Strength of Materials最新文献

Previous studies have shown that with increasing service life of shipbuilding steels, a strong hydrogen charging of their internal near-surface layers occurs, especially if the service life exceeds 3 years or more. This is known to cause changes in the mechanical properties of steels. Therefore, there is a need to conduct additional experimental studies on the effect of hydrogen in a wide temperature range on the degradation of the structural and phase states of steels, in particular, on changes in the crystal lattice and redistribution of cementite, which directly leads to a decrease in the ductile and deformation properties of the metal, especially at subzero air temperatures. The effect of service life and subzero temperatures on the stress state of the a-matrix lattice and its parameters for the 10KhSND and D32 steels was investigated. A tendency to increase in the value of lattice distortion stresses and decrease in the mass fraction of cementite in specimens of these steels after long-term operation was revealed. Metallographic studies showed that with decrease in the temperature of the cooling medium, the volume fraction of hydrides increases significantly, which leads to embrittlement and softening of steels.

This paper proposed a modeling method of the thermo-fluid-solid coupling simulation for the hot water pipeline with the insulation layer outside in the nuclear reactor. The temperature and stress distributions are analyzed and compared with the tests; the effects of the insulation layer thickness and support interval are investigated. The maximum stress occurs at the support points. The maximum stress increases approximately linearly with the increase of the support interval. The maximum deflection occurs at the middle of the two supports. The maximum deflection increases rapidly with the increase of the support interval, which is approximately in a parabolic relationship. The axial elongation ratio increases with the insulation layer thickness. The simulation results agree with the test results very well with regard of both stress and temperature distributions. The simulation method can also be used for the stress and temperature analysis in various cases, such as chemical-vessel and gas-oil pipeline with various thermal fluids inside.

The results of a comparative analysis of five models of multiaxial fatigue based on the concept of the critical plane are presented. The Fatemi–Socie, Wang–Brown, Wu–Hu–Song, and augmented generalized strain energy models were studied. The durability calculated by these models was compared with experimental data obtained for 10 metal alloys and six multi-axis loading paths. The data analysis showed that the prediction of durability under multiaxial loading can be improved by using a fatigue damage parameter that includes the maximum shear strain and the square of the linear strain at the maximum shear site. The proposed model can be considered a new variant of the Brown-Miller model, where for the first time the fatigue damage parameter was presented as the sum of the maximum shear strain and the linear strain at the maximum shear site. It is shown that this model correlates well with the experimental data for both proportional and non-proportional loading.

Aluminum alloy plates with varying properties were prepared through the superposition friction stir processing of three different aluminum alloys with friction stir joining technology and subjected to solid solution aging. Experimental results showed that friction stir joining yielded a relatively smooth aluminum alloy surface gradient change plate. The plate surface presented a circular superposition trajectory with a clear outline and a rough surface with a flying edge during preparation. The three superimposed aluminum alloy plates achieved good fusion. In particular, fusion was most evident between the uppermost 5052 aluminum alloy and the middle layer of the 6061 aluminum alloy. The fusion of the middle layer of the 6061 aluminum alloy and the lower layer of the 7075 aluminum alloy weakened, and an obvious dividing line was observed. The average Vickers hardness values of the uppermost and undersurfaces of the samples heat-treated at 540°C were the highest, which were 1.34 and 119.06% higher than those of the samples without heat treatment, respectively. The most notable cross-section Vickers hardness gradient was obtained at the solution temperature of 500°C.

The Armed Forces of Ukraine pay great attention to the development of personnel protection armor. However, despite the availability of protective equipment, the practice of warfare shows that the tasks of protecting military personnel are not fully resolved. The task of developing protective structures against bullets and shrapnel remains urgent. The analysis of scientific and technical information showed that the processes that occur during the impact interaction of elements of mechanical systems have not been fully studied, and the development of means to protect against kinetic damage has not been fully covered in scientific papers. The process of interaction between the impactor and the protective barrier may change if nested structures are used as a protective barrier. The possibility of developing a new method for increasing the strength of armor protection by using nested structures (bullet–cylinder) is investigated. A new analytical dependence for determining the depth of penetration of a bullet into a cylinder is obtained. The novelty of the analytical dependence lies in the absence in the scientific literature of a mathematical model of bullet penetration into a cylinder whose inner diameter is smaller than the outer diameter of the bullet, taking into account the effective friction coefficient between the bullet and the inner surface of the cylinder, and the fact that the depth of penetration of a bullet into a cylinder can be equal to the length of the bullet, or be greater or less than it. On the basis of the proposed analytical dependence for determining the depth of penetration of a bullet into a cylinder, the dependences of the depth of penetration of a bullet into a cylinder on the value of tension (between the bullet and the cylinder) and the thickness of the cylinder are constructed. The research results indicate the fundamental possibility of reducing the depth of penetration of a bullet by using nested structures.

In experimental investigations, conventional measuring instruments such as dial gauges, linear variable differential transformers, and extensometers are used to measure the displacement at a specific gauge length (i.e., global deformation). They are contact-based instruments which are not able to evaluate the displacements within the gauge length (i.e., localized deformations). The image analysis techniques are able to evaluate the displacements at each point within the gauge length. In the present study, the applicability and effectiveness of the image analysis technique are assessed for four different materials, i.e., concrete, steel, wood, and geomembrane which are widely used in the field of geotechnical engineering as construction material for geo-structures. For this, four different tests, i.e., compression testing of the concrete cube, tensile testing of the steel plate and wooden strip, and wide-width tensile testing of the geomembrane are performed in a systematic manner. The image analysis technique is used to evaluate the displacement fields for each material under given loading conditions. The results obtained from the image analysis technique are compared with those of conventional measuring instruments to emphasize the applicability and effectiveness of the image analysis technique for different materials having different properties.

This study conducted modernization of the VK-350 instrumented drop weight impact testing machine (DWITM) developed at the Pisarenko Institute of Problems of Strength of the National Academy of Sciences of Ukraine, equipped with a high-speed strain and force recording system that allows recording a load diagram with a sampling rate of up to 20 MHz, and heating and cooling systems for testing specimens in a wide temperature range. Modernization was carried out to expand the functionality of the DWITM, namely, a new dynamometer and specimen fixing units for impact shear, compression and dynamic pushing tests were installed. The new dynamometer was calibrated for two ranges of amplification and test specimens of various types were tested. The DWITM was also equipped with a video recording system, which includes a high-speed PHOTRON FASTCAM NOVA S9 camera that allows recording the deformation and fracture process at a speed of up to 200,000 frames per second. The camera is equipped with a 12x zoom lens and stroboscopic high-frequency lamps for illumination with the GSVITEC MultiLED G8 control system. Based on the results of the impact bending and shear tests, video signals of the deformation and fracture of the specimens were obtained, which made it possible to determine the time of onset of plastic deformation and formation of adiabatic shear zones in the impact shear specimens, as well as the moment of crack appearance on the side surface of the Charpy specimen and to estimate the rate of its propagation.

In rotary wing aircraft, composite lug structures are perforated structures that connect the rotor and blades. They play a significant structural role in rotary wing aircraft. In this study the effect of lay-up parameters on mechanical strength of a thick composite part that represents a blade root. This part is autoclave manufactured polyetherketoneketone matrix carbon fiber reinforced (CF/PEKK) composites, which have recently been researched for usage in defense and aerospace is investigated. The chosen material carbon-fiber-reinforced polyetherketoneketone composites (CF-PEKK) have an excellent mechanical, physical, thermal performance. Aerospace sector has a special interest on this material due to material’s low density and versatility. In this study, static and fatigue performance of rotorcraft lug part were investigated by applying non-destructive-test (NDT) methods and the mechanical strength values were discussed according to the experimental results. It has been observed that the component parts with ∓45° layup has the highest mechanical strength. As a result of the static and fatigue tests, it is seen that satisfactorily performance in the view of the both static and dynamic loading states.

The effectiveness of the previously proposed improved approach for determining the non-proportional cyclic hardening coefficient in predicting the maximum level of strain hardening and durability of metallic materials was tested. The approach is based on the correlation between static and cyclic strain hardening of metallic materials, takes into account the amplitude of cyclic deformation, and does not require fatigue experiments under non-proportional loading. The calculated and experimental values of this coefficient were compared for structural materials with different cyclic and physical properties. For the 27 analyzed materials, the maximum level of strain hardening was predicted using the obtained calculated coefficient, and a good agreement with experimental data was demonstrated. Using the strain criterion for assessing durability, which includes the calculated non-proportional cyclic hardening coefficient, the durability for circular cyclic trajectories of non-proportional deformation was predicted on the basis of the basic uniaxial fatigue diagram. Satisfactory results of durability prediction (in comparison with the experiment) were obtained for materials with FCC metal lattice structure. For materials with BCC structure, the agreement between the calculated and experimental data was somewhat worse. It is shown that for this type of materials, the use of an alternative method for determining the non-proportional cyclic hardening coefficient can improve the results of durability prediction.

Using the created experimental equipment for testing metals by the method of instrumented indentation, a procedure for determining Brinell hardness during the nondestructive testing of structural elements has been developed. In contrast to the conventional method, the hardness is determined by the proposed procedure using the proportionality parameter of plastic indentation, a, which is equal to the slope of the instrumented indentation diagram in the coordinates maximum force of cycle F_max – residual indentation depth h_p after removing the test load of this cycle. Based on the results of a series of tests by the method of instrumented indentation of hardness standards, a linear correlation dependence was obtained between the Brinell hardness value and the proportionality parameter of plastic indentation in a wide range of measured hardness values of 110–650 HBW. The peculiarities of using this procedure and its limitations are analyzed on a number of structural carbon, heat-resistant, and high-strength steels. It is shown that the Brinell hardness measurement results, obtained by the improved procedure, agree within the permissible error with the results of the conventional DSTU ISO 6506-1:2019 method. The difference between their values does not exceed 3.9%. The presented improved procedure can be used in the laboratories of research and educational institutes, central factory laboratories and specialized divisions of various subordination, and other organizations involved in monitoring the condition of the operating critical equipment and setting its further service life both in the laboratory and in the field.