Strength of Materials最新文献

The method of linear conjugation of analytical functions of complex variable was used to solve the problem of circular transversally isotropic plate bending hinged on the edge and loaded over the outer surface by the force distributed along the concentric curve. The complex potentials employed for registering the stress and deformation characteristics of the problem can possess the specific features at the concentrated force loading points, their nature was investigated and applied to the existing loading as conditionally concentrated. For getting the solution, the equation for the refined transtropic plate bending model was used that includes transverse shear strains and cross-sectional reductions, and, unlike other refined theories, the formulas with those refinements are advanced. The constants in the complex potentials were established with the boundary conditions and conjugation conditions for the moments and generalized angles of cross-section rotation along the loading line. With the approach by Timoshenko and Woinowsky-Krieger, from the circular loading solution, as a particular case, the solution for the centered concentrated force-loaded plate was obtained. For both cases, the refined normal radial and circumferential stresses were calculated in the center and on the edge of the plate. The data are summarized in tables and graphs. The model and numerical results show that an increase in the transverse plate anisotropy can radically change stress distribution patterns in its transverse cross-sections, up to the change in the radial stress signs on the outer surfaces. The classical model of plate bending and refined models such as by Timoshenko and Reissner are inapplicable in this case.

To investigate the relationship between service temperature ranging from –40~250℃, and size effect on tensile properties of thin ASS-304 sheets with nine different thicknesses (40~500 μm), uniaxial tensile tests were performed on thin ASS-304 sheets of the same average grain size in the present study. Within the thickness range of 40 to 300 μm, corresponding to η = t/d values from 1.1 to 8.1. The ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) of ASS-304 exhibits a dimensional effect of “the thinner, the stronger”. For example, as the η increases from 1.1 to 8.1, the UTS rapidly decreased from 1798.8 to 839.0 MPa at 20℃, from 1703.1 to 526.9 MPa at 150℃, and from 1661.2 to 478.9 MPa at 250℃, attenuation of 53.36, 69.06, and 71.17%, respectively. Meanwhile, the YS at 20℃ are separately 1768.9 to 418.7 MPa with 1695.2 to 343.3 MPa at 150℃ as well as 1645.7 to 330.1 MPa at 250°C, decrease the proportion of 76.33%, 79.75% and 79.94% respectively. Notably, the UTS, YS, and EL at 150 and 250℃ are lower than those at 20℃. The true stress value of ASS-304 was enhanced at –20 and –40℃, and the true strain increases first and then weakens as the thickness increases, the reason is the transformation- induced-plasticity (TRIP) effect of ASS-304 in stretching. The asymptotic function describes the relationship between strength and the values of η, while the Chapman function represents the relationship between elongation and the η. A linear variation exists between service temperature and tensile properties. And relevant empirical equations including T-η- and T-η-tensile properties were established, which can predict the UTS, YS, and EL of thin ASS-304 sheets under different service temperatures.

During the operation of a steam turbine, its structural elements are subjected to significant thermal and mechanical loads. The consequence of the long-term effect of such a load is the accumulation of scattered fatigue damage to the material of the structural elements of the steam turbine, which is localized over time in the form of fatigue cracks of various types. Evidence of this is several accidents and catastrophic failures of steam turbines due to significant fatigue damage to the shafting. The localization of damage in turbine rotors is facilitated by stress concentration in the gouges and fillets, as well as damage to the surface layer of the rotors during the thermomechanical treatment stage since all metal processing operations (forging, turning and milling, heat treatment) are accompanied by plastic deformation of the material. One of the reasons for the long-term accumulation of fatigue damage in the structural elements of steam turbines is thermal stresses, which can reach dangerous values during turbine startup operations. In certain parts of the rotors, these stresses are sufficient to cause scattered fatigue damage to the material (the so-called thermoplasticity), especially when starting the turbine from a cold state. In the case of crack initiation, the thermal stresses are all the more sufficient for its further intensive development even when starting the turbine from uncooled and hot states, which are less damaging than starting from the cold state. To study the intensity of crack growth in the turbine rotor due to thermal stresses arising during turbine startup, a computational model based on using a finite element model of the shaft of the K-200-130 steam turbine and fracture mechanics approaches is proposed. Studies based on the proposed computational model have demonstrated the ability to predict the process of crack growth in the rotor due to turbine startup from different thermal states and assess its danger to structural integrity. The initial size of the internal annular crack was determined, which has the potential for further intensive growth under the influence of thermal stresses.

In engineering practice, conservative approaches are used to assess the technical condition of pipelines with typical elements (branches, tees and adapters) using reference data on the mechanical properties of the metal in its original state. The use of such approaches to assess the technical condition without taking into account changes in the mechanical properties of the metal during the manufacturing process and long-term operation can lead to significant errors. As an example, this paper presents the results of experimental studies of the mechanical properties of the metal of the characteristic zones of typical pipeline elements and the steam generator housing. It is shown that the differences in the characteristics of resistance to the short-term static and cyclic loads of individual sections of typical pipeline elements are associated with the influence of technological and operational factors. It is recommended to use nondestructive testing methods for the rapid evaluation of differences in the structural state of the metal of different portions of the product, which determines its mechanical properties.

Silk fibroin (SF) is a natural polymer with excellent biocompatibility and mechanical properties and moderate human body degradability, making SF an interesting candidate for regenerative medicine. Composite materials of SF and polyethylene glycidyl methacrylate (PEGDMA), a biocompatible polymer, attract attention as scaffold materials for regenerative medicine. To the authors’ knowledge, SF–PEGDMA composite hydrogels have thus far not been manufactured using optical fabrication methods, and the change in their compressive properties during their degradation has not been studied. In addition, cellulose nanofiber (CNF), a plant-derived nanomaterial with excellent mechanical properties and biocompatibility, was added to the SF–PEGDMA hydrogels to enhance their mechanical properties. SF–PEGDMA composite hydrogels were three-dimensionally printed using digital light processing. The compressive strength of the obtained hydrogels stored in pure water or phosphate buffer solution temporarily increased and decreased after 4 days. However, after 7 days, the strength decreased to a level similar to that of the specimens which did not contain CNF. In the formability tests, the reproducibility of the model changed with the intensity of the light and the CNF concentration.

The mechanism of fracture propagation from the circular blast holes with two symmetrical cracks in rock blasting operations was investigated by the displacement discontinuity method. The stress intensity factors at the crack tips of the pre-existing cracks were numerically determined using the displacement variations near the crack tips. In this paper, the variation of the displacement discontinuities along pre-existing cracks emanating from blast holes are studied and compared using linear, quadratic, and cubic collocations. For the crack tip behavior, cubic collections are used, and also the special crack tip element is employed to reduce the singularity at the crack tips. The numerical results of the normalized stress intensity factor for linear, quadratic, and cubic collocations of the displacement discontinuities compared with the analytical results. These sets of results are compared with one another, and good agreements have been reached between them.

The results of experimental studies on the optimal method of testing materials for hardness depending on physical and mechanical properties are presented. The possibility of evaluating the structural parameters, i.e., the homogeneity coefficients and the variation of the reliability of various devices for obtaining the hardness values of rolled metal products, is considered. The results obtained experimentally have shown the effectiveness of such evaluations. The reliability of the methods for testing the hardness of materials of different plasticities was controlled by determining the degree of dispersion of the obtained results by the LM-hardness method. The influence of the material’s physical and mechanical characteristics on the hardness test’s accuracy is analyzed. The possibility of choosing the most reliable Brinell, Vickers, or Rockwell method for determining hardness characteristics depending on the level of plastic properties of the sheet material was determined. Following Fisher’s theory of small samples in terms of the general population, a statistical assessment of the reliability of measurement methods was performed on a sample of a limited size, which makes it possible to assess the reliability of the results of hardness determination by various methods and determine the optimal variant of testing materials, including high-strength ones, for hardness. According to the methodology for analyzing variance to select the optimal variant for conducting studies of material hardness, the statistical assessment of the reliability of hardness measurements was carried out using a random sample from the general population in its central part in the number of five measurements following each other. The variance check is applied using the Cochrane criterion to determine the uncertain members of the variation series. Based on the research results, the predominant expediency of one or another Brinell, Vickers, or Rockwell method of conducting hardness tests depending on the level of plasticity of materials, especially high-strength ones, given their structural state as assessed by the parameters of hardness characteristics scattering – homogeneity coefficients m and variation v. The availability of this information will make it possible to determine the mechanical characteristics of materials by hardness values more reliably.

The paper analyzes the current state of the grain transportation freight car fleet. The results of the analysis show that grain rail cars have exhausted their initially assigned service life, which is set by the manufacturer, and have the same type of defects in load-bearing structures in the area of the riveted joint of the backstop of the car hitch to the girder beam. A 3D model of a grain rail car of a typical design was developed. The stress distribution near the holes of the riveted joint of the backstop of the car hitch with the girder beam under the simultaneous action of normative forces was calculated by the finite element method using the SolidWorks program according to the 3rd theory of strength by the established design modes I and III. Implementing the proposed improvement justified the need to conduct relevant research work. These works included the study of the places and causes of cracks, and their result became the basis for the proposed modernization of the car frame.

In the course of combat operations, accidental projectile detonation in the mortar barrel channel can occur through the faulty fuze actuation or detonation of two projectiles for the violation of safety measures, viz double charging of the mortar. Barrel rupture occasions pose new challenges for the developers of this weapon to improve the safe operation of mortars. The literature analysis revealed that among the current studies on the stress-strain state of mortar barrels during the projectile explosion in their channel, the results of determining the stresses in the barrel structures capable of withstanding the explosive gas pressure in the channel were absent. Existing mathematical models for evaluating the stress-strain state of a mortar barrel on the projectile detonation in its channel need to be improved. The potentials of developing new approaches to mortar barrel strengthening for combat operations are substantiated. For this, the theory of insert liquid-filled cylinder structures (pipes) is proposed. The internal pressure for those structures is calculated. The mortar barrels can be modified by applying the optimum combination of new materials and modern design circuitry.

During the tunneling process of high-abrasion stratum with strong impact by tunnel boring machines (TBM), disc cutter rings made of H13 and DC53 steels are prone to failures like wear and chipping. To improve the service life of the cutter ring, a new kind of Cr-Mo-W-V medium carbon alloy cutter ring steel (DQ1) with better hardness and toughness was developed. The effects of the heat treatment process on the microscopic structure and mechanical properties of DQ1 steel were studied by the spectrometer, optical microscope, Rockwell hardness tester, and impact tester. The wear resistance of DQ1, H13, and DC53 steel was compared and analyzed through the abrasive wear test and rock breaking test. The results indicate that outstanding mechanical properties and wear resistance of DQ1 steel were obtained after quenching at 1040°C and tempering at 540°C, the hardness was 4.4 HRC higher than that of H13 steel, and the impact absorption energy was 85.7% higher than that of DC53 steel. In excavating strong impact and high abrasion formations, the average service life of the DQ1 steel cutter ring was increased by 24.6% compared with the H13 steel cutter ring, and the brittle fracture problem common in the DC53 cutter ring did not appear. The research achievements can facilitate to improve the excavation efficiency and reduce the cost of TBM in high strength and high erosion strata.