چكيده به لاتين
Sleeper in ballast railway lines perform an important role as a holder of rails in one plane to provide and maintain the geometry of the railway line and transfer the loads from the rails to the ballast layer. The first sleepers used in railway lines were wooden sleepers, after which metal sleepers and concrete sleepers were used in recent decades. Also, in the last decade, the production and use of composite sleeper is developing and growing due to its better characteristics than other types. The reason for changing the type of sleeper and replacing them with other materials from wood to metal and then prestressed concrete is to overcome the weaknesses that were in each of them. Composite sleeper are made by adding or replacing materials in the base composition of concrete sleeper. These sleepers, which are also known as engineered sleepers in scientific texts and railway regulations; Because the purpose of producing composite sleepers is to improve its technical characteristics compared to conventional sleepers (especially concrete traverse which is the most common type of sleeper). It is necessary to explain that the simultaneous improvement of all technical specifications is almost impossible; Because the research conducted in this field has shown that usually the improvement of one or more characteristics is accompanied by the weakening of one or more other characteristics, which indicates the high importance of the selection of materials and additives. Research has shown that the main weakness of concrete sleepers used in railway lines around the world is cracking and crack growth in them, which reduces the useful life of the traverse and imposes excessive costs of excess maintenance and repairs on the owners of railway lines. In Iran, concrete sleepers are widely used in all railway lines, and the most common weakness of this sleeper is its cracking and out of service cycle. Examining the regulations for the design of concrete sleepers has shown that despite paying special attention to satisfying the minimum mechanical and technical specifications mentioned in these regulations, addressing the issue of controlling the noncracking of concrete sleepers to the extent of visual inspection during their delivery and use is limited. and there is no standard for measuring the resistance of concrete sleepers against cracking and its growth.
Cracks can act as a critical core for sleeper failure, and from their interlacing and alternate growth, ultimate failure occurs, Therefore, the study of the creation, expansion and joining of these cracks play an important role in predicting the failure process.
A crack in an object can be exposed to three different types of loading, when the crack surface is involved in displacement. The mechanical behavior of a solid body including a crack with a specific size and shape can be predicted by evaluating the stress intensity coefficient (KI, KII and KIII). In the science of fracture mechanics, the critical condition of a cracked part is estimated by using stress intensity coefficients, and to know the state of crack initiation and growth and ultimately the failure of the part, it is necessary to determine the stress intensity coefficients completely. According to the researches, many of failure mechanics problems are in the form of mixed mode. The combined mode is created in two ways: the applied load is combined or there is a non-zero angle between the crack extension and the direction of the applied load.
The use of non-reinforced concrete is practically not useful except for heavy structures due to its brittleness. This major defect of concrete can be solved by arming it with steel bars or reinforcement; But since the reinforcement only forms a small part of the cross-section, the idea that the concrete cross-section is an isotropic and homogeneous cross-section will not be very correct. In order to optimize the performance of the reinforcement, there are fibers that are randomly dispersed among the concrete mix. In order to create isotropy conditions and to reduce the brittleness and brittleness of the concrete body as much as possible, in the last few decades, thin strands of fibers are used, which are homogeneously dispersed in the entire volume of concrete. The physical properties of fiber-reinforced concrete are widely affected by the properties of fibers, which include the volume ratio of fibers, the type and arrangement of fibers, and the bond between fibers and the concrete mixture.
In this thesis, the effect of two types of steel fibers and their combination with polypropylene fibers with different percentages on the fracture toughness of the composite state of concrete has been investigated. The purpose of the investigations was to observe the delay or lack of delay in the initiation of crack growth in concrete, its expansion and, as a result, the failure of the concrete.
