Shear Band Formation Research Articles

Effect of fines loss on the microstructure and shear behaviors of gap-graded soils: A multiscale perspective

Natural sand is typically composed of both coarse and fine particles and are susceptible to suffusion erosion under seepage effect. Fines loss caused by suffusion is a critical factor affecting the mechanical and hydraulic properties of soils. This paper investigates the effect of fines loss on the microstructure and shear behaviors of gap-graded soils using a three-dimensional finite element and discrete element coupling method. A modified particle removal scheme based on the internal moment tensor is first presented, which is utilized to generate several specimens with varying degrees of fines loss. The macroscopic shear responses of both eroded and non-eroded specimens are benchmarked using the responses of representative volume elements (RVEs) in pure discrete element method (DEM). The results reveal that the eroded specimen exhibits a reduced peak stress ratio yet almost unchanged critical state values compared to the non-eroded specimen. Fines loss restrains the formation of shear band and results in more uniform stress and strain distribution inside the specimens. In addition, fine particles in gap-graded soils only sustain a small proportion of external loads, particularly for the specimens with more fines loss. The RVEs at different locations of the specimens are found to exhibit distinct packing assemblies and force networks at the end of shearing.

Introducing a new approach for designing advanced epoxy film adhesives with high mechanical, adhesion, and thermal properties by adding hybrid additives for structural bonding

BackgroundEpoxy-based adhesives are widely used in different industries in various forms. Among them, epoxy film adhesives are particularly applicable in constructing honeycomb structures in aerospace systems. However, these adhesives suffer from brittleness, low flexibility, and thermal stability. In the current paper, the effect of pure and functionalized SiO2 and ZrO2 nanoparticles (NPs), the addition of a block copolymer (butyl acrylate-block-styrene), and Novolac resin on the mechanical, thermal, and adhesive properties of the epoxy film adhesive were thoroughly studied. MethodsThe mechanical properties, thermal stability, and surface morphology of the epoxy adhesive samples were evaluated using tensile, thermogravimetric analysis (TGA), and field emission scanning electron microscopy (FE-SEM) images, respectively. More importantly, the Lap shear and Peel tests were utilized to investigate the adhesive properties. Significant findingsPure and functionalized NPs were added to the prepared epoxy adhesive samples. Intriguingly, compared to the non-functionalized NPs, by only adding 1 phr of each functionalized NPs to the formulation, the tensile modulus, tensile strength, and toughness increased to 66 %, 197 %, and 553 %, respectively. More importantly, the single lab shear and T-peel strength were increased as high as 121 % and 101 %, respectively, indicating a synergistic effect. After optimizing the parameters effective on the properties, FE-SEM images of the fractured surface of optimized samples were used to study the possible fracture mechanisms, including crack deflection, crack pinning, nanoparticle debonding, and shear band formation. Also, this synergistic effect was observed in the thermal stability where the only addition of 1 phr of functionalized NPs increased the decomposition onset temperature up to 40 ℃ higher than the neat one. This simple and applicable method can pave the way for using functionalized NPs in modifying epoxy adhesives.

Susceptibility of adiabatic shear band formation in AZ31B magnesium alloy during high strain rate impact

Susceptibility of adiabatic shear band formation in AZ31B magnesium alloy during high strain rate impact

Deformation behavior and microstructure evolution induced by nano-sized Al6Mn phase particles in a homogenized Al–6Mg–0.8Mn alloy during hot compression

In the present work, hot compression deformation behavior and microstructure evolution in a homogenized Al–6Mg–0.8Mn alloy with nano-sized Al6Mn phase particles were investigated at the temperature ranging from 300 to 500 °C with tha strain rate of 0.001–1 s−1. The constitutive equation of the Al–6Mg–0.8Mn alloy was established and the average deformation activation energy was calculated to be 167.07 kJ/mol. The flow stress of the alloy decreased with increasing the compression temperature and raised with increasing the strain rate, which was closely associated with the competitive relationship between work hardening and dynamic recovery (DRV). The low compression temperature accelerated the formation of shear bands in the alloy. The low strain rate was beneficial for the generation of low-angle grain boundaries (LAGBs), and the high compression temperature and strain rate promoted dynamic recrystallization (DRX). The interaction between the nano-sized Al6Mn phase particles participating near the initial grain boundaries and mobile dislocations facilitated the generation of LAGBs. Discontinuous DRX through grain boundary bulging mechanisms and continuous DRX with the LAGBs transforming to HAGBs led to the preferred formation of recrystallized grains along the initial large-sized grains. Furthermore, this interaction also resulted in the dissolution of Al6Mn phase particles.

Fractal analysis on the uniformity between the shear bands and serrated flows of a Zr-based bulk metallic glass

It is challenging to unravel the relationship between the serrated flows and complex shear bands of bulk metallic glasses (BMGs) when they are evolving. In this work, fractal analyses of the one-dimensional load-drop signals in time domain and the two-dimensional shear-band patterns were conducted at different strain stages. With the increase of strains, the fractal dimensions of both shear-band patterns and serrated flows show similar continuous increasing trends. From the perspective of chaos degree, a positive correlation between the load-drop distributions and shear-band patterns was obtained. The Hurst exponents less than 0.5 show that the distribution of load drops has a strong negative correlation behavior and there is a negative feedback mechanism in the development of shear bands. Moreover, at different strain stages, the fractal dimensions and Weibull moduli have indicated that the load drops demonstrate a relatively-high uniformity initially and then stabilize at a low uniformity, which is related to the regular shear-band patterns with fractal structure. The findings provide further understandings between the formation of shear bands and serrated flows of BMGs, which also open a new window to uncover the plastic deformation mechanisms of BMGs using fractal analysis.

“Implicit” vs “Explicit” gradient plasticity models: Do they always remove mesh dependence in softening materials?

Finite element quasi-static solutions of rate-independent softening plasticity models are known to depend on the mesh size (i.e., are unreliable) due to loss of ellipticity of the governing partial differential equations, which allows for the development of non-smooth solutions, such as shear bands of zero thickness. A “regularization” scheme that has been proposed is the use of “non-local” (Bažant et al., 1984) or “strain-gradient” theories (Aifantis, 1984). We examine in detail the families of “explicit” and “implicit” gradient plasticity models and show that use of such theories does not always eliminate mesh dependence of finite element solutions in rate-independent softening materials. Depending on the value of the non-local hardening modulus, the mathematical problem may still lose ellipticity and allow for the development of solutions with discontinuous spatial velocity gradients, which are responsible for the mesh dependence of numerical solutions. As an example, a rate-independent “implicit” gradient version of the von Mises yield condition with the associated flow rule is implemented in ABAQUS, and the formation of shear bands in plane strain tension is analyzed numerically; it is shown that, unless the material parameters in the non-local model are such that ellipticity is retained, the finite element solutions depend on the mesh size. The example of isotropic porous metal plasticity with a hardening matrix material is considered (e.g., the Gurson (1977) model); it is shown that regularization is achieved if an implicit non-local model is used, in which the local porosity f is replaced by its non-local (neighborhood average) counterpart f̄ in the yield condition. In all non-local plasticity models, dimensional consistency requires the introduction of one or more “material lengths” ℓi, which multiply the additional terms with the higher-order spatial derivatives introduced by the non-local models. The boundary layers associated with the singular perturbation problem when the material lengths ℓi→0 are examined. A linear stability analysis is carried out to study the stability of homogeneous problems under small harmonic perturbations; it is shown that the stability range of the non-local models is always larger than that of the corresponding local models.

Plasticity in cyclic indentation of a Cu-Zr-based bulk metallic glass after tensile loading: An experimental and molecular dynamics simulation study

In this study, we combined experiments and molecular dynamics simulations to investigate the deformation of metallic glasses subjected to previous tensile deformation. The experimental investigations were conducted on a Zr67Cu10.6Ni9.8Ti8.8Be3.8 (wt%) sample and the molecular dynamics simulations were performed on a Cu64.5Zr35.5 sample. Initially, plasticity was induced in the specimens by tensile deformation through the formation of shear bands, and we studied the further evolution of plasticity using cyclic indentation. We found that the macroscopic properties measured in experiments, such as hardness and hysteresis width, qualitatively agree with simulations. We use the molecular dynamics simulations to analyze the microscopic properties resulting from cyclic indentation and determine the polyhedral packing structures (‘motifs’) after tensile deformation in regions outside and around the shear bands. We then followed their evolution after each indentation cycle. Our findings indicate that areas outside the shear bands experience higher shear strain during cyclic indentation and that all motifs increased in fraction but evolved differently inside and outside the shear bands. The first indentation leads to the strongest change in microstructure, affecting both the material within and outside of shear bands. Repeated indentation mostly acts on the shear bands. These changes lead to a stabilization of the material in the sense that motifs with high coordination number and hence bond formation are established.

Micro-Segregation induced strain inhomogeneity in >900 MPa UTS martensitic hot rolled advanced high strength steel

High strength martensitic strip steels used in the yellow goods industry require good ductility and high strength. This paper examines how local compositional inhomogeneity of alloying elements (Mn, Mo, and Cr), caused by interdendritic microsegregation during casting and subsequent deformation, affect local straining behaviour in a >900 MPa autotempered martensite hot rolled strip steel. The alloy rich and alloy depleted bands have been characterised using micro-XRF and SEM-EDX through thickness for a 10 mm thick strip. Tensile samples were selectively machined from areas with a range of microsegregation severity in the longitudinal and transverse orientations. Digital Image Correlation (DIC) during tensile testing was used to examine local straining behaviour and shear band formation. Differential straining of up to 2.5x between solute enriched (lower strain) and solute depleted (high strain) bands was found in the strongly microsegregated transverse samples due to the lower hardness of the solute depleted bands (reduced solid solution strengthening). The high local strain in solute depleted bands in the strongly segregated sample led to premature failure (at lower global applied strain) compared to the less segregated samples. No significant strain partitioning was observed for the longitudinal oriented samples regardless of segregation level.

In-situ compression with acoustic emission detection of high pressure torsion deformed Cu38Zr54Al8 metallic glass micropillars

In-situ compression of Cu38Zr54Al8 metallic glass micropillars in a scanning electron microscope was performed together with continuous acoustic emission recording on samples in the as-cast state and after pre-deformation by high pressure torsion. A size effect was detected in both shear band operation and acoustic emission signal, irrespective of the preliminary deformation. This phenomenon was explained by the size dependent compliance of the pillar/indenter system. Differences between acoustic emission signals from the as-cast and pre-deformed states indicated changes in the shear band formation mechanism due to high pressure torsion. These differences were clear in the elastic regime of the in-situ compression and supported the role of rejuvenation in stabilization of unstable shear bands.

Influence of Deformation of Metal Matrix on the Mechanical Properties of Poly(Ethylene Terephthalate) Inside It

The influence of different types of deformation of the metal matrix on mechanical behavior of the amorphous PET film upon its transverse compression in the ductile metal matrix has been investigated. Three deformation modes have been probed. In the first case, a disc-shaped polymer specimen has been put between two 5 mm thick discs of the lead–tin alloy and squeezed in the press. In the second and the third cases, planar elongation has been performed in the so called “dead channel”, i.e., the channel with fixed side walls, the film being elongated due to decrease in the width or thickness, respectively. The plots of true yield stress at different draw ratios have formed a common master curve. At high total draw ratio Λ, the true yield stresses have been close for the considered three types of drawing in the metal. At the draw ratio Λ 2.6, the neck has not appeared, and the specimen has been deformed uniformly. When the film in the channel is elongated due to the decrease in thickness at constant width, the specimen width has been mainly decreased during subsequent elongation in the testing machine. When the film in the channel is elongated due to the decrease in width at constant thickness, further elongation has mainly led to the decrease in the specimen thickness. The true stress Σ has been expressed as Σ = Σ0 + KΛ3, with K being a constant. Deformation of the polymer in the channel occurred with the formation of shear bands. At the preliminary draw ratio Λ = 1.82, the bands have been oriented at the angle 21.5° with respect to the stretching axis. The planar elongation has led to abnormally strong deformation softening of the polymer. The drawing has been accompanied by an increase in the elasticity modulus of the polymer. The obtained results have suggested that the macromolecules orientation is the main reason for strain hardening of the polymer.

Mechanisms of Shear Band Formation in Heterogeneous Materials Under Compression: The Role of Pre‐Existing Mechanical Flaws

AbstractShear bands critically govern the shear failure processes and associated geophysical phenomena, for example, faulting, in Earth's crust. Earlier studies on homogeneous materials recognized temperature and strain rate as principal factors controlling the band growth. However, how inherent mechanical heterogeneities can influence their growth mechanisms and complex internal structures needs further investigations. The present article revisits this problem by combining field observations and laboratory experiments, supported by numerical simulations considering geological strain‐rates. Field studies in the Chotanagpur Granite Gneissic Complex of eastern India reveal varied types of macro‐scale shear zone localization. We conducted plane‐strain compression experiments on rock‐analogue models to investigate their origin. The experiments show that mechanical heterogeneities develop wide bands, localized preferentially in their neighborhood, unlike uniformly distributed, conjugate sets of closely spaced narrow bands in a homogeneous system. The wide bands eventually attain a composite structure, containing a core of densely packed band‐parallel sharp secondary bands, flanked by linear zones of closely spaced, narrow bands. We also demonstrate the effects of global strain‐rate on the band evolution in heterogeneous systems. Decreasing strain‐rates replace the composite bands by well‐defined homogeneous shear bands, containing a core of uniform shear, bordered by weakly sheared narrow zones, grading into almost undeformed walls. The experimental results are complemented with numerical models based on visco‐elasto‐plastic rheology, which establish appropriate scaling of the rock analogue to mid‐crustal deformations. This integrated approach finally leads us to conclude that inherent heterogeneities can result in large variations of shear band structures in geological terrains.

Effect of processing route on ballistic performance of Ti-6Al-4V armour plate

Titanium alloys are a key part of an armour designer’s toolset due to their high ballistic mass efficiency. However, titanium’s high cost has restricted its use in military land-based vehicles. In this study, the effect of different processing routes on the microstructure, texture, ballistic performance V50 and deformation modes of Ti-6Al-4V armour plate is investigated. Plates were fabricated via conventional hot rolling and the powder metallurgy techniques of HIP and FAST. The plate failure modes varied significantly with the microstructure while the formation of adiabatic shear bands seems to be influenced by crystallographic texture. The rolled plate exhibited the best performance due to a strong transverse-type texture perpendicular to the projectile direction.

Structural mechanisms of enhanced mechanical property in ZrCu metallic glass at low temperatures

The application of traditional crystalline materials as structural materials cannot meet critical design challenges due to low-temperature brittleness. To address this issue, this study investigates the deformation behavior of Zr2Cu metallic glass (MG) model at ambient and low temperature states by molecular dynamics (MD) simulations to unravel the governing mechanisms associated with mechanical performance. Structurally representative amorphous models of the MG were developed and their evolutions under tensile loadings at ambient (300 K) and low temperatures (250 K, 200 K, 150 K, 100 K, 50 K) were captured. Notably, significant variations in maximum stress and corresponding strains characterized the stress-strain behavior as elastic-plastic properties increased with decreasing temperatures. Stress activations of the atomic mass were rapidly incipient at the ambient condition, but largely constrained at low temperatures by which the density variations of activated zones and the emerging strain of first shear transformation zones (STZ's) established a negative correlation with temperature as inferred by the wider strain range of deformation. Disparities in strain thresholds accounted for the higher free volume content at the ambient state, but as temperatures declined, fine structural differences stimulated the formation of densely-packed structures in the amorphous model at low temperatures by the spontaneous annihilation of excessive free volumes. Consequently, the higher growth rates of the average atomic potential energy and the retarded emergence of high-potential-energy atomic mass suggest that at low temperatures, there is a higher resistance to shear band nucleation and higher barrier for atomic migration. As a result, the generation of free volumes, formation of STZs and shear bands were retarded, leading to the enhanced ductility. Thus, at low temperatures, unique tuning mechanisms of structural performance were evolved in the amorphous model that suitably enhanced the constitutive properties of Zr2Cu MG.

Numerical studies of self-organization of shear bands in one and two dimensions

In the present manuscript we perform a numerical analysis of the self-organization processes of adiabatic shear bands formation in depleted uranium, aluminum alloy, and high-strength steel in one- and two-dimensional cases. Since the processes of shear bands formation are strongly nonlinear, three materials with significantly different parameters considered allow us to generalize obtained dependencies. For all materials, we investigate the evolution of stress, temperature and velocity fields from initial to final stage of localization. It is well known that localization processes occur during the high shear rate loads in the presence of initial microstructural defects in metallic materials. Starting with a random distribution of initial stress, which models microstructural defects, we obtained new dependencies for such significant parameters of the problem considered such as localization time and spacing between shear bands. The influence of initial plastic strain rate on these parameters was also studied. We obtain the linear dependence of localization time on initial strain rate and show that spatial dimension significantly influences on its value. Also, we introduce the method of shear bands selection during computations and present new statistical distributions for band spacing. We show that the interaction between shear bands significant influences on the spacing between them.

Time-Varying Tool-Chip Contact in the Cutting Mechanics of Shear Localization

Abstract Shear localization is the dominant chip formation mechanism in machining of high performance metallic components, such as those made of titanium and nickel-based alloys. This paper presents an analytical thermo-mechanical model considering a new tool-chip contact mechanism due to shear localization. First, it is experimentally shown that the sticking and sliding contact lengths fluctuate with the frequency of shear localization. Second, a cutting mechanics model is developed considering the shear band formation, its rolling on the tool’s rake face, and the time-varying tool-chip contact length with experimental validation. Finally, the transient temperature at the tool-chip interface is predicted by taking the rolling phenomenon and the time-varying heat sources at the tool-chip interface into account. The proposed model shows that at the beginning of each segmentation cycle, the entire tool-chip contact length is dominated by sliding condition with negligible sticking length. When the tool advances, new workpiece material piles up in its front with an increase in the sticking length. Meanwhile, the sliding length decreases due to the drop in the load-bearing capacity of the shear band. When enough material piles up in front of the tool, a new shear band forms, and the entire contact length returns to the sliding condition. This process repeats each time a shear band occurs, causing the cyclic formation of shear bands and time-varying nature of the tool-chip contact length, therefore influencing the temperature and stress evolution at the tool-chip interface.

Revisiting the hot adiabatic shear band paradigm

• The reason for temperature rise in shear bands was studied using SCS 0 specimen made of Ti6Al4V • Coupled SHPB experiments with high speed infrared and optical cameras were carried out • Upon shear band formation only a moderate temperature rise was detected • The high temperature rise in shear band was caused by the friction between sliding crack flanks.

Investigation of the twin-induced adiabatic shear bands evolution in a Mg-Al-Mn alloy under ballistic impact

The formation and evolution of twin-induced adiabatic shear bands (ASBs) in a Mg-Al-Mn alloy under ballistic impact were investigated by analyzing the microstructures at three representative locations with different distances from the crater rim. Results show that localized shear deformation occurred at the twin boundaries, resulting in the formation of ASBs. The ASBs thicken at the expense of the deformation twins with increasing plastic deformation, forming ASBs which are mainly composed of subgrains, ultra-fine recrystallized grains and Mg17Al12 precipitates. The appearance of the twin-induced ASBs is mainly attributed to the twin-twin intersections caused by high strain rate deformation. The twin-twin intersections under high strain rate deformation lead to a different twinning activity in which the subsequent plastic deformation mainly occurs by shear deformation along the twin boundaries rather than thickening the twins. These results and analysis can shed new light on understanding the nucleation and evolution of ASBs in Mg alloys under dynamic loadings.

Unexpectedly formed strong basal texture in a rolled Mg-Zn-Ca-Mn alloy sheet

The reheating at high temperature, which contributed to the significant texture weakening in rolled Mg alloys, was used to control the textural features of a Mg-Zn-Ca-Mn alloy sheet. The grain coarsening was apparent when the alloy was reheated at high temperature of 450 °C, facilitating the formation of double twinning and shear bands during rolling. Double twinning and shear bands generally cause the weakening of basal texture after post-annealing; however, the basal texture became strong in the Mg-Zn-Ca-Mn alloy sheet rolled with reheating at 450 °C. The electron backscattered diffraction revealed that the non-uniformly distributed dislocations caused such unexpected textural development. In contrast, when the reheating was performed at 350 °C, the alloy formed uniform deformation-microstructure after rolling, contributing to the apparent texture weakening by the static recrystallization at double twinning during post-annealing.

Temperature-dependent deformation and fracture properties of low-carbon martensitic steel in different stress states

The temperature-dependent ductile-brittle transition (DBT) in low-carbon martensitic steel is investigated in different stress states. To capture the effects of the stress state, tensile tests have been performed with flat specimens and three-point bending tests have been conducted with Charpy specimens at dynamic (impact) and quasi-static loading rates, covering a broad temperature range of 77 K–423 K. The results reveal that ductile-brittle transition temperature (DBTT) is significantly affected by stress triaxiality, wherein the transition temperature in uniaxial tension is ∼142 K lower than that in plane strain condition of Charpy specimens. The energy absorption capacity in the ductile resistance domain during three-point bending tests is sensitive to the loading rate. Moreover, dynamic strain aging poses a decrease in ductile resistance upon quasi-static loading when the temperature exceeds 323 K and thus without a specific upper shelf plateau. Tensile strength and ductility tend to increase with decreasing temperature, and the associated failure mode changes from shear-dominated to nearly pure tension due to the specific discrepancy of temperature influence on the critical normal and shear strengths, wherein the formation of shear bands that dominate the tensile deformation could be strictly inhibited by the formation of nanoscale dislocation cells based on dislocation analysis. The failure mechanisms in tensile tests correlate with the tensile specimens' macroscopic fracture angle. In the end, a new local parameter is proposed for temperature-dependent toughness estimation in tensile tests.

Unveiling the effect of interface on torsional behavior of crystalline Al-Al90Sm10 metallic glass nanolaminates

ABSTRACT Influence of configurational design of single crystal Al-Al90Sm10 metallic glass nanolaminates on torsion deformation behaviour of Al/Al90Sm10 nanolaminate (Configuration 1) and Al90Sm10/Al nanolaminate (Configuration 2) from a structural evolution aspect have been analysed by employing Molecular Dynamics for a torsion speed of 1/600 revolution/ps. Adaptive common neighbour (a-CNA) analysis, Dislocation extraction algorithm (DXA), atomic shear strain analysis, and Voronoi Polyhedral (VP) analysis have been carried out to reveal the structural evolution in the nanolaminates specimen subjected to torque. As a consequence of dislocation density localisation under torsional loading in Al/Al90Sm10 nanolaminate high atomic strain gradient is developed in the nanolaminate specimen causing torsional buckling of the Al/Al90Sm10 nanolaminate. The localisation of dislocation density rings induces the formation of dislocation substructure in Al/Al90Sm10 nanolaminate. The crystalline/amorphous interface serves as a free surface and encourages the formation of such dislocation substructure. The collective nucleation, coalescence, and growth of shear transformation zones (STZs) leading to the formation of thick shear bands on either end of Al90Sm10/Al nanolaminate inducing an almost homogenous atomic strain gradient across the surface of the nanolaminate specimen thereby averting torsional buckling. The C/A interface serves as a nucleation site for the generation STZs in Al90Sm10/Al nanolaminate. VPs such as <0, 0, 4, 6>, <0, 3, 6, 4>, <0, 3, 6, 5> <0, 2, 8, 2> have the load bearing capacity and are resistant to fragmentation under the subjugation of torsion loading.