Physical Mesomechanics最新文献

The paper studies nitride coatings of the TiAlN and CrAlSiN systems with a thickness of 0.8…4.0 μm deposited by the vacuum ion-plasma technology. Specimens of nitrided 38Cr2MoAl and cemented 12Cr2Ni4 steels were used as substrates for coating deposition. Experimental data are derived on the physical and mechanical properties of the coatings by various indentation methods, including scratch tests, as well as on their tribological properties in sliding friction tests. The results of microstructural (SEM) and energy-dispersive X-ray (EDX) analysis of the coatings are also presented, including electron microscopic data on coating wear in tribological tests. It is shown that none of the physical and mechanical characteristics (hardness H, elastic modulus E, and their ratios H/E, H³/E²) determined by continuous or dynamic (for example, critical load F^c_N for coating spallation in scratch tests) indentation can separately describe the coating resistance to wear under the test conditions. This is the methodological problem that does not allow an accurate prediction of the coating wear. The problem is solved by the joint use of the calculated specific work of coating spallation from the substrate G and the coating resistance to plastic deformation H³/E², which together determine not only the coating degradation process, but also the loss of stability of the entire coating–substrate system.

A theoretical model is proposed to describe the micromechanism of plastic deformation in an ultrafine-grained Al-Mg-Zr alloy structured by high-pressure torsion (HPT) with grain boundary segregations of Mg atoms formed during HPT. In the model, plastic deformation is realized due to the emission of lattice dislocations from triple junctions of grain boundaries (GBs), which contain arrays of extrinsic grain boundary dislocations that are pinned by Mg atoms segregated at GBs. These segregations act as obstacles to gliding of extrinsic grain boundary dislocations, thus hindering the formation of dislocation pile-ups near the triple GB junctions and reducing the stress concentration at them, which leads to significant strengthening of the alloy. This model is used to calculate the yield strength of the ultrafine-grained Al-Mg-Zr alloy after HPT and after additional thermomechanical treatment consisting of low-temperature annealing and slight deformation by HPT. An increase in the alloy plasticity due to such thermomechanical treatment is discussed. The proposed model agrees well with the available experimental data.

Atomically ordered gold-copper alloys have technological applications, which makes the search for ways to improve their mechanical properties an urgent scientific and practical task. The present paper studies the effect of tensile and compressive stresses on the formation of an ordered structure, texture, and physicomechanical properties of an equiatomic CuAu alloy. All experiments were carried out on Ø1.5 mm wire specimens, which were initially disordered by quenching from 600°C or plastic deformation by 75%. An ordered structure was formed at a temperature of 350°C for 24 h; compressive stresses during annealing were 7 and 11 MPa; tensile stresses were 7 and 20 MPa. Comparison was made with the specimens ordered in a free state. It is shown that annealing in the compressive stress field causes a significant part of the short c axes of the ordered lattice to align along the force direction. Annealing under tension forms a different texture, i.e. most of the short c axes lie in the cross section of the specimen. The estimation of the degree of long-range order (S) showed that the specimens annealed in a free state had the maximum atomic order (S ≈ 0.95). According to the dilatometric study, the specimen quenched and then ordered in the compressive stress field demonstrates a sharp (by ~0.7%) increase in the length at the temperature of order → disorder phase transformation. It is found that compressive loads during ordering of the quenched specimens increase their strength and ductility, while tensile loads decrease these characteristics. It is shown that the mechanical properties of the specimens ordered after preliminary deformation are almost independent of the load direction. This phenomenon is explained by the absence of a clear texture (small loads do not cause rotation of c domains during ordering of high-strength deformed specimens).

This paper presents a new method for accurately determining the crack length in damaged steel beam structures. The proposed method combines the geometric updating technique of the finite element model (FEM) with a new variant of atom search optimization (ASO) called Lévy–ASO. The key feature of the Lévy–ASO algorithm is that it generates random step lengths determined by the Lévy distribution. Based on these step lengths, Lévy–ASO can achieve wider movements to expand the search space or narrower movements to exploit the potential search spaces, which is close the global optimum. It leads to a new search strategy within the ASO algorithm, effectively improving its ability to find the global optimum solution and escape the local optimum. To compare the effectiveness of Lévy–ASO with the original ASO, 23 classical benchmark functions are used as the first example. The comparison results show the superiority of Lévy–ASO over the original ASO in both accuracy and convergence rate. Then, a series of experiments were conducted on damaged steel beams with the crack lengths of 2 mm, 4 mm, 8 mm, and 10 mm to demonstrate the effectiveness and reliability of Lévy–ASO in determining the crack length of steel beams. Based on the vibration frequencies measured in these experiments and obtained from the finite element (FE) model, an objective function is established. The process of finding the crack length is carried out using the Lévy–ASO algorithm to optimize the objective function, which is established based on the analysis of the FEM where the geometric coordinates of the crack length are adjusted. This study proves the effectiveness of the proposed method, and the Lévy–ASO algorithm is recognized as a promising optimization algorithm for solving various engineering optimization problems.

On the basis of the analysis of deformation of a representative volume element, a micromechanical model is derived to describe the elastic modulus of a unidirectional short-fiber composite under tension in the reinforcement direction. The analysis includes an exact solution to the hyperelastic equations for the deformed matrix and an approximate solution to the equations for the fiber material. The solution is provided for a neo-Hookean material. Formulas are derived to relate the elastic strain energy to the macroscopic longitudinal strain of the composite and to describe the longitudinal and radial deformation of the matrix and fiber material. The main result is a formula that relates the initial tangential elastic modulus of the composite (an analog of Young's modulus in linear elasticity) to the mechanical characteristics of the composite constituents (namely, the ratio of the elastic modulus of the matrix material to the elastic modulus of the fiber), as well as to the geometric characteristic (fiber length-to-diameter ratio) and volume fraction of fibers in the composite. The derived results are compared with other analytical models, as well as with the known results of finite element and boundary element modeling. The results generalize the well-known shear lag (SL) model to hyperelastic materials and are obtained via a more rigorous analysis than the original model.

The paper studies the effect of the cooling rate during solidification on the microstructure of the quasi-binary Al-6Cu-3Gd alloy after casting and homogenization. Different cooling rates are implemented by laser surface melting (LSM), solidification in a cold or heated mold and with a furnace. It is shown that an increase in the cooling rate from 0.02 K/s to 10⁵–10⁷ K/s leads to a significant refinement of dendritic cells from 126 to 0.5 μm and intermetallic phases from 0.24 μm to 0.05–0.1 μm, which improves the hardness of ingots from 25 to 75 HV. The dependence of the dendritic cell size is accurately described by an empirical equation obtained for hypoeutectic silumin. The microstructure contains dispersed eutectic (Al) + Al₈Cu₄Gd (τ1) and individual inclusions of the (Al, Cu)₁₇Gd₂ (τ4) phase, which demonstrate high thermal stability during homogenization at 590 °C. The microstructure after LSM contains a network of larger particles about 1 μm in size, while the main proportion of 0.1–0.2 μm particles is uniformly distributed throughout the volume. In the alloys obtained at the intermediate cooling rates 1–15 K/s, which are close to industrial ones, the processes of fragmentation and spheroidization occur almost identically: the particle size changes from 0.1–0.2 μm in the as-cast state to 0.5–3 μm after 1–24 h of homogenization. In the alloy cooled at the minimum rate of 0.02 K/s, the particle morphology remains almost unchanged.

The paper reviews numerical models that explicitly consider structural features of additively manufactured austenitic stainless steel 316L on different scales. Experimental studies on hierarchical structures and the corresponding mechanical properties of the material are summarized. Key methods for generating two- and three-dimensional structures are discussed. Numerical studies on the features of plastic strain localization and stress concentration associated with interfaces of different nature, geometry, and scale in additively manufactured stainless steel 316L are overviewed.

The paper deals with the fatigue strength of rods made of the Ti-10V-2Fe-3Al alloy with different carbon concentrations (0.031 and 0.063 wt %) under uniaxial cyclic loading. The alloy with 0.063 wt % C contains titanium carbide particles similar in morphology to those of the primary α phase with an average size of about 2–3 μm. It is experimentally found that the 3 million cycle fatigue strength of the alloy with titanium carbide particles is 1000 MPa. Analysis of fracture surfaces reveals that the specimens experience subsurface fracture near the fatigue strength, and fracture sites are represented by facets where particles of the primary α phase accumulate and cracks are initiated. It is experimentally shown and numerically confirmed that the presence of titanium carbide particles in the alloy does not affect its fatigue strength.

A basic model is proposed for the mesoscopic dynamics of a magnetically active elastomer (MAE). The MAE unit cell consists of a pair of linearly magnetizable spherical particles embedded in a Kelvin-type viscoelastic elastomer. Forced oscillations of this system under a magnetic field with both constant and variable components are investigated within a specific amplitude–frequency range. In this range, the pair exhibits a distinctive behavior, which consists in a sudden transition from a finite distance between the particles to close contact (collapse). This phenomenon, known as bistability, is described in statics as magnetomechanical hysteresis, where strain as a function of the applied field shows an ambiguous region. It is demonstrated that, depending on the material parameters and field characteristics, various stationary oscillation cycles are possible. In addition, increasing the frequency of the variable field component reduces hysteresis effects. The system behavior at high oscillation frequencies is described qualitatively.

High-speed railway not only meets the demand for capacity, but also saves energy and reduces emissions, and helps economic development. As the core component of the high-speed electric multiple unit (EMU), the bogie plays the role of bearing, steering, braking, driving, and shock absorption. The bogie side beam is a thick composite structure, which is prone to delamination failure during loading. Prediction of structural delamination by the finite element method can effectively improve the design efficiency. In this paper, a model is established for the short 14T and 16T wheelbase and the lightweight bogie frame structure, and the macroscopic mechanical properties are calculated and predicted. The zero-thickness cohesive element in ABAQUS is used to simulate the delamination damage in the component. Static strength analysis is carried out for the given operation condition, and then the strength of the component is obtained. Due to optimization of the ply design, there is no damage to the bogie under operation conditions. It is illustrated that equivalent modulus theory for composites is suitable for the numerical analysis of delamination damage, and the effective ply design increases the interlayer strength.