Strength of Materials最新文献

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 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 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 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.

The application and comparison of different fracture mechanics concepts are discussed to be used in computing stress intensity factors (SIF) from the numerical solutions of model crack theory problems with a mixed scheme of the finite element method. Approximation of displacements rests on piecewise-linear interpolation functions with triangular elements, and strain and stress distributions are approximated by the linear combination that includes the piecewise-linear interpolation and interior bell function. The latter ensures the stability and convergence of the approximate discrete problem solution. The solution results for linear elastic and elastoplastic model plane central mode I crack-strip tension problems under different loading and plane strain state conditions are presented. Elastoplastic calculations were made with an ideal elastoplastic material model. The application of the energy balance and G-integral concepts to the calculation of the specific work of fracture at the stationary crack tip is substantiated. It is shown that on condition of uniform plate partition in the vicinity of the crack tip, the application of those concepts to SIF calculation for one loading stage is consistent with the Irwin plastic zone correction, maintaining this approach in further mesh thickening. Elastoplastic calculations on repeated loading demonstrated that tensile stresses ahead of the crack tip were about the same as on the initial one, but the opening at the crack tip on the former was larger than on the latter, and this effect was most pronounced for the first half of active loading values. Several aspects of SIF calculations on repeated loading are presented.

Firstly, this paper used a laser particle size analyzer to test the particle size distribution characteristics of four specific surface areas of phosphate slag powder particles. Then, it used the powder fractal theory to determine the fractal dimension of the different specific surface areas of the phosphate slag powder particle group. Finally, the effect of the fractal dimension of the phosphate slag powder particle group on the mechanical strength and activity index of the phosphate slag powder-cement cementitious system at 3 d, 28 d, and 180 d ages were studied. The results showed that the phosphate slag powder with different specific surface areas has self-similar fractal characteristics; the larger the specific surface area of phosphate slag powder, the higher the fractal dimension, and the fractal dimension of phosphate slag powder particle can characterize the properties of phosphate slag powder’s fineness and particle gradation in the process of grinding and refining. The phosphorus slag powder particle fractal dimension, the mechanical strength, and activity index of the phosphorus slag powder-cement cementitious system have linear correlation characteristics; the larger the fractal dimension, the higher the mechanical strength and activity index of the cementitious system with phosphate slag powder.

To date, most WWER-1000 nuclear power units in Ukraine have reached the end of their design life (30 years). In view of this, work is under way to extend the service life of critical equipment elements beyond the design life (to up to 60 years). In addition to reactor vessels, these critical elements include vessel internals (VI). One of the key approaches in such works is a predictive assessment of the structural integrity of VI structures using mathematical modeling of physical processes typical for power unit operation under intense radiation exposure of structural material. The service life prediction methodology constantly requires refining and taking into account more factors affecting structural integrity. The paper considers the kinetics of changes in the stress intensity factor for postulated cracks in the reflection shield of WWER-1000 VIs during long-term operation and at the time of reactor emergency during the rupture of primary coolant circuit pipelines with a nominal diameter of 100 to 850 mm in the most dangerous areas in terms of brittle fracture resistance. A significant impact of taking into account the residual stresses formed as a result of welding and heat treatment of the reflection shield during manufacturing on the results of calculating the brittle fracture resistance was revealed, which may affect the conservatism of assessment methods when justifying long-term operation.