Papers by Keyword: Crack Formation

Abstract: On the basis of theoretical and experimental studies, the prerequisites and the method of calculation of bent and compressed-curved reinforced concrete structures with zone reinforcement made of steel fiber, working under static and short-term dynamic loads, are formulated. In the developed method for calculating the strength of normal and inclined sections, a nonlinear deformation model is implemented, which is based on the actual deformation diagrams of materials. The developed calculation method is brought to the program of calculation of reinforced concrete structures with zone reinforcement of steel fiber under short-term dynamic loading, taking into account the inelastic properties of materials. The numerical studies made it possible to determine the influence of various parameters of steel-fiber reinforcement on the strength of reinforced concrete elements. To confirm the main results of the developed calculation method, experimental studies of reinforced concrete beam structures reinforced with conventional reinforcement and a zone steel-fiber layer are planned and carried out. Experimental studies were carried out under static and short-term dynamic loads. As a result of the conducted experiments, data were obtained that characterize the process of destruction, deformation and cracking of steel-reinforced concrete elements under such types of loading. The dependences of changes in the energy intensity of reinforced concrete structures with zone reinforcement made of steel fiber in the compressed and stretched cross-section zones under dynamic loading are obtained. The effectiveness of the use of fiber reinforcement of normal and inclined sections of bent and compressed-curved elements to improve the strength and deformative.

Abstract: The ability to control the mechanical properties of metal alloys is an urgent task in materials science. For formation of certain operational properties, in most cases, it is enough to treat the working surface of the product by laser radiation. Classical processing methods are ineffective in relation to multicomponent amorphous-nanocrystalline metallic alloys. This is due to their limited use. Usually, this treatment leads to the loss of unique properties the amorphous-nanocrystalline material. Increasing crack resistance and microhardness is not an easy problem. The structure of an amorphous nanocrystalline material can be destroyed under the action of laser processing. Laser nanosecond treatment, as result of a complex effect on the surface, slightly affects the structure of material. The treated material is characterized by increased microhardness and crack resistance, while at the same time such changes may be controlled.

Abstract: Cement-concrete pavement not only has a long service life even at high loads but also has competitive production costs and fewer significant maintenance costs. The concrete road surfaces, thus, are rather economical. In this article, the Vietnamese Standard TCVN 9382 - 2012 was used to determine the heavyweight concrete composition for rural road construction. Assessment of the crack appearance in the concrete block body was made by the temperature field analysis, the thermal stress and cracking index. The conducted studies' result provided with the possibility of obtaining heavyweight concrete from Vietnam local raw materials regarding to the concrete mixture workability of 11-13 cm standard cone, 31-36 MPa compressive strength of heavyweight concrete at the age of 28 - day - normal hardening and 0.30 - 0.42 MPa average water resistance of samples. Using natural pozzolan to replace 20% of mass cement in the concrete mixture leads to a decrease in the concrete strength characteristics at different ages. The concrete compressive strength of composition No2 decreased mostly by 23% at the age of 3 days and least by 14% at the age of 28 days in comparison these values of composition No1. However, all of these concrete compressive strengths at the age 28 days are higher than 30 MPa. Replacing 20% of the mass Portland cement by natural pozzolan in a concrete mix will decrease price for 1 m³ concrete of 219.96 rubles. By applying the computer program MIDAS CIVIL, the maximum temperature in the concrete block center which was determined after 12 hours from the commencement of mixing of raw materials with water, equals to T_max = 34.61 ⁰С. At the same time, the structure temperature difference between the center (node793) and surface (nodes 120 and 898) of the concrete pavement can be neglected because of its insignificance. Besides, the cracking indexes at three hazardous locations of investigated structure are higher than 1, the cement-concrete pavement will be considered as non-appearance of cracks. However, the cracking index at center (node 793) is always less than this on the surface (nodes 120 and 898), equally to higher thermal crack occurrence at center. Therefore, it is necessary to monitor the development and expansion of thermal cracks to ensure the concrete mixture proper care during the hardening process.

Abstract: With millions of square meters of ageing exposed concrete surfaces throughout the world, concrete patch repair is becoming a major component of the civil construction industry. To ensure effective and efficient repairs and rehabilitation of deteriorating concrete surfaces, patch materials with excellent workability during the repair phase and durability during its design life cycle are required. The addition of polymer emulsions to cementitious repair mortars increase the setting time, crack resistance while negatively affects the strength properties of the mortar. Polymer emulsions include natural rubber, polyacrylonitrile and polivinylacetate, to name just a few [1]. This paper focus on the effect of adding different polymer emulsions at varying dosages to cementitious repair mortars to obtain a better understanding of the influences on setting times, crack formation properties and curing regime requirements for optimal strength development. Ambient temperature and relative humidity were kept constant during the testing cycles. The results obtained indicated that the introduction of a polymer emulsion tend to reduce both the initial and final setting times of the repair mortars but that is extends the duration between initial and final set when compared to unmodified repair mortars. Compressive strength reductions were observed when adding polymer emulsions to the repair mortar but it was clear that crack formation was eliminated with the addition of the emulsions

Abstract: In modern manufacture, like in automotive industry, high quality products and high output rates as well as low costs are achieved by highly efficient processes. Optimized tool design represents a key factor for such processes, leading to long tool life and hence to low tooling costs. Early in the industrial manufacturing chain of roller bearings for example, hot bars are sheared into billets, which are subsequently transported automatically to the first forming stage of a press. The shear blades should have a high wear resistance at high temperatures. In this study the first bi-metal composite shear blade made by spray-forming has been developed and tested in industrial environment. The composite tool has been deposited in a co-spray forming process to directly combine a hard-facing alloy layer with a hot working steel body in order to take advantage of the high microstructural homogeneity and the low segregation generated in spray forming. After machining, heat treating and quality inspection of the new material composite, the hot working tool was used in manufacture to prove its wear resistance and durability. The results show that the interface properties of the composite are of high quality and the material has a lower vulnerability to cracks after use in production than the conventional tool, respectively material. Only the porous zone near the interface leads to fissures which are partially going deep into the tool. Hence the parameters of the co-spray forming process need to be improved.

Abstract: In the considering problem usual tasks of designing ground dams (evaluation of filter strength, compressibility and ground permeability) are overlaid by new tasks, that have never touched upon earlier, that provide reliability of earth dams under extreme conditions and that are exposed by new additional forces such as intensive deformation and curvature of base. Here are dependences allowing determination of accepted values of earth surface deformation of ground dams on undermined territories. Using this methodology, you can compute required constructively technologic protection measures. Solution of such problems as predicting behavior and degree of reliability from external influence of a dam at given expected deformations, - can be brought into action. Prediction of conventional dams reliability may been carried out without underworking.

Abstract: Experimental works were carried out to investigate microstructure and crack formation during compression tests of 1.9wt%Cultrahigh carbon steel according to temperature and strain rate. As-received ultrahigh carbon steel is composed of precipitated cementites and pearlite matrix. In addition, numerous voids were observed in the matrix of as-received material. The compression tests at 800 ^oC showed that the voids within the matrix are closed with increase of reduction ratio. On the other hand, when the reduction ratio increased numerous micro-cracks were newly formed in the bulky cementites and at the interfaces between hard cementite and soft matrix. It was also observed that because the volume fraction of cementite is reduced when temperature increased, volume fraction of newly formed micro-crack significantly decreased. Cast microstructures were observed after compression test at 1130 ^oC due to local melting. From experimental results and microstructure anlayses, it was concluded that the forging temperature should be controlled at the temperature of more than 900 ^oC and less than 1130 ^oC.

Abstract: The Rigid-Body-Spring-Model was applied for analyzing fire-damaged concrete beam with voronoi random mesh. Material models of fire-damaged concrete was defined by the experiment. To evaluate the performance and effectiveness of the RBSM with voronoi random mesh, four models of fire damaged concrete beam at various temperatures covered 20°C and 100 to 800°C were analyzed. According to the results, heated specimen 800°C has the lowest strength capacity and internal stress comparing to the other specimens. Main crack of 800°C specimen appears earlier than other specimens. Strength capacity of heated specimen 400°C was higher than the results of unheated specimen because of hydration reaction of large cement clinker was accelerate during heating.

Abstract: The δ→γ phase transformation in the steel was studied in detail, and the relationship between δ→γ phase transformation and the crack formation in continuous casting slabs was discussed as well. The results indicate that the micropores are formed at the positions where δ-phase transformation terminates during the δ→γ phase transformation for low-carbon non-peritectic steel. The micropores will also be formed at the positions where the peritectic reaction of the steel terminates, then the pores are remained on the grain boundary of γ-phase when γ-phase becomes granulated. The micropores distributing on the grain boundary of γ-phase is one of the results for the crack formation of continuous casting slabs and the obvious plasticity decrease of δ-phase zone in the steel. The theoretical analysis results are basically consistent with the experimental results.

Abstract: The Charpy impact test、hardness test、microstructure and morphology analysis of impact fracture by SEM were introduced to research the difference of impact toughness on 25Mn, which were in the station of hot-rolled、normalized and quenched & tempered. The resulted showed that the changes of load、deformation and energy exhaust in difference stage of deformation and fracture could be gained by Charpy impact test. 25Mn in quenched & tempered has more deformation resistance and deformation property than which in other stations. The total impact values was 6J higher than which in normalized in average, however, the crack propagation values was 16.78J higher in average. Ductile-brittle property of the material can be estimated by the analysis of crack formation values、crack propagation values and fracture morphology on samples.