Strength of Materials最新文献

In the development and construction of the power grid, the transmission line corridor inevitably passes through the area with dense vegetation. Forest fires are common in the dry season, seriously affecting the normal operation of the transmission line. Due to the sharp increase in air temperature and a large amount of smoke during combustion, the steel-core aluminum strand without a sheath is directly exposed to the air of the fire field. Due to the high temperature and adhesion, the structure of the wire changes and the tensile strength decreases accordingly, which can lead to serious damage to the wire, including broken strands and fractures, and it can no longer be used. To solve the problem that it is difficult to assess the operational characteristics of transmission lines after a fire, the mechanical characteristics of transmission lines under forest fire conditions are studied through theory and simulation so that the mechanical characteristics of transmission lines after a forest fire can be quantitatively expressed. It is proposed that a mechanical design manual for conductors under mountain fire conditions be created and the online monitoring system for transmission conductors under mountain fire conditions be improved.

A novel class of electro-magneto-elastic (EME) materials comprises electro-active and magneto-active particles in the polymer matrix that change their elastic behavior with an applied electromagnetic field. The material response for such a material class is usually formulated using Lagrangian strain tensor and Lagrangian electromagnetic field vectors as “push forward” to the current configuration. This article presents a novel formulation of an electro-magnetoelasticity in terms of an Eulerian strain tensor and Eulerian electromagnetic field vectors referring to the current configuration. Such an Eulerian formulation is often favorable from both theoretical and computational standpoints, which avoids the “push forward” operation to get the current configuration. An exercise to deduce the constitutive relation for an EME material class available in the existing literature from the newly proposed relation is also illustrated.

Numerical results are presented for the low-temperature serrated deformation process in tension induced by a suspended load of 03Kh20N16AG6 austenitic steel and AMg5 aluminum alloy specimens in liquid helium at 4 K. In practice, large loads at cryogenic temperatures are met with liquefied gas tanks, in particular in hydrogen tanks of launch vehicles. The local one-dimensional multiparametric nonlinear mathematical model of the low-temperature serrated metal deformation process was constructed, with its adequate display and quantitative estimates based on mechanical material properties and loading system characteristics. This effect manifests the local thermomechanical metal deformation instability under adiabatic conditions. The mathematical problem was formulated as a nonlinear differential equation of second order with certain initial and other conditions. It represents the dynamic equilibrium of the specimen-loading device system and describes the process of serrated specimen deformation as the system motion. The model is specified for 03Kh20N16AG6 steel and AMg5 aluminum alloy specimens creep-tested in liquid helium. The numerical experiment demonstrated adequate accuracy with the computational method. The qualitative similarity of the process was revealed for the materials of different classes, with the strain levels achieved differing markedly. Comparative computations established that potential energy of the gravitational field induced a much larger localized deformation of the specimen than potential elastic energy, even in combination with additional factors, viz operation of an electric or hydraulic drive, when the deformation rate is two orders of magnitude higher than the standard one for static metal tests in tension. A very large strain arising and localized under slow loading relaxation inevitably fails before the serrated process is complete.

In the process of use and manufacture, carbon fiber foam sandwich structures were often damaged by low-energy impact, resulting in performance degradation. Therefore, it was necessary to study the damage caused by the low-speed impact of composite sandwich structures. Based on the Hashin failure criterion, this paper established an equivalent finite element model of carbon fiber foam sandwich panels under low-velocity impact. The model was used to simulate the damage of the foam sandwich panel with [±45°/±45°/(core)/±45°/±45°] ply structure under the impact energy of 10.58, 21.17, 31.75, and 42.34 J. The simulation results of impact damage depth were compared with the experimental results. The error was less than 10%, which proved the rationality of the impact equivalent model. The model was used to predict and analyze the damage of foam sandwich panels with [±45°/(core)/±45°], [±45°/ (0°, 90°)/(core)/±45°], and [±45°/(0°, 90°)(core)/(0°, 90°)/±45°] ply structures under 21.17J impact energy. The low energy impact resistance was analyzed by comparing and analyzing the damage situation, impact force response time, and impact velocity response time. The results showed that increasing the number of ply layers [±45°] can reduce the impact damage degree and improve the bearing capacity of sandwich panels.

The research intends to produce conventional and intricate shapes using a deep drawing operation experimental work and finite element analysis (FEA). So, to perform experiment work, the dies of deep drawing were designed, manufactured, and then employed to produce the mugs of cylindrical and polygonal shapes from low-carbon steel (1008-AISI). In addition, a commercial software program, ANSYS (workbench), was applied to perform the numerical analysis. The research aim is to create conventional (cylindrical) and intricate (polygonal has eight edges) shapes in a deep drawing process and compare the experimental work results and the FEA of both shapes. The comparison saw that with the intricate shapes, the maximum drawing force demanded to create a polygonal mug registered at 39.865 and 33.675 kN with the cylindrical mug. The maximum effective strain registered was 0.4542 with mugs of intricate shapes. Conventional shapes (cylindrical) are easier than the production of intricate shapes (polygonal has eight edges) by employing the deep drawing operation.

Crack tip creep is a key parameter affecting the stress corrosion cracking (SCC) growth rate of nickel base alloy structural materials, significantly impacting the stress corrosion cracking rate of austenitic stainless steel and other materials. To explore the variation law of the crack tip creep field at different positions from the weld-seam of the welded joint, a numerical calculation model of the welded joint under different crack initiation positions was established based on alloy 600, and the crack tip stress field and creep were analyzed in detail. Results show that the unevenness of the material will cause obvious stress discontinuity at the crack tip of the material boundary; as the distance between the crack position and the weld interface increases, the stress of the crack tip before creep is affected by the mechanical properties of the base metal decreases. The creep rate of the weld side cracks gradually increases with the distance from the material interface. The minimum crack growth rate appears when the crack is located at the interface between the Ni-based alloy and base metal. The increased distance from the material interface increases the crack growth rate gradually. When the distance from the interface (d>) 0.1mm, the influence of mechanical properties on the growth rate is weakened.

It has been established that due to the peculiarities of the recrystallization process, the structure of welded joints of steam lines made of heat-resistant pearlite steels 15Kh1M1F and 12Kh1MF under creep conditions of more than 270 thousand hours undergoes recovery processes. The process of resting with an increasing service life of welded joints accelerates after their service life exceeds 270 thousand hours and begins to transition to recrystallization. During the recrystallization process, individual grain boundaries gradually crumble, which leads to an increase in their size and a change in shape. The speed of recrystallization processes in welded joints’ heat-affected zone (HAZ) differs significantly. The recrystallization processes in the heat-affected zone areas’ structure are more intense compared to the base metal of welded joints that have not been exposed to welding heat. When a common boundary between two contacting grains is crumbled, a single grain is formed, eventually acquiring the same crystallographic orientation. In the area of overheating of the zone of thermal influence of welded joints made of 12Kh1MF steel, the average initial grain size corresponded to the 9th point (DSTU 8972:2019). After 280 thousand hours of operation (about 40 years), it is the 7th. Such processes are accompanied by a decrease in the density of dislocations in a-phase grains and along their boundaries, which does not significantly affect the decrease in mechanical properties. The dependence of the recrystallization on the self-diffusion of alloying elements, the distribution of dislocations in the grains of the a-phase, and the coagulating carbides was established. In this case, the recrystallization process results in a decrease in the mechanical properties of welded joints. Recrystallization in incomplete recrystallization, overheating, and fusion of the filler material is more intense than in the weld metal and the base metal of the welded joints. Recrystallization contributes to damage to welded joints by creep and fatigue mechanisms.

This research is part of a series of previous studies aiming to advance technology for aluminum alloys, particularly focusing on AA2024-T4 alloy. The main objective encompassed predicting the fatigue life of a standard specimen under varying experimental conditions associated with the shot peening process. The study delved into forecasting the fatigue life stemming from the cyclic impacts of shots, followed by an endeavor to enhance the specimen’s surface longevity through numerical simulation techniques under varying temperatures, including room and high temperatures. The study predicted the fatigue life resulting from cyclic shot impacts, followed by an attempt to improve the specimen’s surface longevity using numerical simulation techniques under different temperatures, including ambient and elevated temperatures. Interestingly, the research also revealed that static stress becomes apparent after the first 10⁶ cycles, consistent with the results observed in different test cases with different forces applied. Indeed, to guarantee the validity of the results, the interpretations and conclusions drawn have been rigorously validated both at room temperature and at an elevated temperature of 250°C. In summary, this research successfully investigates the numerical analysis of the structure of the AA2024-T4 alloy, providing valuable information on its fatigue behavior at different temperatures.

The paper proposes a method for determining diagrams of the limiting cycle amplitudes of welded joints with steady-state residual tensile stresses based on the test results of small-sized specimens and presents the corresponding calculation dependencies. This technique is an express method for estimating the fatigue resistance characteristics of welded joints if full-scale tests are not feasible. The analysis of literature data for various types of welded joints shows a good fit of the experimental and calculated values of the endurance limits of welded joints of low-carbon steels with low-alloy, low-strength steels. It is shown that the inclined section of the diagram with steady-state residual stresses is shifted in parallel relative to the diagram of cycle amplitudes of welded specimens without residual stresses. It has been established that the diagrams of the limiting cycle amplitudes of welded joints with different values of the steady-state residual stresses end on a line where each point at different average cycle stresses corresponds to the minimum limiting cycle amplitude of the welded joint with its particular value of the ultimate steady-state residual stress, which ensures the realization of the ultimate stress cycle. With the same value of the steady-state residual stress, the endurance limit of welded joints decreases with an increase in the yield strength. It is shown that low values of steady- state residual stresses result in nearly the same endurance limit reduction of butt welded joints of steels of different strengths. In contrast, their higher values aggravate the endurance limit deterioration of steels with higher mechanical characteristics.

The potentials of nondestructive test methods are experimentally substantiated to get a cyclic loading fracture mode of ferromagnetic steels against their structural anisotropy, determined by coercive force measurements. Elastic static or cyclic loading was revealed to be consistent with a stable structure with certain anisotropy kinetics due to applied stresses. After plastic deformation, a new stable structure is formed induced by residual stresses. The anisotropy factor depended on the level of active relative stresses under elastic and elastoplastic, static, or cyclic loading. The change in kinetics direction for the anisotropy factor with elastic or elastoplastic loading defines the safe range of mechanical loading caused by reversible damage processes, as well as the ranges of accumulation risks for irreversible fatigue, high- and low-cycle fatigue and quasistatic damages giving rise to corresponding fracture modes. The nondestructive coercimetric method permits setting the metal endurance, yield limit, and transition stress from low-cycle fatigue to low-cycle quasistatic fracture.