Physical Mesomechanics最新文献

Seismic emission is the spontaneous emission of waves in a porous or fractured medium. There is still no explanation for the dependence of these phenomena on the medium parameters and pore structure, as well as for the transition from slow to fast movements. The paper proposes a description of the transition process from statics to dynamics using a model of a structured continuum. The most important parameters of the model are the specific surface area and porosity of the medium. The reason for the emission is that the forces caused by internal stresses are only on average equal to zero over a representative volume, being different at each point of the medium. It is shown that the model of a structured continuum predicts the emission of waves under static load and gives an estimate of the vibration periods. In this case, the characteristic vibration periods depend neither on the characteristic time of destruction of particles, nor on the loading time.

A computational approach is proposed for assessing the oriented curvature of vortex-like atomic structures using regression analysis by component-wise search for regressors. The studied discrete vector fields of atomic displacements of such structural states are obtained based on the results of molecular dynamics simulations of a defect-free deformed crystallite. Within the given approach, a continuous representation of the desired discrete vector field is obtained. The redistribution of the free volume and lattice reorientation are analyzed. It is found that the value of oriented curvature outside specific regions (vortex centers and regions of compensated atomic displacements) is about |K| ≈ 0.286 nm^–1. The relationship is shown between the vortex-like structures and model concepts of a continuous disclination-type defect.

This paper investigates the effect of thermomechanical treatments, including deformation by radial shear rolling or severe plastic deformation by abc pressing with subsequent aging at 773 K, on the structural-phase state, deformation behavior, and mechanical properties of commercial near β titanium alloy VT22 (Ti–5Al–5Mo–5V–1Cr–1Fe). The structure of the alloy after radial shear rolling and subsequent aging consists of transformed β grains with a lamellar α + β structure and primary α-phase particles. Severe plastic deformation of the alloy followed by aging causes the formation of a grain-subgrain α + β structure with an average characteristic size of 0.23 µm. It is found that, after the thermomechanical treatments, the strength characteristics of the alloy at room temperature increase by ~40% compared to the as-received alloy. The alloy after radial shear rolling and aging retains a 40–20% higher strength in the temperature range of 293–823 K. The strength of the alloy after severe plastic deformation and aging becomes lower than that of the as-received alloy already at a temperature of 773 K. Analysis of creep parameters at 743 K shows that the creep deformation of the alloy in the state after radial shear rolling and aging occurs by the motion of dislocations (glide + climb). The creep deformation of the alloy in the state after severe plastic deformation and subsequent aging is largely contributed by grain boundary sliding.

A comprehensive study is reported on high- and hypervelocity impacts of a steel ball simulating a space debris particle on shielded targets. Experimental studies of high-velocity impact of a steel ball were performed in the velocity range up to 2500 m/s. The obtained data were used to verify the mathematical model and numerical algorithm. Numerical simulation of the space debris impact on a shielded target was carried out in the impact velocity range 1400–7000 m/s using the finite element method implemented in the original EFES software package. The proposed failure algorithm can describe the material fragmentation and the formation of new contact boundaries without computational mesh distortion. The specific features of shock wave processes and the destruction of the target and ball were investigated at different impact velocities.

Due to the different superior properties of lightweight and high-strength aluminum and high-conductivity copper metals, the joining of the two is very common and important in today’s industrial applications. Generally, there is no formula to follow for the setting of welding parameters, and the setting is completely based on the past knowledge and experience of experts. Once the range of expert experience is exceeded, the optimal parameters cannot be effectively set, which may easily lead to poor welding quality. This research aims to develop an economical and effective Taguchi experimental design method for achieving the highest shear strength value for aluminum/copper friction stir spot welded joints. Three independent welding process variables were considered including the pin rotation speed, dwell time, and downward pressure. Different optimization techniques such as Taguchi, TOPSIS, artificial neural network, genetic algorithm, and their combinations were utilized for obtaining the best ranges of input welding parameters to achieve the maximum shear strength values. The optimal combination of process parameters was found at the rotation speed of 1800 r/min, the dwell time of 15 s, and the downward pressure of 0.2 mm. The results showed that the integration of the TOPSIS method, neural network, and genetic algorithm provides the best combination of parameter values for the verification of shear strength experiments. According to the performed analyses, the degree of influence of the independent variables on the shear strength of bi-material joints can be ranked as: dwell time > pin rotation speed > downward pressure.

This paper introduces a new shear deformation theory, employing the hyperbolic sine function, for exploring the free vibration properties of a novel functionally graded (FG) shell structure. The proposed theory ensures a parabolic distribution of shear strains and stresses across the thickness, with zero values at the top and bottom surfaces, eliminating the requirement for any shear correction factor. This is the first time such an approach has been utilized for studying this type of FG structure. The material properties are assumed to vary gradually across the thickness in the form of a trigonometric function. The proposed FG material stands out due to its excellent rigidity and smooth and continuous variation of the material components through the thickness. This composition has the potential to compensate for the deficiencies found in conventional FG sandwiches. Two types of functionally graded shells are considered: the trigonometric FG-A shell and the trigonometric FG-B shell. The governing equilibrium equations of the FG shell are derived in detail with the principle of virtual work and are solved analytically by the Galerkin method that can cover different boundary conditions. The proposed solution is constrained to rectangular and straight FG plates of uniform cross-section. A wide range of comparative studies is carried out to establish the accuracy and the performance of the present analytical model. A detailed parametric analysis is performed to highlight the influence of the material inhomogeneity parameter, geometry and various boundary conditions on the vibration response. The proposed model has an important role in the design of various vessels and shells.

The paper presents some multidisciplinary research results on the structure of slip surfaces in segments of tectonic faults in the Baikal region and Mongolia. The properties of subsurface (modern) and deep slickensides exposed after many-kilometer denudation of the Earth’s upper crust are studied at different levels—from macroscale to nanocrystals. Other types of heterogeneities characterizing the structure of fault slip zones are also considered. The presented data indicate a heterogeneous structure of tectonic faults. Their slip zones show both low-friction regions where strong mineral phases are replaced by weak minerals and potentially unstable spots with high friction resistance. Results of the comprehensive study of geological conditions under which different-scale heterogeneities emerge in exhumed fault segments should be taken into account when developing rock mass models suitable for numerical simulation of earthquake preparation processes at the micro-, meso- and macroscales.

Many models for analyzing the dynamic propagation of seismogenic ruptures are based on solving classical problems of fracture mechanics. It is assumed that the fault is a shear crack with uniformly distributed friction and stress concentration at the crack tip. Known fracture mechanics theories do not describe the formation of damage zones in the lateral direction, i.e. perpendicular to the crack plane. Observational data indicate the presence of a fairly extensive zone of damaged material in the vicinity of the fault. This is the zone of dynamic influence where the material has an increased fracture density, higher permeability and lower elastic wave velocities. A correct assessment of the properties and sizes of zones of dynamic influence is crucial for constructing adequate earthquake preparation models. This paper analyzes regularities of development and quantitative characteristics of the damage zone during dynamic earthquake rupture and quasi-static evolution of the fault. The size and mechanical characteristics of the near-fault damage zone produced by movement along the slip surface can be conveniently estimated by the second invariant of the deviatoric stress tensor (shear intensity). Matching of the calculated value with a certain degree of rock mass damage can be done using measurement data from large-scale explosions, by comparing them with the calculation results. It is shown that coseismic movement along the fault leads to insignificant changes in the properties of the host rock. However, the longitudinal wave velocity near the fault decreases markedly by 30–35%, the permeability increases only by approximately a factor of three, and the increase in the degree of fracturing is almost unnoticeable. This means that the properties of the rock mass change due to the opening of preexisting cracks. Repeated movements do not radically change the characteristic dimensions and properties of the damage zone. It is concluded that the fault-affected zone is formed mainly at the quasi-static stage of the formation of the main fault through the coalescence of individual macrofractures, and future seismogenic movements renew the already existing fractures.

A generalization of a physically nonlinear Maxwell-type viscoelastoplastic constitutive equation with four material functions is formulated, whose general properties and range of applicability were discussed in a series of our previous studies. In order to expand the range of rheological effects and materials that can be described by the equation, it is proposed to add a third strain component expressed by a linear integral Boltzmann–Volterra operator with arbitrary functions of shear and volumetric creep. For generality and for the convenience of using the model, as well as for fitting the model to various materials and simulated effects, a weight factor (degree of nonlinearity) is introduced into the constitutive equation, which allows combining the original nonlinear equation and the linear viscoelastic operator in arbitrary proportions to control the degree of different effects modeled. Equations are derived for families of creep curves (volumetric, shear, longitudinal, and transverse) generated by the proposed constitutive equation with six arbitrary material functions, and an expression is obtained for the Poisson ratio as a function of time. Their general properties and dependence on loading parameters and characteristics of all material functions are studied analytically and compared with the properties of similar relations produced by two combined constitutive equations separately. New qualitative effects are identified which can be described by the new constitutive equation in comparison with the original ones, and it is verified that the generalization eliminates some shortcomings of the Maxwell-type viscoelastoplastic constitutive equation, but retains its valuable features. It is confirmed that the proposed constitutive equation can model sign alternation, monotonic and nonmonotonic changes in transverse strain and Poisson’s ratio under constant stress, and their stabilization over time. Generally accurate estimates are obtained for the variation range, monotonicity and nonmonotonicity conditions of Poisson’s ratio, and its negativity criterion over a certain time interval. It is proven that neglecting volumetric creep (the postulate of bulk elasticity), which simplifies the constitutive equation, greatly limits the range of possible evolution scenarios of Poisson’s ratio in time: it increases and cannot have extremum and inflection points. The analysis shows that the proposed constitutive equation provides ample opportunities for describing various properties of creep and recovery curves of materials and various Poisson’s ratio evolution scenarios during creep. It can significantly expand the range of described rheological effects, the applicability of the Maxwell-type viscoelastoplastic equation, and deserves further research and application in modeling.

Statistical data analysis was performed based on the results of standard tests with metallic materials. The relation was found between the so-called fatigue limit, which is considered as the boundary between micro- and mesoscale fatigue fracture processes, and mechanical characteristics under monotonic tension. Using heat-resistant alloy EI698 and titanium alloy VT22 as examples, it was shown that the mechanisms of fatigue crack initiation and growth in materials with different ratios of fatigue limit to yield stress are different. The mechanism of slip band formation determines the fatigue crack initiation and its early growth stage for materials with the σ_–1/σ_0.2 ratio close to 1 and higher, as was shown for EI698. For materials with the σ_–1/σ_0.2 ratio below 1, the fatigue crack initiation is associated with mechanisms other than slipping, as demonstrated for VT22.