Shear Banding Mechanism Research Articles

Dynamic compressive behavior and fracture modeling of Titanium alloy IMI 834

A comprehensive study has been performed for understanding the constitutive behavior of titanium alloy IMI 834 under different strain rates, stress triaxialities and temperatures. Results confirmed an improvement in its strength with increase in strain rate and stress triaxiality. Experimental results obtained from the specimens of materials were employed to calibrate the material parameters of Johnson–Cook (J-C) strength and fracture model. The microstructure study has been undertaken to discuss the shear band and grain growth mechanisms. Three constitutive models: Johnson-Cook (J-C), Zerilli–Armstrong (Z-A) and Cowper-Symonds (C-S) models are established to determine the flow stress using dynamic compression data. The ballistic tests were performed in which 8 mm thick target plates were impacted by 7.62 AP projectiles with a velocity of 830 m/s. The ballistic test results were replicated through numerical simulations carried out using ABAQUS finite element code. It is observed that the residual velocities generated by simulations were in good agreement with the experimental results.

The effect of ultrasonic impact treatment on the deformation behavior of commercially pure titanium under uniaxial tension

The deformation behavior of commercially pure titanium specimens subjected to surface hardening by ultrasonic impact treatment followed by uniaxial tension was investigated experimentally and numerically. The microstructure of the ultrasonically treated ~100μm thick surface layer undergoing uniaxial tension was revealed, using transmission electron microscopy and electron backscatter diffraction. Non-equiaxed 100–200nm α-Ti grains composed of 2nm diameter TiC and Ti2C nanoparticles, ω- and α″-phase crystallites were found in the 10μm thick uppermost layer. Fine and coarse α-Ti grains containing dislocations and twins were observed at depths of 20 and 50μm below the specimen surface, respectively. A non-crystallographic deformation (shear banding) mechanism at work in the nanostructured surface layer of the specimens under study was revealed. The evolution of shear bands was studied by the finite difference method, with the fine-grained structure being explicitly accounted for in the calculations. Shear band self-organization was described, using the energy balance approach similar to that based on Griffith's energy balance criterion for brittle fracture. The tensile deformation of the hardened layer lying at a depth of 50μm was implemented by the glide of dislocations and growth of deformation twins induced by preliminary ultrasonic impact treatment.

Notch fatigue behavior: Metallic glass versus ultra-high strength steel

Studying the effect of notch on the fatigue behavior of structural materials is of significance for the reliability and safety designing of engineering structural components. In this work, we conducted notch fatigue experiments of two high-strength materials, i.e. a Ti32.8Zr30.2Ni5.3Cu9Be22.7 metallic glass (MG) and a 00Ni18Co15Mo8Ti ultra-high strength steel (CM400 UHSS), and compared their notch fatigue behavior. Experimental results showed that although both the strength and plasticity of the MG were much lower than those of the UHSS, the fatigue endurance limit of the notched MG approached to that of the notched UHSS, and the fatigue ratio of the notched MG was even higher. This interesting finding can be attributed to the unique shear banding mechanism of MG. It was found that during fatigue process abundant shear bands formed ahead of the notch root and in the vicinity of the crack in the notched MG, while limited plastic deformation was observed in the notched UHSS. The present results may improve the understanding on the fatigue mechanisms of high-strength materials and offer new strategies for structural design and engineering application of MG components with geometrical discontinuities.

Elucidating the flow-microstructure coupling in entangled polymer melts. Part II: Molecular mechanism of shear banding

In this study, we have elucidated the molecular origin of shear banding in the entangled polymeric melts. Specifically, it is shown that the inflection point corresponding to the stress-overshoot indicates the possibility of inhomogeneity and this combined with slow orientation relaxation in step-strain start-up experiments will lead to formation of local inhomogeneity along the velocity gradient direction. Once the aforementioned inhomogeneities are created, a localized jump in entanglement density and a commensurate jump in normal stress and viscosity will lead to formation of the incipient shear banded flow structure. To this end, number of step-strain and start-up simulations with different deformation rate ramp times in the planar Couette flow with entanglement densities ⟨Zk⟩ ≥ 17 were performed to demonstrate the effect of deformation rate ramp time on the formation of local inhomogeneities and occurrence of shear banding. It has been demonstrated that if the time scale for the deformation rate to reach its steady value is larger or on the order of the orientation relaxation time of the chain, local inhomogeneities in the velocity gradient direction will not form and the linear velocity profile will prevail. Overall, the molecular mechanism of incipient shear banding as well as its evolution to steady shear banding or to a linear velocity profile (transient shear banding) is described for the first time. Moreover, our findings are in agreement with a host of prior step-strain and start-up experiments with synthetic and natural Deoxyribonucleic acid (DNA) entangled polymeric fluids.In this study, we have elucidated the molecular origin of shear banding in the entangled polymeric melts. Specifically, it is shown that the inflection point corresponding to the stress-overshoot indicates the possibility of inhomogeneity and this combined with slow orientation relaxation in step-strain start-up experiments will lead to formation of local inhomogeneity along the velocity gradient direction. Once the aforementioned inhomogeneities are created, a localized jump in entanglement density and a commensurate jump in normal stress and viscosity will lead to formation of the incipient shear banded flow structure. To this end, number of step-strain and start-up simulations with different deformation rate ramp times in the planar Couette flow with entanglement densities ⟨Zk⟩ ≥ 17 were performed to demonstrate the effect of deformation rate ramp time on the formation of local inhomogeneities and occurrence of shear banding. It has been demonstrated that if the time scale for the deformation rate to r...

Formation of adiabatic shear band and deformation mechanisms during warm compression of Ti–6Al–4V alloy

Adiabatic shear band (ASB) was narrow region where softening occurred and concentrated plastic deformation took place. In present study, the effects of height reduction and deformation temperature on ASB were investigated by means of optical microscopy (OM) and scanning electron microscopy (SEM). And the deformation mechanisms within the shear band were discussed thoroughly with the help of transmission electron microscopy (TEM). There is a critical strain for the formation of ASB during warm compression of Ti–6Al–4V alloy. The width of ASB increases with height reduction increasing. Elongated alpha grains within shear band grow up with deformation temperature increasing. Some ultrafine grains that confirm the occurrence of dynamic recrystallization are observed within shear band during warm compression of Ti–6Al–4V alloy. There is a critical strain for the formation of adiabatic shear band during warm compression of Ti–6Al–4V alloy. Some ultrafine grains within shear band are observed during warm compression of Ti–6Al–4V alloy; it confirms the occurrence of dynamic recrystallization of α phase.

Texture and microstructure evolution during non-crystallographic shear banding in a plane strain compressed Cu–Ag metal matrix composite

We studied the texture and microstructure evolution in a plane strain compressed Cu–Ag metal matrix composite (MMC) with a heterophase microstructure using crystal plasticity finite element simulations. Lattice reorientations induced by both crystallographic (dislocation slip and twinning) and non-crystallographic (shear banding) mechanisms are addressed. First, simulations on a polycrystalline composite are made. Quite similar texture trends are observed for the composites and for the individual single-phase materials, namely, copper-type texture components in the Cu phase and brass-type texture components in the Ag phase. This result differs from experimental data that show less copper-type and more brass-type textures in both phases for the composite materials. To explore co-deformation mechanisms that lead to the specific crystallographic textures in the MMC, bicrystal simulations for the composite with specific initial orientation combinations are performed. The bicrystal simulations reproduce the experimentally observed trends of texture evolution in the respective phases of the composite, indicating that the localized stress and strain fields as well as the co-deformation mechanisms within the actual heterophase microstructure are well captured. The modeling shows that to accommodate plastic deformation between adjacent phases in the bicrystals, pronounced shear bands are triggered by stress concentration at the hetero-interfaces. With further deformation the bands penetrate through the phase boundaries and lead to larger lattice rotations. The simulations confirm that the shear banding behavior in heterophase composites is different from that in single-phase metals and the texture evolution in composite materials is strongly influenced by the starting texture, the local constraints exerted from the phase boundaries and the constitutive properties of the abutting phases.

Microstructural evolution of a model, shear-banding micellar solution during shear startup and cessation.

We present direct measurements of the evolution of the segmental-level microstructure of a stable shear-banding polymerlike micelle solution during flow startup and cessation in the plane of flow. These measurements provide a definitive, quantitative microstructural understanding of the stages observed during flow startup: an initial elastic response with limited alignment that yields with a large stress overshoot to a homogeneous flow with associated micellar alignment that persists for approximately three relaxation times. This transient is followed by a shear (kink) band formation with a flow-aligned low-viscosity band that exhibits shear-induced concentration fluctuations and coexists with a nearly isotropic band of homogenous, highly viscoelastic micellar solution. Stable, steady banding flow is achieved only after approximately two reptation times. Flow cessation from this shear-banded state is also found to be nontrivial, exhibiting an initial fast relaxation with only minor structural relaxation, followed by a slower relaxation of the aligned micellar fluid with the equilibrium fluid's characteristic relaxation time. These measurements resolve a controversy in the literature surrounding the mechanism of shear banding in entangled wormlike micelles and, by means of comparison to existing literature, provide further insights into the mechanisms driving shear-banding instabilities in related systems. The methods and instrumentation described should find broad use in exploring complex fluid rheology and testing microstructure-based constitutive equations.

Influences of residual stresses on the serrated flow in bulk metallic glass under elastostatic four-point bending – A nanoindentation and atomic force microscopy study

The effects of residual stress on the deformation behavior of a Zr-based bulk metallic glass (BMG) during nanoindentation were studied by atomic force microscopy. The residual stress was introduced by elastostatically preloading a beam-shaped BMG sample by four-point bending up to tensile and compressive stress levels of ±2.0GPa for up to 14days. Strain-rate-controlled nanoindentations were performed on the four-point bent samples at various times during loading and after unloading to analyze the serrated flow during indentation. The hardness of the alloy, the pile-up behavior as well as the serrations strongly depend on the magnitude and sign of the applied residual stresses. Tensile stresses suppress pile-up formation, decrease the hardness but increase the jump width of the serrated flow during nanoindentation. In contrast, increased pile-up formation with increased hardness occurs along with a successive serrated flow behavior on the compression side.The discrepancy of pile-up and serrated flow is explained by a difference in the shear banding mechanism. The results suggest that for compressive stress individual shear planes are successively activated, leading to localized shear steps on the surface. For tensile residual stresses, the plastic volume is more widely spread, leading to vanishing pile-up together with an intermittent activation of a big number of shear events, causing big serrations. Due to the widely varying pile-up behavior, a hardness correction was performed. This strongly reduced the apparent hardness variations across the beam. For this specific testing arrangement, only reversible mechanical property variations with time due to long-time prestraining at high elastostatic stresses were observed.

In situ observation of bending stress–deflection response of metallic glass

Understanding the shear banding mechanism is of significance not only for improving the mechanical performance of metallic glassy components but also for designing new metallic glassy materials with better combinations of strength and toughness. In this study, through an in situ bending experiment under a scanning electron microscope, the shear banding behavior and the bending stress-deflection response were investigated. We found that the flexural strength was higher than either the tensile strength or the compressive strength of the identical metallic glass. The initial appearance of shear bands was observed at a stress lower than the flexural yield strength. As shear band propagating stably, the flexural stress increases with increasing the deflection, which is similar to but analytically found different with the traditional “work-hardening” behavior of metallic crystalline materials. At the peak stress, a shear banding instability with an abrupt increment of plastic deformation localization and shear cracking was found. Some shear band zones were observed and the size of the zone in tension side of the bending sample was found larger than that in compression side.

Adiabatic shear banding and scaling laws in chip formation with application to cutting of Ti–6Al–4V

The phenomenon of adiabatic shear banding is analyzed theoretically in the context of metal cutting. The mechanisms of material weakening that are accounted for are (i) thermal softening and (ii) material failure related to a critical value of the accumulated plastic strain. Orthogonal cutting is viewed as a unique configuration where adiabatic shear bands can be experimentally produced under well controlled loading conditions by individually tuning the cutting speed, the feed (uncut chip thickness) and the tool geometry. The role of cutting conditions on adiabatic shear banding and chip serration is investigated by combining finite element calculations and analytical modeling. This leads to the characterization and classification of different regimes of shear banding and the determination of scaling laws which involve dimensionless parameters representative of thermal and inertia effects. The analysis gives new insights into the physical aspects of plastic flow instability in chip formation. The originality with respect to classical works on adiabatic shear banding stems from the various facets of cutting conditions that influence shear banding and from the specific role exercised by convective flow on the evolution of shear bands. Shear bands are generated at the tool tip and propagate towards the chip free surface. They grow within the chip formation region while being convected away by chip flow. It is shown that important changes in the mechanism of shear banding take place when the characteristic time of shear band propagation becomes equal to a characteristic convection time. Application to Ti–6Al–4V titanium are considered and theoretical predictions are compared to available experimental data in a wide range of cutting speeds and feeds. The fundamental knowledge developed in this work is thought to be useful not only for the understanding of metal cutting processes but also, by analogy, to similar problems where convective flow is also interfering with adiabatic shear banding as in impact mechanics and perforation processes. In that perspective, cutting speeds higher than those usually encountered in machining operations have been also explored.

Shear banding mechanism of plastic deformation in LiF irradiated with swift heavy ions

The effect of ion irradiation on the behavior of plastic deformation at micro- and nanoindentation on (001) face of LiF has been investigated. The irradiation was performed using heavy ions (U, Au, Ti and S) with energy in the range from 3 MeV to 2 GeV at fluences up to 5x1013 ions/cm2. In non-irradiated LiF, the indentation produces dislocation gliding on the {110} planes along the <100> and <110> directions. At high fluence irradiation, the resource of the dislocation slip along the preferable directions becomes exhausted due to immobilization of dislocations by radiation defects and their aggregates. The present study demonstrates the change of the mechanism of plastic deformation from homogenous dislocation slip to localized shear banding in samples irradiated to high fluences. The factors facilitating of the localization of deformation have been analyzed.

Kinetic and structural interpretations of post-yield crack resistance behavior of polyamide-6/polyolefin-g-maleic anhydride blends

Binary blends of polyamide-6 (PA-6)/polypropylene-grafted-maleic anhydride (PP-g-MA) and PA-6/low density polyethylene-grafted-maleic anhydride (LDPE-g-MA) were prepared with varied concentration (0–30 wt%) of maleic anhydride-grafted polyolefinic (PP) moiety as the impact modifier. The microstructural attributes and thermal properties of the blends were characterized by WAXD, FTIR, SEM, DSC, and TGA. The WAXD/DSC studies have revealed that the crystallinity of the blends decreased with the increase in the PP modifier whereas the onset of degradation temperature remained nearly unaffected. Comparative assessment of the crack toughness behavior of the blends has been carried out following the essential work of fracture (EWF) approach based on post-yield fracture mechanics (PYFM) concept. The kinetics of crack propagation of the blends has been discussed in the realms of structural and compositional parameters. An enhancement in the toughness (resistance to stable crack propagation) as indicated by a maximum in the non-EWF (βw p) values have been observed at 10 and 20 wt% followed by a sharp and consistent drop in the composition regime of 10–20 and 20–30 wt% of PP-g-MA and LDPE-g-MA, respectively; conceptually implying possible ductile-to-semiductile transitions in the blend systems. The equivalence of PYFM–EPFM fracture parameters have been discussed following inequality criterion. Fractured surface morphology investigations revealed that the failure mode of the blends undergo a systematic transition from matrix-controlled homogeneous flow/deformation in the PP/polyamide phase to blend composition-dependent changes in the modes and extent of fibrillation via cavitation and shear-banding mechanisms.

Effect of silica nanoparticle size on toughening mechanisms of filled epoxy

The addition of silica nanoparticles (23 nm, 74 nm, and 170 nm) to a lightly crosslinked, model epoxy resin, was studied. The effect of silica nanoparticle content and particle size on glass transition temperature (Tg), coefficient of thermal expansion (CTE), Young's modulus (E), yield stress (σ), fracture energy (GIC) and fracture toughness (KIC), were investigated. The toughening mechanisms were determined using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and transmission optical microscopy (TOM). The experimental results revealed that the addition of silica nanoparticles did not have a significant effect on Tg or the yield stress of epoxy resin, i.e. the yield stress and Tg remained constant regardless of silica nanoparticle size. As expected, the addition of silica nanoparticles had a significant impact on CTE, modulus and fracture toughness. The CTE values of nanosilica-filled epoxies were found to decrease with increasing silica nanoparticle content, which can be attributed to the much lower CTE of the silica nanoparticles. Interestingly, the decreases in CTE showed strong particle size dependence. The Young's modulus was also found to significantly improve with addition of silica nanoparticles and increase with increasing filler content. However, the particle size did not exhibit any effect on the Young's modulus. Finally, the fracture toughness and fracture energy showed significant improvements with the addition of silica nanoparticles, and increased with increasing filler content. The effect of particle size on fracture toughness was negligible. Observation of the fracture surfaces using SEM and TOM showed evidence of debonding of silica nanoparticles, matrix void growth, and matrix shear banding, which are credited for the increases in toughness for nanosilica-filled epoxy systems. Shear banding mechanism was the dominant mechanism while the particle debonding and plastic void growth were the minor mechanisms.

Three-dimensional drained behavior of normally consolidated anisotropic kaolin clay

A series of consolidated-drained true triaxial tests with a constant mean effective stress and constant Lode angles during shear is performed on cross-anisotropic kaolin clay. All tests are performed in a fully automated true triaxial testing apparatus. The relative magnitude of the intermediate principal stress, expressed in terms of the b-value, and initial cross-anisotropy show significant influence on the stress–strain, strength, and shear band formation characteristics of the clay. Shear bands form and appear to cause failure in all true triaxial tests performed, except in triaxial compression. The initiation and development of shear bands is observed to take place, when the clay undergoes volumetric contraction. The lower strength exhibited in the shear bands is caused by alignment of the clay particles. This shear band mechanism is different from that observed in granular materials, in which the lower shear strength is reached due to dilation in the shear bands.

Plastic Deformation in an Amorphous Ni-P Coating

An experimental and numerical investigation of the hardness and associated plastic deformation in as-deposited and as-annealed nickel-phosphorus (Ni-P) coatings was conducted. In addition to the indentation-deformation behavior, the deformation morphology underneath the indenter was examined. The yield strength extracted from the indentation data is as high as 5.6 GPa, indicating pressure-sensitive plasticity. Results show that the as-deposited Ni-P coating was deformed appreciably through the shear-band mechanism with semicircular and radial shear-band morphologies. From the incremental loading-unloading cyclic experiments, the phenomena on hardening and recovery, which have scarcely been recognized in amorphous materials at room temperature, were observed in the amorphous coating using instrumented nanoindentation. A numerical simulation of the interfacial indentation test between the Ni-P coating and the substrate reveals the pileup and shear bands of the Ni-P coating that were observed during the indentation tests.

Non-crystallographic shear banding in crystal plasticity FEM simulations: Example of texture evolution in α-brass

We present crystal plasticity finite element simulations of the texture evolution in α-brass polycrystals under plane strain compression. The novelty is a non-crystallographic shear band mechanism [Anand L, Su C. J Mech Phys Solids 2005;53:1362] that is incorporated into the constitutive model in addition to dislocation and twinning. Non-crystallographic deformation associated with shear banding leads to weaker copper and S texture components and to a stronger brass texture compared to simulations enabling slip and twinning only. The lattice rotation rates are reduced when shear banding occurs. This effect leads to a weaker copper component. Also, the initiation of shear banding promotes brass-type components. In summary the occurrence of non-crystallographic deformation through shear bands shifts face-centered-cubic deformation textures from the copper type to the brass type.

Stick-slip dynamics and recent insights into shear banding in metallic glasses

Abstract

Enhancing strength and ductility of Mg–12Gd–3Y–0.5Zr alloy by forming a bi-ultrafine microstructure

By means of equal channel angular pressing (ECAP) technique, an Mg–12Gd–3Y–0.5Zr alloy was processed at 300 °C for one and four passes. A bi-ultrafine (BU) microstructure of Mg-alloy, consisting of matrix grains and the second-phase particles with sizes smaller than 1 μm, was obtained after the fourth pass of ECAP. The formation of this BU microstructure originated from the complex interactions between the matrix grains and second-phase particles during the ECAP processing at elevated temperature. A simultaneous improvement of both strength and ductility was achieved in the sample with the BU microstructure, compared with the original sample. The increase in strength is resulted from the refinement of both matrix grains and second-phase particles, while the improved ductility is attributed to the considerable plasticity provided by both dislocation mechanism and profuse shear bands in the BU microstructure. An enhanced plasticity induced by the shear-band mechanism is proposed for the magnesium alloy with the BU microstructure.

Self-consistent modeling of rolling textures in an austenitic–ferritic duplex steel

Rolling textures of the constituent phases in an austenitic–ferritic duplex stainless steel are measured by X-ray diffraction experiments, showing that the brass-type texture, typical of f.c.c. materials with low SFE, is developed in the austenitic phase, and the rotated-cube and brass-R textures are developed in the ferritic phase. On the basis of the experimental texture components and fibers at different reductions, rolling textures of the respective phases in the duplex steel are simulated using a self-consistent model. After considering various micromechanical interactions within the steel, a reliable prediction of the evolution of grain orientation distributions for the phases at small reductions is achieved. An attempt in modeling the brass-type texture for the f.c.c. metallic phase is also performed by incorporating the shear banding mechanism into the presented model.

Numerical Analysis of Shear Banding Flow of Wormlike Micelle Solutions between Parallel Channels using a Network Scission Model

Shear banding emerges in shear flows of wormlike micelle solutions and is observed as band velocity profiles in velocity measurement experiments. In our previous experiments, multi-band velocity profiles were captured in shear flows of a CTAB/NaSal solution between parallel plates. The relationship between the band velocity profile and the micellar network structure is an interesting issue. In the present study, the orientation behavior of the micelle network was numerically analyzed using a network scission model. Brownian dynamics simulations were carried out for a two-species model of micelle networks derived in the Vasquez-Cook-McKinley model. The numerical results indicate that the effect of the orientation of short micelles on the behavior of the entire system is small, whereas long micelles tend to align well in the flow direction, their degree of orientation is relatively high, and their orientation behavior strongly affects the orientation of the entire system. Numerical analysis using the BD simulation provides useful information on the flow-induced structures of the micelle network, and hence the present approach is effective for the analysis of the mechanism of shear banding.