جستجوی مقالات مرتبط با کلیدواژه « ترک » در نشریات گروه « مکانیک »

In recent years, there has been a substantial growth in the utilization of CFRP (Carbon Fiber Reinforced Polymer) composite materials for repairing and reinforcing pipes. This method offers notable advantages, including rapid installation, the feasibility of implementation under service conditions, and cost-effectiveness. The repair and reinforcement of pressurized pipes, particularly in cases where taking the pipe out of service is not feasible, present challenges. Traditional options such as replacing damaged sections or welding on pipes in service become impractical. CFRP emerges as a valuable solution in such scenarios. Nevertheless, the distinct properties of steel and composite materials pose significant challenges to their efficacy. This research employs numerical modeling to investigate the repair and strengthening of pipes with longitudinal cracks under different internal pressures and service conditions. The findings indicate that the presence of initial pressure before applying CFRP diminishes the pipe's capacity. This reduction is attributed to the limited contribution of CFRP to load-bearing at low pressures, owing to the disparate modulus of elasticity between CFRP and steel. However, the rate of capacity reduction decreases with higher initial pressures, highlighting the complexities involved in effectively utilizing CFRP for repairing pressure pipes.

This paper implements the J-integral method to numerically analyze a crotch corner crack in the shell-nozzle junction of an internally pressurized cylindrical pressure vessel, which is used in oil and gas industry. Firstly, by using the three-dimensional domain integral method and for linear elasticity, the equations to calculate the J-integral have been derived. The distinct advantage of using this integral in fracture mechanics is its ability to accurately estimate the stress intensity factor of the crack at points far from its tip, where the stress and strain gradients are high. Secondly, the implementation of the methodology in Abaqus software is described in detail. The methodology can be extended to other locations of vessels and various types of cracks. In order to verify the model, numerical results have been compared with the analytical ones, and a good conformity between them was observed. In fact, the largest difference of 30% is due to correction and safety factors which are considered in WRCB-175. The path independence of the J-integral was also observed for all contours with the error less than 0.15%. In general, the outputs of this research will help inspectors and safety engineers involved in the oil, gas and petrochemical industry to make a more accurate assessment of the safety and health status of equipment.

The presence of cracks in fuel vessels and pipelines against intermittent loads will cause them to grow in the damaged area and cause problems in the fuel transfer process. Using composite fibers to repair the damaged area can increase the life of the structure and part and prevent further damage during unloading and refueling. In this research, a steel tank is subjected to monotonic loading with an increase rate of 1 MPa per minute. The main material of these types of structures is API 5L X65 steel, and the repair conditions are made according to ISO24817 and ASME PCC-2 standards. Numerical analysis of these structures according to the type of defect in it and comparing it with the repaired tank can predict the optimal working conditions. Also, the presence of cracks in this type of tank due to the type of corrosion will cause a lot of stress concentration and change their shape. Using composite fibers and spacers will reduce the stress level and prevent the growth of cracks caused by corrosion. Among the advantages of repairing with the help of composite fibers are the non-interruption of fluid flow inside the pipe during the repair, easy installation, reduced costs, and improved safety. Therefore, this method reduces the stress concentration in the corrosion area.

Analyzing and modeling damage and crack growth in bodies and structures is one of the important issues in designing methods to prevent crack growth or stop it in order to avoid sudden fracture and increase the lifetime of structures. Extensive research has been performed in the field of modeling fracture, crack growth, and damage in bodies and structures. However, there are still many problems in modeling crack growth and damage in bodies with points of singularity and discontinuities. In recent years, a new theory called peridynamic has been proposed to model and analyze such problems. The formulation framework of peridynamic theory is based on integral equations. In addition, points of singularity and discontinuities and damage in the body and modeling them are another type of deformation and part of the structural equations of this theory. As a result, the peridynamic theory is used directly, without the need for additional relations to model crack growth in problems involving points of singularity and discontinuities. In this paper, a bond-based peridynamic modeling for unidirectional carbon fiber reinforced polymer material(UDCFRP) orthogonal cutting process is proposed, and the corresponding composite material bond failure criterion is also investigated for better revealing the machining mechanism of UD-CFRP machining. From comparing between simulation and experimental results, it can be indicated that the peridynamic modeling is capable for predicting the chip formation and surface crack and damages in UD-CFRP machining.

In this research, an analysis of the stress and stress intensity factor in a cracked plate under thermal and mechanical loading is done. For this purpose, using MATLAB coding, a plate with a center crack was modeled by the isogeometric method (IGA) based on the Bezier extraction operator. In this method, NURBS basis functions are generated as a linear combination of Bernstein polynomials using the Bezier extraction operator. Using this operator Bezier elements with Co continuity (similar to elements of the finite element method) were produced and Bernstein polynomials are defined on these elements. In order to model the crack, the extended isogeometric method (XIGA) was used. In this method, the control points that exist along the crack and at the tip of the crack are identified and extracted using the proper level set functions. Therefore, there is no need to re-meshing or modify the elements. By enriching the extracted control points with appropriate enrichment functions and applying boundary conditions, the analysis process was carried out and the strains and stresses were calculated. Finally, the value of the first mode stress intensity factor was obtained based on the interaction integral method. To check the accuracy of the obtained results, a similar analysis was performed using the finite element method and the results obtained from both methods were compared. These studies showed that the considered isogeometric method provides more accurate solutions with a much smaller number of elements and computational costs.

Bonded repair of cracks by gluing patches in metal structures has been widely used due to high strength and speed, and easiness. Obviously, the efficiency of this repair is affected by the shape of the patch. In this research, the shape of the patch with constant thickness is optimized in the unsymmetric bonded repair of an aluminum plate with a central crack, under uniaxial tension perpendicular to the crack. For this purpose, a topology optimization algorithm based on the strain energy density homogenization has been used. The two-dimensional three-layer finite element model of Naboulsi and Maal has been used to model and analyze the repair system. The results show that the optimal shape efficiency of this algorithm in reducing the strain energy released at the crack tip per unit of crack extension is affected by the dimensions of the design initial geometric space relative to the crack length and its volume relative to the patch material volume. It is a efficiency criterion of repair. Therefore, with the appropriate selection of the initial space, the optimal shape of this algorithm is desirable both from the point of view of the repair efficiency criterion and from the point of view of construction.

Large amounts of oil remain in the reservoir in conventional oil production. Strategies for extracting residual oil in the reservoir are called enhanced oil recovery(EOR). One of the usual methods of EOR is the WAG injection. The study of this injection method in a two-dimensional porous medium has been less considered. In this study, two porous mediums are made using a 3D printer. One is simple(S), and the other has a horizontal fracture(F). Nitrogen gas and water have been used in three different scenarios, in the form of a single injection of gas, a single injection of water, and the alternating injection of both, in order to increase the oil recovery. Then, by image processing, the recovery rate of each injection scenario is calculated. This study shows that the WAG injection in S sweeps about 55% of the media, which is more than 6 times the injection of gas alone and more than 2.5 times the injection of water alone. WAG injections in the F also sweep about 38% of the media, which is more than 2.5 times than the other injections scenario. The reason for the difference in recovery rate between the two media in WAG injection is the negative impact of fracture. Also, the difference between WAG injection and single injections can be found in the pressure gradian diagram.

The aim of this research is investigation of surface enerfy effet on free axial vibration of cracked functionally graded nanorods modelled based on the Rayleigh theory of rods. In Rayleigh theory, the effect not only of the axial inertia but also of the inertia of lateral motions are considered. It is assumed that the material of nanorod is functionally graded in its length direction and varies as power low relation. The crack is also modelled as a linear spring in which its stiffness is proporsional to crack severity. The surface energy is included effects of the surface density, surface stress and surface Lame constants parameters. Due to considering the effect of surface energy parameters, the governing equations of motions and corresponding boundary conditions become inhomogeneous in which to solve them, they are converted to homogeneus ones using an appropriate change of variable, firstly. Then, the natural frequencies of fixed-fixed and fixed-free nanorods are extracted using the method of harmonic differential quadrature. In addition to type of boundary condition, effects of parameters like length and radius of nanorod, severe and location of crack, and mode number on natural axial frequencies of cracked functionally graded nanorods in presence of surface energy are investigated.

Sudden failure in structures is one of the results of the presence of defects in parts, which have led to many economic and human damages. Since the creation and growth of cracks can lead to component failure, many researchers have investigated methods to detect the presence of cracks in the structure. Due to the widespread use of pipes in various industries, inspection of pipes is one of the most important issues in the industry. The purpose of this research is to provide a method for crack detection (investigating crack depth and location) in pressurized pipes using modal analysis and differential quadrature method. For this purpose, first calculating the natural frequencies of the vibrations of the defective pipe, the effect of the characteristics of the crack on the vibrations of the structure is investigated, then using the Harris hawk optimization method, the location and depth of the crack is calculated using the frequencies of the cracked pipe vibrations. Examining the results obtained for cracked pipes under fluid pressure confirms the correctness and accuracy of the presented method for direct and inverse solution.

The goal of this study is to analyze the interplay of mechanical and thermal properties and the applied thermomechanical cyclic load combined with the fatigue crack numerical simulation of a thick cylinder. The applied boundary conditions are similar to the working gun barrel during continuous firing. Four stress conditions in 25-950°C and 100-400 MPa pressure has been investigated. Conditions include first, without autofrettage and cracking; second, with autofrettage and without cracking; third, without autofrettage and with cracking; and fourth with autofrettage and with cracking has been investigated. A comparison of the results obtained from simulated models of the autofrettaged and non-autofrettaged barrels has information about the evolution of strains and stresses in the barrel at different points under thermo-mechanical loading cycles in both cases. The materials in the barrel were ST50 steel and SiC/Ti-24Al-11Nb metal matrix composite in three different diameter ratios. The results showed that autofrettage softened the inner surface of the barrel. This phenomenon was seen as a decrease in the hardness of the inner surface of the barrel. The maximum stress of thermomechanical cyclic loading there was until 9 mm of depth. This depth is the active length of crack propagation.

In this paper, the behavior of a stationary crack in a generalized diffusion-thermoelasticity medium under temperature and concentration shock has been investigated. Cracks have been modeled using the extended finite element method and stress intensity factors have been obtained using the interaction integral method. To study the phenomenon of heat dissipation and concentration, generalized Green-Naghdi and non-Fickian theories have been used. The extended finite element method has been developed to discrete equations in space and the Newmark implicit method has been used to calculate time integrals. For different loads (heat shock and concentration), stress intensity factors and temperature and concentration distribution at the crack tip have been studied. The effect of stress wave velocity, concentration wave and temperature wave on stress intensity factors for straight and oblique cracks has also been investigated. It is observed that for the case where the speed of the stress wave, and the temperature wave are the same and greater than the speed of the concentration wave, the increase of the stress intensity factors is faster and higher in other states.

Bearings are one of the main components used in the drive-train of electrical machines. Early fault diagnosis and classification of bearing fault for maintenance of electromechanical system are very important. With progresses in measurement and digital systems, extensive range of real-time data can be available in electrical machines. Since fault diagnosis based on signal processing methods from extracted signals may not be possible due to different reasons such as noise level, natural frequencies of system, saturation of core,, severity of fault and load torque, deep learning methods have been considered in recent years. In this paper, time series deep learning method for condition monitoring of bearing in electrical machine for the purpose of detection and classification of fault is considered. Obtained results by means of proposed method have been compared with pervious techniques. Experimental results show that proposed deep learning method can detect and classify bearing fault with accuracy above 90%.

The free and forced vibration analysis of a rotating large flexible structure with a single crack is investigated using the Homotopy Perturbation Method (HPM). The crack is modeled with a torsional spring element on a structure that follows the Euler-Bernoulli theory. The nonlinear equations of motion of the co-rotational system considering centrifugal forces are derived using the calculus of variation and the Assumed Mode Method (AMM). Applying the Galerkin method, the spatial domain is extracted and the time domain is transformed into a second-order nonlinear differential equation. The results of time response, phase plane, and bifurcation diagrams for different functional parameters variations such as base angular velocity, crack position and stiffness have been analyzed. Moreover, it is shown that as the base angular velocity increases, a tensile force appears along the cracked structure axis, stiff it, and shifts the backbone to the right, this can highly affect the nonlinear features of the system.

One of the most common defects in rotary equipments is cracks. Creation and growth ofcracks can cause irreparable damages as well as occurrence of unwanted vibration in rotatingequipments. On the other hand, increasing the damping of the system is one of the mostpractical solutions to control vibrations in rotating equipment. One of the most importantmethods to increase the damping is the use of squeeze film dampers. Despite the highefficiency of these elements in controlling the amplitude and damping of system vibrations,their nonlinear behavior has created many limitations in the use of this equipment.In this paper, the bifurcation analysis of a flexible rotor with a floating-ring squeeze filmdamper with centralizing spring and considering the occurrence of cavitation in the damper oilfilm and transverse cracks in the shaft is been investigated.

Adhesive joints have been employing in engineering structures such as marine, aerospace, automotive, oil and gas industry. Since the joints are the weakest part of engineering structures, the fracture possibility in adhesive joint is high. Therefore, structural health monitoring of adhesive joint is an important issue. The aim of this paper was condition monitoring of damage in the single lap adhesive joint by multiwall carbon nanotubes. This work is carried out by recording the electrical resistance change during the mechanical loading. This damage propagation is observed as a crack extension in adhesive joint. Firstly, MWCNTs with 9 wt.% are dispersed in an epoxy resin by ultrasonic device. Then, the hardener is added to the nonoadhesive. The obtained material is immediately poured into single lap adhesive joint mold. The defective adhesive joints were manufactured with circular and square defects in different sizes i.e. 10%, 30% and 70% overlap area. The specimen is subjected to tensile test and electrical resistance change are recorded during the shear load. The results showed that the maximum value of relative resistance change is occurring in the adhesive joint comprises of square defect with 30% overlap area of defect. In this situation, the crack was propagated in nanoadhesive and a small part of a crack was extended from defect boundary.

In this research, the dynamic analysis of composite functionally graded thick beam despite transverse cracking with the help of Tymoshenko theory and higher - order shear deformation theory has been studied. The governing equations and boundary conditions for composite functionally graded beam and thick using Timoshenko and Reddy theories and also, the principle of system energy minimization has been obtained. It is assumed that the inhomogeneous mechanical properties of the beam as a function of thickness as a function of power in which the thickness of the beam is variable. The boundary condition as clamp-clamp are also considered. According to these conditions, the closed form solution for natural frequencies of composite functionally graded beam despite the crack is obtained. Then, the results were analyzed and by those published in the literature and with the finite element results obtained by ABAQUS are validated. Finally, the results showed that the percentage of improvement of the answer obtained from Reddy's third order shear theory compared to Tymoshenko's theory for thick beam is about 24% and for thin beam the answers are close.

Freezing water in the surface cracks of wind turbine blades can remarkably deteriorate its mechanical properties. In order to investigate this phenomena, a finite element model is developed to simulate the icing process in a small scale fiber reinforced composite beam with a sharp notch. The load exerted from the ice onto the beam is calculated by using the model taking into account the expansion of water at freezing temperature. Based on this technique, a new parameter, called the ice expansion effect, is introduced and employed to study the influence of notch opening angle and the beam thickness on the distribution of stress and strain along the notch faces. Compromising between the actual problem, in-service damaged blades, and the existing standard which is commonly applied for characterization of fracture behavior of composite materials, an optimum geometry is designed for experimental investigations. The experimental results acquired from small scale composite beams based on the designed geometry and simultaneously imposed to humidity and freezing temperature demonstrate the significance of ice expansion factor on the monotonic response of the materials and structures conditioned at different number of freeze-thaw cycles.

In this research, for the first time, the free vibration of the functionally graded beam containing transverse crack under clamped-clamped boundary condition studied based on Reddy high order theory. It was assumed that mechanical properties of beam vary exponentially in the thickness direction. To drive the closed form solution for natural frequencies of functionally graded beams containing a transverse crack, Ritz method was used. The effects of slender ratio, the location of crack, depth of crack and material gradients on natural frequency were also examined. Finally, the results of the analytical method were analyzed and compared with results of modeling by ABAQUS software and other studies. This research shows that due to more complex equations of Reddy beam theory, using Reddy beam theory for a thick beam and Timoshenko beam theory for a thin beam are more suitable. In addition, a relation between the depth of crack and the necessity of using functionally graded beam is confirmed.

During the assembly process of aluminum structures with welded joints, usually the life of the structure due to the unwanted stress of the welding process and possibly the presence of undesired defects due to the strength of the initial design of the reduced structure will cause cracking and eventually cause fracture near the support sections. ؛ In this paper, to prevent the propagation of microscopic cracks, first, based on field investigations, the exact location of crack damage in rigid floating structure with specific application is identified, and then numerical methods are used to investigate the behavior of model floating structure under cyclic or periodic impact loads. Due to the knocking of waves and firing c Has been discarded. In addition, by analyzing the data output, by changing the component parts of the rigid float superstructure, and by modifying the new positioning of the structural member locally, the optimal design has been achieved, and the new geometry design responds to the needs of the floating user and stops Growth is cracking.