Pile-up Mechanism Research Articles

Force modeling of vertical surface grinding considering wheel-workpiece contact geometry

Vertical surface grinding (VSG) is an effective method that can adapt to multiple machining tasks, including stock removal machining to get high productivity and finish machining to obtain high-quality surfaces. Force prediction can reveal some mechanisms in the VSG process, conducing to the improved machining quality and efficiency. This paper aims to develop a novel analytical force model considering the actual wheel-workpiece contact geometry (WWCG). In the modeling process, the cup grinding wheel was divided into many micro-cutting layers along the wheel axis, and the expressions of geometrical-kinematic parameters were derived to characterize the material removal at different positions of the primary grinding zone (PGZ). A single-grain grinding force model was proposed based on chip formation and plastic pile-up mechanisms from an energy perspective. Based on the analysis of multiple-grain interactions at each micro-cutting layer, equations of the local and total grinding forces in the VSG process were then derived by synthesizing the single-grain forces. Findings show that the developed model could accurately predict the magnitudes of total grinding forces and provide the distributions of local grinding forces. The WWCG variation could significantly affect the grinding forces by altering the material removal rate of different micro-cutting layers. This work provides a new method for predicting grinding forces and theoretical guidance for optimizing machining parameters and tailoring the edge profiles of cup grinding wheels to adapt to specific machining tasks.

Effect of Robotic Pile-Up Mechanism on Cooperative Transportation for Versatile Objects

In recent years, cooperative transportation systems using multiple mobile robots have been studied. The advantage will be to transport objects that cannot be dealt with by a single robot and transport them by using smaller robots. Although cooperative transportation by a group of robots has been studied, the conventional transportation targets are limited to objects whose posture is stable. In this paper, we propose a system in which robots pile up on each other to support an object, aiming at a system for more versatile object transportation, including unstable objects. After deriving the conditions by modeling the support system in object transportation, we verify the transporting performance including the robotic pile-up effect through actual robot experiments.

Effect of proppant sizes and injection modes on proppant transportation and distribution in the tortuous fracture model

Particle-fluid transport and placement mechanism in tortuous fracture played a crucial role in unconventional reservoirs. Currently, most studies focused on mono-size proppant with fluid transport processes in tortuous fractures. However, the mixture-size proppant with fluid movement mechanism in tortuous fracture was still uncommon. Therefore, this study designed and applied a series of experiments with a physical analog model of a tortuous fracture with 120° and 90°-angled bends and combined high-speed camera-based equipment. This experimental system was used to track different-mixture-sized proppant particle motion trajectories for a series of proppant injection schemes; The following conclusions were drawn from this study:1. The pile-up processes mechanism in all investigated schemes were similar and could be reduced to four main stages. 2. The packing structure at both sides of the fracture wall had different variation rates, which were controlled by the mix ratio (change from 1:1–1:5) of proppant size. 3. Some new packing patterns, such as Zebra Stripe, had occurred, controlled by the different proppant injection sequences. 4. Small-sized mono-proppant (30/50 mesh) had the highest transport efficiency in the tortuous fracture, followed by the mixed-sized multi-proppant (10/20 mesh:30/50 mesh), large-sized proppant (10/20 mesh) was the worst. 5. An optimized alternating injection mode was recommended as injecting small-sized proppant first (30/50 mesh) and followed by mixed-sized multi-proppant (10/20 mesh:30/50 mesh), which could contribute to obtaining the optimal both proppant packing height and travel distance in tortuous fracture.6. Two correlations were developed for predicting the proppant packing height and transportation distance.

A new insight into the dynamic impact between geophysical flow and rigid barrier

Understanding the interaction between complicated geophysical flows and barriers remains a critical challenge for hazards countermeasures in engineering design. In this study, the impact of geophysical flows on a rigid barrier is studied numerically using coupled computational fluid dynamics and discrete element method (CFD-DEM). Three types of fluid: Bingham, Herschel-Bulkley, and water are chosen as the fluid phase to mix with particles in the simulation, corresponding to Case B-P, Case HB-P, Case W-P, respectively. The results of two-phase simulations are then compared with initial volume controlled dry granular flow (Case P) under the same Froude condition. Two impact mechanisms, namely pileup and runup mechanisms, are recognised in flow-barrier interaction. The effects of fluid properties including viscosity and density on the impact force against the flume base and barrier and inter-particle contacts are also systematically examined. In addition, the dynamic pressure coefficient α is also predicted by a newly proposed analytical model, which has been validated by the data from experimental and numerical tests. Findings indicate that the new model may provide a useful reference to calculate the impact force for the design of a rigid barrier in intercepting hazardous geophysical flows.

Interaction Between Edge Dislocation and Single-Layered Graphene in Aluminum Matrix Investigated by Molecular Dynamics Simulation

The influence of graphene on dislocation movement and subsequent mechanical response of aluminum is investigated by the computational method of molecular dynamics simulation. A Lennard–Jones potential describing Al-C interaction was obtained through ab initio calculation. It was observed that the 2D graphene could reinforce Al matrix similar to the traditional Orowan mechanism. The Al/graphene interface first attract the gliding dislocation to reduce the system energy, which is unlike the grain boundary to repel gliding dislocations through pile-up mechanism. With the increase of stress, dislocation attracted and trapped at the front of graphene could glide along the interface and finally bypass it through climbing when graphene is orientated out of the shear plane. In addition, the strengthening ability of graphene is size dependent, showing a linear relationship between strength increment and graphene size.

Effect of <i>β</i> lath arrangement on activation of <a> slips in single-colony lamellar structures of Ti-6Al-4V alloy

Micro-tensile tests using single-colony specimens of Ti-6Al-4V alloy with a fine lamellar microstructure were employed in the temperature range between 298 and 423 K to understand the effects of geometrical orientation of β lath on <a> slip behaviour. The critical resolved shear stress (CRSS) for the basal slips was lower than that of the prismatic slips, in addition, the temperature dependence of the slip behaviour was greater in the basal slips than the prismatic slips. In the case of single α phase excluding β lath, the CRSS and temperature dependence are independent of the slip modes. In the lamellar microstructure, however, the basal slip propagates simultaneously through all the lamellar layers (divider type) and the prismatic slip propagates sequentially from one layer to another (arrester type). Therefore, considering that dislocation motion is restricted through the pinning and pile-up mechanisms in divider and arrester orientations, respectively, the CRSS and temperature dependence in the basal and prismatic slips can be explained by the geometrical orientation of β lath included in lamellae. This finding suggests that the geometric configuration of β lath dominates the <a> slip in single-colony lamellar structure of the Ti-6Al-4V alloy.

Interfacial deformation and failure mechanisms at the single-splat length scale revealed in-situ by indentation of cold sprayed aluminum microparticles

Cold spray deposition exploits the phenomenon of impact bonding for solid-state consolidation of metallic microparticles. However, the particle interfaces in the deposits are susceptible to crack propagation under mechanical stresses, which results in inferior ductility. In this work, we seek to develop insights into splat-substrate interface bonding by in-situ micromechanical investigations. A miniaturized mechanical testing approach is reported here, which relies on micromachining, targeted indentation, and real-time scanning electron microscopy to probe deformation and failure at buried interfaces. Investigations at the “single splat length scale” enabled us to distinguish deformation mechanisms associated with 6061Al splats with globular and pancake-shaped morphologies. We observed a transition from mechanical interlocking to metallurgical bonding with an increase in the degree of particle flattening during deposition. The mechanically interlocked splats debond from the substrate via crack propagation and splat sliding. On the other hand, metallurgically bonded splats do not fail under indentation stresses exceeding 380 MPa; instead, displaying shear band propagation and pile-up mechanisms. A four-fold enhancement in the critical stress for crack propagation in mechanically-interlocked splats is achieved after a two-step annealing-aging heat-treatment cycle. We demonstrate that interface bonding plays a more dominant role than the inherent plasticity of splats in influencing bulk deposits' ductility, underscoring the importance of interface engineering in cold sprayed materials.

Quantifying the transition of impact mechanisms of geophysical flows against flexible barrier

Flexible barriers have been increasingly used worldwide for mitigating natural hazards involving various geophysical flows. The analysis and design of flexible barriers need to distinguish two major impact mechanisms, runup and pile-up, which remain poorly understood due to complicated interactions between the impacting flow and the deformable barrier. In this study, a unified computational approach based on coupled computational fluid dynamics and discrete element method (CFD-DEM) is employed to examine the impact mechanisms of a wide spectrum of geophysical flows against a deformable and permeable flexible barrier. We consider geophysical flows including rock avalanche, debris avalanche, debris flow, debris flood and mud flow, and a flexible barrier consisting of a barrier net, cables and brakes. The signatures of runup and pile-up mechanisms are thoroughly analyzed, in terms of flow features and barrier responses. The effects of Froude numbers (Fr = 0.46–7.40), solid volume concentrations (0.1–1), and fluid rheologies (Newtonian and non-Newtonian fluids) on the impact mechanism transitions are systematically examined. Two nondimensional indices, the static-peak load ratio of the barrier and the momentum reduction ratio of the flow, are proposed for identifying the transition of impact mechanism from pile-up to runup. The transition is found to occur with either an increase in Fr or solid volume concentration of the flow. Increased viscosity of the fluid in a solid-fluid mixture may result in a transition occurring at a higher Fr. This study helps gain insights into the two impact mechanisms and their transitions and may provide a useful reference for the future design of flexible barriers in mitigating hazardous geophysical flows.

Experimental evaluation of loading mode effect on plasticity of microscale single-crystal copper

Abstract This study aims to experimentally elucidate the effect of loading mode on plastic behavior of microscale single-crystal copper. Tensile and bending tests involving in situ observations in a scanning electron microscope are carried out by fabricating dog-bone shaped specimens and cantilever beams, respectively, with the same size (2 μm) and the same single-slip orientation. The experimental results show that for microscale single-crystal copper the maximum stress at global failure under bending (288 MPa) is much higher than that of tension (82 MPa), while the yield stress at which the first group of dislocations are generated is similar for both tension (76 MPa) and bending (69 MPa). For tensile tests, the stress increases linearly and monotonically before yielding. The dislocations go through the tensile specimen and synchronously form surface steps at two opposite sides. However, the slip lines in bending specimens are initiated from the surface edges and terminated somewhere inside, and these local strain bursts lead to several intermittent stress drops. Assuming dislocation pile-up mechanism inside of bending specimens, the pile-up stress in the bending is evaluated as about 200 MPa, which agrees well with the experimental results.

Froude characterization for unsteady single-surge dry granular flows: impact pressure and runup height

The impact and pileup mechanisms of unsteady granular flows impacting a rigid barrier are governed by the Froude conditions (Fr). Velocity and depth vary along the length of the flow. There is currently no widely accepted approach for characterizing Fr for impact and runup problems. In this study, a discrete element method (DEM) model was calibrated against a physical flume test. Eighty-six simulations were performed using the DEM model to investigate the equivalent Fr governing pileup height and impact pressure for unsteady single-surge dry granular flows against a rigid barrier. Fr and the grain diameter were varied. Results reveal that Fr within the frontmost 5% of a flow governs both pileup height and impact pressure. Thus, taking frontal velocity and maximum flow depth within the frontmost region is crucial for properly characterizing the runup height and impact load. Consistent characterization of Fr is possible near the longitudinal centre of a flow; the frontmost Fr can then be extrapolated from calibration curves. Results imply that existing studies that predict impact pressure based on nonfrontal Fr values may underestimate impact pressure by a factor of up to 2.

Dislocation interactions and crack nucleation in a fatigued near-alpha titanium alloy

Dislocation interactions at the crack nucleation site were investigated in near-alpha titanium alloy Ti-6242Si subjected to low cycle fatigue. Cyclic plastic strain in the alloy resulted in dislocation pile-ups in the primary alpha grains, nucleated at the boundaries between the primary alpha and the two-phase regions. These two phase regions provided a barrier to slip transfer between primary alpha grains. We suggest that crack nucleation occurred near the basal plane of primary alpha grains by the subsurface double-ended pile-up mechanism first conceived by Tanaka and Mura. Superjogs on the basal 〈a〉 dislocations were observed near the crack nucleation location. The two phase regions showed direct transmission of a3 dislocations between secondary alpha plates, transmitted through the beta ligaments as a[010], which then decompose into (a/2)〈111〉 dislocation networks in the beta. The beta ligaments themselves do not appear to form an especially impenetrable barrier to slip, in agreement with the micropillar and crystal plasticity investigations of Zhang et al.

Influence of debris flow solid fraction on rigid barrier impact

The dynamics of debris flows are fundamentally governed by the interaction between the solid and fluid phases. However, current approaches used to estimate impact load treat debris flow as an equivalent fluid without considering solid–fluid interaction separately from other factors. In this study, a series of centrifuge tests was carried out to investigate the influence of interaction between solid and fluid phases on single-surge debris flow impact on a rigid barrier. The effect of solid–fluid interaction was studied by varying the solid fraction of the flows. A model rigid barrier was instrumented to capture induced bending moment and impact pressure. Test results demonstrate that the transition from a pile-up mechanism to a run-up mechanism is governed by the solid fraction and thus the grain contact stresses. The rigid barrier design for the impact with a pile-up mechanism is mainly dominated by the static load. Contrary to the hydrodynamic approach, which assumes that the frontal impact is the most critical, the frontal impact of a run-up mechanism contributes less than 25% of the total force impulse. The consideration of static loading leads to the development of a new impact model with a triangular distribution of the impact pressure.

Geophysical flows impacting a flexible barrier: effects of solid-fluid interaction

Flexible barriers undergo large deformation to extend the impact duration, and thereby reduce the impact load of geophysical flows. The performance of flexible barriers remains a crucial challenge because there currently lacks a comprehensive criterion for estimating impact load. In this study, a series of centrifuge tests were carried out to investigate different geophysical flow types impacting an instrumented flexible barrier. The geophysical flows modelled include covered in this study include flood, hyperconcentrated flow, debris flow, and dry debris avalanche. Results reveal that the relationship between the Froude number, Fr, and the pressure coefficient α strongly depends on the formation of static deposits called dead zones which induce static loads and whether a run-up or pile-up impact mechanism develops. Test results demonstrate that flexible barriers can attenuate peak impact loads of flood, hyperconcentrated flow, and debris flow by up to 50% compared to rigid barriers. Furthermore, flexible barriers attenuate the impact load of dry debris avalanche by enabling the dry debris to reach an active failure state through large deformation. Examination of the state of static debris deposits behind the barriers indicates that hyperconcentrated and debris flows are strongly influenced by whether excessive pore water pressures regulate the depositional process of particles during the impact process. This results in significant particle rearrangement and similar state of static debris behind rigid barrier and the deformed full-retention flexible barrier, and thus the static loads on both barriers converge.

Oxidation kinetics of Si and SiGe by dry rapid thermal oxidation, in-situ steam generation oxidation and dry furnace oxidation

The fabrication of ultrathin compressively strained SiGe-On-Insulator layers by the condensation technique is likely a key milestone towards low-power and high performances FD-SOI logic devices. However, the SiGe condensation technique still requires challenges to be solved for an optimized use in an industrial environment. SiGe oxidation kinetics, upon which the condensation technique is founded, has still not reached a consensus in spite of various studies which gave insights into the matter. This paper aims to bridge the gaps between these studies by covering various oxidation processes relevant to today's technological needs with a new and quantitative analysis methodology. We thus address oxidation kinetics of SiGe with three Ge concentrations (0%, 10%, and 30%) by means of dry rapid thermal oxidation, in-situ steam generation oxidation, and dry furnace oxidation. Oxide thicknesses in the 50 Å to 150 Å range grown with oxidation temperatures between 850 and 1100 °C were targeted. The present work shows first that for all investigated processes, oxidation follows a parabolic regime even for thin oxides, which indicates a diffusion-limited oxidation regime. We also observe that, for all investigated processes, the SiGe oxidation rate is systematically higher than that of Si. The amplitude of the variation of oxidation kinetics of SiGe with respect to Si is found to be strongly dependent on the process type. Second, a new quantitative analysis methodology of oxidation kinetics is introduced. This methodology allows us to highlight the dependence of oxidation kinetics on the Ge concentration at the oxidation interface, which is modulated by the pile-up mechanism. Our results show that the oxidation rate increases with the Ge concentration at the oxidation interface.

Optimization of Surface Properties for Nanoindentation Studies on Nuclear Structural Materials

Nanoindentation using static loading of indenter tip is one of versatile methods used for evaluation of materials in smaller volume. It includes investigation of structural phases and thin layers on substrates etc. In the field of nuclear core and structural materials, nanoindentation has become an important tool to assess mechanical properties and correlate to the level of radiation damage at elevated and room temperatures. Nanoindentation, ideally in combination with Transmission Electron Microscopy, can describe the extent of damage and behaviour from the nanoand micro scale. Due to the high sensitivity of nanoindentation system, that typically uses loads in the range of tens of nN up to several tens of mN, the precise sample preparation is challenging and to be performed especially to understand behaviour of bulk materials. In the current study, samples from Austenitic Stainless Steel (ASS) 321, which is the representative structural material used for reactor internals, were prepared by standard methods - fine polishing, chemical etching, electrolytic etching, electrolytic polishing, electrolytic polishing &amp; etching and ion polishing. Firstly, non-irradiated samples were used for optimization of the sample preparation methodology and then it will be applied on irradiated samples to obtain local mechanical properties. After each preparation step, nanoindentation was performed and load was optimized leading to the minimum standard deviation, also taking into account an indent size and pile-up mechanism. Scanning Probe Microscopy (SPM imaging) and nanoindentation results showed the multi-grained austenitic structure with minimal roughness. Local mechanical properties can be measured according to the knowledge of radiation damage type and location, with focus on grain boundaries to be evaluated. This study shows that advanced methods such as ion polishing are not suitable for ASS preparation, but standard methods based on chemical reaction with structure, especially electrolytic polishing and etching, are highly recommended.

Preparation and properties of TiB2 particles reinforced Cu–Cr matrix composite

Cu–0.5Cr–1.5TiB2 composites prepared by different melting processes were investigated in the present study. The optimum reaction temperature for the dispersed distribution of TiB2 particles is 1300°C and a higher temperature is not appropriate due to the high loss of solute elements. Holding time should be 10min to make TiB2 particles exhibit a relatively homogeneous distribution in the matrix. An argon shield is necessary to ensure obtaining the desired chemistry. The typical TiB2 particles with equiaxed and symmetric morphology are observed in the matrix, while particles with irregular morphology are obtained when the growth process is inhibited by adjacent particles. After aging, the better mechanical properties are obtained by composite prepared at the optimum melting process. Strengthening is comprised of a dislocation pile-up mechanism of TiB2 particles, precipitation strengthening of Cr particles and strain strengthening. TiB2 particles slightly decrease the electrical conductivity, which indicates a minor contribution from interface scattering. The conductivity exhibits an obvious increase after aging because of the sharp decline of impurity scattering.

Flume investigation of landslide granular debris and water runup mechanisms

The heights of rigid debris flow barriers are designed to provide adequate retention and prevent debris from over-spilling. Designers need to predict potential runup height against a vertical wall to account for potential over-spilling. Experimental investigations of debris flow runup have previously been conducted using dry sand. A 5 m long rectangular flume was used to conduct runup experiments using both dry sand and water, separately. A combination of high-speed imagery, photoconductive sensors and laser sensors was used to study runup along the vertical face of a rigid barrier. The effect of Froude number (Fr) on runup was examined by varying the channel inclination. Commonly adopted energy and momentum approaches for predicting runup were compared with experimental results. The results reveal that runup mechanisms are dependent on approach Fr conditions and whether the flow medium is frictional in nature. For water, subcritical flows did not exhibit significant runup (reflective wave mechanism), whereas supercritical flows led to a vertical jet runup mechanism. Supercritical sand flow resulted in a pile-up mechanism instead of distinct runup.

An investigation on the mechanical property changing mechanism of high speed steel by pulsed magnetic treatment

An investigation was carried out to analyze and quantify the mechanical property changing mechanism of high speed steel by pulsed magnetic treatment. Specifically, a pulsed magnetic treatment system was designed and applied to the tool material treatment. The micro-hardness and red hardness were measured to quantitatively investigate the influence of magnetic treatment on the mechanical properties. Results showed that high speed steel material possessed better mechanical properties using the proposed pulsed magnetic treatment, of which the maximum increase of micro-hardness was 50HV. Also, the red hardness with low temperature could be improved. Moreover, the influence of magnetic treatment conditions on the micro-hardness change rate was investigated. It was shown that the micro-hardness change rate first increases and then stabilizes with the increase of magnetic field intensity, and it decreases with the increase of magnetization time. Ultimately, the dislocation densities were calculated by XRD data analysis according to the Williamson–Hall method and the interaction between magnetic field and dislocation was discussed in detail. Results revealed that the dislocation density of high speed steel material increased by 24% after pulsed magnetic treatment. The dislocation multiplication and motion induced by magnetic field led to the increase of dislocation density, which was the main reason of the increase in micro-hardness. The dislocation pile-up mechanism was responsible for the treatment effects of different magnetic treatment conditions. The dislocation configuration determined from TEM micrographs was in good agreement with the X-ray results and the discussion of dislocation mechanisms.

Dynamic crack surface instabilities initiated at dopants

The origin of micron-scale ridge-like fracture surface instabilities on the {1 1 1} low-energy cleavage plane of silicon crystal was elucidated by fracture experiments and surface analyses by scanning tunneling microscopy under ultra-high vacuum. These investigations show that when a low-speed crack collides with individual boron atoms along the crack front, atomic-height jogs are generated, gradually growing by pile-up mechanisms to over three orders of magnitude in height by forming ridges.

A systematic investigation of fracture mechanisms in Al–Si based eutectic alloy—Effect of Si modification

The fracture characteristics of Al–Si based eutectic alloy are investigated in the unmodified and modified conditions under compression. The investigations are carried out at different strain rates and temperatures. Fracture of the alloy starts with eutectic Si particle fracture and modification plays an important role in particle fracture. The fraction of fractured particles is found to be always lesser in the modified condition than in the unmodified condition. Particle fracture increases with increase in strain. It is found that the Si particle fracture shows an increase with increase in strain rate and decreases with increase in temperature at 10% strain. Large and elongated particles show a greater tendency for fracture in the unmodified and modified conditions. Particle orientation plays an important role on fracture and the cracks are found to occur almost in a direction normal to the tensile strain imposed upon the particles by the deforming matrix in the unmodified alloy. The modified alloy shows a random distribution of fractured particles and crack orientation. The criteria of fracture based on dislocation pile-up mechanism and fiber loading explain the observed difference in particle fracture characteristics due to modification. The particle fracture for the modified alloy is also discussed in terms of Weibull statistics and the existing models of dispersion hardening. Particle/matrix interface decohesion is observed at higher strain rates and temperatures in the modified alloy. Dendritic rotation of 10° is also observed at higher strain rates, which can increase the amount of particle fracture.