Evolution Of Adiabatic Shear Bands Research Articles

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.

Experimental study on fracture mechanism transformation in chip segmentation of Ti-6Al-4V alloys during high-speed machining

Ti6Al4V alloy is one of the typically difficult-to-machine materials which is widely used in aerospace industry. During the machining process of Ti6Al4V alloys, serrated chips are easily formed and would influence cutting forces significantly. In this study, the variation of cutting forces of Ti-6Al-4V alloys during high-speed milling from 50–500 m/min is investigated, and the fracture mechanisms and microstructure evolution of chips are analyzed by optical microscope (OM), scan electron microscope (SEM) and transmission electron microscope (TEM). The results show that cutting forces and serrated degree of chips both increase first and then decrease with the increase of cutting speeds, especially when adiabatic shear bands appear. The microstructure evolution inside adiabatic shear bands also show a significant difference with different chip morphology at different cutting speeds, which finally cause the variation of cutting forces due to the change of fracture mechanisms between chip segmentations. As a result, the evolution of adiabatic shear bands is analyzed, and replica method by polydimethylsiloxane (PDMS) is used to reveal the relationship between cutting forces variation and fracture mechanism transformation in chip segmentation. It shows the relationship between cutting forces and microstructure evolution in adiabatic shear bands, and the transformation of fracture mechanism between type-I and type-II would occur around the gap between chip segmentations, where cutting forces are higher under type-I fracture.

Dynamic Behavior of AA2519-T8 Aluminum Alloy Under High Strain Rate Loading in Compression

In this study, the effects of strain rate on the dynamic behavior, microstructure evolution and hence, failure of the AA2519-T8 aluminum alloy were investigated under compression at strain rates ranging from 1000 to 3500 s−1. Cylindrical specimens of dimensions 3.3 mm × 3.3 mm (L/D = 1) were tested using the split-Hopkinson pressure bar integrated with a digital image correlation system. The microstructure of the alloy was assessed using optical and scanning electron microscopes. Results showed that the dynamic yield strength of the alloy is strain rate dependent, with the maximum yield strength attained by the material being 500 MPa. The peak flow stress of 562 MPa was attained by the material at 3500 s−1. The alloy also showed a significant rate of strain hardening that is typical of other Al–Cu alloys; the rate of strain hardening, however, decreased with increase in strain rate. It was determined that the strain rate sensitivity coefficient of the alloy within the range of high strain rates used in this study is approximately 0.05 at 0.12 plastic strain; a more significant value than what was reported in literature under quasi-static loading. Micrographs obtained showed potential sites for the evolution of adiabatic shear band at 3500 s−1, with a characteristic circular-shaped surface profile comprising partially dissolved second phase particles in the continuous phase across the incident plane of the deformed specimen. The regions surrounding the site showed little or no change in the size of particles. However, the constituent coarse particles were observed as agglomerations of fractured pieces, thus having a shape factor different from those contained in the as-received alloy. Since the investigated alloy is a choice material for military application where it can be exposed to massive deformation at high strain rates, this study provides information on its microstructural and mechanical responses to such extreme loading condition.

Microstructural evolution of adiabatic shear bands in pure copper during impact at high strain rates

A systematic study was conducted on copper of commercial purity (99.9% purity) to elucidate the sequence of events that leads to the evolution of adiabatic shear bands and the structure of the evolved shear bands after strain localization during deformation at high strain rates and large strains. A direct impact Hopkinson Pressure Bar was used to deform different specimens at increasing impact momentum and strain rate followed by microstructural characterization using metallographic techniques and transmission electron microscopy. It was observed that sequential occurrence of emergence of dislocations from grain and twin boundaries, formation of dislocation cell structures and substructures with varying cell sizes and cell walls, dislocation-nucleation controlled softening and extensive micro twinning characterized the structure of the evolved adiabatic shear bands as a function of impact momentum and strain rate. The dislocation cell structures and substructures were typically made up of high-density dislocation walls surrounding low-density dislocation cell interiors. The microhardness distribution within the evolved shear bands increased up to a peak value at a critical impact momentum and strain rate (≥ 45kgm/s and ≥6827s−1). Above this threshold, microhardness of the regions within the evolved shear bands decreased because of the occurrence of softening. However, the first specimen that exhibited softening had high-density dislocation cell walls surrounding dislocation-free cell interiors with no observed recrystallized grains. Despite the onset of softening both within and outside the shear bands, the regions within the shear bands were always harder than the regions outside the shear bands. The shear bands that exhibited significant softening were clad with vast distribution of microtwins in addition to evolved refined grains and sub-grains. It is discussed that thermally activated dislocation processes as a result of the rise in temperature during impact, dynamic recovery and dynamic recrystallization do not necessarily result in strain localization in the impacted copper specimens because their effects are observed in the structure of the evolved shear bands at a latter stage when strain localization had already occurred.

Adiabatic shear banding and the micromechanics of plastic flow in metals

This article studies the conditions that dislocation generation and motion must fulfil to promote the development of adiabatic shear bands in crystalline metals. First, we derive a stability criterion for the formation of shear bands, by linearising the conservation equations of thermo-plasticity. We then apply this criterion on the micromechanics-based Orowan equation for plastic flow, introducing a number of increasingly sophisticated constitutive assumptions on the model. It is found that there are two crucial promoters of shear band formation: the unfettered generation of dislocations that may be found in stage I plasticity; and the softening of the elastic constants with increasing temperature. In turn, we show that limiting the speed of dislocations tends to inhibit the formation of dislocations, even when the temperature dependence of the dislocation’s drag is accounted for. This leads to the existence of an upper temperature limit above which shear band formation appears unlikely.

List of Abstracts

. Production efficiency as well as the reduction of energy consumption and waste are the main drivers for the improvement of large-scale production in chemical industry. In addition, the demand for new processes – these days especially related to the chemical energy conversion as basis for the Energiewende – are additional triggers for innovation. Catalysis, well recognized by the public e.g. in form of the 3-way automobile catalyst, plays an important role for about 90% of all processes in chemical industry. The development of new and improved catalysts is a challenge for chemists – as is the search for new structural materials for mechanical engineers. Due to the often-necessary noble and thus expensive metals (e.g. palladium) in many catalysts, structural and chemical function have been developed separately in the two communities. Recent progress in catalytically active materials allows replacing expensive noble metals in selected catalytic processes by significant cheaper elements [1]. This offers the new perspective to combine structural and chemical functionality by innovative materials. Within this area, intermetallic compounds with their peculiar combination of crystal and electronic structure [2], represent an interesting class of materials to address this vision as will be shown in the contribution.

High temperature and dynamic testing of AHSS for an analytical description of the adiabatic cutting process

In the automotive industry, advanced high strength steels (AHSS) are widely used as sheet part components to reduce weight, even though this leads to several challenges. The demand for high-quality shear cutting surfaces that do not require reworking can be fulfilled by adiabatic shear cutting: High strain rates and local temperatures lead to the formation of adiabatic shear bands (ASB). While this process is well suited to produce AHSS parts with excellent cutting surface quality, a fundamental understanding of the process is still missing today. In this study, compression tests in a Split-Hopkinson Pressure Bar with an initial strain rate of 1000 s-1 were performed in a temperature range between 200 °C and 1000 °C. The experimental results show that high strength steels with nearly the same mechanical properties at RT may possess a considerably different behavior at higher temperatures. The resulting microstructures after testing at different temperatures were analyzed by optical microscopy. The thermo-mechanical material behavior was then considered in an analytical model. To predict the local temperature increase that occurs during the adiabatic blanking process, experimentally determined flow curves were used. Furthermore, the influence of temperature evolution with respect to phase transformation is discussed. This study contributes to a more complete understanding of the relevant microstructural and thermo-mechanical mechanisms leading to the evolution of ASB during cutting of AHSS.

A unified model for coupling constitutive behavior and micro-defects evolution of aluminum alloys under high-strain-rate deformation

High strain rate forming (HSRF) is promising to break through the conventional forming limit of materials and thus to form hard-to-deform components. During HSRF process, long-lasting micro-defects before fracture, i.e. adiabatic shear bands (ASBs) in compression and voids in tension, are significant characteristics existing in a large strain range. These defects bring about the non-destructive softening of stress and continuous exertion of ductility, in return, the stress responses affect the evolution of micro-defects. However, the interaction between stress responses and defects evolution has not yet been reflected in the existing models, resulting in a limited prediction accuracy. Aiming at this issue, the competition between hardening caused by thermal activated dislocation movements and the flow softening brought by micro-defects evolution was well modeled in this work. During the modeling, the relation between the normal strain and effect zone of ASBs was established via the modification of Bai–Dodd model by considering the geometric features of ASBs. Moreover, by introducing a rate-dependent ASB trace angle, the half width of ASBs was expressed as a function of maximum shear strain and critical instability strain. The effect of strain and strain rate to the evolution of voids under tensile conditions was taken into account by combining Hollomon hardening law with Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. Then, the interactions between micro-defects and structure-related athermal stress were characterized by connecting volume fraction of voids and effective bearing area in tension and the intensity of flow localization with ASBs width in compression. As a consequence, a unified model of constitutive behaviors coupled with micro-defects evolution was established with considering rate-dependent hardening and softening. Applied to aluminum alloys, this model predicts the stress responses, evolution of ASBs and voids, and low or negative strain rate sensitivity with high precision in large ranges of strain (0–0.6) and strain rate (0.001–5000 s−1). The proposed model is thus believed to be with a successful application in precise prediction and optimization of HSRF processes.

Microstructural evolution associated with shear location of AISI 52100 under high strain rate loading

The microstructure evolution of adiabatic shear bands (ASBs) in AISI 52100 produced under high strain rate impact loading was investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A full view of the shear band formation and development was presented. The results reveal that no strain hardening effect occurred in the AISI 52100 under the dynamic loading. Two distinct regions of the ASBs and the transition region were observed. Driven by the high shear deformation and assisted by a thermal softening effect, the formation of ASBs started in the core region under the dominant mechanism of dynamic recrystallization. It is followed by the formation of neighbouring region under the co-action of both dynamic recovery and dynamic recrystallization. Then the transition region formed which was controlled by dynamic recovery. The core region of ASBs was consisted of subgrains and fine equiaxed grains. The grains were refined either through partitioning and breaking down the elongated subgrains by entangled dislocations clusters or through splitting and rotating the elongated subgrains into small grains under a synergistic effect of shearing and twisting. The amount and size of cementites were greatly reduced in ASBs due to cementite dissolution induced by large plastic deformation and dynamic recrystallization.

Improving the forming capability of laser dynamic forming by using rubber as a forming medium

Laser dynamic forming (LDF) is a novel high velocity forming technique, which employs laser-generated shock wave to load the sample. The forming velocity induced by the high energy laser pulse may exceed the critical forming velocity, resulting in the occurrence of premature fracture. To avoid the above premature fracture, rubber is introduced in LDF as a forming medium to prolong the loading duration in this paper. Laser induced shock wave energy is transferred to the sample in different forming stages, so the forming velocity can be kept below the critical forming velocity when the initial laser energy is high for fracture. Bulge forming experiments with and without rubber were performed to study the effect of rubber on loading duration. The experimental results show that, the shock wave energy attenuates during the propagation through the rubber layer, the rubber can avoid the premature fracture. So the plastic deformation can continue, the forming capability of LDF is improved. Due to the severe plastic deformation under rubber compression, adiabatic shear bands (ASB) occur in LDF with rubber. The material softening in ASB leads to the irregular fracture, which is different from the premature fracture pattern (regular fracture) in LDF without rubber. To better understand this deformation behavior, Johnson–Cook model is used to simulate the dynamic response and the evolution of ASB of copper sample. The simulation results also indicate the rubber can prolong the loading duration.

Study on the characteristics and thermal stability of nanostructures in adiabatic shear band of 2195 Al–Li alloy

Adiabatic shear bands (ASB) were obtained by dynamic shearing with a split Hopkinson pressure bar in the hat-shaped specimens of 2195-T6 Al–Li alloy. TEM observations reveal that grains in ASB are mainly equiaxed with the grain size from 50 to 100 nm. The kinetics possibility of instant refinement of grains can well be explained with the rotation dynamic recrystallization mechanism. EBSD is used to investigate microstructure evolution in ASB after annealed at 100–400 °C for 1 h. Results show that grain size increases rapidly at higher annealing temperature, and grains grow from 0.22 μm at 300 °C to 1.77 μm at 400 °C. Microhardness measurement indicated that the microhardness value rises slowly with temperature increases and then drops quickly at 300 °C. The study indicates that the nanostructure in ASB is thermally stable below 300 °C.

Study on the Evolution of Adiabatic Shear Band of a High Strength Steel

The evolution process of a high strength steel which subjected with three different heat treatment proceedings and gets different quasi-static tensile properties was investigated in this paper. To precisely control the plastic deformation of the cylinder and capture the development process of adiabatic shear bands, stopper ring was used in Split Hopkinson Pressure Bar (SHPB). Combining the stress-strain curves and microstructures after SHPB tests, the microstructure evolution from the nucleation of adiabatic shear bands to fracture of the cylindrical steel were observed. The experimental results have demonstrated that there are similar fracture procedures of the steel treated through different heat treatments. Shear bands form firstly, then micro-cracks develop from shear bands, and lead to macro-crack finally. However, the critical strains for nucleation of ASBs and the time spending on the fracture procedure of the steel treated at different heat treatments are different. Samples treated at 900°C/AC exhibit the best resistance to adiabatic shear sensitivity when compressed under high strain rates.

Dynamic shear response and evolution mechanisms of adiabatic shear band in an ultrafine-grained austenite–ferrite duplex steel

The dynamic properties of an intercritically annealed 0.2C5Mn steel with ultrafine-grained austenite–ferrite duplex structure were studied under dynamic shear loading. The formation and evolution mechanisms of adiabatic shear band in this steel were then investigated using interrupted experiments at five different shear displacements and the subsequent microstructure observations. The dynamic shear plastic deformation of the 0.2C5Mn steel was observed to have three stages: the strong linear hardening stage followed by the plateau stage, and then the strain softening stage associated with the evolution of adiabatic shear band. High impact shear toughness was found in this 0.2C5Mn steel, which is due to the following two aspects: the strong linear strain hardening by martensite transformation at the first stage, and the suppressing for the formation of shear band by the continuous deformation in different phases through the proper stress and strain partitioning at the plateau stage. The evolution of adiabatic shear band was found to be a two-stage process, namely an initiation stage followed by a thickening stage. The shear band consists of two regions at the thickening stage: a core region and two transition layers. When the adjoining matrix is localized into the transition layers, the grains are refined along with increasing fraction of austenite phase by inverse transformation. However, when the transition layers are transformed into the core region, the fraction of austenite phase is decreased and almost disappeared due to martensite transformation again. These interesting observations in the core region and the transition layers should be attributed to the competitions of the microstructure evolutions associated with the non-uniformly distributed shear deformation and the inhomogeneous adiabatic temperature rise in the different region of shear band. The 0.2C5Mn TRIP steel reported here can be considered as an excellent candidate for energy absorbers in the automotive industry.

Developing the Processing Maps Using the Hyperbolic Sine Constitutive Equation

Hot compression tests were performed on a duplex stainless steel at temperatures ranging from 1223 K to 1473 K (950 °C to 1200 °C) and strain rates from 0.001 to 100 s−1. The constitutive analysis of flow stress was carried out using the hyperbolic sine function, and the material constants were determined at two typical strains of 0.3 and 0.7. The power dissipation map, instability map, and processing map for the material were developed for strains of 0.3 and 0.7. The developed processing maps were based on the hyperbolic sine as well as the conventional power-law constitutive equations. The efficiency of power dissipation (η) varied from 12 to 60 pct over the studied temperature and strain rate. The highest value of η was obtained at strain rates below 0.01 s−1, whereas the lowest value of η was observed at the intermediate strain rates. The instability region in sin h-based processing map was only observed in the range of 1423 K to 1473 K (1150 °C to 1200 °C) and at a strain rate of 100 s−1, while the conventional processing map did not predict any instability region. Optical microscopy observations were more consistent with the results of the sin h-based processing map and indicated that the instability regime at high temperatures and high strain rates was due to the development of adiabatic shear bands.

Calorimetric consequences of thermal softening in Johnson–Cook’s model

At high loading rates, the development of adiabatic shear bands in metals is conventionally attributed to the strong interactions induced by viscoplastic dissipation within the bands and thermal softening effects. The rheological equation proposed by Johnson and Cook takes both viscoplastic hardening and thermal softening into account. The present paper reviews and includes this equation into a thermodynamic framework in order to analyse the energy impacts of thermal softening. Indeed this latter implies the existence of a thermomechanical coupling source, probably non-negligible and which must be considered when estimating temperature variations induced by shear band development.

Formation and evolution of adiabatic shear bands in zirconium alloy impacted by split Hopkinson pressure bar

The formation and evolution of the adiabatic shear bands in the zirconium alloy impacted by split Hopkinson pressure bar were investigated by means of the experimental design. The different strain levels in the zirconium alloy subjected to high strain rate loading were first designed by two times impact at almost same strain rate. Only deformed bands were formed in the zirconium alloy impacted once, and two types of adiabatic shear bands, namely deformed bands and transformed bands, were distinguished in the zirconium alloy impacted twice. Microstructural observation shows that the deformed bands showed the severe strain localization, and the transformed bands composed the ultrafine and equiaxed grains. The formation of the ultrafine and equiaxed grains in the transformed bands could be attributed to the rotational dynamic recrystallization mechanism. Based on the characterization of the ASBs formed at different strain stages, the evolution process of the ASBs is proposed. The deformed bands are suggested as the initial stage of the transformed bands formation, and the transformed bands are considered as the further development of the deformed bands. The microhardness measurements show that the microhardness values in the transformed bands were higher than that of the deformed bands and original sample because of strain hardening and grain refining.

Annealing effect on the evolution of adiabatic shear band under dynamic shear loading in ultra-fine-grained iron

In the present study, evolution of adiabatic shear band (ASB) and annealing effect on the evolution of ASB in ultra-fine-grained (UFG) Fe under dynamic shear loading were 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 ∼500 nm, as confirmed by means of Transmission Electron Microscopy (TEM). Examination of micro-hardness and grain size of UFG Fe shows a transition from recovery to grain growth at annealing temperature of approximately 400 °C. The high-strain-rate response of UFG Fe was characterized by hat-shaped specimen set-up in Hopkinson bar experiments. With increased shear deformation, the evolution of ASB was found to be a two-stage process, namely a nucleation stage followed by a thickening stage. In the thickening stage, ASB evolution is accompanied by increasing in both thickness and micro-hardness of ASB. The increased micro-hardness in ASB and TEM observations inside ASB indicate that grains in the shear band are further refined. Once the shear band is initiated, cracks also nucleate and propagate within ASB, which produces failure in the material. After 450 °C post-ECAP annealing, development of ASB and crack propagation were inhibited under dynamic shear loading. The results indicated that the ductility under dynamic shear loading can be significantly improved by appropriate post-ECAP annealing without losing the strength of the UFG Fe.

Microstructural examination of quasi-static and dynamic shear in high-purity iron

Numerous studies have examined the microstructural evolution of adiabatic shear bands through the utilization of the forced shear or “tophat” test specimen. While the geometry of this specimen does not allow for the microstructure to play a dominant role in the location of a shear band, the forced shear specimen has been shown to be particularly useful for characterizing the influence of parameters such as strain rate, temperature, strain, and load on the microstructural evolution within a shear band. Additionally, many studies have also utilized this geometry to advance the understanding of shear band development in a number of materials.In this study we systematically examine the influence of integrated loading states on the dynamic shear localization response of high-purity Fe by varying the geometry of the forced shear specimen. Post-mortem characterization was performed to quantify the width of the localizations and to examine the microstructural and textural evolution of shear deformation in a body centered cubic (BCC) metal. Increased instability in mechanical response is strongly correlated with development of enhanced intergranular misorientations and high angle boundary evolution. Stress state was also critical to the localization process. Single-component, simple shear configurations were found to promote instability over multi-component stress states. In addition, these geometries resulted in traditional BCC deformation shear textures, while multi-component stress states led to less developed textures.

Analytical solution for shear bands in cold-rolled 1018 steel

AbstractCold-rolled 1018 (CR-1018) carbon steel has been well known for its susceptibility to adiabatic shear banding under dynamic loadings. Analysis of these localizations highly depends on the selection of the constitutive model. To deal with this issue, a constitutive model that takes temperature and strain rate effect into account is proposed. The model is motivated by two physical-based models: the Zerilli and Armstrong and the Voyiadjis and Abed models. This material model, however, incorporates a simple softening term that is capable of simulating the softening behavior of CR-1018 steel. Instability, localization, and evolution of adiabatic shear bands are discussed and presented graphically. In addition, the effect of hydrostatic pressure is illustrated.