Papers by Keyword: Crack Resistance

Abstract: The article is devoted to the analysis of the operational suitability of the aircraft shelter structure under the action of an explosion, taking into account the nature of their spread through the soil backfills. The intensity of the decrease in the bearing capacity of the covering structure, which depends on the content of reinforcement in the structural element, is considered. Including, preventive measures are proposed for the progressive collapse of the hangar roof slabs in the event of a possible missile hit. The article considers various scenarios of explosive effects, under which the worst-case conditions for ensuring the reliability of the hangar structure during emergency effects were checked. Wave propagation during a ground explosion was analyzed. An example of calculating the arched hangar structure for a blast wave in the LIRA-FEM PC under the action of a surface explosive effect is provided. The results of the nonlinear calculation of the arch and the roofing slabs with an additional impact load of up to 20 tf/m² are presented. The modeling of the nonlinear loading process was carried out taking into account the damping of the soil base of up to 10% on an area of up to 80 m² with a dynamic coefficient of 1.2 in the PC LIRA-FEM, the calculation of the emergency load was performed with the specified TR (type of reinforcement).

Abstract: The problems of stress corrosion or hydrogen blistering of gas pipelines are relevant and require careful study of the causes and factors that cause this type of corrosion-mechanical destruction of pipelines. The analysis of numerous publications on this problem revealed contradictions of information regarding the mechanism of stress corrosion and a lack of experimental materials on the substantiation of the nature and peculiarities of the nature of destruction on gas pipeline networks. Systematic experimental studies using different brands of pipe steels allowed to determine the brands of steels, which according to their characteristics are the most resistant to VBR in harsh operating conditions, including even in the most aggressive NACE environment with H₂S and CO₂ additives at a pressure of 10-15 atm. Moreover, the experimental studies were as close as possible to the operating conditions of pipelines of the gas transportation network. The obtained results of experimental studies can serve as a basis for developing methods of technical diagnostics and forecasting the actual state of pipelines, which will significantly prevent the occurrence of sudden destruction caused by stress corrosion. The influence of the service life of gas pipelines on the degree of flooding and microhardness of pipe steels was established, which made it possible to substantiate the embrittlement of the metal with the increase of service life. The values of impact toughness on samples with sharp and round notches and the amount of work of crack growth depending on the service life of the pipe steels were determined, which made it possible to choose steel grades characterized by the highest resistance to brittle fracture. It is shown that with the service life, the destruction occurs according to a brittle mechanism, which is confirmed by the increase in the share of the fibrous component in the fractured samples after impact tests. It was established that the lowest corrosion rate is possessed by new grades of improved steel grades 20А and 08 KhMChA.The PRFNV parameter proposed in the paper makes it possible to assess the susceptibility of pipe steels to stress corrosion cracking and provides an opportunity to regulate the corrosion crack resistance of pipelines by metallurgical methods.

Abstract: The article is devoted to the issues of modeling and analysis of the protective structures’ designs under the influence of explosive loads. An analysis of shock waves is carried out, the nature of their propagation and impact on buildings, as well as methods for their modeling in the LIRA-SAPR software. Some proposals for preventing the collapse failure of the structure due to the impact of missile fragments are given. The article discusses different types of point explosions - air and ground. The diagram of the formation of waves during an air explosion and a characteristic wave pattern during a ground explosion is given. The propagation of waves during a ground explosion is analyzed. A mathematical model for determining shock wave parameters is given. An example of the calculation of the protective structure constructions against a blast wave in PC LIRA-SAPR (LIRA-FEM) under the action of a surface blast effect is provided. The results of the non-linear calculation of the roof slab for resistance to collapse failure in the case of an emergency explosive impact on an area up to 80 m² are presented.

Abstract: The results of determining the bearing capacity, deformability and crack resistance of full-size airfield slabs made of reinforced concrete and fiber-reinforced concrete are presented on the basis of experimental studies conducted in laboratory conditions. The slabs were manufactured by Velikodolinsky Reinforced Concrete Plant LLC by order of the Odessa State Academy of Construction and Architecture. The authors have developed a stand for testing slabs using a cantilever loading scheme in accordance with the regulatory documents in force in Ukraine. Two slabs were studied: a serial reinforced concrete airfield slab and the same slab, but with additional steel fiber reinforcement in the amount of 1% of the volume of the concrete mixture. It has been shown that the load-bearing capacity of airfield slabs with additional reinforcement with steel fiber increases significantly; at the same time, deflections are reduced and crack resistance increases. The bearing capacity of airfield slabs with additional dispersed reinforcement increased by 29%. The maximum deflection decreased by 12.7%. The nature of crack formation changed qualitatively: the process began at a higher load, and the final crack opening width decreased significantly. These results indicate the high efficiency of additional reinforcement of airfield slabs with steel fiber.

Abstract: In fracture mechanics, there is a rather complex theory associated mainly with the choice of model and the correct choice of fracture criteria. There may be models and criteria for the fracture mechanics of bodies with stable and unstable properties, or the choice of a fracture model for an viscoelastic material. When calculating, you can choose the critical crack opening criterion or the local energy criterion. But in practice, it is possible to carry out more simplified initial theoretical calculations, which can be performed by an engineer who has the necessary theoretical knowledge and practical experience, for example, in the operation of pipelines. If the material is considered completely solid, then the interatomic bonds are destroyed. On this basis, the theoretical strength limit of the material is introduced. In this article we study normal tensile cracks, since this is the most dangerous situation (when the tensile stress is perpendicular to the plane of the crack). The main goal is to reduce a real crack detected on an object to simple standard solutions. That is, to imagine it as a crack that has a solution. This means that we must calculate the load with a margin and all our assumptions must be related to the safety margin. In reality, cracks are the most unpredictable (unpredictable shape, unpredictable location).

Abstract: The essential task of modern industry is to increase the reliability and durability of products. One of the promising ways to increase the products operational durability is the ceramics and cermets powders functional coatings application to the working surfaces by the method of high-speed multi-chamber detonation spraying. The objective of the given paper is to determine the regularities of the influence of structural-phase features in the formed material of functional cermets coatings of metal parts on their strength characteristics and crack resistance, while taking into account structural criteria that will provide the required set of strength and crack resistance properties. The method for deep and detailed analysis of the structure features, morphology and distribution of phase particles, their stoichiometric composition, substructure parameters, dislocation density in the coating material obtained due to multi-chamber detonation spraying has been developed in this paper. The research of detonation coating material was carried out using transmission electron microscopy (TEM) on JEM-200CX instrument (produced by JEOL company) with the accelerating voltage of up to 200 kV. The prospects of using the method of multi-chamber detonation spraying on various materials (steel, copper, aluminum, titanium) and alloys are shown on the basis of the research results. A number of composite coatings made of aluminum and zirconium ceramics; chromium, tungsten carbides have been obtained. It has been defined that changes occur in the ratio of the following parameters: microhardness, pore volume fraction, phase composition, distribution of dispersed phases, grain, subgrain, dislocation structures, etc, under different processing modes in the surface layers and corresponding change in the modes of detonation spraying.

Abstract: Shear behavior of reinforced concrete beams using steel lathe scrap waste and end hooked steel fibers as fully or partially web shear reinforcement replacement was studied. Steel lathe scrap waste is generated from industrial steel waste and can be used as recycled fibers offering additional advantages towards environmental pollution reduction. To investigate their effect of reinforced concrete beam under shear behavior, ten reinforced concrete beam specimens with 1200 mm long, 200 mm wide, and 300 mm high were tested under quasi-static loading (two-point loading). The studied parameters in this investigation were types of fibers such as steel lathe waste fiber and traditional hooked steel fiber ratio and the web shear reinforcement ratio are zero% and 50%. Results observed were initial and post-cracking stiffness, maximum capacity load, vertical displacements, modes of failures, and the ductility of the specimens. It was concluded that using of steel lathe scrap waste and hooked steel fibers in concrete are advantageous, they changed the mode of failure of the beam from a brittle to a ductile mode of failure due to the ductility of steel used, whether traditional steel fiber or steel lathe waste fiber. The optimum ratio of the steel lathe scrap waste was found equal 0.5% while the optimum ratio of traditional hooked steel fiber is 1% as fractional volume.

Abstract: When multicomponent molten pool nonequilibrium solidification, the interrelationship of location-dependent dendrite tip undercooling on low heat input and optimal growth crystallography is progressively discussed over planar interface morphology stability range to unidirectionally facilitate epitaxial growth by single-crystallinity control during laser repair of nickel-based superalloy to inhibit microstructure heterogeneity. Suppression of disoriented dendrite growth and crystallography orientation deviation along columnar interface is necessary for crackless repair. Axis-symmetrical (001)/[100] welding configuration kinetically reduces dendrite tip undercooling, nucleation and subsequent disoriented dendrite growth rather than unsymmetrical (001)/[110] welding configuration. When comparison between low heat input, within which laser power is limited and welding speed is rapid, and high heat input, within which laser power is considerable and welding speed is insignificant, the former attenuates dendrite tip undercooling and morphology transition between columnar and equiaxed dendrites to stabilize epitaxy and ameliorate dendrite growth with advantageous solidification conditions, especially drastic temperature gradient and small dendrite growth velocity. Axis-symmetrical growth crystallography and low heat input are favored to mitigate size of high-undercooling region, where stray grain formation are dominant, for homologous single-crystallization of epitaxial growth with satisfactory growth kinetics of dendrite tip, and are capable of elimination of undercooling-induced overgrowth for high quality weld, instead of aggressive unsymmetrical growth crystallography and high heat input. Additionally, the achievement of low heat input with axis-symmetrical welding configuration possesses stronger resistance to unstable interface morphology and solidification cracking. When comparison between growth regions of [100] and [010] crystalline orientation, where identical heat input is kept on both sides, wider dendrite tip undercooling is mainly located on the right side than left side to insidiously exacerbate crack-vulnerable dendrite growth, which is a ubiquitous phenomenon in the adverse (001)/[110] welding configuration. The effect of low heat input on dendrite tip undercooling is spontaneously smaller than growth crystallography. Hence, during nonequilibrium solidification of weld pool, the important mechanism of crystallography-induced microstructure heterogeneity obviation due to undercooling-limited epitaxial growth is consequently provided. The theoretical predictions cogently explain the experiment results in a concise way to properly illustrate microstructure degradation phenomena in the both sides of weld by reproducible calculation of mathematical modeling.

Abstract: This article is devoted to establishing the relationship between the state of the material, phase transformations, and the formation of the microstructure of structural steels widely used in mechanical engineering, as well as for the manufacture of welded sealed bulk structures including capsules for hot isostatic pressing.

Abstract: The paper presents a quantitative assessment of the brittleness of highly hardened silicon carbide ceramics. This characteristic is important for predicting the strength characteristics of the material. The analysis of existing methods for determining the brittleness of the material is carried out. During the research, it was assumed that the production of composite materials in the SiC-Al2O3 and SiC-Al2O3-AlN systems would combine high chemical resistance, wear resistance, and low coefficient of friction. These properties inherent in aluminum oxide and nitride, along with high thermal conductivity and strength, are characteristic of silicon carbide. Materials in the SiC-Al₂O₃ system containing 20, 50 and 80 percent of aluminum oxide – Al₂O₃ were selected as objects of research. Their advantages and disadvantages are shown. The results of an experimental study of samples made of highly hardened silicon carbide ceramics are presented. Based on experimental studies, analytical and graphical dependences are proposed that allow determining the crack resistance of ceramics from the crack length at the corners of the Vickers pyramid. It is shown that these dependences change exponentially, which makes it possible to predict the appearance of cracks at various stages of mechanical processing of products made of highly hardened ceramic materials.