Shear Band Width Research Articles

Improved technical guide from physical model tests for TDR landslide monitoring

Time-domain reflectometry (TDR) can help observe the initiation and evolution of localized shear planes in both rock and soil slopes, and has temporal and spatial resolutions suitable for landslide monitoring. An improved and standardized technical guide for TDR cable installation and data interpretation is needed to utilize it better. TDR technique involves sending an electromagnetic pulse into a coaxial cable grouted in a pre-drilled borehole, and capturing the reflected signal from cable deformity, which is triggered by the localized shear deformation in the underground. This study developed a large direct shear box for testing TDR responses in various cable-grout-ground assemblies, in order to reliably model the cable-grout-ground interaction at the localized shear. Different combinations of cable type, grout condition, soil type, and shear bandwidth were tested using the new physical model to re-examine TDR's response to ground deformation in sliding mode. The results revealed some misconceptions in previous studies that used cable-grout assemblies without considering the entire interaction with the ground. The implications of the experimental results are carefully considered and new technical recommendations are provided for cable installation and data interpretation to foster the improved use of TDR. A new data reduction method that involves data filtering, differential waveform, and three-sigma rule is proposed to support the recommended installation setup for robust early detection of a localized shear plane.

Analytical Modeling of Shear Localization in Orthogonal Cutting Processes

AbstractThis paper presents an analytical thermo-mechanical model of shear localization and shear band formation in orthogonal cutting of high-strength metallic alloys. The deformation process of the workpiece material includes three stages: homogeneous deformation, shear localization, and chip segmentation. A boundary layer analysis is used to analytically predict the temperature, stress, and strain rate variations in the primary shear zone associated with the shear localization. The predictions of shear band spacing and width from the proposed model are verified by experimental characterization of the chip morphology. The rolling of shear bands on the tool rake face is discussed from the experimental observations. The cutting tool temperature, which is influenced by the heat generated during the shear band formation, is simulated and compared with finite element simulations. The proposed analytical model reveals the fundamental mechanism of the complete shear localization process in orthogonal cutting and predicts the stress and temperature variations with high computational efficiency.

A Comparison of in- and ex-situ Shear Bands in Metallic Glass by Transmission Electron Microsopy

Plastic deformation of metallic glasses performed at temperatures well below the glass transition proceeds via the formation of shear bands. In this contribution shear bands originating from in situ tensile tests of Al88Y7Fe5 melt-spun ribbons conducted in a transmission electron microscope are compared with ones which had formed ex situ during cold rolling. The observed contrasts for shear bands generated in situ at the edges of thin electron-transparent foils are shown to be related to a meniscus-like foil thickness reduction. In comparison with ex situ samples, alternating contrast changes due to volume changes are missing and the shear band width is a factor of 15 larger. The meniscus-like shear band profile is accounted for by the thin foil allowing volume increase due to shear deformation to annihilate via the free surfaces of the thin foil and the sheared zones to widen.

Finite element analysis of deformation and failure mechanisms in nanoscale hexagonal cellular structures of metallic glasses

Cellular metallic glasses (MGs) have been found to be a potential candidate for structural and functional applications due to their attractive properties such as high strength to weight ratio, excellent energy absorption and enhanced plastic deformation. The experiments have shown transition in deformation mode from global failure caused by localization in a shear band to the local failure by damage confined to few cells with reduction in relative density of specimen from a large to moderate value. The mode of deformation again changes over to the collective buckling of ligaments through row by row collapse when the relative density is decreased to a sufficiently lower level. The atomistic simulations on nanoscale cellular MGs have also reported transition from localized but confined to few cells to almost homogeneous deformation with increasing cell size. They have also shown strain localization in a dominant shear band for cell spacing along diagonal direction above a threshold value which was correlated to shear bandwidth in monolithic MG. However, it is not clear as to why and how the shear band thickness in monolithic MG controls the threshold cell-spacing. Therefore, 2D plane strain finite element (FE) simulations of compressive loading are performed on nanoscale cellular MGs using thermodynamically consistent finite strain non-local plasticity model. The present FE simulations successfully predict the two transitions in deformation mode as observed in MD simulations and experiments. It is found that the interaction stress associated with the flow defects such as shear transformation zones (STZs) plays an important role in the deformation response of cellular MGs. Results show that the transition in deformation behavior is governed by the ratio of cell-wall thickness to the intrinsic material length associated with interaction stress. Also, the moderate change in sample size has marginal effect on the deformation response of MG cellular structure. The present work may provide guidelines for designing cellular MG structures capable of showing enhanced plastic deformation for practical engineering applications.

Shear banding behavior of AA2099 Al-Li alloy in asymmetrical rolling and its effect on recrystallization in subsequent annealing

AA2099 Al-Cu-Li alloys were rolled by both asymmetrical and conventional rolling to a total thickness reduction of 80% at room temperature. Microstructure and texture evolution through thickness of rolling sheets were comprehensively investigated. Results show that large shear stress in ASR sample gave rise to a distortion of grain structure and a remarkable work hardening at region close to fast rolled surface. Shear banding behavior was largely promoted by ASR and various types of shear bands with higher density were observed. The shear bands triggered a kink of grain structure during rolling process. The kink behavior was related to the tilt angle and width of shear bands. The shear bands exhibited large tilt angle and width gave rise to a more significant kink of the grain structure nearby. Recrystallization during subsequent annealing was also promoted by ASR. Shear bands became favorable nucleation sites for rolled material. Growth advantage of some nuclei inside shear bands had a stronger influence on the recrystallized texture in AA2099 rolled sheets.

Phase-field modeling of spontaneous shear bands in collapsing thick-walled cylinders

In this work a physical mechanism-based phase-field model for adiabatic shear bands (ASBs) in collapsing thick-walled cylinders (TWCs) is addressed within the framework of the unified phase-field theory for damage and fracture (Wu, 2017; Wu and Nguyen 2018). Owing to the properly constructed energetic degradation function and crack geometric function, this model is capable of characterizing the three-stage failure process typically for ASBs. After the theoretic aspect is introduced, the model is extensively verified by two representative benchmark tests of TWCs, i.e., a smaller specimen under electromagnetic loading and a larger one subjected to explosive loading. It is found that the proposed model reproduces the experimental results fairly well with calibrated model parameters. In particular, evolution of multiple shear bands and morphological characteristics unique to the failure of TWCs, e.g., intersections and counterchecks, etc., are well captured. Compared to previous works based on local damage models, the proposed model is objective with respect to mesh discretization -- a challenge issue been rarely achieved so far in the modeling of ASBs in TWCs. Moreover, the predicted shear band width is also consistent with analytical results, making it possible to calibrate the phase-field length scale provided test data are available.

Strain localization of a frozen sand under different test conditions

In this paper, a series of plane strain tests were conducted on a coarse frozen sand under different temperatures and loading conditions of constant strain rate and stress. With the overall strain fields obtained with particle image velocimetry (PIV) technique, the influence of loading conditions, temperature and strain rate on the strain localization characteristics of the frozen sand was analyzed. It is indicated that, under the same loading conditions, the temperature and strain rate have little influence on the shear band formation process. Both of the single and X shape shear band are observed under different test conditions. The X shape shear bands are more likely to be generated under strain rate lower than the magnitude of 10−3 min−1. When the sand samples are at failure, most part of the strain field is at dilatancy state. The shear band inclination angle and width are inversely proportional to the temperature and independent of strain rate and loading conditions. Further analysis indicated that, within the testing temperature range in this study, the widely accepted Coulomb's solution of unfrozen soils is applicable as the upper bound of the measured shear band inclination angle of the tested frozen sand, and the measured shear band width falls in the range of 10 to 20 times of mean particle size.

Nanocrystalline structure fabricated by cryogenic temperature rolling of AA 2099 aluminum alloy

Solid solution treated AA 2099 alloy was rolled by a total reduction of 80% at both room and cryogenic temperature. Results show that nanocrystalline grain structure with a size around 20–100 nm was only observed in the cryogenic temperature rolled sample at high rolling strain. The formation of those nanograins related to shear banding behavior. Shear banding behavior was greatly promoted by cryogenic temperature rolling, which had a strong influence on mechanical homogeneity of rolled samples. The width of these high-density shear bands is around 0.2–8 µm. Shear strain partitioning between slip and shear banding was depended on slip behavior in deformed grains. Strain concentration inside shear bands may give rise to an obvious temperature increase, which caused significant grain refinement in or near the shear bands. The growth of nano-grains is retarded due to limited heat and pinning effect of nanosized precipitation.

Geotechnical stability analysis considering strain softening using micro-polar continuum finite element method

Geotechnical stability analyses based on classical continuum may lead to remarkable underestimations on geotechnical safety. To attain better estimations on geotechnical stability, the micro-polar continuum is employed so that its internal characteristic length (4) can be utilized to model the shear band width. Based on two soil slope examples, the role of internal characteristic length in modeling the shear band width of geomaterial is investigated by the second-order cone programming optimized micro-polar continuum finite element method. It is recognized that the underestimation on factor of safety (FOS) calculated from the classical continuum tends to be more pronounced with the increase of 4. When the micro-polar continuum is applied, the shear band dominated by 4 is almost kept unaffected as long as the adopted meshes are fine enough, but it does not generally present a slip surface like in the cases from the classical continuum, indicating that the micro-polar continuum is capable of capturing the non-local geotechnical failure characteristic. Due to the coupling effects of 4 and strain softening, softening behavior of geomaterial tends to be postponed. Additionally, the bearing capacity of a geotechnical system may be significantly underestimated, if the effects of 4 are not modeled or considered in numerical analyses.

Effect of grain size distribution on the shear band thickness evolution in sand

Triaxial experiments were conducted on granular materials presenting uniform, graded and fractal particle distributions in order to investigate how the broadness of the distribution affects the phenomenon of strain localisation. The shear band thickness evolution is assessed by digital image correlation (DIC) using three cameras placed at different angles around a transparent triaxial cell. From the field of deformation, Gaussian distributions have made it possible to fit the data satisfactorily and determine the shear band width evolution. The latter exhibits a rapid decrease in the softening regime until a residual value is reached in all cases. In the conditions of the experiments, it is shown that the residual shear band thickness scales with the mean grain size and the ratio between the two increases with the broadness of the distribution. Samples with uniform distribution exhibit an average residual thickness of ∼ 10D50, samples with graded distribution exhibit an average residual thickness of ∼ 12·5D50and samples with fractal distribution exhibit an average residual thickness of ∼ 17D50.

Studies on chip morphology and modes of tool wear during machining of Ti-6Al-4V using uncoated carbide tool: application of multi-walled carbon nanotubes added rice bran oil as nanocutting fluid

In this work, machining performance of Ti-6Al-4V is studied in consideration with cutting force, tool-tip temperature, tool wear mechanisms, chip morphology and roughness of the machined surface. Machining experiments are carried out by varying cutting speed (Vc) with constant feed and depth-of-cut. Uncoated WC-Co insert is used as cutting tool. Multi-walled carbon nanotubes (MWCNTs) added rice bran oil is utilized to produce nanofluid minimum quantity lubrication (MQL) environment. Chip morphology along with measurable geometrical parameters including chip-tool contact length, equivalent chip thickness (H ch), segmentation spacing, segmentation frequency, shear band width, shear angle, chip hardness, etc. are studied in detail. Effects of Vc on cutting force, tool-tip temperature, depth of flank wear, and area of crater wear are discussed. Different tool wear mechanisms are identified as well. It is experienced that nanofluid MQL (NFMQL) outperforms traditional dry cutting as well as conventional MQL in purview of improved process performance.

The effects of Al2O3 coating on serrated chip geometry and adiabatic shear banding in orthogonal cutting of AISI 316L stainless steel

Al2O3 coatings serve as a thermal barrier during a cutting operation and lead to a different chip formation due to changes in heat sharing and friction conditions, thus affect tool life substantially. In this study, the effects of Al2O3 coating on chip formation were investigated in orthogonal cutting of AISI 316L stainless steel. For this purpose, the resulted chip morphologies after the tests performed using uncoated and Al2O3 coated cemented carbides inserts at different cutting speed (75, 150, 250m/min) and feed (0.05, 0.1, 0.2, 0.3mm/rev) values were examined. In addition, a number of 2D modelling studies on chip formation were also conducted. In numerical analyses, the cutting forces and chip morphologies were obtained and compared to experimental results to confirm the validity of the employed material model. According to the results, the increased cutting speed increases the chip segmentation frequency. At high cutting speeds (Vc≥150m/min), the Al2O3 coating leads to a reduction in the segmentation frequency. Increasing cutting speeds and Al2O3 coating also result in the significant decreases in shear bandwidth. In addition, it was found that simulation results of cutting forces and shear bandwidths are in a good agreement with experimental ones at medium and high cutting speeds. On the other hand, the good prediction of the chip geometry was achieved only under specific cutting conditions.

Dynamic Shear Deformation of a Precipitation Hardened Al0.7CoCrFeNi Eutectic High-Entropy Alloy Using Hat-Shaped Specimen Geometry.

Lamellar eutectic structure in Al0.7CoCrFeNi high-entropy alloy (HEA) is emerging as a promising candidate for structural applications because of its high strength-ductility combination. The alloy consists of a fine-scale lamellar fcc + B2 microstructure with high flow stresses > 1300 MPa under quasi-static tensile deformation and >10% ductility. The response to shear loading was not investigated so far. This is the first report on the shear deformation of a eutectic structured HEA and effect of precipitation on shear deformation. A split-Hopkinson pressure bar (SHPB) was used to compress the hat-shaped specimens to study the local dynamic shear response of the alloy. The change in the width of shear bands with respect to precipitation and deformation rates was studied. The precipitation of L12 phase did not delay the formation of adiabatic shear bands (ASB) or affect the ASB width significantly, however, the deformed region around ASB, consisting of high density of twins in fcc phase, was reduced from 80 µm to 20 µm in the stronger precipitation strengthened condition. We observe dynamic recrystallization of grains within ASBs and local mechanical response of individual eutectic lamellae before and after shear deformation and within the shear bands was examined using nano-indentation.

Shear band widening mechanism in Ti–6Al–4V under high strain rate deformation

Abstract

Dislocation damage and adiabatic shear mechanisms of 7055 aluminum alloy during cutting process

This paper is intended to examine the dislocation and adiabatic shear mechanisms of 7055 aluminum alloy during cutting process with different cutting speeds. The result indicates that, at low cutting speeds, isometric dislocation cells appear, the dislocation cells are interconnected into dislocation cell blocks, and the dislocation movement is controlled by thermal activation; at high cutting speeds, dislocations mostly come in the form of elongated or not fully closed dislocation cells, and the dislocation movement is controlled by phonon drag; the width of an adiabatic shear band increases with the cutting speed, and low cutting speeds are more likely to result in microcracks. The precipitation-free zones on the grain boundary display a discontinuity under all cutting speeds; distribution and grain size of the precipitates also vary significantly.

3D study of plastic flow localization at a void-sheet

A single layer of voids in a ductile material is considered to study the effect of this material imperfection on the onset of shear band localization, and to study the failure evolution in the void-sheet mechanism. The layer is inclined relative to the main tensile direction, and the initially spherical voids are taken to be uniformly spaced both along the inclined direction and transverse to it. The material is elastic-viscoplastic with isotropic hardening. When the solid is subjected to a prescribed strain-rate and the shear band width is negligible compared to specimen dimensions, an infinite ratio of the strain-rate in the band relative to the overall prescribed strain-rate defines the onset of localization. For each stress-state analyzed a limited number of initial inclinations of the void layer are considered, with an angular spacing of 5 degrees, and the angle of inclination giving the smallest critical strain is used as a good approximation of the critical band orientation. Different initial void sizes, relative to the void spacing, are considered. It is shown that increased void spacing in the transverse direction significantly delays localization. Also the effects of different levels of strain hardening are compared. The evolution of the void shape during shear localization is studied, to be able to follow the void-sheet failure mechanism. The critical strain for the onset of void coalescence is compared with the critical strain for shear bands.

On dynamic response and fracture-induced initiation characteristics of aluminum particle filled PTFE reactive material using hat-shaped specimens

Dynamic response and fracture-induced initiation characteristics of aluminum particle filled polytetrafluoroethylene (PTFE/Al) reactive material was studied in two experimental configurations: (a) thin disks for compression and (b) hat-shaped specimens with predesigned shear bands of different widths for compression-shear, at elevated strain rates and temperatures. Compression stress-strain curves demonstrate typical elasto-plastic behavior with prominent strain hardening, strain rate strengthening, as well as thermal softening effects of the material. While growing oscillations and marked strength decrease due to shear band widths decrease observable on the shear stress-strain curves illustrate the localized shear induced adiabatic thermal effect and size effect of polymer materials. Post compression-shear to initiation phenomenon recorded by high-speed photography reveals the necessity of fracture process to initiation, apart from the intense shear work and loading rate effect. Employing the constitutive parameters determined at elevated strain rates and temperatures, 3D full size finite element modeling was performed. On the basis of a good agreement of the simulated and tested shear stress-strain curves for specimens of different shear band widths, a more direct insight into the deformation and stress evolution at the vicinity of the predesigned shear zones was analyzed and obtained.

Structure of deformed Al-based amorphous alloys

The structure of deformed Al-based amorphous alloys was studied by X-ray diffraction, scanning, transmission and probe electron microscopy. It has been found that the material in the shear bands has lower values of Young’s modulus and density. The shear band width is about 50 nm. In the band, no redistribution of the alloy components typical for phase separation is observed. The size of the regions with reduced Young’s modulus around the shear band is in the range from several tenths of a micron to 1 µm.

The effects of ultra-fine-grained structure and cryogenic temperature on adiabatic shear localization in titanium

The deformation at low temperatures (173 K and 77 K) in ultrafine-grained (100 and 500 nm) titanium is investigated and its effect on adiabatic shear localization is established. In comparison with coarse-grained titanium, the strength of ultrafine-grained titanium is higher due to the classic Hall-Petch effect while the strain hardening approaches zero. Our results show that shear localization in dynamic deformation is also altered. The width of the shear band of coarse-grained titanium decreases from 30 to 18 μm (by 40%) with decreasing the initial deformation temperature to 77 K. In contrast, for 100 nm titanium, the width of shear band decreases more significantly, from 4 μm at room temperature to 1 μm (a 75% decrease) at 77 K. This difference is attributed to the combined effects of the decrease in the thermal conductivity and specific heat capacity, and the increase in thermal softening rate. These changes in the width are consistent with the predictions of the Grady and Bai-Dodd theories. Ultrafine- and nano- recrystallized grains are observed inside the bands which are dependent on initial grain size and initial deformation temperature. The dislocation evolution is evaluated for the different conditions using a Kocks-Mecking formulation; the rotational dynamic recrystallization mechanism responsible for forming the ultrafine/nanosized grains (40–250 nm) is successfully modeled incorporating the differences in initial temperature and grain size. Our results and analysis are important in enhancing the understanding of the structural evolution processes under high strain-rates and cryogenic temperatures.

Dense dispersed shear bands in gradient-structured Ni

During tensile deformation, nanostructured (NS) metals often fail soon after yielding by forming a localized shear band. Here we report the observation of high density of shear bands that are homogeneously dispersed in the NS layer of a gradient Ni sample. These shear bands were nucleated at early elastic/plastic strain stage, reached number saturation at ∼3% strain, and remained arrested by the central coarse-grained (CG) matrix during the entire plastic deformation, resulting in a uniform tensile plasticity comparable to that of CG matrix. The formation of dispersed shear bands was promoted by the elastic/plastic interaction between NS surface layer and CG matrix, and affected by the surface roughness and the hardness variation in the NS surface layer. The width of shear bands remained constant, but the intensity of strain accumulation increased almost linearly with applied tensile strain, suggesting a stable shear banding process. Microstructure examination revealed that the strain in shear bands was accommodated by mechanically driven grain boundary migration and grain coarsening. These results clarify the fundamental questions: why/how does the NS layer supported by CG matrix achieve large uniform elongation? Moreover, the findings demonstrate the possibility of activating dispersed stable shear bands by synthesizing gradient architecture for optimized mechanical performances, i.e., a new strategy for evading the strength-ductility tradeoff in NS metals.