Critical Strain Rate Research Articles

Mesoscopic numerical simulation of dynamic size effect on the splitting-tensile strength of concrete

Under static loadings, concrete has been demonstrated to exhibit obvious size effect due to the heterogeneity of mesoscopic composition. Under dynamic loadings, the strain rate effect has a significant influence on the failure of concrete. It is therefore of great importance to explore the size effect of concrete under dynamic loadings. In this study, the focus is on the size effect in dynamic splitting-tensile strength of concrete. A mesoscopic numerical model for the simulation of the splitting-tensile failure and size effect at different strain rates (10−5/s ∼ 200/s) was established. The mesoscopic simulation results indicate that the size effect on the dynamic splitting-tensile strength of concrete has an obvious discrepancy with the static one. There is a critical strain rate in dynamic splitting-tensile strength of concrete. As the applied strain rate below the critical strain rate the size effect behavior is restrained and weakened gradually with the addition of strain rate. As the applied strain rate exceeds the critical strain rate the dynamic strength enhances linearly as the structural size increases. Furthermore, considering the contribution of strain rate on the dynamic size effect, a Static and Dynamic unified Size Effect Law (i.e. SD-SEL) for splitting-tensile strength of concrete was developed. The proposed SD-SEL was verified by the numerical results and the available test data.

Effects of loading rate on strength and deformation characteristics of gypsum mixed sand

It has been unanimously acknowledged that the strength and deformation characteristics of bounded geomaterials, viz. cemented soils and natural rocks, are predominantly governed by the rate of loading/deformation. Rational evaluation of these time-dependent characteristics due to viscosity and ageing are vital for the reliable constitutive modelling. In order to study the effects of ageing and loading/strain rate (viscosity) on the behaviour of bounded geomaterials, a number of unconfined monotonic loading tests were performed on Gypsum Mixed Sand (GMS) specimens at a wide range of axial strain rates; ranging from 1.9E-05 to 5.3E+00 %/min (27,000 folds), and at different curing periods. The results indicate shifts in the viscous behaviour of GMS at critical strain rates of 2.0E-03 and 5.0E-01 %/min. In the light of this finding, the results are categorized into three discrete zones of strain rates, and the behaviour of GMS in each of these zones is discussed. A significant dependency of peak strength and stress-strain responses on strain rate was witnessed for specimens subjected to strain rates lesser than 2.0E-03 %/min, and the effects of viscosity/strain rate was found to be insignificant at strain rate higher than 5.0E-01%/min.

An asymptotic assessment of non-premixed flames fed with porous biomass particles in counter-flow configuration considering the effects of thermal radiation and thermophoresis

In this research, a comprehensive mathematical analysis is conducted to evaluate the effects of thermal radiation, porosity and thermophoresis on combustion characteristics of multi-zone counter-flow non-premixed flames fed with moisty porous biomass particles taking into account non-unity fuel and oxidizer Lewis numbers. Volatile lycopodium particles are the biofuel and gaseous fuel evolving from the vaporization of these particles is considered to be methane. In this model, combustion system is composed of preheat, drying, vaporization, reaction and oxidizer zones. To describe the drying and vaporization processes, an asymptotic concept is applied. In order to model the radiative heat transfer, Mie theory is used. Governing conservation equations of mass and energy are solved using Matlab and Mathematica software. Finally, the influences of fuel and oxidizer Lewis numbers, effective equivalence ratio, particle size and porosity, fuel and oxidizer mass fractions, thermophoresis and critical strain rate on temperature and mass fraction distributions in presence/absence of thermal radiation are studied. Current results are compared with reliable findings from previous reports under the same conditions. Maximum flame temperatures in presence and absence of radiation are found to be 1995 K and 1955 K, respectively. Maximum absolute value of thermophoretic force is found to be 1.44×10-8 N.

Meso-scale modelling of the size effect on dynamic compressive failure of concrete under different strain rates

Great progresses have been made in static size effect of concrete, while almost no efforts have been conducted in dynamic size effect of concrete. The macro nonlinearity and size effect of concrete should be attributed to the inner heterogeneities. Herein, a meso-scale simulation method was built to study the dynamic failure and size effect of concrete. In the approach, the concrete heterogeneities were explicitly described, and the strain rate effect for the meso components were also taken into account. Taking squared concrete specimens as examples, the dynamic compressive failure and the size effect of concrete under different strain rates from 10−5/s to 200/s were modelled and investigated. The effect of strain rate on the size effect in compressive strength of concrete was also explored. The simulation results indicate that, there exists a critical strain rate (ɛ˙cr=1/s). When the applied strain rate is less than the critical strain rate, the compressive strength decreases with increasing the specimen size and the size effect on the dynamic compressive strength is weakened as the strain rate increases. The dynamic compressive strength turns to be independent on the structural size of the specimen for the critical strain rate. When the applied strain rate is more than the critical strain rate, the compressive strength increases with the increase of specimen size and the size effect on the dynamic compressive strength is enhanced as the strain rate increases. Finally, based on the influencing mechanism of strain rate effect to dynamic strength and size effect, a Static and Dynamic unified Size Effect Law (i.e. SD-SEL) for compressive strength of concrete was established. The proposed size effect law was also calibrated by the meso-scale simulation results.

Serrated flow during inhomogeneous bending deformation of bulk metallic glasses: From self-organized critical to chaotic dynamics

In this research, the dynamic behavior of serrated flows observed in the flexural stress-deflection curves during three-point bending (3 PB) tests on Zr-, Ti- and La-based bulk metallic glasses (BMGs) is statistically analyzed. Furthermore, the effect of strain rate on the serrations is studied. Based on the ductility of alloy, two distinct types of dynamics are detected. For more ductile alloys (Zr- and Ti-based BMGs) the stress drop magnitude of serrations obeys a power-law distribution of shear avalanches, which is an indicator of the self-organized critical (SOC) state in dynamics. In this case, the size of avalanches has no characteristic scale. In contrast, the size of serrations corresponding to the less ductile alloy (La-based BMG) exhibits a characteristic length scale and implies that the underlying dynamics is chaotic. Results indicate that the plasticity in less ductile alloy can be related to the stick-slip process of a few shear bands, but in the case of more ductile alloys, it can be attributed to the simultaneous nucleation and interaction of a large number of shear bands. Above a critical strain rate, the serrations on flexural stress-deflection curves do not appear and a transition occurs from the serrated to non-serrated plastic flow. The comparison of apparent activation energy for the onset of serrated flow and the STZ activation energy indicates that these two processes are interrelated. Using these values, it is possible to determine the dynamic behavior of serrated flows in BMGs.

Characterization of coarse and ultrafine-grained austenitic stainless steel subjected to dynamic impact load: XRD, SEM, TEM and EBSD analyses

A comparative study of the dynamic impact responses of coarse (37 µm) and ultrafine-grained (0.24 µm) AISI 321 austenitic stainless steel was conducted using a split Hopkinson pressure bar system. An increase in firing pressure of the system's projectile results in the corresponding increase in impact momentum, strain rate, total strain and maximum flow stress of the specimens. Although ultrafine-grained (UFG) specimens show higher compressive strength, coarse-grained (CG) specimens possess higher strain hardenability than the UFG specimens under the same impact loading conditions. UFG specimens exhibit lower critical strain and strain rate at which adiabatic shear bands (ASBs) are observed; hence, UFG specimens are more susceptible to ASB formation than CG specimens. Using the XRD, SEM, TEM and EBSD characterization techniques, the underlying deformation and strengthening mechanisms were studied. Slip and twinning are the active deformation mechanisms in CG specimens and they are highly suppressed in UFG specimens due to spatial restriction effect. At higher strain rates, CG specimens are only susceptible to the formation of multiple transformed shear band (MTSB), while UFG specimens are susceptible to both MTSB and TSB bifurcation. Although the mechanism behind the formation of MTSB is not completely clear, bifurcation of TSB was geometrically necessary and the presence of precipitate at the bifurcation point could play major role. EBSD analysis of inside, interface and outside the TSB revealed the development of equiaxed ultra-fine grain structure (∼0.17–0.21 µm in CG and ∼0.14–0.15 µm in UFG specimens) inside TSB by rotational dynamic recrystallization mechanism.

Mechanical behaviour of auxetic cellular structures built from inverted tetrapods at high strain rates

Auxetic cellular structures build from inverted tetrapods were experimentally tested at high strain rate compression loading for the first time. The strain rates up to 10,000 s−1 were achieved with gas powder gun, where the shock deformation mode is predominant. The deformation localizes in the deformation front between the impacting specimen and the fixed plate due to the inertia effects. This deformation mode results in stiffness increases in comparison to the quasi-static response. The results from experimental testing were used for validation of developed computational models in finite element explicit code LS-DYNA. Furthermore, the validated computational models were used for critical strain rate analysis, determination of critical loading velocities and analysis of deformation modes together with analytical constitutive crushing models of cellular structures.

An apparatus-independent extinction strain rate in counterflow flames

Resistance to extinction by stretch is a key property of any flame, and recent work has shown that this property controls the overall structure of several important types of turbulent flames. Multiple definitions of the critical strain rate at extinction (ESR) have been presented in the literature. However, even if the same definition is used, different experiments report different extinction strain rates for flames burning the same fuel-air mixture at very similar temperatures using similarly constructed opposed-flow instruments. Here we show that at extinction, all these flames are essentially identical, so one would expect that each would be assigned the same value of a parameter representing its intrinsic resistance-to-stretch-induced-extinction, regardless of the specifics of the experimental apparatus. A similar situation arises in laminar flame speed measurements since different apparatuses could result in different strain rate distributions. In that instance, the community has agreed to report the unstretched laminar flame speed, and methods have been developed to translate the experimental (stretched) flame speed into a universal unstretched laminar flame speed. We propose an analogous method for translating experimental measurements for stretch-induced extinction into an unambiguous and apparatus-independent quantity (ESR∞) by extrapolating to infinite opposing burner separation distance. The uniqueness of the flame at extinction is shown numerically and supported experimentally for twin premixed, single premixed, and diffusion flames at Lewis numbers greater than and less than one. A method for deriving ESR∞ from finite-boundary experimental studies is proposed and demonstrated for methane and propane experimental diffusion and premixed single flame data. The two values agree within the range of ESR differences typically observed between experimental measurements and simulation results for the traditional ESR definition.

A micro-macro dynamic compressive-shear fracture model under static confining pressure in brittle rocks

A new dynamic compressive-shear fracture model under static confining pressure is proposed in brittle rocks containing numerous initial microcracks. The formulation of this model is based on the wing crack model, the Mohr-Coulomb failure criterion, the crack-strain relation, and the established dynamic fracture toughness relating axial strain rate. The crack-strain relation is obtained by correlating the micro- and macro damages. The dynamic fracture toughness relating axial strain rate is established by combining the crack-strain relation and dynamic fracture toughness relating crack growth speed. The intact dynamic stress-strain constitutive relationship describing pre-peak hardening phase and post-peak softening phase is presented. The effects of strain rate on dynamic stress-strain relationship, compressive strength, shear strength, cohesion and internal friction are predicted. A novel critical strain rate making the dynamic crack initiation stress equal dynamic compressive strength is found. Exceeding this critical strain rate, the dynamic compressive strength, shear strength, cohesion and internal friction angle have a larger change. The sensitivities of confining pressure, crack size, crack friction, crack inclination angle on dynamic stress-strain relationship, compressive strength, shear strength, cohesion and internal friction angle under high strain rate are also discussed. Rationality of this proposed compressive-shear fracture model is verified by comparing the experimental data.

Temperate ice in the shear margins of the Antarctic Ice Sheet: Controlling processes and preliminary locations

The flux of grounded ice from the Antarctic Ice Sheet (AIS) primarily occurs through narrow, fast-flowing outlet glaciers and ice streams. Shearing generates heat in the lateral glacier margins, where there is a transition from fast-flow to near-stagnant ice or rock. This heat warms the ice and can form zones of water-saturated porous ice at the melting temperature, known as temperate ice. Here we derive a one-dimensional thermomechanical model to infer which AIS shear margins may contain temperate ice. Our model provides expressions for the critical shear strain rate at which a zone of temperate ice initiates and the thickness of the temperate zone. Both of these quantities are described by two nondimensional numbers—the Brinkman and Péclet numbers—that are functions of observable parameters such as shear strain rate, surface temperature, surface accumulation rate, and ice thickness. The development of temperate zones is incited by shear strain rates (Brinkman number) and suppressed by the advection of cold ice from the surface (Péclet number), which scales with surface accumulation rate. We also include a parametrization for lateral advection, which can suppress the formation of temperate ice. Applying recent observations and outputs from a regional climate model, we show that many active glacier shear margins could contain temperate ice. The spatial distribution and thickness of temperate zones are controlled largely by shear strain rates and surface accumulation rates.

The interaction between fuel inclination and horizontal wind: Experimental study using thin wire

Forward flame spread behavior changes significantly with both the fuel inclination and the wind velocity, but interactions between the inclination and horizontal wind is still not well understood. In this work, a controlled laboratory experiment is conducted in a wind tunnel providing a horizontal wind ranged from 0 (quiescent) to 2.5 m/s, and a thin electrical wire as fuel that can be inclined from a horizontal (0°) to a vertical (90°). An electrical wire of 0.8-mm diameter with copper core and polyethylene insulation was used as the characteristic thin fuel. The flame-spread rate, as well as the flame geometrical characteristics, was quantified as a function of the wind velocity and wire inclination. Results show that as the horizontal wind velocity is increased, the flame-spread rate over the wire for all inclination angles first increased to a maximum, and then, slightly decreased until blow-off. The critical inclination for flame acceleration decreases from 45° to 15° as the wind velocity increases. The fastest flame spread along the inclined wire occurs when the wind pushes the flame parallel to the fuel. The interaction between the fuel inclination and wind velocity are quantified using the Froude number. Moreover, the blown-off wind velocity decreases as the fuel inclination is increased, which can be explained by a critical strain rate (536 s−1). This work provides valuable information for fire behaviors under wind not only on the inclined wires and roofs in structure fires, but also on inclined branches in wildland fires.

Stress corrosion cracking of biodegradable Mg-4Zn alloy in simulated body fluid at different strain rates – A fractographic investigation

Degradable magnesium based implants are predisposed to failure by stress corrosion cracking (SCC) when subject to tensile loads in the human body. Slow strain rate (SSR) tests are used in SCC analysis and the applied strain rate is a critical factor to determine if the material failed by stress corrosion or mechanical overload. In this paper, the SCC susceptibility of Mg-4Zn alloy in simulated body fluid (SBF) is investigated at different strain rates. It was found that elongation to failure (ε) and ultimate tensile strength (UTS) decreased by 80% and 58% respectively when extension rate in SBF decreased from 3.6 × 10−4 s−1 to 3.6 × 10−6 s−1. When the Mg-4Zn alloy is strained in SBF, multiple surface cracks appear on the gauge, initiated at the corrosion pits that induce mechanical overload and failure at higher strain rates. As the strain rate decreases, the increased contact time between the surface and SBF results in hydrogen embrittlement, micro crack formation and progressive transgranular failure that is evident in the fractography. The reduction in UTS and elongation even at the highest strain rate tested indicate that the critical strain rate for stress corrosion cracking of Mg-4Zn alloy in simulated body fluid could be as high as 3.6 × 10−4 s−1.

Influences of in-situ ZrB2 and TiB2 particles on Portevin-Le Chatelier effect for AlSi9Cu1 alloy

In this paper, the plastic instability phenomenon Portevin-Le Chatelier effect (PLC) was investigated in AlSi9Cu1 alloy and the in-situ (ZrB2+TiB2)/AlSi9Cu1 composites fabricated from the Al-K2TiF6-K2ZrF6-KBF4 system. Microstructure observation and X-ray diffraction testing revealed that the ZrB2 and TiB2 particles were generated via direct melting reaction. Mechanical property testing showed that 3 wt.% (ZrB2+TiB2) particles reinforced composites owned the highest ultimate tensile strength (226 MPa). Meanwhile, the effects of mass fraction of reinforced particles, strain rate on the critical strain were also systematically studied. The results indicated that both normal and inverse behavior of critical strain vs. strain rate existed during the tensile deforming for AlSi9Cu1 alloy and (ZrB2+TiB2)/AlSi9Cu1 composites.

Effect of transport processes on ignition of stretched diffusion flames using laser spark

Laser spark ignition of methane diffusion flames, in the stagnation region in counter-flow configuration, is studied with a range of transport properties achieved by using different levels of inert dilution with helium and argon as diluents in the fuel stream. The effects of transport properties on ignition strain rate and ignition delay time are investigated and possible effects of effective Lewis number (Lee) are also discussed and compared with those for flame extinction. The following results are obtained:(1) The role of Lee on the critical global strain rate, beyond which ignition is not possible, is qualitatively similar to that on the extinction strain rate. That is, with the same level of dilution, the inert diluent with smaller Lee (therefore relatively smaller heat loss rate) yields larger critical global strain rate.(2) For successful ignition, the ignition delay time decreases with increasing level of helium dilution, while the opposite is true with argon dilution – the experimental results correlated with the calculated results of the diffusion times with these two inert diluents, suggesting that the successful ignition and the ignition delay time is limited by the thermal diffusion time across the flammable layer in the counter-flow region, rather than resulting from the Lewis number effect.

Mechanical and microstructural properties of 2024-T351 aluminium using a hat-shaped specimen at high strain rates

This paper investigates adiabatic shear bands (ASBs) in 2024-T351 aluminium and their effect on the mechanical properties and microstructure of this aluminium alloy. Using hat-shaped specimens in a split Hopkinson pressure bar (SHPB) and universal testing machine, resolved shear stress-nominal plastic shear strain curves were presented at shear strain rates between 1 × 10−4 and 15.4 × 103 s−1. A revised formula for shear stress in hat specimens is proposed. Microstructural observations were completed on deformed specimens, with hardness results, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) presented. A critical shear strain rate range for ASB initiation in these specimens of approximately 8500–10500 s−1 was determined within impacted specimens.

Stress bifurcation in large amplitude oscillatory shear of yield stress fluids

Large amplitude oscillation shear has been an important method to investigate the yielding and flow behavior of yield stress materials. However, there are great uncertainties in determination of the yield stress from the shear stress (or shear strain) dependence of the apparent dynamic moduli or the relative harmonic intensity using Fourier transform rheology. The yield stress from these dynamic methods is also inconsistent with the steady shear and transient shear measurements. We propose a new method, namely, stress bifurcation, based on the geometric average of elastic and viscous Lissajous curves to study the yielding transition of different yield stress fluids in large amplitude oscillatory shear flow. The results prove that typical yield stress fluids such as concentrated emulsions, polymer nanocomposites, microgels, and particulate gels all exhibit stress bifurcations, both inter and intra cyclically, in large amplitude oscillatory shear experiments. Such stress bifurcation phenomena between the average stress-strain (or strain rate) curves are independent of the type of input signal, i.e., stress-controlled versus strain-controlled. A start yield stress (strain) (related to strain) and an end yield stress (strain rate) (related to strain rate), instead of a single critical variable, were suggested to characterize yielding transitions. The frequency dependences of critical stresses, critical strain, and critical strain rate determined by the new method were also investigated systematically for the different kinds of yield stress fluids. A visco-elastic-plastic model, the Kelvin-Voigt-Herschel-Bulkley model, was also adopted to understand the stress bifurcation and frequency dependencies of critical variables in large oscillatory shear flow.Large amplitude oscillation shear has been an important method to investigate the yielding and flow behavior of yield stress materials. However, there are great uncertainties in determination of the yield stress from the shear stress (or shear strain) dependence of the apparent dynamic moduli or the relative harmonic intensity using Fourier transform rheology. The yield stress from these dynamic methods is also inconsistent with the steady shear and transient shear measurements. We propose a new method, namely, stress bifurcation, based on the geometric average of elastic and viscous Lissajous curves to study the yielding transition of different yield stress fluids in large amplitude oscillatory shear flow. The results prove that typical yield stress fluids such as concentrated emulsions, polymer nanocomposites, microgels, and particulate gels all exhibit stress bifurcations, both inter and intra cyclically, in large amplitude oscillatory shear experiments. Such stress bifurcation phenomena between the av...

Self-restraint hot cracking test for aluminum alloys using digital image correlation

The susceptibility to hot cracking depends on the alloy composition, the welding parameters and the clamping conditions. We combined the simplicity of the fan shaped cracking test introduced by Kutsuna with the digital image correlation method to evaluate the local strain rate at the solidification zone. The analysis gives quantitative information, which strain rate is needed to stop the centerline crack propagation. By this approach, the hot cracking susceptibility was quantified by measuring the critical strain rate for four different AlMgSi aluminum alloys at three different weld speeds.

Stochastic characteristics and Second Law violations of atomic fluids in Couette flow

Using Non-equilibrium Molecular Dynamics (NEMD) simulations, we study the statistical properties of an atomic fluid undergoing planar Couette flow, in which particles interact via a Lennard-Jones potential. We draw a connection between local density contrast and temporal fluctuations in the shear stress, which arise naturally through the equivalence between the dissipation function and entropy production according to the fluctuation theorem. We focus on the shear stress and the spatio-temporal density fluctuations and study the autocorrelations and spectral densities of the shear stress. The bispectral density of the shear stress is used to measure the degree of departure from a Gaussian model and the degree of nonlinearity induced in the system owing to the applied strain rate. More evidence is provided by the probability density function of the shear stress. We use the Information Theory to account for the departure from Gaussian statistics and to develop a more general probability distribution function that captures this broad range of effects. By accounting for negative shear stress increments, we show how this distribution preserves the violations of the Second Law of Thermodynamics observed in planar Couette flow of atomic fluids, and also how it captures the non-Gaussian nature of the system by allowing for non-zero higher moments. We also demonstrate how the temperature affects the band-width of the shear-stress and how the density affects its Power Spectral Density, thus determining the conditions under which the shear-stress acts is a narrow-band or wide-band random process. We show that changes in the statistical characteristics of the parameters of interest occur at a critical strain rate at which an ordering transition occurs in the fluid causing shear thinning and affecting its stability. A critical strain rate of this kind is also predicted by the Loose–Hess stability criterion.

The Effects of Specimen Geometry on the Plastic Deformation of AA 2219-T8 Aluminum Alloy Under Dynamic Impact Loading

The effects of specimen geometry on shear strain localization in AA 2219-T8 aluminum alloy under dynamic impact loading were investigated. The alloy was machined into cylindrical, cuboidal and conical (frustum) test specimens. Both deformed and transformed adiabatic shear bands developed in the alloy during the impact loading. The critical strain rate for formation of the deformed band was determined to be 2500 s−1 irrespective of the specimen geometry. The critical strain rate required for formation of transformed band is higher than 3000 s−1 depending on the specimen geometry. The critical strain rate for formation of transformed bands is lowest (3000 s−1) in the O5 mm × 5 mm cylindrical specimens and highest (> 6000 s−1) in the conical specimens. The cylindrical specimens showed the greatest tendency to form transformed bands, whereas the conical specimen showed the least tendency. The shape of the shear bands on the impacted plane was also observed to be dependent on the specimen geometry. Whereas the shear bands on the compression plane of the conical specimens formed elongated cycles, two elliptical shaped shear bands facing each other were observed on the cylindrical specimens. Two parallel shear bands were observed on the compression planes of the cuboidal specimens. The dynamic stress–strain curves vary slightly with the specimen geometry. The cuboidal specimens exhibit higher tendency for strain hardening and higher maximum flow stress than the other specimens. The microstructure evolution leading to the formation of transformed bands is also discussed in this paper.

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.