Strength of Materials最新文献

In this study, using laser cladding technology, three different coatings with varying contents of spherical tungsten boride (WB) were completed on the EH40 steel substrate. These coatings included the Co coating, Co+15%WB coating, and Co+45%WB coating. The electrochemical corrosion performance of these three coatings was investigated in a low-temperature environment. The findings indicated that the phases present in the WB-reinforced Co-based coatings are mostly Cr₂₃C₆ and Cr₇C₃, WB, and WO₃, as well as γ-Co. The study showed that as the amount of tungsten boride in the coatings rose, their corrosion resistance increased and gradually dropped. Among them, the Co+15%WB coating exhibited the best corrosion resistance in a neutral solution. In the low-temperature (–20°C) immersion test, the main corrosion products for the Co+45%WB coating in simulated seawater solution were Co(OH)₂ and Co₃O₄, along with the presence of WO₂ and WO₃ oxides. Overall, the spherical tungsten boride coatings significantly enhanced the corrosion resistance of the EH40 steel substrate, providing an effective approach to improving the steel’s performance in low-temperature environments.

This research aims to investigate approximate process parameters in severe surface mechanical treatments, which play a main role in producing good surface quality, inducing residual stress, and less damage to material during surface treatment of materials. The Taguchi orthogonal array and ANOVA are utilized to find the impact of process parameters and their significant contribution. It is observed that shot diameter and speed of revolution of the shaft have a significant effect on surface hardness. The optimum condition, i.e., an 8 mm shot diameter, a 750 rpm speed of revolution, and a 45 min treatment duration, contribute a higher surface hardness of 124 HV confirmed with the predicted value, and the obtained surface hardness is 35% higher than the untreated specimen. Compressive residual stress is calculated using the depth-sensing indention method, which is about 126 MPa for the optimum condition of hardness. The depth of the deformed layer is around 350 μm from the top surface towards the metal core. The nanohardness is improved from 1.311 to 1.464 GPa for the optimum condition which is 10% higher than the unpeened specimen.

The mechanical properties, alloy microstructure, and fatigue resistance under cyclic loading of a 1.5 mm thick D16 sheet with a stress concentrator in the form of a central hole were investigated. In the process of fatigue damage accumulation in the area of the stress concentrator, changes in the microdeformation relief in the form of peaks and troughs were periodically recorded to quantitatively assess the parameters of the discrete deformation relief on the surface of specimens. To analyze mesostructural deformations, the surface relief of specimens was observed by illuminating their surface with a coherent irradiation source with a wavelength of 0.45 μm. Kinetic plots of the deformation parameters of the surface of specimens at the stages of high-cycle deformation were constructed, and the parameters of the deformation relief of the surface layer, which correlates with the scattered damage, were determined. Based on the measurements of the spatial parameters of local peaks and troughs of the deformed specimen surface, kinetic plots of fatigue localization were constructed. It is shown that these plots can be used to judge the residual life of a cyclically loaded material.

Disk cutters, as the main component of rock fragmentation, are prone to various types of breakages during rock cutting due to the unevenness of rocks in nature, which can affect rock breakage efficiency. This study has numerically simulated stress and strain changes for different rocks under rock breaking using disk cutters. This study has used several behavioral models, including elastic- plastic constitutive, plastic-kinematic, and elastic constitutive models, to analyze stress and strain in rock breakage. The aim was to determine different rocks’ strength and deformation ability under the action of TBM disk cutters. In three simulation models, the maximum stress in the elastic model exceeded those of plastic-kinematic and elastic-plastic models by 15.5 and 8.7 times, respectively. The plastic model is very susceptible to deformation and damage. On the other hand, the elastic model requires significant stresses to produce partial strains and releases the stress faster as the disk shear continues.

A new method to suppress the galloping of electric conductors in high-power transmission lines based on isochronous roller vibration dampers is proposed. The mathematical model of the dynamic process of suppressing the conductor galloping was constructed, including the interaction effect of the damper and string of insulators as a bearing body. This model differs from the previous ones since it accounts for the string rotation together with the damper due to its deviation from the vertical and special shaping of the damper grooves. The mathematical vibration protection model was devised with two nonlinear differential equations, which, after their linearization and integration, can find the amplitude-frequency response equation in the linear statement of the problem. The formula was analytically derived for the natural vibration frequency of the working body center of mass in the isochronous damper, which does not include the roller radius. The critical control damper parameters, which affect the suppressing quality and efficiency of forced conductor vibrations, are defined. The graph-numerical method was advanced to determine the optimum parameters of the damper adjustment. The damper offers the advantage of its natural frequency independence of the working body amplitude. This property provides high adjustment accuracy of its parameters and best performance over the low-frequency range. The isochronous damper can greatly decrease the conductor galloping level: the angle of the insulator string deviation from the vertical can be reduced fivefold.

A new numerical-and-analytical method is developed for solving geometrically and physically nonlinear problems of bending shallow shells of complex shapes made from materials with different resistance to tension and compression. To linearize the initial nonlinear problem, the method of continuous continuation in the parameter associated with the external load was used. For the variational formulation of the linearized problem, a Lagrange functional was constructed, defined at kinematically possible displacement velocities. To find the main unknowns of the problem of nonlinear bending of a hollow shell (displacements, deformations, stresses), the Cauchy problem for a system of ordinary differential equations is formulated. The Cauchy problem was solved by the Runge-Kutta– Merson method with automatic step selection. The initial conditions are found in the solution to the problem of geometrically linear deformation. The right-hand sides of the differential equations at fixed values of the load parameter corresponding to the Runge-Kutta–Merson scheme were obtained from the solution of the variational problem for the Lagrange functional. The variational problems were solved by the Ritz method in combination with the R-function method. The latter makes it possible to present an approximate solution in the form of a formula, which solution structure exactly satisfies all (general structure) or part (partial structure) of the boundary conditions. The problems of nonlinear deformation of a square cylindrical shell and a shell of complex shape with combined fixation conditions are solved. The influence of the direction of external loading, geometric shape, and fixation conditions on the stress-strain state is investigated. It is shown that failure to consider the different behaviors of the material in tension and compression leads to significant errors in calculating the stress-strain state parameters.

This work presents the Charpy energy values (CVN) in the ductile-to-brittle transition (DBT) temperature zone for different directions with respect to the longitudinal direction of API 5L X52 seamless pipeline steel. The material-processing directions were L-T, T-L, S-T, T-S, L-S, S-L, and 45°. At the same time, the temperature variation was –100, 0, 25, 50, and 100°C. Charpy impact specimens were machined in accordance with the ASTM E23 standard, with dimensions of 10×10×55 mm. In addition, the microstructures and fracture surfaces were examined using scanning electron microscopy. The energy values for all directions show lower shelf behavior at –100°C, a transition zone from –100 to 50°C, and an upper shelf behavior above 50°C as the characteristics of this type of grade pipeline.

The methodological foundations of assessing the fracture toughness of elastic-plastic materials are considered. A methodology is proposed based on the results of assessing the fracture toughness of a material based on the structure state in the region of the crack tip. It is shown that the state of the material structure in the fracture zone of a uniaxially loaded specimen under the action of a stress close to the material’s ultimate strength is adequate to the structure state in the region of the crack tip before crack initiation and is a structural characteristic of this material. Based on the above correlation of material states, a structural parameter is proposed for assessing the fracture toughness of elastic-plastic materials with a crack. A statistical parameter, determined by the LM-hardness method from the Weibull homogeneity coefficient m, which characterizes the degree of scattering of hardness characteristics obtained by indenting the test portion of the loaded specimen, was taken as the fracture toughness index. This characteristic is invariant to the type of stress state and the loading method, i.e., the state of the material structure in the region of the crack tip does not depend on the loading method, which affects only the rate of reaching the ultimate structural damage in the region of the crack tip and the direction of crack propagation. It has been experimentally proved that the structural criterion of fracture toughness can be the same material characteristic as the force or strain characteristic of material fracture toughness. The difference between materials in their ability to resist crack initiation and propagation is determined by their initial structure. It depends only on the value of the thermal and force load parameters at which the material reaches a level of damage (loosening) of the structure at the crack tip sufficient for crack initiation. For comparison, we consider possible options for assessing the fracture toughness of elastic-plastic materials about the damageability of the structure: based on the scattering of the hardness values of the material of the specimen with a crack, on the state of the material structure in the test portion after the failure of the loaded specimen or after its reloading, as well as based on the state of the material structure of the specimen under loading to the ultimate strength of the material.

One of the most common ways to assess the activity of acoustic emission (AE) sources and their hazard under mechanical loading of materials and structures is to use the local-dynamic criterion. To verify the latter, specimens of various materials, namely fiberglass, corundum refractory, and steel, were tested under static loading during three-point bending. A special software AE-Criterion was developed to determine the AE parameters, including the current parameters of source activity, based on the local dynamic criterion using both the force factor and the time since the beginning of the test. Taking into account the multiplicity and stochastic nature of AE events during the loading of materials, it is proposed to determine the average values of M_S, M_T, and the values of the indices I_AS and I_AT of the activity degree of AE sources when applying the force parameter and time, respectively. The analysis of the data obtained during the bending tests of the above-mentioned specimens and, accordingly, the calculation of the parameters of source activity showed that the value of the index I_AS, when using the force factor, satisfactorily reflects the deformation and fracture processes of these materials. The study’s results indicate the suitability of the local dynamic criterion for diagnosing the deformation and fracture processes of the above materials.

The mechanical properties of the crack tip are vital for material safety evaluation. The weld material of welded joints is prone to work hardening during processing and installation, which changes the material mechanical properties and seriously affects the safety assessment of nuclear power welded joints. At the same time, nuclear power pipelines will creep under high temperatures and cause changes in the mechanical field at the crack tip. This paper obtains the mechanical properties of nickel-based alloy 182 in a nuclear power water environment. With the help of ABAQUS software, the change rules of the crack tip stress field and creep field under different hardening degrees are analyzed, and then the influence on crack tip creep and crack propagation rate is analyzed. Results show that increasing pre-hardening will increase the Mises stress around the crack tip, and the creep around the crack tip is expressed under pre-deformation, indicating that pre-hardening can accelerate the cracking of stress corrosion to a certain extent. Research has shown that the work hardening induced by the pre-deformation has an important influence on the creep rate at the crack tip, and the high creep zone is mainly concentrated in the direction of crack propagation. Under a certain work hardening rate, the crack growth will be accumulated.