جستجوی مقالات مرتبط با کلیدواژه « ضریب شدت تنش » در نشریات گروه « عمران »

The purpose of this paper is to reinforce the pressurized pipe containing a fully circumferential crack (internal and external) using composite patches due to their remarkable properties. In the research procedure, in order to validate the proposed models, firstly, a cylindrical pipe with a circumferential crack subjected to the uniform tension was modeled and the results of stress intensity factors were compared with an existing paper. Then, in order to examine the modeling process of composite patch and adhesive, a damaged plate with a central crack was considered and the results of stress intensity factors were investigated and evaluated. Stress intensity factor at the crack tip was calculated using the J-integral. In this regard, using the 3D finite element method, the stress intensity factors in the crack before and after the pipe repair were evaluated with boron/epoxy, carbon/epoxy and glass/epoxy composite patches. The crack geometries correspond to different relative depths from 0.4 to 0.8. The results showed that the use of composite patches significantly decreased the stress intensity factors, and this reduction in the external circumferential cracks is more than the internal ones. Also, in the research process, the effect of the patch thickness, the angle of the fiber and the adhesive material on the stress intensity factors has been investigated.

The constant ``F'' is of significant importance when it comes to the design and theoretical understanding of crack expansion under fatigue or impact loads. Even though numerous papers on the failure of materials via the cracked disc have been published, almost no method vis−acutea-vis vis the optimization of the shape factor ``F'' has been proposed. Thereupon, this study strives to determine the stress intensity factor (SIF) for the semi-circular disc model under uniform compressive load and also to calculate the optimized shape factor. In this research, inspired by the geometry of the Brazilian disc, the stress intensity factor (SIF) and the optimized shape factor (F) under uniform compressive load have been proposed. In this paper, to assess KI under uniform compressive load, the experimental results of photo-elastic materials and also the finite element results have been employed. The shape factor of the Edge Cracked Semicircular Disk (ECSD) under uniform compressive load can experimentally be tested more conveniently and therefore by altering the value of the factor beta, the optimal shape factor is obtained.The angle beta varies from zero (purely vertical load) to 10 degrees. Also, the length of the crack changes from 1 to 29 mm. After the carried-out investigations, it can be ascertained that the value for KI reduces as the angle increases. The reason behind that is the reduction of the tensile area in the edges of the semi-circular disc. Further, in a particular angle, as the crack length increases, KI initially rises and then, due to the tip of the crack approaching the compressive area of the disc, it undergoes reduction. The'' zero degree'' diagram is considered to be the limit state and is perpetually increasing and it should not be crossed by any other diagram. To achieve the optimal shape factor, the values of ``SIF'' and the length of the normal crack should first angles, the optimum geometric shape factor of the cracked semi-circular disc was determined to be equal to 1.325 at the angle beta=20

Due to cyclic nature of applied pressure on engineering structures like airplane fuselages, storage tanks, and ship hulls, members evolving the through crack, results in the sudden failure; so-called as fatigue fracture. Investigation of this type of fracture is an important issue for a successful design of these structures. On the other hand, the finite volume method (FVM) as a well-known method in the computational mechanics, is found as a powerful method for the bending analysis of beams and plates, with no matter of thickness varies from very thin to moderately thick beams and plates. Usually in this method, according to the classic approximation of the field variables, the displacement at a favorite point on the face of control volume is approximated linearly in terms of displacement of cell centers adjacent to that face. However, accuracy of the approximation can be increased by using of the higher-order approximation techniques like Moving Least Squares (MLS) approximation which is well suited in the mesh free methods. In this research the MLS approximation is used in the finite volume method, and the Reissner plate with a through-the-thickness crack (through crack) is analyzed for the computation of the stress intensity factors (SIFs) at the crack tip. First, the influence of the size of support domain as one of the key parameter of the MLS approximation is investigated. By using of the obtained value of the support domain size, the SIF values of several benchmark cracked plates are calculated. The obtained results compared with the values obtained by the other analytical and numerical methods presented in literature where good agreement can be seen. These comparisons indicate the capability of the presented FVM for studying of the cracked plates. Furthermore, the size of support domain found in this research can be considered as the convenient values for the FVM analysis of cracked plate in the future researches.

Tensile cracking is one of the main modes of deterioration and failure in rigid pavements. Hence, in this paper the effects of roller compacted concrete (RCC) characteristics on their fracture behavior in both pure mode I and II are experimentally investigated. A set of 288 edge cracked beam specimens endured the four-point bending (4PB) test and were tested for different concrete mixtures made with waste materials such as reclaimed asphalt pavement (RAP) and crumb rubber. The influence of aggregate gradation, cement content, RAP, and rubber content on the critical stress intensity factors in pure fracture mode I and II were studied. The fracture toughness results showed that concrete mix specifications had a noticeable influence on the cracking initiation of RCC and the effect of substituting of waste materials with natural aggregates was found to be negative, but their behavior was still comparable to those obtained for control mixes.

High-speed heavy-haul pre-stressed concrete sleepers are used in railway lines with bearing capacity up to 30 tons and with an allowable speed up to 200 km/hr due to their high performance capability. In this study, it is tried to analyze the fractural behavior of pre-stressed concrete sleepers based on fracture mechanics of concrete. To determine the parameters, a positive three-point bending load is applied to the rail seat of a sleeper with 6 different initial crack (notch) lengths, starting from 0 to 40 mm, with an increasing increment of 10 mm, and also 45 mm. The modelling of high-speed heavy-haul sleeper is done by ABAQUS finite element software. In this paper, some paramount parameters of fracture mechanics for concrete are considered for analyzing and designing e.g. the initial and final stress intensity, load-displacement diagram, final load and the sleeper energy. The results indicate that the fracture parameters of pre-stressed concrete sleeper are different from plain concrete. Unlike plain concrete which the stress intensity factor does not depend on the initial length of the notch, in pre-stressed concrete by increasing the crack length, both of the initial and final stress intensity factors are increased. Also, the results of the time-displacement diagram show that by increasing the crack length, the final load and energy of the pre-stressed concrete sleeper decreases almost linearly. Results represent that by increasing the notch length up to 25%, the area under load-displacement curve decreases up to 37%; while the final load decreases up to 22%.

The existence and extension of the cracks in structural materials is one of the issues to be considered to prevent the devastating effects of cracks. Cracks can be subjected to different types of fracture modes that considering these modes help us to predict the behavior of cracks. This paper investigates the effects of the fracture modes (opening, shearing and tearing) on the annular crack in an infinite transversely isotropic solid. In each mode, by substituting the boundary conditions into governing equations of the medium, the problem reduced to triple integral equations. With the aid of Hankel and Abel integral transforms, the triple integral equations reduced to two Fredholms integral equations which are amenable to numerical solutions. The inner and outer stress intensity factors of the annular crack are obtained for different ratios of inner-outer radius of the annular crack. Some limiting cases such as the penny-shaped crack and external crack are considered. From the results, it can be concluded that the stress intensity factors (SIFs) are independent of material properties; additionally, loads play major role in the variation of SIFs which may lead to change in the direction of crack extension.

Understanding initiation and growth of cracks leads to better evaluation of rock masses behavior. Investigation of crack growth and propagation is noteworthy in geotechnical engineering, petroleum engineering, geology, seismology and many other sciences which encounter with rock fracture mechanics problems. Evaluation of rock slopes stability, design of retaining structures, design of tunnels, and prediction of flow path through rock masses, are some example of application of crack growth study in geotechnical engineering. By evaluating cracks growth, propagation and coalescence, faults seismic behavior can be understood. In general faults are considered as quasi-static cracks, so study of crack propagation under various loading conditions brings important information about faults. Many researchers studied crack growth in Brazilian disk shape specimen under diametrically compression loading. Previous research focused on open crack propagation and closed crack were infrequently included. In this paper using molded gypsum, as a model material, crack growth in Brazilian disk shape specimens with pre-existing open and closed flaws are investigated. The dimension of specimens is 100 mm in diameter and 50 mm in thickness. Inserted open and closed flaws length is 30 mm. Open and closed flaws are inserted using 1 mm thickness steel shims and 0.05 mm thickness silicon tapes, respectively. In this study, the specimens engineering properties were determined by conducting uniaxial compression test and indirect tensile test on Brazilian disks. The experiments were conducted according to ISRM and ASTM standards. Uniaxial compressive strength, Poisson ratio, elastic modulus, and tensile strength of specimens are, respectively, qu=38 MPa, υ=0.25, E=8.5 GPa, and σt=7 MPa. Diametrical compressive loading was applied to the specimens in various inclination angles of flaws with respect to the loading direction. In this paper, influence of open and closed cracks on strength of the specimen is investigated. Fracture toughness of mode I, II and mixed mode I-II, are also determined using analytical solutions. Throughout the tests, slippage between closed crack surfaces is continually monitored by digital microscope. According to the curves of slip distribution along the crack length derived from laboratory data, there is a good agreement between slip distribution along closed flaws and that recorded from real faults by other researchers. Also, results of this study show that open and closed flaws reduce disks strength, significantly (i.e., more than 50%). Inclination angles of the flaw with respect to the loading direction affects crack propagation patterns. Generally, new wing cracks initiate form the flaw tips and then propagate toward the loading direction. Increase in the inclination angle leads to a reduction in the effect of open flaw on crack growth and for angles more than 27.2˚ (i.e., pure shear, mode II) new crack does not initiate from flaw tips. At inclination angle of 90˚, flaw has no effect on crack propagation pattern and the disk fails under tensile splitting condition. In disks with closed flaws, new cracks initiate from flaw tips expect for the case of 90˚ which is the same as open flaws. For the material used in the study, mode I and II fracture toughness are calculated form experimental data and found to be equal to 0.250 and 0.258, respectively. The obtained values for fracture toughness indicate that used material is brittle. Also, the values of mode I fracture toughness for angels above 27.2˚ are negative which point to the fact that flaw tips in these angles are in compression and not in tension.

Nowadays fracture behavior composites play an important role in geomechanics engineering. Also, it is common knowledge that all existing structural materials contain different inter- and intra-component defect (cracks, delaminations, etc.). On the other hand, analytical techniques can provide a better physical interpretation of problems. In this paper, by using an analytical approach, effects of the fracture modes (opening, shearing and tearing) on a penny-shaped crack in a layer of transversely isotropic solid has been studied. The layer surfaces are fixed from displacement and the system is loaded symmetrically in each mode. In each mode, by substituting the boundary conditions into the governing equations of the medium, the problem reduced to dual integral equations. With the aid some mathematical methods, the dual integral equations are converted to a Fredholm integral equation which is amenable to numerical solution. These Fredholm integral equations are the functions of the thickness of the layer, the radius of crack and the properties of the layer. To evaluate the effect of anisotropic materials on the stress intensity factors(SIFs), several synthetic types of isotropic and transversely isotropic materials are selected. By employing a numerical method the opening, shearing and tearing SIFs for different ratios of layer thickness are obtained. The results for the opening SIF show that by increasing the the SIF decreases substasinaly. On the other hand, an increase in leads to increments in opening SIF. Also, the results demonstrate that the variation in has a negligible effect on the opening SIF. Moreover, an increasing in leads reductions in SIF. For the shearing SIF, has little effect on the results although by decreasing the the shearing SIF increases. Unlike the , the modulus of the young in the plane ( ) of the isotropy has substantial effect on the shearing SIF. An increase in leads increments in the shearing SIF. Also, by increasing the the SIF increases marginally. In the mode III, the tearing SIF is only the functions of (the shear modulus for the plane normal to the plane of isotropy) and . The results show that by reduction in the tearing SIF increases and by increasing the tearing SIF increases. An important point that can be inferred from the results is that by increasing the ratio of layer thickness to the radius of the penny-shaped crack all of the three SIFs increase, this increase for the lower thicknesses is much more in comparison to the greater thicknesses. Additionally, when the layer thickness gets higher, the stress intensity factors for all the materials tend to a constant coefficient. This means that when the layer thickness gets greater and tends to infinity, the SIFs become independent of the material of the layer

Ground facilities are an integral part of the airport, which the most important of them is runway pavements. One of the difficulties and the inevitable failure of the pavement are cracking. Cracking in flexible pavements is a distress seen in the most flight zones. Therefore, it is important to take action to reduce the speed of its spread. In this study, field data of Kerman airport (Iran) were used to achieve the mentioned goal and to adapt the results to the reality. Also, the real values of the thickness and density of the layers and other layer properties obtained from the experiments were used. By using finite element software for runway pavement, a crack was modeled and then its output was used to analyze the pavement in different modes. At first, by loads displacement to the position of the cracking, the place where loading causes the highest stress to the cracking was determined. Next, the different analysis in the targeted area was conducted to investigate the effect of the asphalt thickness changes, crack length changes and the temperature changes in the cracking behavior. The results of change in the crack length showed the stress intensity factor KI increase to 17% with increasing the crack length two times. The amount of stress intensity factor decreases with increasing the asphalt thickness. In other words, the crack growth rates decrease with increasing the asphalt thickness. The results showed that for each degree increase in temperature, the stress reduces by 3%, which reflects the importance of temperature parameter. It is recommended at the temperatures close to zero degrees or below zero, the number of flights to be limited.

Assessment of probability of risk occurrence to the railway system is so important because of severe cost which could be imposed to both railway administration and passengers. Wheel movement on rail is one of the factors which has significant role in overall damage of railway infrastructure. Theses interaction-based defects are divided into several categories such as wheel flat and RCF faults. Nowadays, railway administrations are interested in increasing speed and axle load of rail vehicles due to its economical advantages. Therefore, studies on wheel-rail interaction defect-base become a concern for researchers. Implementing finite element method is a major approach in determining fracture behavior of both wheel and rail. In previous studies, simulations were mainly carried out in 2D domain by considering symmetrical assumptions. The main aim of this paper is to implement FE method based on three dimensional situations with more realistic conditions to assess rail surface crack behavior which is exposed to dynamic moving load. For this purpose, stress intensity factors for different depth of crack was obtained and compared also. The results showed that by increasing crack depth and crack tip position from rail surface, the crack tip stresses and consequently stress intensity factors were decreased.

The use of numerical methods, particularly finite element methods in solving different problems are used in abundance. Because these methods are approximate, having a real understanding of the extent and distribution of the errors is extremely important. Studies show that the mesh used in the finite element method will be an essential error rate. So many different ways to find the optimal mesh and minimize the error of the proposed method. These methods are mainly aimed at reducing the error in the stress field have been obtained. Although one of the aims of reducing errors tension field finite element method is adaptive, but no guarantee is intended to achieve specific issues. These issues can be Craek to the issues noted in the analysis of the elastic parameters of the stress intensity factor will play a key role in determining the direction and Craek the design. The purpose of this study is to measure the stress intensity factor adaptive analysis on the parameter to be modified to accommodate ancillary parameters such as strain, stress intensity factor able to modify the target parameter.

Cracking increases the flexibility and displacement of structures. This paper introduces a triangular element with internal crack considering the increase in flexibility of the element. First, Relationship between strain energy release rate and stress intensity factor by fracture mechanic approach is used to calculated energy released by the crack. Then, additional flexibility matrix is obtained in the independent system. This matrix is added to the compliance matrix of a non-cracked element. Finally, Stiffness matrix is calculated by compliance matrix and transformation matrix for the plane problem. Stiffness matrix obtained with this approach includes the decreasing effect of stiffness due to cracking.