Strength of Materials最新文献

Underwater vehicles have a great deal of tactical significance in the future sea warfare, and their structural forms are mostly cylindrical shells: underwater vehicles. It may be attacked by underwater weapons. Its payload forms include hydrostatic pressure and localized impact. This article uses finite element analysis software ABAQUS to study the dynamic response of thin-walled cylindrical shells under local impact loading and circumferential pressure effect of cylindrical shell under coupling effect of pressure and local shock. Study shows that under impact load alone, the time of action, the maximum deflection at the point of application of the timing is linear with the peak value of the impact load. When the impact load peak is less than 40MPa, the coupling 30% reduction in critical collapse pressure of columnar crucibles due to cooperative use than cumulative ones, and impact load crest satisfy a certain function form and fit the results of this study to get the functional relationship.

In the present research, the Al-Si-Fe hypereutectic alloys with different Ti addition were prepared and the electric field treatment was performed on the alloys to regulate the phase morphology. The microstructure and mechanical properties of the alloys were characterized by OM, SEM, TEM, EPMA and tensile test. The results reveal that the Al-Si-Fe hypereutectic alloy prepared by conventional casting is mainly composed cubic β-Si phase, long rod-like and needle-like β-Al₅FeSi phases. In addition, there are stacking faults in the β-Al₅FeSi phase. Minor Ti addition in Al-Si-Fe hypereutectic alloy could change the needle-like phase into eutectic structure, decrease the size of β-Al₅FeSi phase and homogenize the β-Si phase size. The more Ti addition tends to coarsen the β-Al₅FeSi and β-Si phases, and moreover the needle-like phase precipitate again. The electric field treatment promotes the coarsening of β-Al₅FeSi and β-Si phases in the Al-Si-Fe hypereutectic alloy with 0-1.0 wt.% Ti addition, but results in the refinement of β-Al₅FeSi and β-Si phases in 1.5 wt.% Ti doped Al-Si-Fe hypereutectic alloy. Furthermore, the needle-like phase has been transformed into small-size eutectic structure in the 1.0 and 1.5 wt.% Ti doped Al-Si-Fe hypereutectic alloys. With the synergistical effect of Ti addition and electric field treatment, the 1.5 wt.% Ti doped Al-Si-Fe hypereutectic alloy obtains yield strength of 100 MPa and ultimate tensile strength of 113 MPa, which is about 26% and 37% higher than the conventional-cast Al-Si-Fe hypereutectic alloy.

The rock-breaking force of the disk cutter is affected by many factors. The greater the rock-breaking force applied to the disk cutter, the higher the breakage efficiency and removal in the rock masses, thereby speeding up the excavation speed and improving construction efficiency. The elastic-plasticity of rock materials will greatly affect the disk cutter rock-breaking force, thereby affecting the efficiency and speed of the TBM excavation process. The commercial finite element software LS-DYNA was performed to simulate the rock breakage mechanism by disk cutter, and the influence of three rock behavior models of elastic-plastic, plastic, and elastic models on the rock-breaking force by disk cutter was studied. Finally, the elastic model with the least influencing parameters was selected to study the effects of rock elastic modulus and disk-cutting depth on rock-breaking force.

Hybrid plasma-arc welding with a consumable electrode (PAW-MIG) of sheet aluminum alloy blanks exposed to a fast-moving heat source is involved in the formation of residual stresses and strains that can lead to changes in size and shape, buckling distortion, fatigue strength reduction, and deterioration of other operational properties of the welded product. This necessitates to introduce additional arrangements for reducing welding strains, stresses, and displacements, such as rigid outfits. Finite-element analysis of residual stress-strain state parameters of PAW-MIG welded 1561 aluminum alloy butt joints 5 mm thick with two different process modes: in the free state and in the outfit followed by its removal after cooling, was performed. The PAW-MIG welding after the second alternative compared to the first one was established to reduce the total displacements by 15 times and equivalent stresses by more than 30%, it also provides a decrease in displacements on the weld axis: longitudinal by 1.5 times and buckling distortion by 16.7 times.

Mathematical simulation of stress states arising in butt weld joints of AMg61 aluminum alloy plates (δ = 2, 4, and 8 mm) induced by electrodynamic treatment (EDT) at different temperatures was performed. The vertical velocity V₀ of the indenter electrode (EDT tool), determined by the energy characteristics of EDT equipment, was taken to be V₀ = 5 m/s. The T values were set to represent the EDT conditions after welding (20°C) and during fusion welding (150 and 300°C). The three-dimensional problem was solved by the finite element method using an ANSYS software package. The conditions for the stresses arising in the EDT plates after and during welding were defined by the mechanical characteristics of an AMg61 alloy at 20, 150, and 300°C, which were described by the kinematically-hardened material model. The computational results for kinetics and residual stress states in weld joints are presented. EDT at 150°C (during welding) was established to be more effective than that at 20°C (after weld cooling). EDT of weld joints (δ = 2–4 mm) was found to result in residual compression stresses across the whole width of the plate, with their values being close to the yield strength of an AMg61 alloy. EDT of weld joints (δ = 8 mm) generates residual compression stresses on the outer plate surface and the tensile ones on its back surface. Thus, for optimum residual stress states of weld joints with δ = 2–4 mm, one-sided EDT (at given V₀) is sufficient, while for δ = 8 mm, two-sided EDT would be required.

The influence of kinematic excitation of vibrations on vibration stress in a disk–blade system with a violation of cyclic symmetry when one blade is damaged is evaluated. To assess the trouble-free operation, it is relevant to determine their stress state when the blade shape changes due to erosion damage. The results of calculations of the maximum stresses in the blades under power and kinematic excitation of oscillations are presented. The three-dimensional finite element models of the disk–blade system and the corresponding mathematical software are used to determine the parameters of stationary vibrations and blade stresses. The force effect of a steam flow with a frequency of 2100 Hz (the number of guide blades is 42) and kinematic excitation when the center of the disk moves along an elliptical trajectory in its plane with a frequency of 50 Hz, which is caused by rotor vibration in sliding bearings in stationary operating conditions, is taken into account. The load from the steam flow on each blade was set to be linearly variable from zero at the root of the blades to 1 and 5 kPa at the periphery and for a uniformly distributed 2.5 kPa along their length, acting normally at the points of the blade working surfaces. It is assumed that the physical and mechanical properties of the damaged blade material are preserved after repair and surface treatment. The change in the maximum equivalent stresses in the impeller blades for different loading conditions is determined. The amplitude-frequency characteristics for the maximum stresses in the region of rotational speeds and the action of the load on the blades are given. The results are compared for the system with and without kinematic excitation of oscillations. The studies confirmed the practicality of considering the influence of kinematic excitation when assessing the stress state of the last stage blades of a powerful steam turbine.

The model of Timoshenko’s shell theory of shells was used to analyze the dynamic characteristics of conical shells of variable thickness on a Pasternak elastic bed under nonstationary loading. Based on the Hamilton–Ostrogradsky variational principle, the equations of motion of a conical shell of variable thickness on a Pasternak elastic bed were derived. This system of hyperbolic differential equations is solved by the finite difference method. The numerical algorithm for solving the obtained equations is based on applying the integral-interpolation method for constructing difference schemes in the spatial coordinate and an explicit finite difference scheme for integration in the time coordinate. The influence of geometric dimensions, taper angle, and elastic media on the natural frequencies and other dynamic characteristics of a conical shell of variable thickness under the action of a pulsed load is analyzed using specific examples. New mechanical effects are revealed.

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.