Band Propagation Research Articles

Mechanisms for the Formation of an Exceptionally Gently Inclined Basal Shear Zone of a Landslide in Glacial Sediments—The Ludoialm Case Study

The Ludoialm landslide, which is located in the municipality of Münster in Tyrol, Austria, represents a large-scale translational landslide in glacial soil sediments characterised by an exceptionally low inclined basal shear zone of only 12°. Although a temporal coincidence between meteorological events and slope displacement is obvious, the hydromechanical coupled processes responsible for the initial landslide formation and the ongoing movement characteristics have not yet been identified. This article provides a comprehensive analysis of the predisposition factors and the initial failure mechanism of this landslide from geological and geotechnical perspectives. We use a prefailure geometry of the cross section to simulate the initial slope failure process by a limit equilibrium analysis (LEA), a strength-reduction finite element method (SRFEM), and a finite element limit analysis (FELA). The shape and location of the computationally obtained basal sliding zone compare well with the geologically assumed one. Based on the computational study, it turns out that a high groundwater table probably caused by snow melting in combination with different permeabilities for the different layers is needed for the formation of the exceptionally low inclined basal shear zone. This paper presents the failure mechanism of the Ludoialm landslide and discusses the role of the shear band propagation in the process of slope destabilization.

Effects of pore diameter and B2–NiTi crystal on plasticity of amorphous Ni–Ti alloy based on molecular dynamics simulation

The poor plasticity of metallic glasses (MGs) limits its applicability as engineering materials. Therefore, it is particularly important to find ways to improve the plasticity of metallic glasses. In this study, the effects of pore diameter and B2–NiTi crystal on the compressive deformation of Ni50Ti50 MGs are studied by molecular dynamics simulation. The results demonstrate that pores effectively inhibit rapidly stress reduction. Pore experiences pressure in the direction of the load under compression process, which induce the formation of high shear strain atoms around the pores. The presence of pore induces the formation of shear transition zones (STZs) and shear bands. Specifically, larger pore diameters result in lower yield strengths, flatter stress-strain curves, and increase plasticity of MGs. As the B2–NiTi crystal is highly resilient in MGs, it could not be easily destroyed. In the compression process, the B2–NiTi crystal reduce the deformation of the system and impede the propagation of shear bands. As a result, the shear bands only propagate in the cracks of crystals. The larger number of crystals lead to a larger inelastic deformation interval in the nanocomposites and improve the ability to block the propagation of shear bands. These findings elucidate the relationship between yield strength, shear band, plasticity, pore diameter, and B2-phase crystals, and provide theoretical guidance for the development of MGs with improved plasticity.

Hierarchical strain band formation and mechanical behavior of a heterostructured dual-phase material

Dispersive strain bands have been reported as a characteristic deformation feature of heterostructured materials, which helps to improve ductility. However, their formation mechanism is still not well understood. Here we report the formation of dispersed strain bands through dual-level hierarchical strain banding and its effect on the mechanical behavior of a heterostructured Fe-40Cu model material. Specifically, deformation started by the formation and propagation of dispersed microscale strain bands in the heterostructured Fe-40Cu material. High strain gradient was generated within the microscale strain bands during their propagation and was accommodated by the accumulation of geometrically necessary dislocations (GNDs). The dispersed microscale strain bands were not uniformly distributed, but instead grouped together to form macroscale strain bands that were uniformly distributed over the entire gage section to accommodate the majority of the applied strain. The formation of this dual-level hierarchical strain bands prevented the formation of large strain localization to fail the sample prematurely. It was also found that increasing the strain hardening capacity of soft copper zones provides more room for the accumulation of GNDs, resulting in higher constraint to microscale strain band propagation and consequently higher ductility. These observations suggest the possibility of tailoring microscale strain bands to optimize tensile performance of heterostructured materials.

Research progress on the shear band of metallic glasses

Metallic glasses have received considerable attention in the fields of materials science and condensed matter physics. The disordered structure of metallic glasses gives rise to unique mechanical properties, such as high strength, large elasticity and a distinct deformation mechanism in contrast to conventional crystalline alloys. The plastic deformation of metallic glasses at temperatures well below the glass transition is generally carried out through the formation of highly localized shear bands. Therefore, a thorough understanding of shear banding behavior is vital for improving the mechanical properties of metallic glasses and tailoring the properties of post-deformed metallic glasses. This review article aims to provide an up-to-date overview of the atomic mechanisms underlying shear band nucleation, propagation and interaction. Additionally, the dynamics of shear bands and their correlation with plasticity are also presented. This article also focuses on the progress in the fundamental structure and properties of mature shear bands, such as density fluctuation, medium-range order, shear-band affected zone, and diffusivity. Furthermore, the fracture mechanism based on the development of cavities is elaborated. Finally, we identify a number of key unsolved issues on this subject.

Effects of void shape and location on the fracture and plastic deformation of Cu (crystalline) /Cu64Zr36 (amorphous) composites

Voids are probably the most widely studied type of manufacturing defects since they are often formed during material processing. In this paper, the fracture and plastic deformation of Cu/Cu64Zr36 crystalline/amorphous composites with different void locations and shapes are investigated by molecular dynamics simulations. The results show that changes of the void location and shape in the composites significantly change the dislocation motion, shear transformation zones (STZs) activation, shear band propagation, and fracture strength of the composites. The largest shear strain is always found around the void, where it is most likely to activate dislocation motion, STZs and shear band formation. A stepwise increase of the shear localization is mainly related to the nucleation and emission of dislocations, while the very rapid increase is caused by shear band formation. Moreover, the variation of dislocation density and activity of shear bands are mainly responsible for the stress fluctuations in the composites. This paper provides important information for understanding and improving the plastic and fracture behaviors of composites by changing the location and shape of void from an atomistic perspective.

Mechanisms of types B and C serrations in the tensile flow curve of C-bearing high Mn steel

In the present study, the mechanisms of types B and C serrations in the tensile curves of C-bearing high Mn steel were investigated. Many tensile tests were performed at various temperatures (313–573 K) with two different initial strain rates (ε˙ini = 1 × 10−3 and 1 × 10−4/s). Type B serrations appeared in a lower tensile temperature range compared to type C serrations. Whereas a critical strain for type B serrations (ecB) was similar to yield strain regardless of tensile temperature and strain rate, a critical strain for type C serrations (ecC) increased with increasing tensile temperature, indicating the inverse PLC behavior of ecC. The inverse PLC behavior of ecC was explained by the intensified dynamic strain aging (DSA) model. The repetition of rapid aging and easy breakaway causes the discontinuous propagation of the PLC band with flickering, thereby resulting in type B serrations.

A reformulated non-ordinary state-based peridynamic method for dynamic failure of ductile materials

A non-local correspondence peridynamic model is reformulated to describe the dynamic failure of ductile materials, under the framework of non-ordinary state-based peridynamics. To eliminate the zero-energy mode as well as to improve the numerical stability, the deformation gradient is updated on the bond level, which owns information including both the non-local deformation and the bond deviation. Peridynamic thermo-viscoplastic constitutive formulations are presented for the failure analyses of ductile materials. Moreover, a mixed strain-stress criterion describing the bond breaking and considering the temperature effect is presented on the bond level, in which the bonds can keep intact in calculations. Several comprehensive cases, including a notched tension test and Kalthoff-Winkler impact tests with different velocities, are investigated to verify the applicability of the proposed model for ductile fracture and impact failure. Numerical results, in terms of failure patterns, cracking switching and propagation, as well as adiabatic shear band propagation, show a reasonable agreement with corresponding experimental observations.

The dynamics of shear band propagation in metallic glasses

Understanding the dynamics of shear band propagation in metallic glasses remains elusive due to the limited temporal and spatial scales accessible in experiments. In micron-scale molecular dynamics simulations on two model metallic glasses, we studied the propagation of a dominant shear band under uniaxial tension with a macroscopic strain of 3-5%. For both materials, the shear band can be intersonic with a propagation speed exceeding their respective shear wave speeds. The propagation exhibits intrinsic instability that manifests itself as microbranching and considerable fluctuations in velocity. The shear strain singularity ahead of propagating shear band tip scales as 1/r (r is the distance away from the tip), independent of the macroscopic tensile strain. In addition, we studied the intersection of two shear bands under uniaxial tension, during which path deflection, speed slowing-down, and temperature rise at the junction region were observed. The dynamics of propagating shear band shown here indicate that shear band in metallic glasses can be viewed as shear crack under the framework of weakly nonlinear fracture mechanics theory.

Quantitative investigation of strain partitioning and failure mechanism in ultrafine grained medium Mn steel through high resolution digital image correlation

Ultrafine grained medium Mn steel exhibits simultaneous high strength and ductility, however, often showing Lüders deformation and discontinuous yielding phenomena. The connections between martensite transformation and Lüders deformation as well as the localized failure behavior are quantitatively investigated through high resolution digital image correlation, enabled by nanoscale strain distribution and microstructural alignment. At a low global tensile strain level, strain localization happens in austenite domain resulted from a sequential slip deformation and martensite transformation, which then triggers the yielding of ferrite by generating thin strain channels to connect adjacent hotspots. This microscopic deformation mode promotes the formation of macroscopic deformation bands, resulting in Lüders band initiation and propagation. When increasing the applied load, the externally imposed strain is accommodated by ferrite instead, resulting in pronounced strain partitioning. The triple grains boundary having no further strain hardening capacity is favorable for void initiation and propagation.

Mechanical Properties of Internally Hierarchical Multiphase Lattices Inspired by Precipitation Strengthening Mechanisms.

In metal metallurgy, precipitation strengthening is widely used to increase material strength by utilizing the impediment effect of the second-phase particles on dislocation movements. Inspired by this mechanism, in this paper, novel multiphase heterogeneous lattice materials are developed with enhanced mechanical properties utilizing a similar hindering effect of second-phase lattice cells on the shear band propagation. For this purpose, biphase and triphase lattice samples are fabricated using high-speed multi jet fusion (MJF) and digital light processing (DLP) additive manufacturing techniques, and a parametric study is carried out to investigate their mechanical properties. Different from the conventional random distribution, the second-phase and third-phase cells in this work are continuously distributed along the regular pattern of a larger-scale lattice to form internal hierarchical lattice structures. The results show that the triphase lattices possess balanced mechanical properties. Interestingly, this indicates that introducing a relatively weak phase also has the potential to improve the stiffness and plateau stress, which is distinct from the common mixed rule. This work is aimed at providing new references for the heterogeneous lattice design with outstanding mechanical properties through material microstructure inspiration.

Yield strength of “brittle” metallic glass

“Brittle” metallic glass (MG) usually fractures catastrophically in a shattering mode under macroscopic compression, because cleavage cracking of splitting that originates from extrinsic flaws dominates the failure of such alloys, which brings challenges for studying yield strength. Here we show that the plastic yielding behavior in a brittle Fe-based MG can be successfully activated by decreasing the sample size to micrometer scale to avoid the possible large tensile stress concentrators. The yield strength was found to be at least 33% higher than the fracture strength measured with bulk samples for the present brittle MG. The results further demonstrate that the critical stresses for shear band initiation and propagation are size-independent, while the required stress for cleavage cracking increases with decreasing sample size. The competition of thermodynamic driving forces between the two processes of shear banding and cleavage cracking hence leads to the size-induced brittle-to ductile-transition. These findings clarify the physical nature of the strength of “brittle” MG, implying the great opportunity for using high-strength brittle MGs in devices with small dimensions.

Millimeter-Wave Channel Measurements and Path Loss Characterization in a Typical Indoor Office Environment

In this paper, a path loss characterization at millimeter-wave (mmWave) frequencies is performed in a typical indoor office environment. Path loss results were derived from propagation channel measurements collected in the 25–40 GHz frequency band, in both line-of-sight (LOS) and obstructed-LOS (OLOS) propagation conditions. The channel measurements were performed using a frequency-domain channel sounder, which integrates an amplified radio over fiber (RoF) link to avoid the high losses at mmWave. The path loss was analyzed in the 26 GHz, 28 GHz, 33 GHz and 38 GHz frequency bands through the close-in free space reference distance (CI) and the floating-intercept (FI) models. These models take into account the distance dependence of the path loss for a single frequency. Nevertheless, to jointly study the distance and frequency dependence of the path loss, multi-frequency models were considered. The parameters of the ABG (A-alpha, B-beta and G-gamma) and the close-in free space reference distance with frequency path loss exponent (CIF) models were derived from the channel measurements in the whole 25–40 GHz band under the minimum mean square error (MMSE) approach. The results show that, in general, there is some relationship between the model parameters and the frequency. Path loss exponent (PLE) values smaller than the theoretical free space propagation were obtained, showing that there are a waveguide effect and a constructive interference of multipath components (MPCs). Since the measurements were obtained in the same environment and with the same configuration and measurement setup, it is possible to establish realistic comparisons between the model parameters and the propagation behavior at the different frequencies considered. The results provided here allow us to have a better knowledge of the propagation at mmWave frequencies and may be of interest to other researchers in the simulation and performance evaluation of future wireless communication systems in indoor hotspot environments.

Dynamic strain localization into a compaction band via a phase-field approach

We present a new phase-field formulation for the formation and propagation of a compaction band in high-porosity rocks. Novel features of the proposed formulation include (a) the effects of inertia on the rate of development of compaction bands, and (b) degradation mechanisms in tension, compression, and shear appropriate for dynamic strain localization problems where disturbances propagate in time in a wave-like fashion to induce micro-cracking, grain crushing, and frictional grain rearrangement in the rock. We also present a robust numerical technique to handle the spatiotemporal formation and evolution of the compaction band. We validate the model by simulating a benchmark problem involving a V-shape notched cylindrical specimen of Bentheim sandstone tested in conventional triaxial compression. The model is shown to reproduce different geometric styles of deformation that include pure compaction, shear-enhanced compaction, and a combination of pure and shear-enhanced compaction, where the combination mechanism consists of a straight primary compaction band surrounded by secondary chevron bands.

Dual precipitate simultaneous enhancement of tensile and fatigue strength in (FeCoNi)86Al7Ti7 high-entropy alloy fabricated using selective laser melting

Recent studies have indicated that precipitation-strengthened high-entropy alloys (HEAs) show superior mechanical performance and have been successfully fabricated by additive manufacturing. However, the lack of fatigue and fracture research has limited the engineering applications of additive manufacturing HEAs. This work explored a dual precipitation-strengthened (FeCoNi)86Al7Ti7 HEA with excellent tensile and fatigue strength, prepared through selective laser melting and heat treatment. Compared with the as-built samples, the tensile properties and fatigue endurance limit improved through aging by 48.7% and 30%, respectively. The strengthening mechanism and enhanced fatigue performance were clarified in detail. The improvement in fatigue strength was attributed to the improved resistance of the L12 and L21 precipitates. During deformation, the dislocation shear coherent L12 precipitates reduced slip band energy and inhibited slip band expansion, while the L21 particles acted as obstructions for further slip band propagation, severely limiting the rapid formation and propagation of crack growth. In-situ TEM cyclic tensile-tensile testing also clarified the fatigue crack growth behavior, demonstrating that crack deflection due to L21 precipitate obstruction slowed down the crack growth rate and efficiently promoted the closure of the microcrack tips. This work offers implications for a new strategy to develop additive manufacturing HEAs.

Retrogressive failure pattern and retrogression distance in sensitive clays induced by river erosion

AbstractThe failure patterns and retrogression distances in sensitive clays induced by river erosion are two significant factors of concern in landslide risk assessment. For the former, empirical criteria, based on past events and numerical results, can hardly produce a unified guideline and explain their complex mechanism. For the latter, the capacity of the river cross section is rarely considered in existing studies, even though the failed soil would deposit on the riverbed and further affect upslope failure extension. The present study proposes an analytical method to quantitatively assess the failure patterns and retrogression distance by combining the shear band propagation method (SBP) with limit analysis on active block failure. A closed‐form active earth pressure (CAEP) and the shape function of the failure surface are proposed with the variational calculus method (VCM), providing a tool to understand the transformation mechanism of the failure patterns. The criteria for initiation failure positions and stages are formulated following the competition results between the induced minimum slope‐parallel force by foot unloading and the proposed CAEP. Further, the terminal retrogression distance is given considering two possible river cross sections. The proposed CAEP solution is validated against Buss's solution and further applied to a parametric study on the failure patterns. Finally, three practical cases are analyzed with the proposed method, and the predictions are shown to agree well with the field observations.

Serrated flow accompanied with dynamic type transition of the Portevin–Le Chatelier effect in austenitic stainless steel

• By introducing high-temperature DIC analysis, the PLC effect in an Fe-19Cr-13Ni-0.2C austenitic stainless steel was investigated. • Various types of serrated flow associated with the diffusion of interstitial C atoms appeared at temperatures from 473 K to 623 K, and the type dynamically changed even at an applied strain rate. • High-temperature DIC analysis revealed the dynamic type transition of serrated flow obeyed the variation in PLC band nucleation and propagation behavior. • Numerical modeling proves that the spatial-temporal coupling effect plays an important role in the microscopic nucleation manner of the PLC band. In order to further understand high-temperature deformation behavior and its-related mechanical properties, the Portevin–Le Chatelier (PLC) effect in an Fe–19Cr–13Ni–0.2C austenitic stainless steel was investigated using high-temperature digital image correlation (DIC) analysis. Under the tensile testing at temperatures from 473 K to 623 K, different types of serrated flow appeared even at a constant applied strain rate, and the type transition took place dynamically in a certain order with deformation time. DIC analysis revealed that the dynamic type transition of the serrated flow obeys the PLC band propagation behavior, and that the transition of the PLC band propagation behavior could be attributed to the PLC band nucleation manner. Numerical modeling also proved that the nucleation manner of PLC band nucleation is determined by the spatial-temporal coupling effect.

Dynamic hardness and formation of Portevin-Le Chatelier bands during impact indentation

The effect of shock indentation on the formation and propagation of the Portevin-Le Chatelier deformation bands in an aluminum-magnesium alloy was studied experimentally using acoustic emission and high-speed video recording methods. Dynamic and nonlinear responses to shock indentation are revealed: the first include high values of dynamic hardness and local strain and loading rates, and the second - threshold and multiple nature of force and acoustic responses. It is shown that the bands of macrolocalized deformation represent a latent bulk type of erosion damage, which reduces the mechanical stability and durability of the alloy. Keywords: contact shock, Portevin-Le Chatelier effect, deformation bands, fracture, aluminum-magnesium alloy.

Динамическая твердость и образование полос Портевена-Ле Шателье при ударном индентировании

The effect of shock indentation on the formation and propagation of the Portevin-Le Chatelier deformation bands in an aluminum-magnesium alloy was studied experimentally using acoustic emission and high-speed video recording methods. Dynamic and nonlinear responses to shock indentation are revealed: the first include high values of dynamic hardness and local strain and loading rates, and the second - threshold and multiple nature of force and acoustic responses. It is shown that the bands of macrolocalized deformation represent a latent volumetric type of erosion damage, which reduces the mechanical stability and durability of the alloy.

MECHANICAL PROPERTIES OF SINTERED AL-SN-FE ALLOYS

Plastic flow under compression was studied in samples of Al-Sn-Fe alloys produced by sintering from elemental powder mixtures. Some of the sintered composites were additionally compacted at 250°C in a closed mold at a pressure of 300 MPa, and some were processed via equal-channel angular pressing using route A (ECAP-A) at the same temperature. A key structural feature of the sintered alloys is the presence of agglomerates of solid Al3Fe particles cemented with tin, formed in place of iron powder particles as a result of interaction between Al and Fe during sintering. The agglomerates have high strength but are sufficiently ductile owing to tin. They do not break during deformation and effectively suppress the propagation of localized flow bands and microcracks. In compression tests of three-phase Al-20Sn-17Al3Fe composite samples, the particle agglomerates retained their shape and moved as solid structures, while the samples showed good ductility during testing. In ECAP processing via route A, the agglomerates were pulled in the direction of material flow, which negatively affected their ductility in subsequent compression tests.

In situ investigation of the mechanism of propagation of Portevin-Le Chatelier deformation bands

Based on the analysis of high-speed video data of propagating deformation bands in an aluminum-magnesium alloy, it was found that the movable deformation band contains an excess of dislocations of one mechanical sign, and the main mechanism for the propagation of the localized plastic deformation front along the sample axis is a relay transmission of sliding in the antiparallel direction to relax the bending moment created by the primary deformation band. It is shown that the "instantaneous" rate of plastic deformation during ~1 ms in the growing band reaches a value of ~103 s-1, comparable to the rate of deformation during shock tests. Keywords: intermittent deformation, vocalization, dislocations, mechanical charge, aluminum-magnesium alloy.