Strength of Materials最新文献

The influence of kinematic excitation of rotor vibrations of a powerful steam turbine without and with disturbance of blade vibration frequencies on the extension of their trouble-free operation is evaluated. The results of determining the maximum equivalent stresses of the blades under the condition of power and kinematic excitation of stationary oscillations are presented. A system with cyclic symmetry is considered. The three-dimensional finite element models of the disk–blade system and the corresponding mathematical support for calculating stationary harmonic oscillations are used. Computational studies to determine the maximum equivalent stresses of the blades were carried out under the condition of simultaneous action of power excitation of oscillations from the steam flow with a frequency of the forcing force of 2100 Hz (with the number of guide blades of 42) and kinematic excitation due to rotor vibration on sliding bearings with a frequency of 50 Hz. The load from the steam flow on each blade was set to be linearly variable from zero at the root to 1 kPa and 5 kPa at the apex, as well as a uniformly distributed 2.5 kPa along the blade, acting normally at points on their working surface. The kinematic excitation was set as an ellipse describing the motion of the disk center in its plane. It is assumed that the physical and mechanical properties of the blade material are preserved after their repair and surface treatment. The change in the maximum equivalent stresses for different variants of blade loading in a cyclic-symmetric disk–blade system under kinematic excitation of oscillations is evaluated. The obtained results are compared with the data for the system with all damaged blades after restorative repair in their lower part under the condition of kinematic excitation of vibrations and without repair. These results confirm the practicality of assessing the stress state of the last stage blades of a powerful steam turbine, considering the disk–blade system’s kinematic excitation when determining their operation’s reliability.

The addition of fibers helps to increase the performance of concrete in terms of resistance and flexibility. Different types of fibers that have different mechanical properties may change the behavior of concrete. Fiber-reinforced concrete with varied combinations of fibers (steel, macro synthetic, and polypropylene fibers) in 1% volume is investigated in this research. Concrete samples were fabricated from a combination of two types of fibers with different percentages to measure the compressive strength with the approach of determining the optimal ratio of fibers. The results showed that the hybrid samples containing steel fibers provide higher compressive strength compared to the samples containing macro synthetic and polypropylene fibers. Macro synthetic and polypropylene fibers in concrete samples have played a significant role in increasing the flexibility and efficiency of concrete, as well as significantly reducing cracking and increasing durability and toughness. In these hybrid models, coherence is preserved in the event of failure. The combination of polypropylene fibers with both steel and macro synthetic fibers significantly reduces the compressive strength of concrete samples. In concrete samples with hybrid fibers, samples with a combination of macro synthetic and steel fibers had higher compressive strength than samples with a combination of steel and polypropylene fibers.

To improve the high-temperature erosion wear resistance of 304 stainless steel, this study designed and fabricated bionic samples based on the erosion wear resistance characteristics of the desert scorpion body surface. Uniformly distributed ridged bionic units were fabricated on the surface of 304 stainless steel by laser cladding 25% WC-NiCrBSiFe. The experiments were conducted using self-designed high-temperature erosion wear equipment to compare the erosion rates of bionic and untreated samples from room temperature to 1000ºC. The results showed that the erosion rate of the bionic samples was significantly lower than that of the untreated sample at different temperatures. The erosion rate of bionic units slowly increased from room temperature to 1000ºC, showed a decreasing trend at 400–600ºC, and reached a maximum value at 1000ºC, which is 50% of the untreated sample. The bionic unit’s wear mechanism mainly includes chiseling, plowing, and removal of the oxide film, as well as hard phases. Subsequently, the mechanisms of the unit on high temperature erosion wear resistance improvement were suggested: (i) the unique microscopic structure of bionic units with alternating distribution of soft and hard phases can weaken the impact of solid particles, the hard phase resists the plastic deformation and the soft phase absorbs impact energy and hinders crack propagation;(ii) the uniformly distributed ridged units can generate air cushion and shielding effects during high temperature erosion, which can weaken the chiseling and plowing effects of solid particles on the surface of bionic samples.

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.

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.

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.

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.