جستجوی مقالات مرتبط با کلیدواژه "ضریب شدت تنش" در نشریات گروه "مکانیک"

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.

One of the most important analyses in the design of various structures, especially pressure vessels and their safety is the fracture mechanics. One of the important parameters in fracture mechanics is the study of the stress intensity factor of cracks in the tank wall. In the present study, the behavior of a semi-elliptical crack in a spherical pressure vessel made of functionally graded materials has been studied using Abaqus finite element software. The effects of parameters such as crack geometry, simultaneous internal and external cracks, pressure distribution, thermal load distribution, changes in the properties of the functionally graded material, and support conditions on the value of the stress intensity factor have been investigated. In order to model and analyze the stress intensity factor in this type of tank, various power, exponential and linear functions were used in the form of MATLAB code as well as a subroutine code. The results showed that with increasing pressure, the stress intensity factor increases. Crack geometry is also an important factor that has a significant effect on the stress intensity factor. So with the increase of the a/c value, the stress intensity factor also increases. Also, the examination of the support conditions shows that with the increase in the number of foundations, the stress intensity factor also increases.

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.

Crack growth and fatigue life assessment of aviation parts are very important. One of the special and important parts in airplane is the Rib trunnion landing gear, which has been investigated in this research. Due to technological and material problems in I.R. Iran, H13 have been selected instead of HY-TUF material, and diameter of one holes for jointing to landing gear is increased 7 mm greater than the original one. According to these changes, life estimation of manufactured part becomes significant. For this reason, the behavior of fatigue crack growth to obtain the critical crack length, and the trajectory of the crack growth has been analysed. Finally, the fatigue life of the part has been assessed using fracture mechanics-based method considering an arbitrary initial crack. The results showed that the critical crack length for original and manufactured parts is 19.4 and 9.2 mm, respectively. The allowable initial crack length is 6 and 2 mm for the original and the manufactured parts, respectively.

In this research, stress intensity factor of a small radial crack in a rotating thick cylinder made of viscoelastic materials subjected to internal pressure and radial temperature distribution is investigated. The radial crack is assumed to be located at the outer edge of the cylinder. The Zener model (i.e., standard three parameter solid) is applied to simulate the viscoelastic behavior of the cylinder. To obtain the stress intensity factor of the viscoelastic cylinder, the problem of uncracked viscoelastic cylinder subjected to internal pressure and radial temperature distribution is analyzed with plane strain assumptions. Then, the hoop stress distribution is determined. Finally, by assuming the small crack length and applying the superposition method, the problem of cracked viscoelastic cylinder is replaced by the problem of cracked viscoelastic cylinder subjected to determined average stress in the previous step. By employing the proper geometry factor, the stress intensity factor of the viscoelastic cylinder is determined. A parameter study is performed to investigate the effects of various parameters on the stress intensity factor of the cylinder. To validate the results, the problem of viscoelastic cracked cylinder is simplified to an elastic cracked cylinder by neglecting the viscoelastic terms, and the data obtained in the present work are compared with those given in the literature search. The results show that the proposed method is quite capable of estimating the stress intensity factor of small cracks in structures made of viscoelastic materials. A parameter study is performed to investigate effect of various parameters on stress intensity factor of the cylinder. The parameter study shows that a) the stress intensity factor of the viscoelastic cylinder in early moments of loading is higher than its equivalent elastic cylinder, b) increasing the elastic properties of the viscoelastic material results in higher stress intensity factor, and increasing the viscous properties of the material decreases the stress intensity factor.

A fuel cell is an electro-chemical tool capable of converting chemical energy into electrical energy. The high operating temperature of the solid oxide fuel cell (SOFC), (between 700oC to 1000oC), causes thermal stress which is the origin of crack initiation and propagation. Thermal stress causes gas escape, structure variability and SOFC operation cessation before its lifetime. The purpose of the current paper is to present a method that predicts the thermal stress distribution and forecasts the beginning of fissure or crack occurrences in an anisotropic porous electrode of the planar SOFC. The governing coupled non-linear differential equations of heat transfer, fluid flow, mass transfer, mass continuity, and momentum are solved numerically. A code based on computational fluid dynamics (CFD), computational structural mechanics and finite element method (FEM) is developed and utilized. The results show that the highest thermal stress occurs at the lower corners of anode and the upper corners of cathode. The cathode’s thickness at the left side increases by 1.5% and the concentrated temperature and thus the fissure occurs between the top and bottom left corners of the cathode.

Due to various benefits of composite materials such as light weight, high strength and their excellent formability, the external bonded composite patches have been proved to be a preferable method of repairing flaws and cracks in various engineering structures. In this paper, the effect of some parameters such as the composite patch material, the adhesive material and the base plate material on the reduction of the stress intensity factor and the variation of fracture initiation angle has been studied using the 3D finite element method. The results showed that the composite patch material and base plate material have significant effect on reduction of stress intensity factor. However, the adhesive material has insignificant effect on it. One of the other important results of this study is that the use of patch does not have a significant impact on the fracture initiation angle.

Today, because of widespread faults in metallic pipes such as internal and external corrosion, hard deformability, abundant deposition arising from much roughness on internal parietal and profuse salt absorption and high pressure drop because of internal surface roughness, weightiness and installation difficulties and low lifetime etc. the tendency to use polymer pipes in transfer pipelines, industrial, marine, drilling and agricultural zones has increased. Examine the rate of damage the plastic tubes reinforced with the glass fiber under internal pressure from different angles, which might be defeated under creep phenomenon in long term, has not receive the attention to a large extend. Therefore, in the present research, it has been tried to simulate the fracture by Tsai-Wu, Hashin criterions and also the crack inlet opening with J-integral criterion in this pipes with different layering at internal pressure using finite element method and ABAQUS software. This research results show that the fibers under compressive and tensile stress and the resin of tube has not fractured by tensile stress, but the fracture has occurred in the resin of composite tubes that were under pressure. According to the yielded results, the fracture index is less than one for each of ten layers, when layering angle varies from [(30)] to[(60)], but the fracture index is more than one, layering decoration for[(70)], [(80)] and [(90)] of composite tubes has been fractured. However, the value of stress intensity factor was immediately decreased for layering decoration angle of [(55)], but this factor has improved when layering angle increased.

In this paper, the effect of composite patch on reinforcement of cylindrical tube containing a surface crack under internal pressure is investigated using a three dimensional finite element method. The cylindrical tube contains an external circumferential semielliptical crack and the aspect ratio of the cracks is between 0.2 and 1 and its relative depths are 0.2 and 0.8. In order to ensure the accuracy of modelling, first, a cylinder containing a surface crack is simulated under uniform tension loading and the results are compared with the available data. The stress intensity factor at the crack tip is determined using the J-integral. Then, the cracked cylinder under internal pressure is repaired using topical composite patch. In this regard, in order to select the appropriate patch, the effect of different parameters such as length, width, thickness, material and angle of composite multi-layer fibers on the stress intensity factors are examined. The results indicate that the stress intensity factors can decrease about 23% by using the appropriate patches which caused the increase of fatigue life due to the decrease of fatigue crack growth rate.

The purpose of this article is to investigate the effect of composite patch on stress intensity factor (SIF) for internal and external semi-elliptical crack located in spherical pressure vessel. The composite patch is wrapped only around the crack. The three dimensional analysis is done by Finite Element Method (FEM). ANSYS Parametric Design Language (APDL) codes are developed to facilitate modeling of semi-elliptical crack in spherical pressure vessel. By using these codes the effect of some parameters such as crack geometry (crack depth/thickness of vessel and crack depth/ half-length of crack), the ratio of composite thickness to metal thickness, the ratio of composite patch width to half-length of crack and composite material on stress intensity factor values are investigated and discussed in detail. The results show that composite patch has significant reduction in stress intensity factor at crack tip. Also it can be concluded that composite patch is more effective for deeper and slender crack. In addition composite patch is more effective to decrease stress intensity factor of external crack in comparison to internal crack.

In this study, a semi-analytical method based on the Reisner's mixed formulation is presented for the elastic analysis of anisotropic homogeneous solids with an edge or interior crack. In this method, the displacement and the stress fields are represented as the sum of a known function and a finite series of functions with unknown coefficients. The functions are constructed in such a way that the displacement discontinuity across the crack faces and the exact singular behavior of the stress field at the crack tip are captured, moreover all essential and natural homogeneous and inhomogeneous boundary conditions are satisfied exactly. The equilibrium and the compatibility equations are also applied with the desired accuracy using the Reissner's variational principle. Solution of the variational equation leads to a set of linear algebraic equations in terms of the unknown coefficients. After computing of the unknown coefficients, the displacement and the stress fields are obtained and subsequently the stress intensity factors (SIFs) and crack opening displacement (COD) are calculated. The results show that computing of SIFs has high convergence rate and the results of the proposed approach are in good agreement with those of the analytical solutions reported in the literature.

Thin plates are used in marine and aerospace structures. Geometric defects in components and industrial structures usually occur intentionally or unintentionally which hole is one of them. It is common that a crack initiate in the areas of stress concentration. Even if the crack is relatively small, it propagates and can lead to a dangerous situation. Stress intensity factor is one of the important parameters of the crack behavior. In this paper, by using the analytical solution of Muskhelishvili and finding suitable conformal mapping, behavior of two unequal and aligned cracks emanating from a circular hole is investigated. The effect of parameters such as load orientation, crack length and etc. is studied. The hole and the cracks are assumed to be traction free. The infinite isotropic plane is subjected to a uniform tensile loading at infinity in an arbitrary direction. To ensure the accuracy of the method, results are compared with some specific problems. In this paper, an explicit formula based on the geometric parameters of the problem is presented for stress intensity factor. Also and are obtained for various loading and cracks length.

Due to various benefits of composite materials such as light weight, high strength and their excellent formability, the externally bonded composite patches have been proved to be a preferable method of repairing flaws and cracks in various engineering structures. In this paper, the behavior of various patches such as composite and metallic patches has been studied by calculating the stress intensity factor and the T-stress via 3D finite element method. The study of the out-of-plane bending role in repair of plates with single-sided patch is another aims of this research. The results showed that the higher stiffness of the composite patch leads to further reduction in stress intensity factor. It is found that in the studied specimens, the boron/epoxy patch has the better behavior compared with glass/epoxy one. Furthermore, using single-sided and double-sided patches leads to change in the T-stress value. The largest change achieved by the crack angle equals to 0 and the smallest on achieved by the crack angle equals to 45 degrees. Increasing the adhesive thickness leads to increasing the stress-intensity factor in repaired plate. Finally, it is found that the out-of-plane bending has the significant effect on behavior of repair with single-sided composite patch.

Production methods of functionally graded materials cause anisotropic behavior. In this paper, the element-free Galerkin method is used to study of fracture behavior of orthotropic functionally graded materials. In this method, numerical calculation of the main and auxiliary strain fields is simply done while, in finite element and extended finite element methods, it is associated with difficulties. In addition to in this paper, a path-independent integral is presented to compute the fracture parameters in heterogeneous materials which has a unique form for mechanical and thermal loading. This new form of interaction integral includes terms with the certain physical interpretation. Also, the presented form of interaction integral has fixed term for interaction of linear fields on elastic field and it is systematically extensible. In this paper, the stress intensity factors in orthotropic functionally graded materials are obtained by using this interaction integral formula. The element-free Galerkin method is implemented to discrete the governing equations. The effect of materials characteristics on the stress intensity factors is studied in several examples. The obtained results are in good agreement with reported ones in existence papers.

The Griffith crack, a central crack in an infinite plane under uniform loading, is converted to a subsurface one by moving close to a loaded edge of the plane. Subsurface cracks initiate under rolling contact fatigue conditions. In this paper, first, finite element model of the Griffith crack has been developed and validated by calculating stress intensity factors (SIFs) under uniform tension and shear loadings. Then, by moving the crack close to a parallel edge of the plane, mixed mode SIFs of the subsurface crack have been determined for a wide range of the cracks depths. Non-symmetrical geometry with respect to the crack edge causes coupling between fracture modes and so, considerable shear and tension fracture modes under tension and shear loadings, respectively. The ratio of SIF for the coupling mode to the direct mode is creased up to 69% for the length to depth ratio of 20. Also, by fitting third-degree polynomials to the mixed mode SIFs, four geometry correction factors have been obtained for SIFs of subsurface cracks under uniform loadings. These approximate equations can be used easily and efficiently by engineers. Also, the relations can be utilized as a primary estimation for non-uniform loadings, especially when the crack length as well as the load variation along it is small.

In this paper, the closed-form stress intensity factors are calculated at the deepest point of a circumferential semi-elliptical crack located at the inner surface of a cylinder. The cylinder is subjected to pressure (internal and external) and the inner surface of the cylinder is subjected to convection cooling. To solve the problem, initially, a weight function is derived for the deepest point of the circumferential semi-elliptical crack using two reference loads. Then, the steady state solution of the thermoelasticity problem is derived and, finally, the stress intensity factors are extracted using the weight function method. For some special cases of loading, the results of the present theory are compared with available solutions in the literature indicating an acceptable agreement. Moreover, the effects of crack relative depth and aspect ratio and heat transfer type on the thermal stress intensity factors are studied. The extracted results demonstrate that for some cases of loading and crack geometry, shallow cracks are more critical than deep ones and the solution of conduction heat transfer is more conservative than the forced convection one.

In this article, stress analysis of an isotropic infinite cylinder weakened by multiple concentric cylindrical cracks subjected to shear and radial loadings on the lateral surface of the cylinder is accomplished. First, relations for stress and displacement components are given in terms of the Galerkin biharmonic vector potential. Next, using these relations, the dislocation solutions for both Somigliana and Volterra dislocation are derived. Using the distributed dislocation technique, integral equations for the infinite cylinder with arbitrary number of the cylindrical cracks are constructed. The numerical solution of resulting integral equations which are of the Cauchy type singular equations leads to evaluation of Somiglianaand Volterra dislocation densities on the crack surfaces. Using related dislocation densities, the stress intensity factors of crack tips for some examples are attained and variations of them with lengths of cracks are discussed. For some special cases, validation of the results of this study with others available in the literature is done.

In this paper, the extended Finite Element Method (XFEM) is implemented to compute the Stress Intensity Factors (SIFs) for rectangular media subjected to a hygrothermal loading. In governing hygrothermoelasticity equations, the cross coupled of temperature and moisture fields and temperature-dependent diffusion in some cases are considered. Furthermore, an interaction integral for hygrothermal loading is developed to compute the stress intensity factors. The non uniform mesh of isoparametric eight-nod rectangular element is used in XFEM to decrease the absolute error in SIFs computations. In order to numerical results validation, the SIF of mode I is obtained analytically. The coupled governing equations are firstly decoupled in terms of new variables and then solved by the separation of variable method. According to the results, the moisture concentration gradient has a significant effect on the SIFs so should be considered in the model. Up to reaching temperature to its steady state, the cross coupled of temperature and moisture synchronies their time variation which affects on the time variation of SIF. At early time of thermal shock, the SIF for shorter cracks is not necessarily lesser than the longer ones. Also, the mode I SIF for longer and inclined cracks is smaller. On the other hand, considering the moisture concentration as a temperature function increases the time required to reach the moisture steady state.

In this paper¡ the stress intensity factor for an internal circumferential crack in a thick-walled cylinder has been determined. The cylinder has been subjected to an axisymmetric thermal shock on the outer surface according to the dual phase lag theory. The uncoupled¡ quasi-stationary thermoelastic governing equations have been assumed. The temperature and stress fields have been solved analytically in the Laplace domain and its Laplace inversion transform has been obtained numerically. Using weight function method¡ the stress intensity factor for mode-I has been extracted. Temperature¡ stress and stress intensity factor of hyperbolic and dual phase lag theories have been compared and the effects of heat flux and temperature gradient time relaxations on the temperature¡ stress and stress intensity factor have been studied. According to the results¡ the dual phase lag temperature distribution is different in comparison with the Fourier model. Also¡ the stress intensity factor for dual phase lag model is significant larger than Fourier one. Moreover¡ the maximum stress intensity factor in dual phase lag model occurs for a crack that the peak of stress wave reaches to its tip. Results show assumption of adequate heat conduction model for structure design under transient thermal loading is critical.