Maximum Shear Plane Research Articles

Vibration-induced crack tip flipping: A closer look at an unfamiliar phenomenon in pipeline failure analysis

Fracture propagation control is an essential strategy to avoid a catastrophic event involving both economic losses and environmental damage. The Dynamic Tensile Tear Test (DT3) was introduced as an alternative tool to characterise dynamic fracture behaviour of high-strength pipeline steels. Mimicking the in-service loading conditions by imposing a dynamic tensile load, the obtained fracture surfaces closely resemble those observed in full-scale pipeline burst tests. A phenomenon called Crack Tip Flipping (CTF) could also be observed in combination with the formation of Arrowhead Markings (AHMs). The mechanisms provoking these phenomena and their effect on the fracture resistance of the pipeline material have not yet been investigated with respect to pipeline failure. Therefore, a high-speed stereo DIC setup was used to study the fracture behaviour of ×70 grade pipeline steel. The DIC setup identified a mechanical out-of-plane vibration that resulted in an additional torsional loading component. Numerical models were constructed with implementation of the Modified Bai-Wierzbicki (MBW) damage model to validate the experimental observations. Imposing a vibration-induced out-of-plane component resulted in a nearly identical fracture behaviour as observed during experiments. Both the crack path and the final fracture surface could be reproduced. Based on the obtained data, it is suggested that this oscillation causes a rotation of the stress tensor controlled by the through-thickness shear component. As a result, the stress state at the crack front is oriented towards the plane of maximum shear, forming a slanted fracture surface. Notably, the additional torsion load did not affect the load-displacement curve, as the value of the effective stress remains unaffected. Consequently, the fracture resistance, computed from energy values extracted from the load-displacement curve, are not influenced by the occurrence of CTF or the formation of AHMs. Conversely, based on the discussed mechanism of CTF, its presence could indicate the conditions under which pipeline failure occurred.

Extension of a shear-controlled ductile fracture criterion by considering the necking coalescence of voids

An isotropic ductile fracture criterion considering nucleation, growth, shear coalescence and necking coalescence of voids during plastic deformation is developed to model fracture behavior of ductile metals. The innovation of this fracture criterion is that it considers the mechanism of necking coalescence of voids on the basis of a shear-controlled ductile fracture criterion developed by the micro-mechanism (Lou et al., 2014). The damage from coalescence of voids is expressed as the sum of the damages from the shear coalescence of voids and necking coalescence of voids. We assume that the damage from necking coalescence of voids is controlled by the normalized normal stress at the maximum shear plane, the larger positive normalized normal stress at the maximum shear plane will promote the necking coalescence of voids, while the negative gives no effect on the necking coalescence of voids. After the establishment of the proposed fracture criterion, a detailed parametric study is performed to demonstrate the flexibility of the proposed criterion. Then, the proposed ductile criterion is employed to construct fracture loci for three different materials (AA 2024-T351, AA 6060-T6 and A710 steel). And fracture loci constructed are compared with those experimental data points to validate the performance of the proposed fracture criterion. For the purpose of comparison, the predicted results are compared with those predicted by the micro-mechanism-motivated criterion (Lou et al., 2014), the modified Mohr-Coulomb criterion (Bai and Wierzbicki, 2010). The better agreement of the predictions with the experimental data demonstrates that the proposed fracture criterion is able to improve the prediction accuracy for different metals under various stress states and the mechanism of necking coalescence exhibits a noticeable role in predicting the ductile fracture. Moreover, the proposed criterion is expected to be employed in finite element simulation to calculate the ductile fracture for different metals due to the high accuracy.

Composition and size dependent torsion fracture of metallic glasses

The fracture of metallic glasses (MGs) of different compositions and sizes down to micrometers under torsion loading were systematically investigated. Contrary to the flat shear fracture along the circumferential plane as commonly supposed under torsion, we find that the torsion fracture of metallic glasses can deviate from flat shear plane, and the fracture angle is closely dependent on the composition and the size of MG samples. With a conversion method, we show that the torsion fracture of both millimeter- and micrometer-sized MGs can be described by the ellipse fracture criterion as originally proposed for the tension fracture. The deviation from the circumferential shear plane under torsion is further shown to intrinsically relate to the fracture toughness of MGs. The tougher MG tends to have a smaller fracture angle with respect to the maximum shear plane, and vice versa, indicating a correlation between the fracture toughness and pressure/normal stress sensitivity in MGs. Our results provide new insights on the fracture mechanism and are helpful to design and control the deformation and fracture behavior of MGs under torsion loading.

Disparate behaviour of deformation patterns beneath northeast Indian lithosphere inferred from shear wave splitting analysis

New shear wave splitting measurements are obtained from the North East Indian region utilizing the core refracted PKS/SKS/SKKS phases, suggesting the region as more complex and anisotropic in nature. The splitting parameters namely time delays (δt) and fast polarization directions (ϕ) are computed at nine stations and the results are found to be more consistent with the geologic/tectonic structures and present mantle deformation patterns under this particular region. The result streamlines the deformation patterns strictly into three categories. Significant anisotropy in the western fringe of Shillong plateau and surrounding regions implies an asthenospheric flow related strain dominating the splitting direction inline the direction of absolute plate motion (APM) of the Indian plate in a no net reference frame and discards the effects from lithospheric strain. The stations in Himalayan collision zone redefines an actual deformation pattern in ENE-WSW direction in line with the maximum shear plane construing the Himalayan arc at the northeast corner as a consequence of N-S Indo-Eurasian collisional derived lithospheric strain. The anisotropic effect at the Himalayan foredeep section signifies the lithospheric strain induced E-W deformation from N-S continental collision. Mapping the anisotropic layer depth under these stations also strengthens our anisotropic observations and geodynamics understanding of this region with a similar thought of ideas where the deformation patterns are supposed to be dominated by huge forces like APM and lithospheric strains with negligible effects from the local geological structures.

Cyclic shear deformation and fatigue of extruded Mg-Gd-Y magnesium alloy

Deformation and fatigue of extruded Mg-8.0Gd-3.0Y-0.5Zr (GW83, wt%) magnesium (Mg) alloy were experimentally investigated under cyclic torsion using tubular specimen fabricated along the extrusion direction. The controlled shear strain amplitudes ranged from 0.606% to 4.157%. Twinning and detwinning of extension twins are observed to take place during cyclic torsion and the shear stress-shear strain hysteresis loops display a perfectly symmetric shape at all tested strain amplitudes. Marginal cyclic softening is observed when the shear strain amplitude is higher than 1.732%. The strain-life fatigue curve shows two kink points, corresponding to the shear strain amplitude of 1.040% and 1.732%, respectively. When the shear strain amplitude is higher than the upper kink point, early fatigue crack is found to initiate on the maximum shear plane. When the strain amplitude is lower than the lower kink point, fatigue cracking is parallel to the maximum tensile plane. At an identical equivalent strain amplitude, the fatigue life under pure shear is much higher than that under tension-compression. The fatigue life of extruded GW83 alloy is much higher than that of extruded AZ31B alloy at the same plastic strain energy density.

The influence of inherited continental margin structures on the stress and strain fields of the south-central Taiwan fold-and-thrust belt

SUMMARYIn this paper we test whether or not structural and morphological features inherited from the Eurasian continental margin are affecting the contemporary stress and strain fields in south-central Taiwan. Principal stress directions (σ1, σ2 and σ3) are estimated from the inversion of clustered earthquake focal mechanisms and the direction of the maximum compressive horizontal stress (SH) is calculated throughout the study area. From these data the most likely fault plane orientations and their kinematics are inferred. The results of the stress inversion are then discussed together with the directions of displacement, compressional strain rate and maximum shear strain rate derived from GPS data. These data show that there is a marked contrast in the direction of SH from north to south across the study area, with the direction of SH remaining roughly subparallel to the relative plate motion vector in the north, whereas in the south it rotates nearly 45° counter-clockwise. The direction of the horizontal maximum compression strain rate (εH) and associated maximum shear planes, together with the displacement field display an overall similar pattern between them, although undergoing a less marked rotation. We interpret the southward change in the SH, εH and the dextral maximum shear plane directions, together with that of the horizontal displacement field to be related to the reactivation of east–northeast striking faults inherited from the rifted Eurasian margin and to the shelf/slope break. Inherited faults in the basement are typically reactivated as strike-slip faults, whereas newly formed faults in the fold-and-thrust belt are commonly thrusts or oblique thrusts. Eastwards, the stress inversions and strain data show that the western flank of the Central Range is undergoing extension in the upper crust. SH in the Central Range is roughly parallel to the relative plate convergence vector, but in southwestern Taiwan it undergoes a marked counter-clockwise rotation westwards across the Chaochou fault. Farther north, however, there is no significant change across the Lishan fault. This north to south difference is likely due to different margin structures, although local topographic effects may also play a role.

Notch fatigue life prediction considering nonproportionality of local loading path under multiaxial cyclic loading

AbstractAn innovative numerical methodology is presented for fatigue lifetime estimation of notched bodies experiencing multiaxial cyclic loadings. In the presented methodology, an evaluation approach of the local nonproportionality factor F for notched specimens, which defines F as the ratio of the pseudoshear strain range at 45° to the maximum shear plane and the maximum shear strain range, is proposed and discussed deeply. The proposed evaluation method is incorporated into the material cyclic stress‐strain equation for purpose of describing the nonproportional hardening behavior for some material. The comparison between multiaxial elastic‐plastic finite element analysis (FEA) and experimentally measured strains for S460N steel notched specimens shows that the proposed nonproportionality factor estimation method is effective. Subsequently, the notch stresses and strains calculated utilizing multiaxial elastic‐plastic FEA are used as input data to the critical plane‐based fatigue life prediction methodology. The prediction results are satisfactory for the 7050‐T7451 aluminum alloy and GH4169 superalloy notched specimens under multiaxial cyclic loading.

Textural evolution during high-pressure dehydration of serpentinite to peridotite and its relation to stress orientations and kinematics of subducting slabs: Insights from the Almirez ultramafic massif

The Almirez ultramafic massif (SE Spain) preserves the transformation of high-P antigorite (Atg-) serpentinite to chlorite (Chl-) harzburgite (1.6–1.9 GPa; 680–710 °C), a metamorphic reaction that is the primary source of water at the intermediate depth of subducting slabs. We present a detailed μ-CT and EBSD study of oriented samples across the Atg-serpentinite dehydration isograd to investigate the textural evolution during serpentinite dehydration to peridotite and its relation to stress orientations and the kinematics of subducting slabs.Above the Atg-out isograd, Atg-serpentinite shows a prograde mylonitic foliation and a weak Shape Preferred Orientation (SPO) of oxide aggregates defining a N—S stretching lineation. The antigorite Crystal Preferred Orientation (CPO) is characterized by [001]Atg perpendicular to the foliation, and the poles to (100)Atg and (010)Atg distributed in a girdle-like symmetry with [100]Atg nearly parallel to the stretching lineation. The antigorite microstructure and CPO are consistent with deformation by dislocation creep, twinning, and dissolution-precipitation creep. These textures record the long-term shear deformation near the slab interface where the main compressive stress, σ1, was at an acute angle to the foliation.Below the Atg-out isograd, Atg-serpentinite dehydrated to unfoliated, coarse-grainedChl-harzburgite with granofels or spinifex textures distributed in alternating decameter-sized lenses. Crystallization of granofels and spinifex Chl-harzburgite records, respectively, a sequence of slow and fast fluid draining events during serpentinite dehydration under the same orientation of the principal stresses that resulted in the shear deformation of the Atg-serpentinite. Both textural types exhibit coarse-grained textures with systematic mineral CPOs and SPOs and microstructures without evidence of major ductile deformation. The texture of the granofels Chl-harzburgite formed by a topotactic dehydration reaction after Atg-serpentinite coupled to compaction leading to an olivine layering subparallel to the Atg-serpentinite foliation. The olivines of granofels Chl-harzburgite are rounded and display a weak CPO that can be accounted for by the topotactic reaction 〈100〉Atg||〈100〉Ol and (001)Atg||(010)Ol after Atg-serpentinite. Similarly, orthopyroxene shows a marked CPO consistent with the topotactic reaction (100)Opx||(001)Atg and [001]Opx||[100]Atg.Spinifex Chl-harzburgite displays systematic mineral SPO and CPO. Spinifex olivines are tabular on (100)Ol and elongated along [001]Ol (c > b >> a), and define a ESE–WNW platelet lineation. The average texture is characterized by [001]Ol,Opx subparallel to a strong ESE–WNW oxide aggregate lineation, and [100]Ol,Opx and [001]Ol,Opx within a plane of similar orientation to the Atg-serpentinite foliation. The SPOs and CPOs of spinifex Chl-harzburgites are composed of up to four orientation populations of tabular olivines, where one population is volumetrically dominant in all samples. Relative to the main orientation population, the CPO of olivine shows clustered distribution of [100]Ol and [010]Ol maxima rotated at systematic angles around [001]Ol. These clustered CPOs and SPOs show a remarkable correlation with the orientation of the principal paleostresses. The preponderant orientation population of tabular olivines lies on the plane of maximum compression (σ2–σ3 plane) and has [010]Ol and its (100)Ol tabular faces nearly perpendicular to the Atg-serpentinite foliation. This population can be accounted for by oriented growth of platy crystals perpendicular to σ1. The other populations lie in the plane perpendicular to the least compressive stress (σ3) and the planes of maximum shear. The driving force and causes for the oriented crystallization of tabular olivine in these planes is uncertain, but their correlation with paleostresses suggests a cause-effect relationship.Spinifex and granofels Chl-harzburgites show a marked ESE–WNW oxide aggregate lineation that differs in orientation from that of Atg-serpentinite and is approximately parallel to the intermediate compressive stress (σ2). These lineations and the platelet lineation of spinifex Chl-harzburgite may be due to along-strike fluid flow below the permeability barrier that constituted the Atg-out dehydration isograd. Our study shows that the kinematics of the slab, paleostresses, and fluid flow exert a dynamic control on the textures of Atg-serpentinite dehydrating to peridotite in subducting slabs.

Multiaxial Fatigue Crack Orientation and Early Growth Investigation Considering the Nonproportional Loading

The paper presents a comprehensive investigation of fatigue cracking behaviors under various nonproportional multiaxial cycle loading paths based on the critical plane approach. The maximum normal and shear stress/strain fields are presented to analyze the crack orientation and early growth directions in polar diagrams. The experimental observed crack paths and directions were compared with maximum strain loci of each angle to determine multiaxial fatigue failure mode. The results show that crack orientation and growth paths appear in the maximum shear and normal strain plane, respectively. Likelihood cracking regions of various loading paths are predicted according the determined failure mode. Besides, nonproportionality factor is introduced to characterize the degree of multiaxiality on these loading paths.

On the consideration of normal and shear stress interaction in multiaxial fatigue damage analysis

Due to the abundance of engineering components subjected to complex multiaxial loading histories, being able to accurately estimate fatigue damage under multiaxial stress states is a fundamental step in many fatigue life analyses. In this respect, the Fatemi-Socie (FS) critical plane damage parameter has been shown to provide satisfactory fatigue life correlations for a variety of materials and loading conditions. In this parameter, shear strain amplitude has a primary influence on fatigue damage and the maximum normal stress on the maximum shear plane has a secondary, but important, influence. Additionally, in order to preserve the unitless feature of strain, the maximum normal stress is normalized by the material yield strength. However, in examining some data from literature it was found that, in certain situations, the FS parameter can result in better fatigue life predictions if the maximum normal stress is normalized by the shear stress range on the maximum shear plane instead. These data include uniaxial loadings with large tensile mean stress, and some combined axial-torsion load paths with different normal-shear stress interactions. This modification to the FS parameter was investigated by using fatigue data from literature for 7075-T651 aluminum alloy and a ductile cast iron, as well as additional data from 2024-T3 aluminum alloy fatigue tests performed in this study.

Microstructures and mechanical behavior of Inconel 625 fabricated by solid-state additive manufacturing

Here we introduce a novel thermo-mechanical Solid State Additive Manufacturing (SSAM) process referred to as Additive Friction Stir (AFS) manufacturing that provides a new and alternative path to fusion-based additive manufacturing processes for developing fully-dense, near-net shape components with a refined-equiaxed grain morphology. This study is the first to investigate the beneficial grain refinement and densification produced by AFS in IN625 that results in advantageous mechanical properties (YS, UTS, εf) at both quasi-static and high strain rate. Electron Backscatter Diffraction (EBSD) observed grain refinement during the layer deposition in the AFS specimens, where the results identified fine equiaxed grain structures with even finer grain structures forming at the layer interfaces. The EBSD quantified grains as fine as 0.27µm in these interface regions while the average grain size was approximately 1µm. Additionally, this is the first study to report on the strain rate dependence of AFS IN625 through quasi-static (QS) (0.001/s) and high strain rate (HR) (1500/s) tensile experiments using a servo hydraulic frame and a direct tension-Kolsky bar, respectively, which captured both yield and ultimate tensile strengths increasing as strain rate increased. The HS results exhibited an approximately 200MPa increase in engineering strength over the QS results, with the fracture surfaces at both strain rates aligned with the maximum shear plane and exhibiting localized microvoids.

Characterising fatigue macrocrack initiation in profiled steel roof cladding

In Australia and abroad screw-fixed, thin, high strength, profiled steel roof cladding systems are widely used on low rise industrial, commercial and residential buildings. Under extreme wind conditions, such as those produced by cyclones, these systems can fail as a result of low cycle metal fatigue. Current design of such systems is based entirely on standardised cyclic testing of full scale prototype assemblies. This paper describes the mechanism underlying this fatigue behaviour by studying the initiation of cracking in corrugated high strength steel cladding. When subject to repeated loading cracks initiate within the cladding at the fasteners, nucleating in the plane of maximum shear through the cladding thickness prior to propagating into the plane of the cladding. Crack initiation was considered to be the point in which the thickness of the cladding was entirely bisected and the crack was beginning to propagate into the plane of the cladding. Based on this characterisation, a crack initiation criterion dependent on the surface principal strains in the region of crack initiation was developed for both the longitudinal and transverse directions of the cladding. In combination with detailed stress analysis such criteria could be implemented in a numerical model to predict crack initiation in cladding, providing an analytical tool which could greatly assist the fatigue design process and lead to a reduction in the total reliance on prototype testing of each individual system used.

Interaction of shear and normal stresses in multiaxial fatigue damage analysis

Due to the abundance of engineering components subjected to complex multiaxial loading histories, being able to accurately estimate fatigue damage under multiaxial stress states is a fundamental step in many fatigue life analyses. In this respect, the Fatemi-Socie (FS) critical plane damage parameter has been shown to provide excellent fatigue life correlations for a variety of materials and loading conditions. In this parameter shear strain amplitude has a primary influence on fatigue damage and the maximum normal stress on the maximum shear plane has a secondary, but important, influence. In this parameter, the maximum normal stress is normalized by the material yield strength in order to preserve the unitless feature of strain. However, in examining some literature data it was found that in certain situations the FS parameter can result in better fatigue life predictions if the maximum normal stress is normalized by shear stress range instead. These data include uniaxial loadings with large tensile mean stress, and some non-proportional axial-torsion load paths with different normal-shear stress interactions. This modification to the FS parameter was investigated by using fatigue data from literature for 7075-T651 aluminum alloy, as well as additional data from 2024-T3 aluminum alloy fatigue tests performed in this study.

Progress in understanding of intrinsic resistance to shear-mode fatigue crack growth in metallic materials

Recently made experiments allowed measurement of effective data (free of crack tip shielding) and enabled a better understanding of intrinsic behaviour of shear-mode cracks. Influence of crystal structure and microstructure was distinguished. Experimental results for niobium showed that growth micromechanisms and effective thresholds (ΔKIIeff,th≈1.0MPam1/2, ΔKIIIeff,th≈1.4MPam1/2) of modes II and III cracks were different, which was also observed in other metals. Validity of the simple formula for mode II effective threshold for single-phase metals based on the dislocation emission model was confirmed. The cracks propagated in a coplanar manner along the maximum shear plane, which is a characteristic behaviour for bcc metals. Model of propagation of remote mode III loaded cracks by local mode II advances was confirmed.

Crack path in aeronautical titanium alloy under ultrasonic torsion loading

This paper discusses features of fatigue crack initiation and growth in aeronautical VT3-1 titanium alloy under pure torsion loading in gigacycle regime. Two materials: extruded and forged VT3-1 titanium alloys were studied. Torsion fatigue tests were performed up to fatigue life of 109 cycles. The results of the torsion tests were compared with previously obtained results under fully reversed axial loading on the same alloys. It has been shown that independently on production process as surface as well subsurface crack initiation may appear under ultrasonic torsion loading despite the maximum stress amplitude located at the specimen surface. In the case of surface crack initiation, a scenario of crack initiation and growth is similar to HCF regime except an additional possibility for internal crack branching. In the case of subsurface crack, the initiation site is located below the specimen surface (about 200 µm) and is not clearly related to any material flaw. Internal crack initiation is produced by shear stress in maximum shear plane and early crack growth is in Mode II. Crack branching is limited in the case of internal crack initiation compared to surface one. A typical ‘fish-eye’ crack can be observed at the torsion fracture surface, but mechanism of crack initiation seems not to be the same than under axial fatigue loading.

On the use of tensor paths to estimate the nonproportionality factor of multiaxial stress or strain histories under free-surface conditions

Nonproportional (NP) strain hardening is caused by multiaxial load histories that induce variable principal stress/strain directions, activating cross-slip bands in several directions, due to the associated rotation of the maximum shear planes. This effect increases the strain-hardening behavior observed under proportional loads, those with fixed principal directions, and must be considered in multiaxial fatigue calculations, especially for materials with low stacking fault energy, such as austenitic stainless steels. NP hardening depends on the material and on the shape of the multiaxial load history path in a stress or strain diagram as well. It can be evaluated by a nonproportionality factor \({F_{\rm NP}}\) that varies from zero, for a proportional load history, to one, for a \({90^{\circ}}\) out-of-phase tension–torsion loading with the same normal and effective shear amplitudes. Originally, \({F_{\rm NP}}\) was estimated from the aspect ratio of a convex enclosure that contains the load history path, such as an ellipse or a prismatic enclosure, but such convex enclosure estimates can lead to poor predictions of \({F_{\rm NP}}\). Another approach consists on evaluating the shape of the six-dimensional (6D) path described by the six normal and shear components of the stress tensor, where the stress path contour is interpreted as a homogeneous wire with unit mass. The moment of inertia (MOI) tensor of this hypothetical wire is then calculated and used to estimate \({F_{\rm NP}}\). The use of 6D stress paths to estimate \({F_{\rm NP}}\) is questionable, since 6D formulations implicitly include the effect of the hydrostatic stress, while NP hardening is caused by the deviatoric plastic straining, not by stresses alone or by their hydrostatic component. In this work, the NP factor \({F_{\rm NP}}\) of a multiaxial load history is estimated from the eigenvalues of the MOI tensor of the plastic strain path, which are associated with the accumulated plastic straining in the principal directions defined by the associated eigenvectors. The presented formulation assumes free-surface conditions, but allows a surface pressure, covering the conditions of most critical points, which indeed are located on free surfaces. Experimental results for 14 different tension–torsion multiaxial histories prove the effectiveness of the proposed method.

Multiaxial fatigue modeling for Nitinol shape memory alloys under in-phase loading

The realistic loading condition for many components is multiaxial arising from multidirectional loading or geometry complexities. In this study, some multiaxial stress-based classical and critical plane fatigue models are briefly reviewed and their application for martensitic Nitinol under torsion and in-phase axial-torsion loading is evaluated. These models include von Mises equivalent stress, Tresca, Findley, McDiarmid, and a proposed stress-based Fatemi–Socie-type model. As the fatigue cracks appear to be on the maximum shear plane for the martensitic Nitinol, all the models examined here consider the shear stress as the primary damage parameter. Among all the models considered in this study, the proposed Fatemi–Socie-type model provides a better prediction for fatigue lives when compared to torsion and in-phase multiaxial fatigue experimental data from literature. Analyses indicate that critical plane approaches are more appropriate for multiaxial fatigue prediction of Nitinol alloys, at least in martensitic phase. Finally, recommendations are made to calibrate more reliable multiaxial fatigue models for Nitinol.

Electrical charging effects on the sliding friction of a model nano-confined ionic liquid.

Recent measurements suggest the possibility to exploit ionic liquids (ILs) as smart lubricants for nano-contacts, tuning their tribological and rheological properties by charging the sliding interfaces. Following our earlier theoretical study of charging effects on nanoscale confinement and squeezout of a model IL, we present here molecular dynamics simulations of the frictional and lubrication properties of that model under charging conditions. First, we describe the case when two equally charged plates slide while being held together to a confinement distance of a few molecular layers. The shear sliding stress is found to rise strongly and discontinuously as the number of IL layers decreases stepwise. However, the shear stress shows, within each given number of layers, only a weak dependence upon the precise value of the normal load, a result in agreement with data extracted from recent experiments. We subsequently describe the case of opposite charging of the sliding plates and follow the shear stress when the charging is slowly and adiabatically reversed in the course of time, under fixed load. Despite the fixed load, the number and structure of the confined IL layers change with changing charge, and that in turn drives strong friction variations. The latter involves first of all charging-induced freezing of the IL film, followed by a discharging-induced melting, both made possible by the nanoscale confinement. Another mechanism for charging-induced frictional changes is a shift of the plane of maximum shear from mid-film to the plate-film interface, and vice versa. While these occurrences and results invariably depend upon the parameters of the model IL and upon its specific interaction with the plates, the present study helps identifying a variety of possible behavior, obtained under very simple assumptions, while connecting it to an underlying equilibrium thermodynamics picture.

Delineation of large localized damage structures forming ahead of an active mining front by using advanced acoustic emission mapping techniques

We applied advanced mapping techniques to 291 230 acoustic emission (AE) events as small as around M −4 that were recorded over 50 days by an ultra-high resolution network close to the active front of a tabular mining stope being advanced northward at 1km depth in the Cooke 4 Gold Mine in South Africa. We first applied joint hypocenter determination (JHD) to improve absolute locations, and then applied the double-difference relative location algorithm to the JHD output. These steps resolved the seemingly continuous, dense cloud of AEs that extend about 20m ahead of the stope front into several discrete, steeply dipping tabular clusters a few meters thick and 10–30m in dip extent, separated by quiet intervals a few meters thick. The clusters have a strike parallel to the stope face and a dip of about 65°, resembling commonly observed large shear fractures along the plane of maximum shear (Ortlepp shears). In general, the activity of the clusters changed in similar ways as the stope face advanced, but each cluster remained stationary and the gaps between clusters were impressively quiet. This study demonstrates that high-resolution AE mapping can delineate the formation of large structures of localized damage in the highly stressed intact rock mass ahead of the stope face, a process that may culminate in hazardous seismic events.

Interactions between fluid and fibers in a turbulent backward-facing step flow

In this work, a turbulent backward-facing step flow at Reynolds number ReH = 17 520, laden with rod-like particles, is investigated by means of particle image velocimetry. Particle loadings tested are within 0.02% and 0.05% volume fractions; thus, the suspensions may be considered as dilute. The high spatial resolution allows the identification of fiber orientation within the flow, so that mutual interactions between phases can be investigated by considering flow modifications and phenomena related to preferential concentration and orientation of fibers. Fibers’ concentration data show a moderate trend to accumulate at the channel centreline, whereas orientation data display a strong preferential orientation of fibers. Specifically, the local fibers’ orientation is correlated to the flow field in terms of mean flow direction and plane of maximum shear. The occurrence of a symmetrical orientation profile with respect to the channel centre-line is reported as well as the tendency to recover such profile downstream of the step.