied. The fracture process and the expanding velocity of the cylinder were recorded by a high-speed camera and a Photonic Doppler Velocimetry (PDV) probe in real time. The fragments were recovered and analyzed by metallurgical examinations. The fracture mechanism, expanding velocity and fragment size were analyzed respectively. We found that the adiabatic shear band (ASB) plays a key role in the fracture process of an expanding 1045 steel cylinder. As the deformation of the cylinder increases, multiple ASBs firstly initiate near the inner surface of the cylinder and then propagate outward along the maximum shear stress direction. After the ASBs arrive at the outer surface of the cylinder, all of the plastic deformation is concentrated in the ASBs, and meanwhile the cracks initiate near the outer surface of the cylinder and propagate inwardly along the developed ASBs. The terminal velocity of the cylinder was accurately predicted by the Gurney model and was found to be little influenced by the fracture mode. The width and thickness of the fragments were measured and found to be controlled by the momentum diffusion. Furtherly, we modeled the expanding fracture process of the cylinder. A three dimensional simulation results suggested that the propagation of the shock wave along the radial direction can't be neglected although the cylinder is very thin. At an early stage, the material near the inner and outer surface is in a compressive and tensile state, respectively, and the material in the middle of the cross-section is alternately in a tensile and compressive state due to the propagation of the shock waves. At a late stage, the whole cylinder is in a tensile state. In a two dimensional plane strain simulation, the initiation and propagation of multiple ASBs in the expanding cylinder were successfully replicated.

Microstructural characteristics of adiabatic shear localization in a metastable beta titanium alloy deformed at high strain rate and elevated temperatures

The microstructural evolution and grain refinement within adiabatic shear bands in the Ti6554 alloy deformed at high strain rates and elevated temperatures have been characterized using transmission electron microscopy. No stress drops were observed in the corresponding stress–strain curve, indicating that the initiation of adiabatic shear bands does not lead to the loss of load capacity for the Ti6554 alloy. The outer region of the shear bands mainly consists of cell structures bounded by dislocation clusters. Equiaxed subgrains in the core area of the shear band can be evolved from the subdivision of cell structures or reconstruction and transverse segmentation of dislocation clusters. It is proposed that dislocation activity dominates the grain refinement process. The rotational recrystallization mechanism may operate as the kinetic requirements for it are fulfilled. The coexistence of different substructures across the shear bands implies that the microstructural evolution inside the shear bands is not homogeneous and different grain refinement mechanisms may operate simultaneously to refine the structure.

Effect of microstructure on the nucleation and initiation of adiabatic shear bands (ASBs) during impact

While instability may occur homogenously during plastic deformation, the formation of adiabatic shear band (ASBs) does not follow a homogenous instability during impact. Geometrical stress concentration sites and/or microstructural inhomogeneities result in the nucleation and initiation of shear strain localization. In this study, initial microstructural inhomogeneity was found to produce nucleation sites for the initiation of ASBs. It was observed that double misfit interfaces and boundary layers with random arrangement of atomic columns are formed around precipitated carbides and they increase the volume fraction of dislocation sources within the specimens. The AISI 4340 steel specimens which were tempered at the lowest temperature had smaller precipitated carbides with high aspect ratios densely distributed within the matrix and were easily susceptible to the formation of ASBs. As the tempering temperature increased, the relative sizes of the carbides increased with a corresponding reduction in their aspect ratios and their distribution density within the matrix and thus were more resistant to the formation of ASBs. In this study, it is demonstrated that the intersection of an activated dislocation source with the direction of maximum shear (regions of stress concentrations) within the specimens during impact, is a necessary condition for the point of intersection to act as a possible site for the nucleation of ASBs, depending on the rate of dislocation generation, local strain and strain rate. At a constant carbide volume fraction, the higher susceptibility of the tempered specimens to the initiation of ASBs is attributed to the volume fraction of the points of intersection between activated dislocation sources and direction of maximum shear during impact. Additionally, the smaller carbides, with their higher aspect ratios and distribution densities, accentuate the effect of strain gradients and the microstructural inhomogeneities associated with the tempered specimens.

Atomistic-to-particle multiscale coupling method for adiabatic shear banding simulation

An atomistic-to-particle (AtP) multiscale coupling method is proposed to study the initiation of adiabatic shear band (ASB) under shock loadings from both macro and micro aspect. A non-interpolation coupling technique is used to construct the atomistic boundary condition between micro and macro method, and atomistic contribution to internal force is also incorporated in the overlapping region. Combining with the mixture criterion of critical strain and temperature, this AtP method has been successfully applied to study the micro mechanism of ASB onset for hat-shaped metal sample. Multiscale simulation results at the time of ASB onset indicate that highly strain localization and rapid temperature rise are main reasons for ASB initiation. Numerical results are validated by experimental observations.

A novel approach to testing the dynamic shear response of Ti-6Al-4V

Modifications were made on the traditional split Hopkinson pressure bar (SHPB) system to conduct dynamic shear tests. The shear response of Ti-6Al-4V was acquired at a shear strain rate of 104 s−1 by using this modified apparatus. The geometry as well as the clamping mode of the double-notch specimen was optimized by commercial FEM software ABAQUS, and the feasibility of the experiment set-up was validated. A shear stress calibration coefficient of ▪ = 1.03 and a shear strain calibration coefficient of ▪ = 0.50 were obtained. We have employed highspeed photography to record the deformation process, especially the initiation and propagation of adiabatic shear band (ASB), during the dynamic shear test. The frames show that the time duration from ASB initiation to its completion is less than 2 μs, from which we can estimate that the propagation speed of ASB within Ti-6Al-4V is more than 1250 m/s under such loading conditions. The temperature rise within ASB is also estimated to be ΔT2 ≈ 1460 °C based on energy balance. Such high temperature has led to softening of the material within the ASBs, and has intensified the shear localization and finally resulted in fracture of the material.

Temperature field measurement in titanium alloy during high strain rate loading—Adiabatic shear bands phenomenon

This paper concerns the study of the damage and failure mechanisms of metallic materials under dynamic loading. For high strain rates, it appears a strain localization in narrow bands called adiabatic shear bands (ASB). As the loading duration is very short, the dissipation of the mechanical energy into heat generates strong temperature heterogeneities. To study the mechanisms of initiation and propagation of ASB during a dynamic torsion test, we developed an experimental device to measure temperature by pyrometry: for the “low temperatures” ranging between 50 °C and 300 °C, we use a bar of 32 InSb infrared detectors in order to study temperature heterogeneities during the ASB formation. The acquisition frequency is 1 MHz and the spatial resolution is 43 μm. For the “high temperatures” ranging between 800 °C and 1700 °C, we use an intensified camera whose spectral band is in the visible range. The spatial resolution is 2 μm and the aperture time is 10 μs. This device shows that the temperature could exceed 1100 °C with temperature heterogeneity inside the band. The “low temperatures” device allows us to observe the formation of one or two adiabatic shear bands on temperature recording along the torsion axis of the specimen. In order to explain these results, we propose a mechanism of multiple ASB formation followed by interactions and annihilations of the ASB.

Development of adiabatic shear bands in annealed 316L stainless steel: Part I. Correlation between evolving microstructure and mechanical behavior

Adiabatic shear localization in an annealed AISI 316L stainless steel was examined through a forced shear technique using a split Hopkinson pressure bar and hat-shaped specimens. A well-controlled forced shear technique provided the possibility of correlating the microstructural evolution of adiabatic shear localization to its transient mechanical behavior. The initiation of adiabatic shear bands occurred when the shear stress peaked after substantial work hardening. The work-hardening rate was found to play a dominant role in the formation of adiabatic shear localization. The stress drop presupposed the development of the localized deformation. A core structure of shear bands was generated within the shear band, which characterized a narrow-down process in the early stage of the shear band evolution. The continuous expansion of the shear band core to the entire width of the band was seen to correlate with the full development of shear localization.

Consideration of microstructural effects in the analysis of adiabatic shear bands in a tungsten heavy alloy

We use multiscale and multiphysics analyses to approximately account for the microstructure of a composite comprised of tungsten particulates embedded in a nickel–iron matrix and deformed in plane strain tension at a high strain rate. Both materials are assumed to be perfectly bonded to each other, and heat-conducting, microporous, strain- and strain-rate hardening, and thermally softening with thermomechanical material parameters degrading with the evolution of porosity. The square region whose finite thermomechanical deformations are analyzed is divided into a uniform mesh, (for example), of 10 × 10 super-elements or patches, and each patch is subdivided into 10 × 10 uniform finite elements (FEs). Material properties in a super-element are obtained from those of its constituents and their volume fractions by a homogenization technique. Thus the square region is comprised of 100 homogeneous subbodies perfectly bonded to each other. Keeping the total number of FEs fixed, the effect of the number of patches on the time of initiation of an adiabatic shear band (ASB) is delineated, and it is compared with that obtained from the mesoscale analysis of the problem with the 100 × 100 uniform FE mesh and considering each material separately. With an increase in the number of patches, the ASB initiation time converges to that obtained from the mesoscale analysis. The CPU time and other computational resources required for the patchwork analysis are considerably less than those needed for the mesoscale analysis. The proposed technique enables one to consider effects of microstructure in analyzing deformations of a full-scale structure.

Mesoscale Analysis of Shear Bands in High Strain Rate Deformations of Tungsten/Nickel-Iron Composites

ABSTRACT We analyze the initiation and propagation of adiabatic shear bands in a tungsten heavy alloy by modeling each constituent as a heat-conducting, microporous, isotropic, elastothermoviscoplastic material. The two constituents are assumed to be perfectly bonded to each other so that the temperature, heat flux, displacements, and surface tractions are continuous across an interface between a tungsten particulate and the nickel-iron matrix. Three different modes of deformation, namely plane strain tension/compression, plane strain shear, and axisymmetric tension/compression are analyzed. No other defects are introduced. It is found that contours of the rate of temperature rise and/or velocity and/or the specific energy dissipation rate rather than those of effective plastic strain delineate the shear banded regions. For the same volume fraction of particulates smaller diameter particulates enhance the formation of adiabatic shear bands. The time of initiation of an adiabatic shear band also depends upon the particulate arrangement.

Ductility of aluminium alloy AA7075 at high strain rates

Under dynamic loading the stabilising effect of increased strain rate sensitivity of the material restrains neck formation in tension tests and leads to an increase in ductility. On the other hand the adiabatic character of the deformation process reduces the flow stress and promotes instability, localisation and adiabatic shear band initiation. Furthermore, the notch sensitivity of the material increases with increasing strain rate. Dynamic and quasi-static tension and compression tests were carried out on the age hardenable aluminium wrought alloy AA7075. There, dispers distributed precipitations are often the starting point for ductile fracture caused by impact due to the nucleation, growth and coalescence of voids and micro-cracks in case of tension. Neck formation under tensile loading and instabilities like shear bands in case of compression are discussed on the basis of the theory of imperfections under consideration of the increased strain rate sensitivity of the material and the adiabatic character of the deformation process at high strain rates. In case of tensile loading, tests with various notched geometries allowed the study of the influence of degree of multiaxiality. Through combination of experiment and simulation, the influence of strain rate on the local fracture strain could be determined for tensile and compression loading.

Self-organization in the initiation of adiabatic shear bands

The radial collapse of a thick-walled cylinder under high-strain-rate deformation (∼104s−1) was used for the investigation of shear-band initiation and pattern development in titatium. Experiments were carried out in which the collapse was arrested in two stages, in order to observe the initiation and propagation of shear bands. The occurrence of shear bands to accommodate plastic deformation in response to external tractions is a collective phenomenon, because their development is interconnected. The bands were observed to form on spiral trajectories and were periodically spaced. The spacing of the shear bands decreased with the progression of collapse, and was equal to approximately 0.6mm in the final stage of collapse. The shear-band spacing was calculated from two existing models, based on a perturbation analysis and on momentum diffusion. Values of 0.52 and 3.3mm were obtained with material parameters from quasi-static and dynamic experiments. The predictions are found to give a reasonable first estimate for the actual spacings. The detailed characterization of the shear-band front leads to an assessment of the softening mechanisms inside a shear localization region. The initiation of localization takes place at favorably oriented grains and becomes gradually a continuous process, leading eventually to dynamic recrystallization.