The structure of composite materials is considered one of the main factors determining their properties, therefore its study, and ultimately knowledge, are prerequisites for predicting their behavior. Various technological methods for manufacturing products from fiber-reinforced polymer composites lead to differences in fiber distribution. It is known that when pressing with flat punches, the fibers are predominantly oriented parallel to the plane. However, it appears that this process may be influenced by the rheological properties of the matrix, fiber length, pressing height, and possibly the intensity of interaction between the filler and the matrix. Computer tomography is one of the reliable methods for analyzing and non-destructively testing the structure of composite materials. The capabilities of computer tomography for analyzing the structure of polyphenylene sulfide filled with glass fibers of various lengths are investigated. Composite materials were obtained by mixing in an extruder followed by compression molding of blanks. Samples of composite materials reinforced with fibers of various lengths were scanned with a resolution of 2.2 microns. Analysis of fiber distribution after tomography was performed using structural analysis with the VoxTex processing software. It is shown that the arrangement and distribution of fibers depend on the manufacturing method and fiber length. The conducted research shows that micro-computed tomography allows obtaining data on the structure of composite materials, which is a prerequisite for calculating physical and mechanical properties. Since technological factors can significantly affect the final structure of the composite material, this method makes it possible to establish a one-to-one correspondence between the manufacturing technology and the structure of the resulting composite.

The paper presents the results of experiments on studying the effect of long-term UV radiation and temperature on the mechanical properties of composite material. The studied material consisted of unidirectional carbon fibers IMS65 and high-temperature epoxy resin T26. According to the presented experimental program, accelerated degradation of composite material samples was carried out under the influence of elevated temperature +100°C and UV radiation for 360, 720, and 1080 hours. Degradation was carried out on a specially designed test bench, which included a thermal chamber and a UV lamp. This bench provided a controlled stable environment that allowed simulating the combined effect of thermal and UV radiation on the material over a long period. A total of 60 flat samples were prepared on the bench with reinforcement schemes [0]8, [±45]8, [90]8 and with different degradation times. These samples were tested under tension until failure, resulting in the determination of the elastic modulus and ultimate strength of the studied material. Based on the test data, curves of the dependence of elastic modulus and ultimate strength on material degradation time were constructed for different reinforcement schemes. Analysis of the obtained results showed that under the influence of UV radiation and elevated temperature, the elastic modulus and ultimate strength of the material decreased by 10-20%. It was also noted that for samples with reinforcement schemes [±45]8, [90]8, the degradation effect affects the mechanical properties more significantly. This necessitates consideration of the considered degradation factors when performing calculations of structures made of composite materials operated in adverse environmental conditions.

This paper investigates the fracture of a three-layer beam under three-point bending conditions. The condition of exceeding the maximum allowable tensile stress is chosen as the fracture criterion. A linear-elastic formulation within the framework of the Bernoulli-Euler beam model is used for analysis. Due to the formation of a diffusion contact of layers during the SHS (self-propagating high-temperature synthesis) technology, delamination effects are not considered. A relationship determining the ultimate allowable load for a three-layer beam is derived as a function of load magnitude, mechanical and geometric parameters of the beam. The obtained results were applied to the problem of determining the optimal layer proportions that would provide the beam with the best strength under three-point bending, provided that the layer materials are intermetallic – TiAl, ceramic – TiB, and metal – Ti, based on technological necessity, taking into account the dependence of parameters on temperature. The solution takes into account the possibility of fracture initiation from the middle or even the upper layer. With appropriate selection of parameters, when the neutral axis falls into the upper layer, the point of equal strength is investigated. For these materials, it is shown that the cause of failure is the TiB (middle layer) and Ti (lower layer) layers, and to maximize the ultimate allowable load for the composite beam, it is necessary to minimize the thickness of TiAl (upper layer). As examples, the dependence of the limit load on the introduced parameters is considered in the case of linear temperature dependences of Young’s modulus and flexural strength of each layer for such three-layer beams as intermetallic-ceramic-metal and ceramic-intermetallic-ceramic. The change in the influence of the layer ordinal number on the limit load with increasing temperature is shown.

A fully coupled boundary value problem of bending of a plate made of shape memory alloy under conditions of heat exchange with the environment and heat diffusion along the plate thickness, as well as taking into account the influence of latent heat of martensitic transformation, is solved. The problem includes solving the equations of mechanical equilibrium, heat conduction, and constitutive relations for the deformation of shape memory alloy implemented by a microstructural model. This model takes into account the elastic, thermal, and phase deformations of the material, the latter being associated with phase transitions occurring in it. The temperature field in the plate undergoing direct martensitic transformation during its cooling from the surface is calculated. Temperature distributions across the plate thickness are found for a series of successive time instants corresponding to different ambient temperatures. In particular, the time dependences of temperature on the surface and at the center of the plate are found. It is shown that without considering the release of latent heat of transformation, significant errors are obtained in calculating the time required for the completion of the direct transformation. Simulation of rapid bending of the plate at temperature Ms (start of direct martensitic transformation) and subsequent holding under constant bending moment under constant ambient temperature conditions is performed. It is shown that the neutral layer, where stresses are zero, is not located in the middle of the cross-section but is shifted towards the compressed layers due to the asymmetry of the properties of the alloy under consideration with respect to tension-compression. As a result of heat transfer, the temperature decreases in the inner layers of the plate, accompanied by direct phase transformation and strain accumulation over time. This phenomenon can be formally interpreted as creep. During holding under fixed deflection conditions, stress relaxation occurs over time. These effects should be taken into account when designing functional devices with a working body made of shape memory alloy.

The article performs asymptotic homogenization of elastic homogeneous and inhomogeneous in the transverse direction plates up to the fifth approximation. The asymptotic expansion is applied to the three-dimensional equilibrium equations of elasticity theory and reduces them to a series of two-dimensional problems in the plane of the plate and one-dimensional problems in the transverse direction. Consequently, the considered methodology is also a method of reducing the dimensionality of the original equations. Geometrically linear elasticity theory is considered. The asymptotic solution satisfies the boundary conditions on the faces, but not on the lateral surface. Accordingly, the edge effect is not considered. Asymptotic dimensionality reduction has been considered by many authors. The difference of this study from known works in this direction lies in the consideration of the fourth and fifth asymptotic approximations. It is shown that for the fourth approximation, the states of bending and deformation in the reference plane are not separated even for a homogeneous plate. For a homogeneous plate, the asymptotic theory used in the first approximation leads to differential equations analogous to Kirchhoff theory, and in the third approximation—to third-order theory. Reissner’s theory lies between them. For a laminated plate, the first approximation leads to classical laminate theory. As a special case, the performed asymptotic study is valid for strongly orthotropic plates, homogeneous and laminated plates. As an example, cylindrical bending of a homogeneous and three-layer plate is considered. A comparison of the fourth asymptotic approximation with the finite element solution is performed. It is shown that the fourth approximation allows significantly refining the transverse shear stress. Also, the asymptotic representation can be used as a basis for comparing various phenomenological plate theories of third and fifth orders.

The article analyzes the physics of deformation and fracture at the interface of a composite joint based on nanocrystalline/amorphous and crystalline alloys at liquid nitrogen temperature. Layered composites based on nanocrystalline alloy of grade 5BDSR and amorphous metallic alloy of grade 82K3KHSR with fusible alloys were prepared for the study. Mechanical tests of the prepared samples for uniaxial tension at room temperature and liquid nitrogen temperature were carried out. During tension of composite joints, it was found that some samples exhibit a ductile fracture nature regardless of the test temperature. Based on the obtained experimental results, a theory of self-heating at the crack tip of the composite is proposed, explaining the ductile nature of fracture at liquid nitrogen temperatures. Based on the proposed theory, modeling was performed, which showed that the energy released ahead of the crack tip is sufficient to heat the material by 100-200 K and transition to ductile fracture. This process is probabilistic and may be determined by the nature of crack growth at the beginning of the fracture process. If crack growth is preceded by atomic rearrangements, then the fracture will be ductile even at the cryogenic temperature of the composite sample as a whole. A physical model of deformation and fracture at the interface between the nanocrystalline/amorphous and crystalline phase in composite joints “fusible crystalline alloy – nanocrystalline/amorphous films” is proposed, explaining the retention of the ductile fracture nature in the temperature range of 77 K and 293 K under uniaxial tension conditions.

Currently, lattice composite structures are widely used in rocket and space technology as transition sections, rod elements of spacecraft, satellite bodies, payload adapters, etc. For all these structures, along with stiffness and strength characteristics, the minimum natural frequency of vibration is a critically important parameter. During the launch of payload into orbit, dynamic loads act on the launch vehicle and the payload, as a result of which the system experiences vibrations. When the frequency of these vibrations coincides with the natural frequency of the structure (system), the phenomenon of resonance occurs, sharply increasing the probability of structural failure. The use of unified basic structures (spacecraft platforms) can significantly reduce the development time of a new product, increase reliability (a platform already tested on previous products is used), and reduce the cost of delivering cargo to orbit. The paper investigates the influence of the main design parameters of the adapter (adapter height, rib section height, and spiral rib inclination angle) on the first natural frequency. The natural frequency values were obtained using the finite element method implemented in the commercial software “FEMAP with NX Nastran”. A mathematical model is constructed, the predictive capability of which was confirmed by finite element calculation. Using the obtained model, it is possible to determine the optimal design parameters of the adapter to achieve the maximum natural frequency, as well as to predict the first natural frequency for any adapter parameters within the computational domain. A methodology for finding optimal parameters for spacecraft platforms (load-bearing structure and adapter) is presented, allowing meeting the specified requirements for the natural frequency of the platform.

In most known phenomenological models of deformation of shape memory alloys, constitutive relations are proposed that express increments of phase-structural deformations through increments of stresses, temperature, phase composition parameter, and the values of stresses, deformations, temperature, and phase composition parameter themselves. The values of stresses, deformations, and temperature enter the right-hand sides of these relations in a rather complex, significantly nonlinear manner. The dependence of the phase composition parameter on temperature and stress deviator is also significantly nonlinear, and this nonlinearity does not obey A.A. Ilyushin’s particular isotropy postulate, i.e., a dependence is observed not only on stress intensity but also on the third invariant of the stress deviator. Procedures for numerical solution of boundary value problems on deformation of structural elements made of shape memory alloys by the finite element method require the construction of a velocity stiffness matrix. For this, it is necessary to have explicit expressions for increments of stress tensor components through increments of deformation components, temperature increments, and the values of stresses, deformations, temperature, and phase composition parameter themselves. Thus, it is necessary to construct an inversion of the known constitutive relations obtained within the framework of phenomenological models of deformation of shape memory alloys. The construction of such an inversion is complicated by the presence of a significantly nonlinear dependence of the phase composition parameter on the second and third invariants of the stress deviator. The paper presents a methodology for analytical obtaining of such an inversion. The results of numerical solution of the problem of direct transformation in a thick-walled cylindrical shell under constant internal pressure under assumptions of linear and bilinear dependence of material parameters on the stress state type parameter are presented.