Initiation Of Shear Bands Research Articles

Shear band development under simple shear and the intrinsicality of strain softening of amorphous glassy polymers

The development of shear bands that induced by localized strain inhomogeneity is a characteristic deformation mechanism and determines ductility of amorphous glassy polymers. To investigate the essence of intrinsic strain softening and its influence on shear band evolution under shearing and reverse shearing, a full-network constitutive model based on the micro-mechanism is implemented into the material point method and finite element method respectively. The results show that the shear band nucleates and propagates in forward shearing, and gradually fades until disappears in following reverse shearing when the specimen regains its original geometry. As reverse shearing proceeds or there is a second forward shearing, the homogeneous plastic deformation occurs preferentially in the previously deformed band but it doesn't display any shear band like the initial forward shearing. It means the glassy polymers undergoing plastic deformation cannot be recovered by reverse straining. In further shearing, new shear bands tend to be initiated outside the existing band and then propagates to merge with the previous band. The initiation and propagation of shear band along the shearing direction is caused by the intrinsic strain softening, independent of loading histories and boundary conditions. The widening of shear band is related to the strain hardening. • Origin of intrinsic strain softening lies in the saturation of plastic shear strain. • Amount of intrinsic strain softening keeps fixed. • Glassy polymers undergoing plasticity cannot be recovered by reverse straining. • Full-network model can capture the shear band evolution under reverse shearing.

Effect of structural heterogeneity on work-hardening behavior of metallic glass thin film

In this study, three kinds of atomic disordered Cu50Zr50 metallic glass thin films (MGTFs) with different structural heterogeneities are prepared by controlling the substrate temperature in magnetron sputtering, i.e., controlling the thermal history. A work-hardening behavior is observed in the MGTFs by cyclic nanoindentation. The spatial distribution of structural heterogeneities, and the correlation length of the viscoelasticity of heterogeneity in three MGTFs with different thermal histories demonstrate the effect of thermal history on the viscoelasticity of three MGTFs. The activation volume of heterogeneity controlling the initiation of shear band during local deformation correlates the correlation length of viscoelastic heterogeneity, which is used to elucidate the relationship between the structural heterogeneity and the work-hardening behavior. The results show that the increase of the substrate temperature causes the viscosity of structural heterogeneity to be increased, which means the reduction of viscoelastic heterogeneity correlation length. In this case, the activation volume of heterogeneity in the MGTF is decreased, which leads the formation of shear band to be difficult.

Mechanical behavior and deformation mechanism of shape memory bulk metallic glass composites synthesized by powder metallurgy

• Powder metallurgy synthesis of bulk metallic glass composites with TRIP effect. • Quantification of confining effect of glassy phase on martensitic transformation. • Quantification of each phase to the plasticity of bulk metallic glass composites. • Mechanism for shear band initiation in shape memory bulk metallic glass composites. The synthesis of martensitic or shape-memory bulk metallic glass composites (BMGCs) via solidification of the glass-forming melts requires the meticulous selection of the chemical composition and the proper choice of the processing parameters in order to ensure that the glassy matrix coexists with the desired amount of austenitic phase. Unfortunately, a relatively limited number of such systems, where austenite and glassy matrix coexist over a wide range of compositions, is available. Here, we study the effectiveness of powder metallurgy as an alternative to solidification for the synthesis of shape memory BMGCs. Zr 48 Cu 36 Al 8 Ag 8 matrix composites with different volume fractions of Ni 50.6 Ti 49.4 are fabricated using hot pressing and their microstructure, mechanical properties and deformation mechanism are investigated employing experiments and simulations. The results demonstrate that shape-memory BMGCs with tunable microstructures and properties can be synthesized by hot pressing. The phase stability of the glass and austenitic components across a wide range of compositions allows us to examine fundamental aspects in the field of shape memory BMGCs, including the effect of the confining stress on the martensitic transformation exerted by the glassy matrix, the contribution of each phase to the plasticity and the mechanism responsible for shear band formation. The present method gives a virtually infinite choice among the possible combinations of glassy matrices and shape memory phases, expanding the range of accessible shape memory BMGCs to systems where the glassy and austenitic phases do not form simultaneously using the solidification route.

Predicting the location of shear band initiation in a metallic glass

We report atomistic simulation results which indicate that the location of shear banding in a metallic glass (MG) can be ascertained with reasonably high accuracy solely from the undeformed static structure. Correlation is observed between the location of the initiation of shear bands in a simulated MG and the initial distribution of the density of fertile sites (DFS) for stress-driven shear transformations identified a priori based on a deep learning model devised in our recent work [Fan and Ma, Nat. Commun. 12, 1506 (2021)]. In addition, we demonstrated that one can judge whether a glass is brittle or ductile solely based upon its initial DFS distribution. These validate that shear bands in MG arise from non-linear instabilities, and that the as-quenched glass structure contains inhomogeneities that influence these instabilities. This work also reveals an important subtlety regarding the non-deterministic nature of athermal quasistatic shear simulations.

Effect of Ag substitution for Ti on the deformation behaviors of in-situ Ti-based bulk metallic glass composites

Four Ti53−xZr20Nb10Cu5Be12Agx (x = 0, 1, 3 and 5 at.%) bulk metallic glass composites (BMGCs) with various size, volume fraction and morphology of in-situ crystal phase were prepared by the arc-melting method. The shear bands extensions under strain of 5% and 20% were observed to explore the contributions of in-situ crystal phase on the deformation behaviors. The results showed that addition of Ag element significantly increased the glass forming ability. When Ag addition increased from 0 to 5 at.%, the volume fraction of in-situ crystal phase decreased from 74.5% to 38.3%, the morphology transformed from the dense cluster to the spotted shape, resulting in the decrease of dendrites span length (L) and the increase of dendritic arm spacing (DAS). The size, volume fraction and morphology of in-situ crystal phase had great influence on the initiation and extensions of shear bands. In the in-situ BMGCs, the crystal phases with the appropriate size, volume fraction and morphology were conducive to the initiation of multiple shear bands and could effectively restrain the expansion of single shear band, improving the plasticity, and the in-situ BMGCs with Ag element of 3 at.% exhibited the highest plasticity of 17.9%.

The importance of lamellar architecture to obtain ductility in heavily cold-worked pearlitic steels revealed by microbending experiments

In-situ microbending experiments were conducted to understand the effect of composite architecture on the deformation behavior and ductility of heavily cold-worked pearlitic steels. Two extreme architectures were deliberately chosen for this purpose: first, a high-pressure torsion-deformed fully pearlitic steel consisting of a nanolayered lamellar arrangement of ferrite and cementite and second, an even further refined state where this lamellar arrangement is transferred to a carbon-decorated ferritic subgrain structure. Despite the enormous strength of these samples, in-situ microbending experiments reveal an astonishing homogeneous deformation capability of nanostructures – much larger than in uniaxial compression or tension. Plastic strains of up to 20% can be realized without severe failure of the specimens. This outstanding ductility seems to be linked to the inherent strain gradient of the bending beams, hindering early initiation and propagation of shear bands. While this holds true for both tested states, the architecture of the pearlitic steels plays a predominant role in this respect. In case of the lamellar architecture homogenous plastic flow can be realized up to significantly larger plastic strains, while shear band and crack formation occur already at lower plastic strains in case of the carbon-decorated subgrain structure.

Improving fatigue performance of metallic glasses with crystalline metal coating revealed by atomistic simulations

In this work, a comparative study was undertaken to investigate the effects of Cu coating on the deformation behaviour of metallic glass (MG) and crystalline Cu-coated MG under monotonic tensile loading and cyclic compressive-tensile loading by means of atomistic simulations. The results showed that under monotonic tensile loading, yielding first occurs in the Cu coating due to its low elastic limit, while plasticity in the MG core is directly triggered by the transfer of crystal plasticity from the Cu coating across the well-bonded crystalline-amorphous interfaces. The Cu coating serves as an effective medium for nucleating multiple shear bands, thus resulting in an improvement of tensile ductility. However, under cyclic loading, yielding tends to initiate via shear band activation in the interior of the Cu-coated MG. The Cu coating serves as effective barrier to suppress shear band initiation on the free surfaces, thus giving rise to enhanced fatigue life of the Cu-coated MG. The interlay bounding condition is also verified as playing a crucial role on the fatigue performance in the Cu-coated MG. The findings provide valuable insights on the effect of crystalline metal coating on the tensile and cyclic deformation mechanisms of MGs from an atomistic perspective.

In Situ Generated Shear Bands in Metallic Glass Investigated by Atomic Force and Analytical Transmission Electron Microscopy

Plastic deformation of metallic glasses performed at temperatures well below the glass transition proceeds via the formation of shear bands. In this contribution, we investigated shear bands originating from in situ tensile tests of Al88Y7Fe5 melt-spun ribbons performed under a transmission electron microscope. The observed contrasts of the shear bands were found to be related to a thickness reduction rather than to density changes. This result should alert the community of the possibility of thickness changes occurring during in situ shear band formation that may affect interpretation of shear band properties such as the local density. The observation of a spearhead-like shear front suggests a propagation front mechanism for shear band initiation here.

A new idea of modeling shear band in metallic glass based on the concept of distributed dislocation

Shear bands are closely linked with the plasticity and fracture behaviors of metallic glasses (MGs). This work proposes a new idea to predict shear bands by continuously distributed dislocations. The dislocations are unreal and used to model the plastic deformation of shear band. The possible positions of shear band initiation are determined based on the elastic stress field, and the direction and length of shear band propagation are determined by the distributed dislocation technique. Finite element simulations based on constitutive model are carried out to compare with the theoretical modeling. Two examples are considered, i.e., shear bands near a void and a notch under tensile loading. The results show that the theoretically predicted shear band morphology is well consistent with the finite element simulations, which verifies the validation of using distributed dislocations to predict shear bands. This work provides a new way to model shear band, and it has potential applications in predicting shear band morphology and fracture behaviors in MGs.

A Technique for High-Speed Microscopic Imaging of Dynamic Failure Events and Its Application to Shear Band Initiation in Polycarbonate

Abstract An experimental technique is reported, which can image the deformation fields associated with dynamic failure events at high spatial and temporal resolutions simultaneously. The technique is demonstrated at a spatial resolution of ∼1 µm and a temporal resolution of 250 ns, while maintaining a relatively large field of view (≈1.11 mm × 0.63 mm). As a demonstration, the technique is used to image the deformation field near a notch tip during initiation of a shear instability in polycarbonate. An ordered array of 10 µm diameter speckles with 20 µm pitch, and deposited on the specimen surface near the notch tip helps track evolution of the deformation field. Experimental results show that the width of the shear band (SB) in polycarbonate is approximately 75 µm near the notch tip within resolution limits of the experiments. The measurements also reveal formation of two incipient localization bands near the crack tip, one of which subsequently becomes the dominant band while the other is suppressed. Computational simulation of the experiment was conducted using a thermomechanically coupled rate-dependent constitutive model of polycarbonate to gain further insight into the experimental observations enabled by the combination of high spatial and temporal resolutions. The simulation results show reasonable agreement with the experimentally observed kinematic field and features near the notch tip, while also pointing to the need for further refinement of constitutive models that are calibrated at high strain rates (∼105/s) and also account for damage evolution.

Micromechanical Behavior of Granular Soils Characterized by Acoustic Emission

Abstract The study of failure mechanisms from a microperspective demands a comprehensive understanding of the initiation and evolution of shear banding within granular materials. However, this topic is not fully understood due to the technical constraints in continuous quantification of the failure degree and visualization of particle interactions within soil specimens due to their inherent opacity. This paper reports the possibility of acoustic emission (AE) technique in characterizing the micromechanical behavior of saturated coral sands subjected to drained triaxial shearing. Results show that coral sand is less emissive than silica sands, while its AE rate forms concrete mathematic relations with soil mechanical parameters, which agrees with those revealed for silica sands. This allows soil mechanical parameters of coral sands that are difficult to be assessed in the field to be back-evaluated by acoustic measuring. The traced AE source locations confirm their spatiotemporal correspondence with the photographed appearance of deformed specimens, in which complex shear banding process, regarding the initiation and evolution, is traced. Three acoustic precursors are observed before the complete soil failure and are considered resulting from the structural variation that affects the emission and propagation of AEs, which hence could be used as indicators for early warning of soil’s instability.

Experimental Study on the Shear Band of Methane Hydrate-Bearing Sediment

The occurrence of a shear band is often thought as the precursor of failure. To study the initiation of shear banding in gas hydrate-bearing sediments, two groups of triaxial compression tests combined with a CT (computer tomography) scan were conducted by triaxial CT-integrated equipment under two confining pressures and seven hydrate saturations. The macro stress–strain curves and the corresponding CT scanning images of the micro-structure and the distribution of the components were obtained. The geometric parameters of the shear bands were measured based on the CT images at four typical axial strains, respectively. The distribution characteristics of soil particles, water, hydrate and gas were also analyzed. It is shown that the existence of methane hydrate changes the mechanical property of hydrate-bearing sediment from plastic failure to brittle failure when the hydrate saturation is over 13%, which occurs in the range of the tests in this paper. The peak of the deviatoric stress increases with the hydrate saturation. The shear band is in either a single oblique line or inter-cross lines depending on the hydrate saturation, the effective confining pressure and the initial distribution of the gas hydrate. Most of the shear band surfaces are not straight, and the widths of the shear bands are almost non-uniformly distributed.

Shear band evolution related with thermal annealing revealing ductile-brittle transition of Zr35Ti30Be27.5Cu7.5 metallic glass under complex stress state

The ductile-brittle transition (DBT) caused by thermal annealing was systematically investigated by means of small punch test (SPT), accompanied by finite element simulation, uniaxial compression and nanoindentation in Zr-based metallic glasses (MGs) annealed for 1 h in a wide temperature range, i.e., sub-Tg, Tg ∼ Tx and above-Tx. In multiaxial loading mode, three deformation mechanisms, i.e., (ⅰ) circumferential shear bands (CSBs) + radial shear bands (RSBs), (ⅱ) RSBs and (ⅲ) radial cracks (RCs), can be found separately on MGs annealed sub-Tg, Tg ∼ Tx and above-Tx, leading to a transition in deformation mode from ductility to brittleness. First, for MGs with sub-Tg, Tg ∼ Tx annealing, the reduction in the number of CSBs and RSBs, as well as the disappearance of CSBs, are responsible for the ductile-brittle transition, which is attributed to the annihilation of free volume caused by structural relaxation and the reduction of released elastic energy. Based on simulation results, the decrease in plasticity of MGs is accompanied by the reduction in stress distribution area, which is in agreement with the change in the number and distribution area of shear bands. The initiation of multiple shear bands and their strong interaction dramatically improve the plasticity of MGs under the synergistic effect of large stress distribution area and high content of free volume. Second, the embrittlement of MGs with above-Tx annealing is mainly attributed to the rapid formation and extension of RCs induced by the occurrence of crystallization. Meeanwhile, simulation results indicate that stress distribution area decreases gradually and facilitate the formation of stress concentration, leading to the rapid extension of RCs and the occurrence of catastrophic fracture of MGs. In addition, MGs with sub-Tg, Tg ∼ Tx and above-Tx annealing not only exhibit such DBT in uniaxial compression, but also show a soft-hard transition in nanoindentation test.

Size dependence of ductile to brittle transition of Zr-based metallic glasses under multiaxial loading

The size dependence of ductile to brittle transition of Zr-based metallic glasses was systematically investigated by small punch test. It was found that the metallic glasses with thickness ranging from 50 to 700 μm displayed an obvious transition from ductility to brittleness. Compared with thick samples, thin samples exhibit higher strength and better plasticity, which is attributed to the size-dependent transformation from soft state to hard state dominated by the deformation factor ( α = τ / σ ). Specifically, two significant transitions in deformation behavior at sample thickness of 200 and 500 μm are attributed to two critical deformation factors of α c 1 and α c 2 , which are responsible for the initiation of circumferential and radial shear bands, respectively. Specifically, only several micro-cracks can be observed on sample surface with α ≤ α c 2 . While α c 1 ≥ α ≥ α c 2 , multiple radial shear bands are activated and uniformly distributed on the sample surface. Further, when α ≥ α c 1 , multiple circumferential shear bands primarily initiate under high shear stress and large number of radial and circumferential shear bands can be observed on sample surface. The initiation and interaction of multiple radial and circumferential shear bands significantly enhance the ductility of the samples with α ≥ α c 1 .

Development of Zr-based metallic glass matrix composites with hybrid reinforcing structures

The hybrid reinforcing structures, which contain Ta-rich particles, B2–CuZr and B19′-CuZr phases, have been successfully introduced into a Zr-based metallic glass matrix by doping with Ta. Due to the high melting temperature of Ta, the Ta-rich particles precipitate firstly, and contribute to the homogeneous distribution of B2 and B19′-CuZr phases by acting as their nuclei to form a core-shell reinforcing structure. The plasticity of the present hybrid structures reinforced metallic glass matrix composites have been improved a lot when compared with the monolithic counterparts. The rapid propagation of main shear bands in the matrix is considered to be hindered by the presence of the hybrid reinforcing structures. The martensitic transformation of B2–CuZr phases and the deformation of the Ta-rich particles cooperatively contribute to the plasticity and work-hardening behavior of the composites. The simulation results suggest that the core phase (Ta-rich particle) bears the highest strain and triggers the initiation of shear bands.

Shear fracture in bulk metallic glass composites

Fracture behavior of bulk metallic glass matrix composites (BMGCs) with both transforming and non-transforming β-Ti dendrites under shear and opening modes was examined. Experimental results show that the fracture toughness of all three BMGCs is considerably lower in mode II than in mode I, due primarily to the shear dominant stress state in the former, which renders easy shear band initiation. However, stable crack growth in mode I is insignificant whereas it is considerable in mode II. The toughness of BMGCs reinforced with coarse but non-transforming dendrites in both the modes is higher than the respective values in BMGCs with transforming β-Ti inspite of their ability to strain harden and hence enhanced ductility. Fracture surface features and shear band patterns at notch tips of both mode I and mode II specimens indicate that despite the interactions of dendrites with the shear bands, the fracture criterion and mechanism of BMGCs is identical to that in BMGs. The differences in the fracture behaviour of these BMGCs is rationalized by considering the effect of relaxation enthalpy of the amorphous matrices in addition to the length scale and transforming tendency of the dendrites on the transformation of a shear band into a crack. Implications of these results in terms improving the fracture toughness of BMGCs with transforming dendrites is discussed.

Preparation and mechanical properties of tungsten-particle-reinforced Zr-based bulk-metallic-glass composites

Abstract Tungsten-particle (Wp)-reinforced bulk-metallic-glass composites (Wp/BMGCs) with a Wp volume fraction of 0%–50% were prepared by spark plasma sintering (SPS). The sintering process parameters were optimized through an orthogonal experiment. With increasing volume fraction of Wp, the compressive yield strength of the Wp/BMGCs decreased, while their ultimate strength and plastic strain increased. Nanoindentation experiments revealed that the elastic modulus and hardness of the composites increased with increasing volume fraction of Wp. The elastic modulus mismatch between the tungsten particles and metallic glass matrix resulted in a stress concentration at the interface of the two phases, promoting the initiation and propagation of shear bands in the metallic glass matrix. Fracture morphology analysis of the composites showed that the failure mode of the composites was shear fracture mode, which is beneficial to the self-sharpening of the composites. With increasing volume fraction of Wp, the plastic deformation of Wp became more obvious, and the deformation was most obvious near the shear fracture surface. The degree of plastic deformation of Wp in the region far away from the shear fracture surface gradually decreased. In addition, the reliability of the mechanical properties of Wp/BMGCs sintered in the same preparation parameters was verified by three-parameter Weibull statistical analysis. These results help deepen our understanding of the preparation and mechanical properties of Wp/BMGCs and further promote their application.

Effect of strain rates on the plastic deformation behavior and serrated flow of Zr55.7Cu22.4Ni7.2Al14.7 bulk metallic glass

The effects of strain rates on the plastic deformation behavior and serrated flow of Zr55.7Cu22.4Ni7.2Al14.7 bulk metallic glass (BMG) were studied by compression test at room temperature. The results show that the plastic deformation behavior of metallic glass at room temperature is highly sensitive to strain rates. The plastic strain, yield strength, compressive strength and fracture strength were found to increase with decreasing of strain rates. For example, the plasticity and the compressive strength increased from 0.4 ± 0.1%–8.3 ± 0.4 %, and from 1684 ± 11 MPa to 2000 ± 23 MPa, respectively. In addition, the number and density of shear bands on the fracture side also increased gradually. The fracture morphology, which is mainly composed of vein-like patterns, can be understood by the fact that the number and density of vein-like patterns increase with decreasing of strain rates. However, the mechanism of serrated flow was analyzed and discussed from the perspective of the initiation, expansion, and interaction of multiple shear bands correspondingly. With decreasing strain rates, the uniformity of the serrated flow in the first stage is not sensitive to the change of strain rates, while the uniformity of the second stage serrated flow changes. The change of serrated flow from near uniformity to non-uniformity in the second stage is attributed to the interaction between multiple shear bands. In comparison, with decreasing strain rates, the stress drops in the second stage first decreases and then increases due to the interaction of shear bands. The plastic deformation behavior and serrated rheology of bulk metallic glasses (BMGs) at room temperature, which lay a foundation for promoting the engineering application of BMGs, were discussed in details in this study.

Dependence of ductile-brittle transition related with serrated flow on viscosity dominated by glass forming ability in ZrxTi65-xBe27.5Cu7.5 metallic glasses

The transition in deformation mode from ductile to brittle related with glass forming ability (GFA) was systematically investigated via uniaxial compression tests in Zr x Ti 65-x Be 27.5 Cu 7.5 metallic glasses (MGs). As the GFA increases, both the number and amplitude of serrated flow decrease gradually in stress-strain curves. There are two main reasons for the reduction in viscosity during shear deformation. First, the larger elastic energy is mainly dissipated in the form of heat and the temperature rises within shear bands beyond the melting points. Second, the MGs with better GFA contain larger excess quenched-in free volume due to rapid solidification. Based on the Cohen-Grest model, large free volume can effectively decrease the atomic packing density and reinforce the mobility of atoms, greatly reducing the viscosity during shear deformation. Hence, for the reduction of the viscosity during deformation, the coupling effect of temperature and free volume matters. Multiple shear bands are activated by applied stress and interact strongly with each other, significantly improving the plasticity. The initiation of multiple shear bands is an indicative of much serrated events in stress-strain curves. Thus, the change in viscosity associated with GFA dominates the occurrence of serrated flow, facilitating the transition between ductile and brittle deformation.

Links between surface morphology changes and damage in a toughened epoxy adhesive

ABSTRACT With the increased use of toughened epoxy adhesives in current transportation lightweighting efforts, it is critical that damage mechanisms, such as strain whitening, are understood and quantified. Damage quantification is needed for the constitutive models used in structural design; however, thin bond lines in adhesive joints limit direct observation. In this study, microscope observations of bulk toughened epoxy adhesive specimens subjected to tensile loading were linked to damage. Cracks on the surface opened during loading, leading to strain whitening at the crack tips and the initiation and propagation of shear bands. The stresses approximated at the crack tips suggested that particle cavitation could be occurring in these regions. Changes in specimen stiffness were linked to crack growth and the formation of shear bands. Material damage calculated using traditional load-unload stiffness (D ~ 35%) was higher than other methods such as change in material strength (D ~ 18%) and damage from changes in stiffness during load-reload (D ~ 19%). The differences were attributed to short-term viscoelastic effects. A new approach calculated damage from the strain whitening on the free surface (D ~ 21%). Values were in agreement with damage figures from other methods. The technique can quantify damage over the loading history and identify areas of damage localization.