Molecular-dynamics studies of systems of confined dumbbell molecules

We present the results of molecular-dynamics simulations of systems of dumbbell molecules confined by parallel molecular walls. We have carried out systematic studies of three cases: freezing, steady flows, and stick-slip friction. We find that the molecular orientational degrees of freedom cause the surface layers to deviate from a planar configuration. Nevertheless, steady flows, in a channel as narrow as 15 molecular sizes, display continuum behavior. A range of mechanisms in the dynamics of the freezing of a confined fluid is found, as a function of the wall-fluid interactions and the bond length of the dumbbell molecules. The simple order-disorder transition associated with stick-slip motion in the presence of a layer of monoatomic lubricant molecules is supplanted by more complex behavior due to rotational degrees of freedom of the diatomic molecules.

Hidden order in serrated flow of metallic glasses

We report results of statistical and dynamic analysis of the serrated stress-time curves obtained from compressive constant strain-rate tests on two metallic glass samples with different ductility levels in an effort to extract hidden information in the seemingly irregular serrations. Two distinct types of dynamics are detected in these two alloy samples. The stress-strain curve corresponding to the less ductile Zr65Cu15Ni10Al10 alloy is shown to exhibit a finite correlation dimension and a positive Lyapunov exponent, suggesting that the underlying dynamics is chaotic. In contrast, for the more ductile Cu47.5Zr47.5Al5 alloy, the distributions of stress drop magnitudes and their time durations obey a power-law scaling reminiscent of a self-organized critical state. The exponents also satisfy the scaling relation compatible with self-organized criticality. Possible physical mechanisms contributing to the two distinct dynamic regimes are discussed by drawing on the analogy with the serrated yielding of crystalline samples. The analysis, together with some physical reasoning, suggests that plasticity in the less ductile sample can be attributed to stick-slip of a single shear band, while that of the more ductile sample could be attributed to the simultaneous nucleation of a large number of shear bands and their mutual interactions. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Response of Materials during sliding on various surface textures

In the present investigation, basic studies were conducted using Inclined pin-on-plate sliding Tester to understand the role of surface texture of hard material against soft materials during sliding. Soft materials such as Al-Mg alloy, pure Al and pure Mg were used as pins and 080 M40 steel was used as plate in the tests. Two surface parameters of steel plates — roughness and texture — were varied in tests. It was observed that the transfer layer formation and the coefficient of friction which has two components, namely adhesion and plowing component, are controlled by the surface texture of harder material. For the case of Al-Mg alloy, stick-slip phenomenon was absent under both dry and lubricated conditions. However, for the case of Al, it was observed only under lubricated conditions while for the case of Mg, it was observed under both dry and lubricated conditions. Further, it was observed that the amplitude of stick-slip motion primarily depends on plowing component of friction. The plowing component of friction was highest for the surface that promotes plane strain conditions near the surface and was lowest for the surface that promotes plane stress conditions near the surface.

Response of Materials During Sliding on Various Surface Textures

In the present investigation, soft materials, such as Al-4Mg alloy, high-purity Al and pure Mg pins were slid against hard steel plates of various surface textures to study the response of materials during sliding. The experiments were conducted using an inclined pin-on-plate sliding apparatus under both dry and lubricated conditions in an ambient environment. Two kinds of frictional response, namely steady-state and stick-slip, were observed during sliding. In general, the response was dependent on material pair, normal load, lubrication, and surface texture of the harder material. More specifically, for the case of Al-4Mg alloy, the stick-slip response was absent under both dry and lubricated conditions. For Al, stick-slip was observed only under lubricated conditions. For the case of Mg, the stick-slip response was seen under both dry and lubricated conditions. Further, it was observed that the amplitude of stick-slip motion primarily depends on the plowing component of friction. The plowing component of friction was the highest for the surfaces that promoted plane strain conditions and was the lowest for the surfaces that promoted plane stress conditions near the surface.

Confined fluids in a Janus pore: influence of surface asymmetry on structure and solvation forces

Density distribution, fluid structure and solvation forces for fluids confined in Janus slit-shaped pores are investigated using grand canonical Monte Carlo simulations. By varying the degree of asymmetry between the two smooth surfaces that make up the slit pores, a wide variety of adsorption situations are observed. The presence of one moderately attractive surface in the asymmetric pore is sufficient to disrupt the formation of frozen phases observed in the symmetric case. In the extreme case of asymmetry in which one wall is repulsive, the pore fluid can consist of a frozen contact layer at the attractive surface for smaller surface separations (H) or a frozen contact layer with liquid-like and gas-like regions as the pore width is increased. The superposition approximation, wherein the solvation pressure and number density in the asymmetric pores can be obtained from the results on symmetric pores, is found to be accurate for H > 4 sigma(ff), where sigma(ff) is the Lennard-Jones fluid diameter and within 10% accuracy for smaller surface separations. Our study has implications in controlling stick slip and overcoming static friction `stiction' in micro and nanofluidic devices.

Elastic-plastic contact conditions for frictionally constrained bodies under cyclic tangential loading

A Frictionally constrained condition implies dependence of friction force on tangential displacement amplitude. The condition may occur due to chemical, physical, and/or mechanical interaction between the contacting surfaces. The condition, sometimes also referred to as the presliding condition or partial slip condition, is characterized under fretting. Under such conditions, various experimental studies indicate the existence of two distinguishable regions, that is, stick region and slip region. In the present study, frictionally constrained conditions are identified and the evolutions of stick-slip regions are investigated in detail. Investigations have been performed on self-mated stainless steel and chromium carbide coated surfaces mated against stainless steel, under both vacuum and ambient conditions. Contact conditions prevailing at the contact interface were identified based on the mechanical responses and were correlated with the surface damage observed. Surface degradation has been observed in the form of microcracks and material transfer. Detailed numerical analysis has also been performed in order to understand the energy dissipation and the damage mode involved in the surface or subsurface damage. It has been observed that under frictionally constrained conditions, the occurrence of annular slip features are mainly due to the junction growth, resulting from elastic-plastic deformation at the contact interface. Ratcheting has been observed as the governing damage mode under cyclic tangential loading condition.

Conformal Cytocompatible Ferrite Coatings Facilitate the Realization of a Nanovoyager in Human Blood

Controlled motion of artificial nanomotors in biological environments, such as blood, can lead to fascinating biomedical applications, ranging from targeted drug delivery to microsurgery and many more. In spite of the various strategies used in fabricating and actuating nanomotors, practical issues related to fuel requirement, corrosion, and liquid viscosity have limited the motion of nanomotors to model systems such as water, serum, or biofluids diluted with toxic chemical fuels, such as hydrogen peroxide. As we demonstrate here, integrating conformal ferrite coatings with magnetic nanohelices offer a promising combination of functionalities for having controlled motion in practical biological fluids, such as chemical stability, cytocompatibility, and the generated thrust. These coatings were found to be stable in various biofluids, including human blood, even after overnight incubation, and did not have significant influence on the propulsion efficiency of the magnetically driven nanohelices, thereby facilitating the first successful ``voyage'' of artificial nanomotors in human blood. The motion of the ``nanovoyager'' was found to show interesting stick-slip dynamics, an effect originating in the colloidal jamming of blood cells in the plasma. The system of magnetic ``nanovoyagers'' was found to be cytocompatible with C2C12 mouse myoblast cells, as confirmed using MTT assay and fluorescence microscopy observations of cell morphology. Taken together, the results presented in this work establish the suitability of the ``nanovoyager'' with conformal ferrite coatings toward biomedical applications.

Dynamics of intermittent force fluctuations in vesicular nanotubulation

Irregular force fluctuations are seen in most nanotubulation experiments. The dynamics behind their presence has, however, been neither commented upon nor modeled. A simple estimate of the mean energy dissipated in force drops turns out to be several times the thermal energy. This coupled with the rate dependent nature of the deformation reported in several experiments point to a dynamical origin of the serrations. We simplify the whole process of tether formation through a three-stage model of successive deformations of sphere to ellipsoid, neck-formation, and tubule birth and extension. Based on this, we envisage a rate-softening frictional force at the neck that must be overcome before a nanotube can be pulled out. Our minimal model includes elastic and visco-elastic deformation of the vesicle, and has built-in dependence on pull velocity, vesicle radius, and other material parameters, enabling us to capture various kinds of serrated force-extension curves for different parameter choices. Serrations are predicted in the nanotubulation region. Other features of force-extension plots reported in the literature such as a plateauing serrated region beyond a force drop, serrated flow region with a small positive slope, an increase in the elastic threshold with pull velocity, force-extension curves for vesicles with larger radius lying lower than those for smaller radius, are all also predicted by the model. A toy model is introduced to demonstrate that the role of the friction law is limited to inducing stick-slip oscillations in the force, and all other qualitative and quantitative features emerging from the model can only be attributed to other physical mechanisms included in the deformation dynamics of the vesicle. (C) 2014 AIP Publishing LLC.

Damage mechanisms in stainless steel and chromium carbide coatings under controlled environment fretting conditions

Fretting is of a serious concern in many industrial components, specifically, in nuclear industry for the safe and reliable operation of various component and/or system. Under fretting condition small amplitude oscillations induce surface degradation in the form of surface cracks and/or surface wear. Comprehensive experimental studies have been carried out simulating different fretting regimes under ambient and vacuum (10(-9) MPa) conditions and, temperature up to 400 degrees C. Studies have been carried out with stainless steel spheres on stainless steel flats, and stainless steel spheres against chromium carbide, with 25% nickel chrome binder coatings. Mechanical responses are correlated with the damage observed. It has been observed that adhesion plays a vital role in material degradation process, and its effectiveness depends on mechanical variables such as normal load, interfacial tangential displacement, characteristics of the contacting bodies and most importantly on the environment conditions. Material degradation mechanism for ductile materials involved severe plastic deformation, which results in the initiation or nucleation of cracks. Ratcheting has been observed as the governing damage mode for crack nucleation under cyclic tangential loading condition. Further, propagation of the cracks has been observed under fatigue and their orientation has been observed to be governed by the contact conditions prevailing at the contact interface. Coated surfaces show damage in the form of brittle fracture and spalling of the coatings. Existence of stick slip has been observed under high normal load and low displacement amplitude. It has also been observed that adhesion at the contact interface and instantaneous cohesive strength of the contacting bodies dictates the occurrence of material transfer. The paper discusses the mechanics and mechanisms involved in fretting damage under controlled environment conditions. (C) 2015 Elsevier B.V. All rights reserved.

小湾电站坝基岩体卸荷工程地质力学研究

Dam is the key main works in the construction of water power. The success or failure of the construction of the dam mainly depends on the stability of the dam foundation. The double curvature arch dam-XiaoWan Dam is the highest one among the dams with the same type in the world, and the water thrust acted on it reaches ton, so the rock bearing capacity of dam foundation becomes more important. Because of the high and steep valley-side slope, the large scale of excavation and the complex body type of excavation, it is prominent that the problem of stress release of the rock mass in dam foundation. More great attentions should be paid for the stability and the degraded of rock properties of rock mass induced by the stress release. In this paper, the phenomena of stress release of rock mass in XiaoWan Dam foundation and its mechanisms were analyzed based on the collection of data, the detailed field engineering investigations, measurement of the rock mass and the 2D numerical calculations. The rock mass under the foundation is weak-weathered to intact, the quality of which is good. After excavation of the foundation, the rock mass near the slope surface occurred extend, stretch and stick-slip along original textures till the new fracture surface formed. Then platy structure of the rock mass takes on. The rock mass in the dam foundation occur resilience due to stress release towards free faces with the characteristics of time effect and localized deformation. In-situ measurements show that the rock mass near the surface are degraded. The stress release induced by excavation is a process of the interaction between engineering structures and geologic body. The stress release of rock mass in dam foundation is related to the changed degree of geometrical conditions. The rock near excavation surface failed nearly under uniaxial stresses. The bending-breaking mechanism of plate girder can interpret the failure model of the rock mass with platy structure in dam foundation slope. In essence, the stress release is the change of stress field including the change of directions and magnitudes of stress induced by excavation, which can induce the comedown of the safety margin. In this paper, the inducing conditions of stress release were calculated by numerical analyses. Moreover, from the point of view that the change of stress field, the coefficient of K, i.e. the variable load coefficient was proposed. Then the law of the change of it is interpreted. The distributional characteristics of fracture zone were expressed by the coefficient. The stress release of hard rock has the characteristic of localization. The measuring technique of sound wave can not reflect the small cracks in this kind of rock mass due to stress release. So, the spectral analysis method was proposed. At the same time, the application foreground in engineering of the Stockwell Time-Frequency- Spectrum method was discussed with a view to the limitation of it.

Modelling and visualisation of material flow in friction stir spot welding

Abstract The material flow in friction stir spot welding of aluminium to both aluminium and steel has been investigated, using pinless tools in a lap joint geometry. The flow behaviour was revealed experimentally using dissimilar Al alloys of similar strength. The effect on the material flow of tool surface features, welding conditions (rotation speed, plunge depth, dwell time), and the surface state of the steel sheet (un-coated or galvanized) have been systematically studied. A novel kinematic flow model is presented, which successfully predicts the observed layering of the dissimilar Al alloys under a range of conditions. The model and the experimental observations provide a consistent interpretation of the stick-slip conditions at the tool-workpiece interface, addressing an elusive and long-standing issue in the modelling of heat generation in friction stir processing.

Coupled subdaily and multiweek cycles during the lava dome erupt of Soufriere Hills Volcano, Montserrat

Observations of volcanoes extruding andesitic lava to produce lava domes often reveal cyclic behaviour. At Soufriere Hills Volcano, Montserrat, cycles with sub-daily and multi-week periods have been recognised on many occasions. These two types of cycle have been modelled separately as stick-slip magma flow at the junction between a dyke and an overlying cylindrical conduit (Costa et al. 2012), and as the filling and discharge of magma through the elastic-walled dyke (Costa et al., 2007a) respectively. Here, we couple these two models to simulate the behaviour over a period of well-observed multi-week cycles, with accompanying sub-daily cycles, from 13 May to 21 September 1997. The coupled model captures well the asymmetrical first-order behaviour: the first 40% of the multi-week cycle consists of high rates of lava extrusion during short period/high amplitude sub-daily cycles as the dyke reservoir discharges itself. The remainder of the cycle involves increasing pressurization as more magma is stored, and extrusion rate falls, followed by a gradual increase in the period of the sub-daily cycles.

Studying the contact point and interface moving in a sinusoidal tube with lattice Boltzmann method

The multicomponent nonideal gas lattice Boltzmann model by Shan and Chen (S-C) is used to study the immiscible displacement in a sinusoidal tube. The movement of interface and the contact point (contact line in three-dimension) is studied. Due to the roughness of the boundary, the contact point shows "stick-slip" mechanics. The "stick-slip" effect decreases as the speed of the interface increases. For fluids that are nonwetting, the interface is almost perpendicular to the boundaries at most time, although its shapes at different position of the tube are rather different. When the tube becomes narrow, the interface turns a complex curves rather than remains simple menisci. The velocity is found to vary considerably between the neighbor nodes close to the contact point, consistent with the experimental observation that the velocity is multi-values on the contact line. Finally, the effect of three boundary conditions is discussed. The average speed is found different for different boundary conditions. The simple bounce-back rule makes the contact point move fastest. Both the simple bounce-back and the no-slip bounce-back rules are more sensitive to the roughness of the boundary in comparison with the half-way bounce-back rule. The simulation results suggest that the S-C model may be a promising tool in simulating the displacement behaviour of two immiscible fluids in complex geometry.

On the control of a non-ideal engineering system: A friction-driven oscillating system with limited power supply

In this work a switching feedback controller for stick-slip compensation of a 2-DOF mass-spring-belt system which interacts with an energy source of limited power supply (non-ideal case) is developed. The system presents an oscillatory behavior due to the stick-slip friction. As the system equilibrium for a conventional feedback controller is not the origin, a switching control law combining a state feedback term and a discontinuous term is proposed to regulate the position of the mass. The problem of tracking a desired periodic trajectory is also considered. The feedback system is robust with respect to the friction force that is assumed to be within known upper and lower bounds.

Experimental Study on Friction between Saline Ice and Steel

Understanding the interaction of sea ice with offshore structures is of primary importance for the development of technology in cold climate regions. The rheological properties of sea ice (strength, creep, viscosity) as well as the roughness of the contact surface are the main factors influencing the type of interaction with a structure. A device was developed and designed and small scale laboratory experiments were carried out to study sea ice frictional interaction with steel material by means of a uniaxial compression rig. Sea-ice was artificially grown between a stainless steel piston (of circular cross section) and a hollow cylinder of the same material, coaxial to the former and of the same surface roughness. Three different values for the roughness were tested: 1.2, 10 and 30 μm Ry (maximum asperities height), chosen as representative values for typical surface conditions, from smooth to normally corroded steel. Creep tests (0.2, 0.3, 0.4 and 0.6 kN) were conducted at T = -10 ºC. By pushing the piston head towards the cylinder base, three different types of relative movement were observed: 1) the piston slid through the ice, 2) the piston slid through the ice and the ice slid on the surface of the outer cylinder, 3) the ice slid only on the cylinder surface. A cyclic stick-slip motion of the piston was detected with a representative frequency of 0.1 Hz. The ratio of the mean rate of axial displacement to the frequency of the stick-slip oscillations was found to be comparable to the roughness length (Sm). The roughness is the most influential parameter affecting the amplitude of the oscillations, while the load has a relevant influence on the their frequency. Guidelines for further investigations were recommended. Marco Nanetti - seloselo@virgilio.it