A New Combined Model for considering the Plasticity Effects in Contacting Asperities

Wheel-rail contact in railway engineering is an important topic. Due to different materials and surface roughness of wheel and rail, the contact characteristics can alter significantly. This article aims to investigate the effects of surface roughness and asperities on the contact parameters such as contact area, contact force, and contact stiffness. The lateral contacts between asperities are assumed to be the general contact condition. Azimuthal and contact angles distributions are assumed to be spherical harmonic distribution. This assumption is compatible with the asperity distribution on the wheel and the rail surfaces. Besides, a new combined model is developed to cover the stick-slip and the plasticity effects in contacting asperities. The results of the presented model offer very good estimations for the asperities contact characteristics, especially at the small-contact area and separation where high-contact pressure and plastic deformation usually exist.

The influence of the heterogeneous asperity distribution on induced seismicity and permeability evolution during hydraulic fault zone stimulation

<p>Several decameter-scale in-situ stimulation experiments were conducted in crystalline rock at the Grimsel Test Site, Switzerland, with the aim to advance our understanding of the seismo-hydro-mechanical processes associated with deep geothermal reservoir stimulation. To allow comparability between the experiments, a standardized injection protocol was applied for all experiments. Induced seismicity was recorded using acoustic emission sensors and accelerometers, which were distributed along tunnel walls and within four boreholes. Hydro-mechanical responses of the fault zones were measured using grouted longitudinal fiberoptic strain sensors and open pressure monitoring borehole intervals. A total of four ductile shear zones (with brittle overprint) and two brittle-ductile shear zones have been stimulated during these experiments.</p><p>Here we present an analysis of heterogeneous permeability evolution within a target shear zone during ongoing stimulation. The shear zone in question is an originally ductile shear zone which contains a single fracture in the injection interval. The observed planar seismicity cloud indicates that most of the stimulation process was confined within the target shear zone. Hydraulic characterization of the injection interval before and after stimulation revealed an enhancement in interval transmissivity from 8.3<sup>-</sup>10<sup>-11</sup> m<sup>2</sup>/s to 1.5<sup>-7</sup> m<sup>2</sup>/s. Within the reservoir, the seismo-hydro-mechanical data (i.e. seismicity cloud, pressure peaks and local deformation) spatiotemporally coincide, suggesting that permeability enhancement along the shear zone is highly localized and heterogeneous. Thus, we argue that the permeability evolution is linked to asperity distribution and breakdown within the shear zone.</p><p>The conceptual model developed from the experimental analysis is implemented in a three-dimensional numerical model, with which we attempt to simulate the directional permeability creation observed in the experiment. The model accounts for a discrete planar fault zone of finite thickness with distributed low-permeability, brittle asperities embedded in a more permeable damage zone mimicking the ductile shear zone at Grimsel. The hydro-mechanical processes are modeled with the TOUGH-FLAC simulator, which sequentially couples fluid flow and poroelastic deformation within the fault and the surrounding medium. A Mohr-Coulomb failure criterion is used to simulate asperity reactivation, which can lead to permeability enhancement of the reactivated area.</p>

STUDY ON COAL FRACTOGRAPHY UNDER DYNAMIC IMPACT LOADING BASED ON MULTIFRACTAL METHOD

Coal fractography is a powerful tool for interpreting coal fracture behaviors, which is significant for dealing with failure issues encountered in deep coal mining. However, the accuracy of coal fractography highly depends on the method of quantitatively characterizing coal fracture surfaces. In this study, coal fractography under dynamic impact loading was investigated based on a multifractal method, the multifractal spectrum parameters were proposed to quantitatively describe the coal fracture surfaces. The width of the multifractal spectrum [Formula: see text] characterizes the uniformity of the surface asperity distribution, and the spectrum parameter [Formula: see text]–[Formula: see text] characterizes the proportion of dominant asperities on fracture surface. The coal fractography results indicate that larger loading rate leads to more asperities on the coal fracture surfaces, i.e. rougher fracture surfaces, and the fracture surfaces are dominated by small asperities induced by dynamic impact loading. In addition, significant anisotropy effect was found on the fracture surfaces under dynamic impact loading by the spatial distributions of multifractal spectrum parameter [Formula: see text]. The parameter [Formula: see text] was further utilized to determine the macrocrack direction and microfracture markings on the coal fracture surfaces, the results transpire that the multifractal method is feasible for coal fractographic analysis under dynamic loading conditions.

Tangential Contact Stiffness of Rough Cylindrical Faying Surfaces Based on the Fractal Theory

The tangential contact stiffness of cylindrical asperities is investigated using macro- and micro-mechanisms in this study. A microanalysis model is developed and the tangential contact stiffness of elliptically parabolic asperities on the contact surface is determined. The shape influence coefficient of the cylindrical contact is defined, and its rationality is evaluated. The influence of asperity distribution on the rough surface is determined, and the tangential contact stiffness macroanalysis model is constructed based on fractal theory. The mathematical expression to determine the tangential contact stiffness of the macroscopically cylindrical contact is generated, and the effects of influential factors on tangential contact stiffness are explicitly evaluated. Numerical results show that the tangential contact stiffness of asperities is determined by several factors, such as material properties, applied loads, fractal dimension, and surface shape.