Physical Mesomechanics最新文献

In some case, polymer composites can exhibit a significantly nonlinear deformation behavior. The conventional approach to estimating the strength of a composite structure, in which the material is considered linearly elastic up to failure, can lead to significant errors and overloading. In this work, we examine the effectiveness of two relatively simple models taking into account the nonlinear deformation of the fabric composite with a thermosetting matrix. Both models are based on the assumption that the shear stress–strain curve is independent of the 2D stress state. On the one hand, this significantly simplifies the process of determining model parameters, because only standard tensile and shear tests are used. On the other hand, it is necessary to clearly understand the applicability limits of such models. Verification of deformation models was carried out based on the results of off-axis tensile tests on carbon fabric-reinforced composite specimens as well as composite specimens with the ±15° and ±30° lay-ups of the carbon fabric. Comparison of the calculated and experimental stress–strain curves shows that this approach can be successfully used in the analysis of the nonlinear mechanical behavior of fabric composites made of thermosetting polymers under shear strains up to 5% without significant errors.

The investigation of waves propagating in intricate structures encompassing diverse materials and diverse interfacial conditions holds immense significance in multiple fields, including geophysics, nondestructive testing, and sensor technology. Shear horizontal waves propagate out of plane, which is described by the direction of wave propagation and normal to the surface of the medium. The behavior of waves is influenced by material properties, the nature of bonding, and boundary conditions. The interior of the Earth is characterized by heterogeneity, stresses, and imperfect bonding between layers. Thus, the current study delves into examining the behavior of antiplane shear horizontal waves in an intricate geometrical structure comprising a heterogeneous layer lying on the initially stressed substrate. Perfect contact between materials with different properties is nearly impossible to achieve, thus bonding between the layer and the substrate was assumed to be imperfect. The imperfectness is modeled through various conditions including dislocation-like, force-like and spring-like conditions at the interfacial surface. These interfacial conditions are analyzed along with traction-free and rigidly fixed boundary conditions at the free surface of the layer. Dispersion relations are analytically derived under each scenario. Graphical representations are plotted to illustrate the impacts of various parameters, such as heterogeneity, initial stress, layer thickness, imperfectness, and jumping coefficients, on the propagation of antiplane shear horizontal waves.

Biocompatible magnesium alloys offer promise as materials for the manufacture of bioresorbable implants. This work is devoted to establishing rational modes of equal channel angular pressing (ECAP) of the Mg-8.6Zn-1.2Zr alloy in order to form a structural state with high mechanical properties and corrosion resistance. It is found that 1 cycle of ECAP at a temperature of 400°C results in a noticeable increase in the tensile strength (up to 330 MPa), while the corrosion resistance decreases. Immersion tests indicate that, after 1 cycle of ECAP at 400°C, the corrosion rate in Ringer’s solution reaches 9 mm/year. It is suggested that the 2nd cycle of ECAP at a decreased temperature of 250°C should be carried out to maintain the tensile strength at 325 MPa and increase the corrosion resistance to a level corresponding to the as-annealed state (6–8 mm/year). EBSD studies reveal that this behavior is associated with an increase in the number of special Σ13a, Σ15b and Σ17a boundaries in the structure after the 2nd ECAP cycle.

The paper reports on the deformation mechanism (slip and twinning), strain hardening rate Θ(ε), and plasticity of ([bar 111])- and ([bar 144])-oriented single crystals of the Cr₂₀Fe₂₀Mn₂₀Co₃₅Ni₅ (at %) high entropy alloy (HEA) under tension in the temperature range 373–573 K. It is shown that the onset of plastic flow in this temperature range is associated with the slip of a/2‹110› dislocations highly split into a/6‹112› partial Shockley dislocations, and critical shear stresses for slip (tau _{{rm{cr}}}^{{rm{sl}}}) are independent of the crystal orientation. The development of high-temperature twinning at the temperature 373–573 K and strain 5 and 20% is first discovered in the ([bar 111])- and ([bar 144])-oriented single crystals of the Cr₂₀Fe₂₀Mn₂₀Co₃₅Ni₅ HEA, respectively. High-temperature twinning is facilitated by the combination of a low stacking fault energy γ₀ = 14 mJ/m² and heavy lattice distortion as a result of mixing of substitutional atoms in equal or nearly equal atomic concentrations. It is found that, at high temperatures, (tau _{{rm{cr}}}^{{rm{tw}}}) does not depend on the crystal orientation and the test temperature: (tau _{{rm{cr}}}^{{rm{tw}}} = (80...110) pm 5) MPa at the ([bar 111]) orientation and (tau _{{rm{cr}}}^{{rm{tw}}} = (100...110) pm 5) MPa at the ([bar 144]) orientation. The maximum plasticity of 70–90% is realized for the ([bar 144])-oriented crystals when twinning develops mainly in one system. The dependence of the strain hardening rate Θ(ε) is characterized by stages, which are observed during twinning in face-centered cubic polycrystals.

The phenomenology and nature of two-level (matrix and second phases) nanostructuring of complex aluminum alloys during thermomechanical processing based on cold severe plastic deformation are considered. The importance of taking into account the principle of structure heterogeneity optimization at all stages of processing by controlling the parameters of second phases is substantiated. A number of conclusions are formulated that can be used as the basis for the criteria of this principle for the alloy treatment involving severe plastic deformation to ensure its effective nanostructuring and property enhancement. A new approach to classification of nanostructured alloys is proposed.

In the paper, we study the influence of high-temperature low-cycle fatigue on the microstructure of a new low-nitrogen and high-boron 10% Cr steel additionally alloyed with cobalt, tungsten, molybdenum, and rhenium. After heat treatment, the lath structure of tempered troostite with a high dislocation density both within laths and at the boundaries of martensite laths is stabilized by particles of grain-boundary carbides M₂₃C₆ and M₆C, as well as by carbonitrides NbX uniformly distributed throughout the matrix volume. The average width of martensite laths was 380 nm, and the density of free dislocations within the laths was 1.4 × 10¹⁴ m^–2. As the strain amplitude increases from 0.2 to 1% during low-cycle fatigue, the number of cycles to failure decreases by ~2 orders of magnitude, while the contribution of the plastic strain component increases significantly. Maximum softening (24%) is observed at the temperature 650°C and strain amplitude 0.6% in the middle of cyclic loading. After low-cycle fatigue tests, the studied steel contains small recrystallized grains free of lattice distortions. Moreover, the lath structure begins to transform into a subgrain structure, with the lath width and subgrain size being dependent on the strain amplitude. The density of free dislocations is hardly affected by the increase in the strain amplitude compared to the initial state, while the density of dislocations at the lath boundaries decreases significantly with strain amplitude, which is due to shortening of the martensite lath boundaries. Fractography shows that oxide particles act as sources of crack initiation at both temperatures of low-cycle fatigue tests.

Reducing the hazard of sudden rock and gas outbursts in coal seams is one of the key problems of geodynamic safety of mining. The paper presents a mathematical model of hydraulic fracturing of roof rocks as one of the key methods for reducing the outburst hazard. Structural features of the working area are studied by means of an example of the host rock strata of seam 3 of the Alardinskaya mine of the Kondomskoye field of the Kuznetsk coal basin. Three-dimensional finite difference analysis and continuum damage mechanics are used to study the influence of the well spacing on the formation of an effective crack network in the hard-to-cave roof during hydraulic fracturing. It is shown that changing the initial well spacing affects not only the formation time of an effective crack network but also the spacing between damage accumulation zones. Hydraulic fracturing of roof rocks from the working face effectively reduces the abutment stress and outburst hazard.

The influence of additional deformation-heat treatment consisting in annealing at 150 or 230°C and additional deformation by 0.25-revolution high-pressure torsion (HPT) at room temperature on the microstructure, mechanical characteristics, and electrical conductivity of the ultrafine-grained Al-1.17Mg-0.33Zr (wt %) alloy processed by HPT at room temperature is studied for the first time. It is shown that deformation-heat treatment at both annealing temperatures leads to the plasticization effect in the material, i.e. a significant increase in plasticity (by more than an order of magnitude) on retention of high strength (80% of the strength of the untreated alloy). The revealed effect is compared with that in ultrafine-grained Al-Mg-Zr alloys with a lower magnesium concentration. It is shown that the value of plasticity achieved as a result of deformation-heat treatment (annealing at 150°C and additional 0.25-revolution HPT) decreases, and the strength increases as the Mg concentration grows from ~0.5 to ~1.2 wt %. The ultrafine-grained alloy Al-1.17Mg-0.33Zr (wt %) demonstrates a higher thermal stability compared to the ultrafine-grained Al-Mg-Zr alloys with a lower Mg concentration, which allows using a higher annealing temperature (230°C) during deformation-heat treatment. It is found that deformation-heat treatment by 230°C annealing and 0.25-revolution HPT provides the best combination of strength (yield strength ~380 MPa, ultimate tensile strength ~480 MPa) and plasticity (elongation to failure ~9%, uniform strain ~4%), which is not inferior to commercial Al-Mg alloys with ~4% magnesium after conventional strengthening treatment or treatment by equal channel angular pressing. The physical reasons for such combination of properties are analyzed against microstructural changes during deformation-heat treatment.

The elastic properties of a number of binary titanium alloys Ti–Me (Me = V, Nb, Mo, Ta) with a body-centered structure were calculated using the exact muffin-tin orbital method in the coherent potential approximation. It is shown that the elastic constants C₁₁ and C₁₂ increase with concentration of the second component in β-Ti–Me alloys, although the latter weakly depends on the concentration. However, C₄₄ decreases in the presence of V and Nb and increases in the presence of Mo and Ta. According to the calculated densities of electronic states, the concentration behavior of C₁₁ is due to an increase in chemical bonding with the second neighbors, which is most pronounced with an increase in the number of d electrons of the alloying element. It is found that all the studied binary alloys have the lowest Young’s moduli near the β-phase instability region and in the ‹100› direction. With growing tantalum concentration, the anisotropy of Young’s modulus decreases, but its pattern remains unchanged. However, V-, Nb- and Mo-containing alloys become practically isotropic at a certain concentration of the second component, and their anisotropy pattern changes. In general, the obtained elastic characteristics of binary titanium alloys are in good agreement with the available experimental and theoretical data.

This paper examines the nature of h- and S-type plastic flow instabilities within the concept of localized plasticity au-towaves. It is shown that both of these instabilities can be observed in the same ARMCO iron material in the form of switching (h-type instability) or excitation (S-type instability) autowaves. The switching autowave represents the localized deformation front uniformly moving under a constant stress, and the excitation autowave represents the same front, but moving with a constantly decreasing velocity with reducing stress. The switching autowave passes continuously through the object, but the excitation autowave propagates intermittently. The manifestation of one or the other wave is deter-mined by the temperature-strain rate conditions. There is an interval of low temperatures where, regardless of the strain rate, only a switching autowave is generated, and the deformation front velocity increases exponentially with increasing stress. An excitation autowave can generate at high temperatures, when the deformation front moves abruptly during stress drops. This phenomenon can be interpreted in terms of dynamic strain aging. Under such conditions, the front velocity depends linearly on the stress. It is shown that the deformation front velocity is always determined by local strain rates at the front. Using the dislocation approach to dynamic strain aging and by analyzing the dependences of local strain rates on the effective stress, it is established that the switching autowave (h-type instability) is controlled by thermally activated motion of dislocations, and the excitation autowave (S-type instability) is governed by their viscous overbarrier motion.