Papers by Keyword: Crack

Abstract: In-situ evaluation of crack generation using acoustic emission (AE) technique as a non-destructive testing method was applied during laser cladding of WC-Co cemented carbide via a multi-beam laser metal deposition (LMD) system. Especially, this study aims to investigate relationships between the laser output and the number of cracks generated in beads of WC-12mass%Co cemented carbide cladded by the LMD system. The number of cracks was evaluated using an AE sensor attached to a substrate. The number of cracks was also evaluated from SEM images of beads. By comparing these results, problems in the both evaluation methods for crack generation in laser cladding were discussed.

Abstract: In service condition rail joints, especially the weldments are under the action of various loadings which are not only working in multiple axis direction but also time-dependent having a cyclic and mixed-mode in nature and non-relative to each other. The surface of the rail and its weldment is acted by very high repetitive stress through the wheel and because of this contact stress the running surface or subsurface may have cracks or fractures due to fatigue. This work is based on numerical simulation of an aluminum thermite weldment on a UIC 60 rail under multi-axial fatigue crack propagation under the friction with surficial interaction between weldment and wheel with bending load due to vertically applied load through the wheel on the weld. Since contact is highly influenced by vertical load and also for minimizing the simulation time the lateral and longitudinal traction forces are not included in this study. The work formulation and discretization have been done with the finite element method and a non-linear lagrangian algorithm solver is applied. A 3-D rail-weld wheel model assembly and a semi-elliptical crack as a flaw on the weld surface are used to identify 3-Modes of SIFs along with its graphical plot generation. Simulation is performed under multi-axial weld wheel surface contact at different locations on weld running surface, taking into account varying position of fracture crack on weld 3-D model to calculate fracture life of weld joint and observation of fatigue crack propagation. This work involves the numerical and theoretical approach of fracture mechanics on created FE fatigue model using the Linear Elastic Fracture Mechanics (LEFM) method following Paris law for fracture mechanics. All the numerical simulation for critical fracture dimension and cycle count with stress intensity factor for weld failure data is estimated using software ANSYS 2020 academic and plotted, then comparison of predicted and observed transverse crack growth behavior and fatigue life of weld, based on Millions Gross Tonnes (MGT) is discussed.

Abstract: The article presents the results of a single grain scratching simulation of a ceramic workpiece using the smoothed particle Galerkin method. The studies were carried out for 0.1-0.8 μm depths of cut, matching the ductile-brittle transition of ceramics grinding with minimum subsurface damage. The crack depth and scratching force were obtained for scratching speed of 40-140 m/s.

Abstract: This paper considers the problem of the mathematical theory of elasticity on the stressed state of a piecewise homogeneous half-space at an antiplane deformation in the presence of stress concentrators such as cracks and stringers. The piecewise homogeneous half-space consists of an elastic layer and an elastic half-space with different shear modules. The arbitrary finite number of collinear system of tunnel cracks is located at the joint line between the layer and the half-plane, and the upper edge of the layer is reinforced by an arbitrary finite number of collinear system of stringers. It is assumed that under the action of tangential forces applied to the crack faces and stringers, the piecewise homogeneous half-space is in the conditions of antiplane deformation (longitudinal shear). Such problems arise in research in structural mechanics, geomechanics, geophysics, measuring technology and other areas of applied mechanics and engineering practice. Using the Fourier integral transformations, solving the described problem is reduced solving a system of SIE systems. Based on the solution of these systems, important characteristics of the problem such as the dislocation densities on the crack edges, crack opening, breaking tangential stresses outside the system of cracks on their line, stress intensity coefficients (SIF), tangential contact stresses under the stringers are determined. These values are represented by explicit analytical formulas that are convenient for practical calculations. Particular cases are considered.

Abstract: Amorphous metal alloys have unique properties and are widely used. The unique properties of such materials are accompanied by problems of mechanical strength. The existing methods of their processing are not unambiguous and require a certain approach. In practice, laser technologies allow us to optimize the complex properties of such materials. The selection of optimal processing modes, including the influence of the gas phase, allows you to locally affect the material, increase the microhardness in certain areas. The absence of the influence of the processing medium on the mechanical properties is confirmed. Local impact on the surface sample also leads to an increase in crack resistance. In general, nanosecond laser exposure can be an effective tool for controlling the mechanical characteristics of an amorphous nanocrystalline material.

Abstract: The nature and mechanism of stress corrosion cracking have been studied and modeled in laboratory conditions. It was established that the destruction process develops in three stages: the formation of corrosion defects on the pipe surface, birth and subcritical growth of stress-corrosion cracks, and break. Release bands observed in focal fracture at subcritical crack growth stage indicate that fluctuating stresses are involved in the destruction development. Transcrystalline nature of the fracture at subcritical growth stage implies that SCC in pipelines develops in consonance with the hydrogen embrittlement mechanism.

Abstract: Crack formation is a common and generally inevitable phenomenon in the field of concrete structures. On the other hand, the ever-increasing demand for sustainable construction, thus the structures durability, has led researchers to propose and investigate various crack-sealing methods. This study deals with the key aspect of these investigations – the in-vitro creation of cracks. A large number of the conducted studies have been carried out on artificially cracked specimens, and various methodologies of the controlled crack introduction were presented; however, no specific method was clearly preferred. In this paper, several approaches to the crack introduction are applied: cracking through compressive loading, tensile loading, and 3-point bending. Further, different types of specimens are presented: plain concrete, reinforced with short and long steel fibers, and reinforced with steel rod. The achievable crack characteristics, such as widths or its stability over time, are evaluated and compared. This study thus provides valuable overlook of the possible approaches to the controlled crack creation and points out their potential and limitations. Based on the comparisons presented in this paper, the long steel fiber reinforced concrete specimens subjected to 3-point bending are identified as the most appropriate method of crack induction.

Abstract: The article presents the results of a study of use thermographic non-destructive testing to search for cracks in steel structures. Theoretical substantiation of thermal non-destructive method of control for detection of cracks in steel structures. A practical study proving the possibility of using thermal non-destructive testing to detect cracks in steel structures is described. The problems that arise during the thermal non-destructive method of control and possible ways to solve them. The authors conclude that the method of thermographic control can be used during the inspection of steel structures for qualitative assessment of cracks.

Abstract: Active elements are objectively existing necessary structural components, which largely determine the physical and technical properties of the material and the functional stability of the structure in operational conditions. To manifestation the creative (structure-forming) role of the active elements of the structure, it is necessary to ensure that their initial distribution at various levels of structural heterogeneities is capable of initiating metastable elements and ensuring structural changes of the system without losing its functional purpose.

Abstract: Calculations using the example of a reinforced concrete ribbed slab have shown that if, as a result of an explosion, due to cracks that have arisen, a part of the compressed concrete layer is turned off, then even while maintaining the bearing capacity of the slab, its fire resistance is significantly reduced. It is shown that on the basis of the proposed methodology for studying the behavior of bending elements under the combined effect of "explosion-fire", it is possible to take into account the necessary parameters of reinforced concrete ribbed slabs in the design and operation of structures of hazardous operations industrial facilities. Also, the proposed technique makes it possible to predict a relatively safe amount of explosive in the technological process of an hazardous operations industrial facility, without leading to catastrophic consequences.