Modeling, Synthesis and Fracture of Advanced Materials for Industrial and Medical Applications

The mechanical response of the eye to cornea loading by Maklakoff tonometers weighting 5; 7.5; 10; and 15 g is studied in the case of weakening of the cornea in the peripheral annular zone due to hyperopia surgical correction. Firstly sequential measurements of intraocular pressure are carried out, then the dependence curve of the tonometric intraocular pressure (IOP) versus tonometer weight is plotted and analyzed (so-called Maklakoff elastotonometry is used). The mathematical model of measuring the IOP by the Maklakoff tonometer is made in the engineering simulation software ANSYS Inc. The elastic system cornea–sclera is presented as two joint transversely isotropic spherical segments with different radii of curvature and biomechanical properties loaded with internal pressure. The results of elastotonometry for the preoperative stage and two different methods of vision correction LASIK and Femto LASIK are compared.

The paper presents the results of dynamic tests of some titanium alloys. The tests for compression, tension, and shear were caпied out using the Kolsky method and its modifications. In addition, a technique for determining the dynamic coefficient of friction, based on the Kolsky method, was implemented. As a result of the tests, dynamic, and static strain diagrams were plotted. Ultimate strength dependencies versus strain rate (yield stress σ₀₂, ultimate tensile strength σ^B, ultimate plasticity characteristics, axial strain δ and lateral strain ψ at fracture) were determined on the basis of the diagrams. The positive sensitivity of the strength characteristics to the strain rate was noted for the tested materials. At the strain rates more than 5 × 10² s⁻¹ the intensive growth of the strength characteristics is observed. The ultimate plasticity characteristics weakly depend on the strain rate. For the titanium alloy, the value of the dynamic coefficient of friction was obtained, which turned out to be close to its static value. The parameters of the well-known Johnson-Cook model were calculated using the obtained data. Verification experiments were performed, the results of which were compared with the results of virtual (computational) experiments. Good qualitative and quantitative agreement between experimental and numerical results was noted.

This study is devoted to the problem of damage detection in layered media. A package of dissimilar elastic layers is considered. Continuity conditions for the stress and displacement vectors are accepted on the interfaces between the layers. The package can be bonded to elastic half-plane and this interface may be damaged by an interfacial crack that can be of normal mode or shear mode or combined mode. It is suggested that strain/displacement measurements are accrued on the stress-free part of the external boundary. These data are used further on as input into a model that can detect the presence of interfacial cracks and predict delamination. The problem is reduced to the Cauchy initial problem for the complex potentials used in plane problems of elasticity and then solved by applying the Fourier transform. This problem is conditionally ill-posed, as it possesses unstable solutions, which can make impossible crack detection, as the noise in measured strains can be greater than the strain caused by interfacial cracks, and thus they could be indistinguishable. A possibility of crack detection is discussed for a special example.

New carbon nanostructured material is an ideal candidate for bone defects substitution and development of fundamental approaches for conventional orthopaedic surgery. Experiment on rats has been performed, which demonstrated biological compatibility with bone. Some characteristics of biocomposite at the bone-to-implant interface have been described. Mechanical properties of a wedge-shaped implant were tested under static and cyclic loading conditions. The need incompression between bone and implant has been shown. Clinical application of some special-shaped implants for substituting large bone defects and axial bone disorders has been performed. The excellent clinical and economic prospects of new carbon nanostructural implants have been demonstrated.

In the present paper, the mechanical characteristics (reduced Young’s modulus and indentation hardness) were measured for three sound human enamel and dentine samples using nanoindentation. Each sample was subjected to 60 s exposure in citric acid with varying concentration (0.5 wt.%, 3 wt.%, 5 wt.%). Each sample is a longitudinal section obtained from a single human maxillary molar. For each sample, a series of characteristic curves for the dependence of indentation depth on indentation load (load-displacement curves) for both enamel and dentine were obtained.

The process of nucleation of fatigue wear in industrial alloys and ion-plasma coatings under the influence of high-speed discrete droplet flow is investigated. The results obtained relate to the stage of microstructural mechanics of fatigue fracture. This stage precedes the stage of the steady growth of fatigue cracks, which is described by the continuum mechanics of fatigue failure (for example, by the Paris–Erdogan concept). Processes of formation of fatigue defects in materials and coatings with a heterogeneous structure are considered on the base of the dislocations theory. A computational–analytical model for determining the moment of formation of a defect (cracks, pores, microcrater) of critical size depending on the parameters of droplet impacts is proposed and theoretically substantiated. The model is universal in its application to various materials and coatings, in which the mechanisms for the nucleation of defects are of a dislocation nature. The correspondence of the results of numerical experiments in the framework of the developed model to the results of bench tests is shown. The data testifying to the high compliance of the results of the application of the proposed model and the well-known fatigue model of Murakami–Endo for steels and alloys in conditions of droplet impacts are given. At the same time, the results obtained using both models for thin high-strength ion-plasma coatings contradict each other.

Compacts from aluminium and iron powders were compressed under various loads such that their cylindrical surfaces touched that of the concentrically placed sleeve around the compact. Further compression led to the development of frictional conditions between the compact and the sleeve. The variation of frictional force depended on the ductility of the material and the particle shape. The interfacial coefficient of friction showed no dependence on the material, and showed a power law relation with the axial force. The surface roughness (R_a) values varied along the height of the compacted aluminium powder sample from 4.5 to 6.64 microns, while the hardness ranged from 24 Hv to 11 Hv.

Synthetic hydroxyapatite is involved in osteogenesis when using as an implant. Different substitutions in the hydroxyapatite (HAp) can change the low rate of degradation and solubility of pure hydroxyapatite as well as other physical, chemical, and biological properties. We synthesized single-phase Ca₈Gd₂(PO₄)₄(SiO₄)₂(OH)₂ (Gd, Si-HAp) by coprecipitation and investigated its behavior in model solutions: water, simulated body fluid and physiological saline solution (0.9% NaCl). Morphology of samples before and after degradation confirmed that flake-like crystals of initial Gd, Si-HAp is a preferable form for the further precipitation of HAp crystals with needle-like morphology the same as in human body. The degradation rate of substituted Gd, Si-HAp in comparison with pure HAp is higher in different solutions and the rate of nucleation is apparently slower as we can see in a series of in vitro degradation tests.

Microposit S1813 is very popular photoresist due to high resolution, simplicity of deposition methods, and high adhesion to semiconductors. The least controlled parameter of photoresist thin films deposition is the tanning process, where everything depends on the operator and therefore it is quite difficult to ensure perfect reproducibility of the process. In this work, we studied thin films of Microposit S1813, deposited by spin coating on silicon substrates, and then tanned in 30, 45, 60, 75, and 90 s at 110 \(^\circ \)C. The thickness of the films was studied using optical microscopy. The waviness and surface roughness were investigated by atomic force microscopy. To evaluate the mechanical properties of Microposit S1813 thin films, the nanoindentation method in the quasistatic mode was used. The results obtained will help to determine the region of the indenter-sample contact area as well as the measurement error, which is necessary for modern analytical mathematical models used to study the mechanical characteristics of multilayer coatings.

The issue of evaluating the strength and service life of critical engineering facilities with the exploitation properties characterized by multi-parametrical nonstationary thermal–mechanical effects is discussed. The main degradation mechanisms of structural materials (metals and their alloys), characteristic of such facilities, are examined. Basic requirements imposed on mathematical models of the above processes are formulated. In the framework of mechanics of damaged media (MDM), a mathematical model is developed, which describes processes of inelastic deformation and damage accumulation in the presence of creep. The MDM model consists of three interrelated parts: relations defining inelastic behavior of the material, accounting for its dependence on the failure process; equations defining damage accumulation kinetics; a strength criterion of the damaged material. The results of experimentally studying short-term creep of the VZh-159 heat-resistant alloy at constant temperatures and different stresses applied to the specimens are presented. Deformation and damage accumulation processes have been numerically analyzed; the obtained numerical results are compared with the data of full-scale experiments. The results of numerically modeling the carrying capacity of a nuclear power plant (NPP) reactor vessel in the conditions of a hypothetical accident are presented. The emergency was modeled by applying pressure modeling the effect of meltdown, constant internal pressure and temperature varying within the limits of the analyzed part of the reactor vessel. The analysis of the obtained numerical results reveals a number of characteristic features accompanying the process of deformation and failure of such facilities, related with time and location of the forming macrocracks, history of the stressed–strained state (SSS), damage degree in the failure zone, etc. In general, the results of comparing the numerical and experimental data make it possible to conclude that the proposed defining relations of MDM are reliable in accounting for the degradation of initial strength properties of materials according to the long-term strength mechanism and can be effectively used for evaluating long-term strength of structural elements under thermal–mechanical loading.

The problem of nonlinear moment theory of elasticity about the equilibrium of a hollow sphere with distributed dislocations is considered. For an arbitrary isotropic micropolar elastic material and a spherically symmetric distribution of screw and edge dislocations, the problem is reduced to a system of nonlinear ordinary differential equations. In the case of a physically linear micropolar body model, exact solutions are found for the eigenstresses in the sphere due to the spherically symmetric distribution of edge dislocations.