Evgeny V. Shilko

Quasi-fluid nano-layers at the interface between rubbing bodies: simulations by movable cellular automata

Abstract Most of technical surfaces show roughness on different space scales. When pressed against each other, they initially come into contact only in small number of micro-contacts. Our aim was to study the processes occurring in a single micro-contact between two rubbing bodies. The solids were simulated in the frame of the method of movable cellular automata (MCA). The main finding of our simulations is formation of a boundary layer where intensive plastic deformation and mixing processes occur. The boundary layer is well localized and does not spread to deeper layers. We investigated how the thickness of the boundary layer and the friction force stemming from the processes in this layer do depend on parameters of material and loading. To this end, all the parameters involved in the numerical model have been varied and the average friction coefficient as well as thickness of the layer determined for each set of parameter. We found that at velocities much smaller than velocity of sound and normal pressures much smaller than the yield stress, the thickness of the quasi-fluid layer is proportional to effective viscosity of the medium and the friction coefficient does depend only on two dimensionless parameters: κ1=ρv2E/σ02 and κ2=PE/σ02.

Converting displacement dynamics into creep in block media

The possibility of converting displacement dynamics into a creep regime in deformed block media is corroborated by the results of numerical simulation. The model is based on the dynamics of one tectonic platform moving on another. The proposed model describes the characteristic features in the behavior of geological media quite well; in particular, it reproduces the earthquake statistics (as described by the Gutenberg-Richter and Omori laws). An analysis showed that local, relatively low-energy actions could transform the block dynamics from the stick-slip to creep regime. This implies that the statistics of seismic energy evolution can be qualitatively modified using a series of local, periodic, and relatively weak actions so that the average strength of seismic impacts significantly decreases. In this way, it is possible to “suppress” strong earthquakes. The results justify good prospects for a concept previously proposed by the authors, according to which the initiation of displacements in fault-block media can provide a mechanism for releasing local elastic energy and, in particular, decreasing seismic danger.

Influence of the state of interfaces on the character of local displacements in fault-block and interfacial media

We have studied the possibility of producing a directed action upon the process of local stress relaxation in interfacial media occurring in a complex stressed state by changing the state of boundaries between structural elements. The experiments were performed on the ice sheet of Lake Baikal, which represents a hierarchically organized fault-block structure and belongs to the class of interfacial media. It is shown that, by changing the state of boundaries between structural elements, it is possible to influence the regime of deformation of the interfacial medium as a whole. The general features of the observed effect are confirmed within the framework of a theoretical model.

Peculiarities of the mechanical response of heterogeneous materials with highly deformable interfaces

Peculiarities of the deformation and fracture of heterogeneous materials with a large fraction of interfacial regions (interfacial materials) under the action of complex alternating loads have been studied by computer-aided simulation. The dependence of the character of fracture, deformability, dissipation of the applied energy, and strain distribution in the material bulk on the frequency of cyclic loading was determined. High-frequency vibrations at a frequency much greater than that of the natural oscillations may significantly increase the deformability of interfacial materials.

Effect of compression nonequiaxiality on shear-induced dilatation in a block-structured medium

The peculiarities of shear-induced dilatation in block-structured media under nonequiaxial compression are investigated using the movable cellular automaton method. For a characteristic of compression nonequiaxiality (also termed the degree of constraint) a dimensionless parameter representative of the lateral to normal pressure ratio in the deformation plane is used. The main objective of the work is to trace the sequence in which various dilatation mechanisms are involved in deformation depending on the shear stress level and degree of constraint. It is shown that in a block-structured medium as a hierarchically organized system, increasing the degree of constraint changes the dominating dilatation mechanism from slip of discontinuity surfaces to opening and expansion of pores. The dominating dilatation mechanism is changed because the increase in the degree of constraint increases the slip-activating threshold shear stress. Beginning with certain lateral pressures, the slip is impeded giving way to pore space expansion; however, the latter fails to produce the so noticeable volume change as the slip of discontinuity surfaces does, and this lowers the critical dilatation characteristics of the medium, in particular, its volume change and dilatancy coefficient.

Parametric study of the conditions of supershear crack propagation in brittle materials

The paper is devoted to the numerical analysis of the conditions of acceleration of dynamically propagating longitudinal shear cracks from sub-Rayleigh to intersonic/supershear velocities. We showed that an ability of the initial crack to propagate in supershear regime can be predicted with use of the empirically derived dependence of the geometrical criterion of sub-Raleigh-to-intersonic transition on material and crack parameters.

Study of the role of vortex displacement in contact loading of strengthening coatings based on movable cellular automaton modeling

Main attention of the research is focused on the role of vortex-like structures in the velocity fields of the strengthening coating and substrate under contact loading by hard conical indenter. The peculiarities of velocity vortex formation and propagation, as well as its interaction with structural elements are studied. One of possible application of the study is non-destructive technique for detecting nanoscale defects in surface layer of a material using frequency analysis of the friction force. Possibilities of this technique are studied based on 3D simulation.

Theoretical study of peculiarities of unstable longitudinal shear crack growth in sub-Rayleigh and supershear regimes

In the paper we present the results of the theoretical study of some fundamental aspects of mode II crack propagation in conventional sub-Rayleigh regime and transition to intersonic regime. It is shown that development of a sub-Rayleigh shear crack is determined in many respects by elastic vortex traveling ahead of the crack tip at a shear wave velocity. Formation of such a vortex helps to better understand the well-known phenomenon of acceleration of a shear crack towards the longitudinal wave velocity. Simulation results have shown that due to self-similarity of shear crack propagation the conditions of sub-Rayleigh to intersonic transition depend on dimensionless material and crack parameters. Two key dimensionless parameters are proposed.

Strength of shear bands in fluid-saturated rocks: a nonlinear effect of competition between dilation and fluid flow

This study shows the significant and nonlinear effect of the competition between dilation and fluid flow on the shear strength of constrained shear bands in fluid-saturated rocks. This effect is conditioned by the contribution of the pore pressure to the yield stress and strength. The pore pressure is controlled by the dilation of the pore space in the solid skeleton of the shear band during plastic deformation and by squeezing of pores in surrounding blocks by the dilating shear band due to the high stiffness of the host massif. A generalized equation has been derived to describe the dependence of the shear band strength on the ratio of strain rate to fluid flow rate.

Theoretical study of strength of elastic-plastic water-saturated interface under constrained shear

This paper presents a theoretical study of shear strength of an elastic-plastic water-filled interface between elastic permeable blocks under compression. The medium is described within the discrete element method. The relationship between the stress-strain state of the solid skeleton and pore pressure of a liquid is described in the framework of the Biot’s model of poroelasticity. The simulation demonstrates that shear strength of an elastic-plastic interface depends non-linearly on the values of permeability and loading to a great extent. We have proposed an empirical relation that approximates the obtained results of the numerical simulation in assumption of the interplay of dilation of the material and mass transfer of the liquid.