Papers by Keyword: Crack

Abstract: Preliminary mechanical loading at a temperature close to the ductile-brittle transition temperature leads to stress relaxation near cracks in brittle materials due to local plastic deformation at microcrack tips. As a result, such preloading increases the physical and mechanical properties of ceramic materials when tested at room temperature. In the present work, this phenomenon is investigated for silicon and silicon-based ceramics. A thermomechanical treatment (TMT) method of the mentioned materials has been developed to increase their strength and fracture toughness.

Abstract: The article provides some information about gypsum concrete, its applications, and the advantage of using organic fillers compared to mineral ones. The optimal technology for the production of gypsum concrete mix was determined, and an economically attractive type of organic filler in the form of chopped corn stalks was established. The compressive strength of the resulting material was studied depending on the fraction of crushed stone used. Effective methods for combating shrinkage cracks at the stage of manufacturing prototypes have been identified, which allows increasing the bearing capacity of the samples by 2.5 times. The water resistance and water absorption of the material, as well as their effect on strength, were investigated. As a result of experimental studies, it was found that the optimal concrete compositions with filler fractions of 3-5 and 5-10 mm should be considered 1:1 and 1:1.5 by volume (binder: filler), which can provide sufficient compressive strength (13-23 MPa) for blocks and slabs of internal partitions and good water resistance (0.91-1.0), while having good sound-absorbing properties.

Abstract: The paper outlines a method for comparative analysis of the load-bearing capacity and crack formation of reinforced concrete and fiber-reinforced concrete cylindrical shells based on experimental studies. To implement this task, the authors have developed a special stand. The results of tests of reinforced concrete and fiber-reinforced concrete cylindrical shells, which had the same geometric parameters, are presented. The fiber-reinforced concrete shell had additional dispersed reinforcement with steel fiber with curved ends, which was added at the stage of mixing the concrete mixture in an amount of 1% by volume of concrete. The shells were hinged at four points and loaded with a vertical distributed load applied along four strips, each 13 cm wide, and only along the body of the shell. The load-bearing capacity of the reinforced concrete shell was 101.6 kN, and the first crack appeared at a load of 64.5 kN, which is 63.48% of the load-bearing capacity. Before the loss of bearing capacity, 10 cracks with the same initial opening width of 0.05 mm had formed in the shell. The load-bearing capacity of the fiber-reinforced concrete shell was 149.9 kN, and the first crack appeared at a load of 74.9 kN, which is 49.97% of the load-bearing capacity. Before the loss of bearing capacity, 12 cracks with the same initial opening width of 0.05 mm had formed in the shell. The load-bearing capacity of the fiber-reinforced concrete shell turned out to be 1.48 times greater than that of the reinforced concrete shell.

Abstract: Fracture analysis of crack is very essential to ensure the reliability and avoid the catastrophic failure of engineering components and structures since most of the failures start from the crack which leads to loss of life and economy. In the present study, extended finite element method (XFEM) is used to simulate the axial surface crack in bi-material pipe. Bi-material consists of two dissimilar materials with distinct properties. In this study, bi-material pipe consists of inner pipe made of steel alloy and outer pipe made of ceramic. An axial semi elliptical part through crack at different location is used for 3D linear elastic fracture mechanics (LEFM) analysis of bi-material pipe. Bi-material pipe is subjected to internal pressure and stress intensity factor (SIF) is computed at different location of crack front of semi elliptical surface crack using virtual domain extension approach.

Abstract: Bonding metallic structures with composite materials is widely considered to be the most optimal method for joining damaged and fractured structures. This method offers important advantages, such as reducing the stress intensity factor (SIF) and increasing the lifetime of the joined structure. However, hygrothermal aging is a phenomenon that can reduce the lifetime of reinforced structures made of glass fiber-reinforced polymer (GFRP). This study used numerical modeling to investigate a cracked stainless steel pipe operating in a hygrothermal environment and repaired with three patches. The main objective of this work is to determine the effect of adhesive aging due to hygrothermal damage on the repair efficiency of a cracked SA312 type 304 stainless steel pipe. The Finite Element Method (FEM) is used to evaluate the SIF as a function of applied load for different immersion times and at two different temperatures. First, the developed model was validated against literature results. A parametric study was then carried out. The obtained results showed that the adhesive maintains its stiffness for 7.5 months of immersion and that the mechanical properties of the adhesive are acceptable even at temperatures of 90°C or lower and internal pressures less than or equal to 50 bar. However, when the pressure load exceeds 50 bar (p_int > 50 bar), the degradation of the adhesive becomes more significant, and the hygrothermal aging leads to variations in the mechanical properties of the joined structure. It is important to note that these results can contribute to the improvement of the existing composite repair design standard and can provide reliability for the application of GFRP in different humid environments.

Abstract: In order to analyze the stress concentration impact, intensity close to the zone of the crack tip, this work examines the in-plane SIF(SIF) of composite plates utilizing measured crack tip opening displacement (CTOD). The test specimens' E-glass fiber mats were arranged in various ply configurations. The ASTM standards utilized for researching mode I fracture of composite materials served as the foundation for the compact tension (CT) specimen. The mode I, K_I Stress intensity factor (SIF), and critical stress, c, were calculated for each specimen along the fracture length propagation based on the experiments. It was found that the SIF is directly proportional with fracture length, or a/W, for all E-glass fiber laminate cases tested. The K_IC is often higher in thinner laminates. The presence of woven roving increases the SIF and hence the toughness of the laminate.

Abstract: Beam-column joint is the most vulnerable location of a moment-resisting reinforced concrete frame structure. The joint region experiences the maximum shear stress both in vertically and horizontally which is generated due to the shear transfer mechanism from the adjoining beams and columns. The shear capacity and bond stress capacity are the two major factors affecting the strength of a joint core in RC structure. An important discovery recently is the ductile behaviour of the whole structure under repeated loading. The behaviour of the concrete beyond elastic limit which is in the concrete hardening zone can drastically influence the ductility of the concrete. The non-linear stress-strain behaviour after the onset of the initial crack and up to ultimate compressive strength plays an important role in improving ductility. Beyond the ultimate compressive strength, concrete will undergo softening which is neglected in this study as once concrete reaches ultimate stress it is unsafe for service. This material ductility can be fulfilled with the application of high-strength fibres with ductile behaviour. However, the hybridization of two or more fibres can incorporate two different characteristics of the fibre used. The use of ordinary-grade of concrete moreover reduces the shear-resisting capacity of the joint. A hybrid mix of hooked-end steel fibre with basalt fibre and crimpled steel fibre with polypropylene fibre are used with a volume fraction of 1% to 1.4% of the concrete. In this study, ordinary M25 grade concrete and fibre mixed M25 grade concrete is employed under static and cyclic loading. The laboratory tests are also conducted to evaluate the compressive strength, split-tensile strength, and flexural strength of the hybrid mix fibre-reinforced concrete at the age of 28^th days. Five full-scale models of the beam-column joint are designed as per the Bureau of Indian Standards. Numerical models of concrete and steel reinforcement are developed. Numerical analysis is carried out using finite element software ANSYS-v21. The behaviours of the beam-column joint are observed under static as well as cyclic loading. Crack patterns, first crack load, initial displacement, ultimate load, and ultimate displacement are observed under static conditions. And under cyclic loading, hysteresis load vs displacement, energy dissipation, and stiffness degradation are observed. The hybridization of hooked steel with basalt fibre gives better results in mechanical strengths and the hybrid effect of crimpled steel with polypropylene fibre gives better results in mechanical strengths. And also under numerical study, the above specimens show an improvement in energy dissipation capacity. Keywords beam-column joint, hybrid fibre reinforced concrete, numerical concrete model, ANSYS, static, reverse cyclic, energy dissipation, stiffness, crack

Abstract: The methodology of experimental research of long cylindrical shells to determine their stress-strain state, carrying capacity and crack resistance is proposed. To implement the task, the authors have developed a special stand. Eight cylindrical shell models were made for testing - four of reinforced concrete and four of fiber concrete. Fibro-concrete specimens-shells had additional dispersed reinforcement by steel fiber with bent ends in an amount of 1% by volume of concrete. All specimens-shells had a constant length and cross-sectional radius, and varied the thickness of the shell and the size of the cross-sectional section of the board elements. The paper presents the results of tests of reinforced concrete cylindrical shell, which showed that the carrying capacity of the shell was 96.4 kN, and the first crack formed with a load of 42.9 kN, which is 44.5 % of the carrying capacity. Up to the moment of bearing capacity loss, 8 cracks with the same initial opening width of 0.05 mm and maximum final opening width of 0.8 mm had formed in the shell. Computer modeling of the shell and calculations with ANSYS 17.1 licensed software were performed. The bearing capacity determined in ANSYS was 93.0 kN, which is 3.6 % less than in the experiment. The test methodology and the developed stand are universal and will be used for further research.

Abstract: Desiccation is the process of extreme drying that results in cracking in materials such as soils. Cracking is a complex process that has the potential to spread and penetrate deeper into the soil, which could lead to instability in earth structures like embankments. A number of desiccation tests were conducted to observe the crack behaviour of manufactured kaolin clay at laboratory scale. All samples were prepared at variation of initial water content and desiccation tests carried out using aluminium desiccation trays. Their crack behaviour was monitored at different thickness and comparison made to the samples tested on different base materials such as ceramic, steel and wood plates. The drying process, crack propagation and pattern were measured and observed daily. A digital image is captured and the crack intensity factor (CIF) calculated manually using a grid system. It was found that a sample of kaolin clay that was 5 mm thick had the highest CIF with rapid propagation and produced non-orthogonal patterns. It also discovered that the thickness of the samples, type of plates utilised and soil state at the start of the test have an impact on the formation and propagation of cracks.

Abstract: In order to research the relation between fracture and texture conditions of PEEK thrust bearing in water, the rolling contact fatigue, RCF test was carried out. And then, the specimen after test was observed with a laser confocal microscope. Three types of surface damage: single crack, flaking and multiple crack: were ob-served. Arithmetic Average Roughness Height, AARH’s around single crack and no-damaged area were calculated. AARH’s in these two areas have no sig-nificantly different each other. This means the effect of cracks on roughness was negligible, and vice versa. The changing of AARH during the test was also dis-cussed. AARH’s both of failure and non-failure specimens were decreased dur-ing RCF tests. AARH Range of non-failure specimens after test included that of the failure specimen. It indicated AARH was not dominated the condition failure of PEEK bearing in water. On the other hands, the load had a clear threshold. This means the failure of PEEK bearing in water is strongly affected by load.