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In this paper, the effects of reflected waves of the facets on the internal optical intensity of semiconductor DFB lasers are investigated. The uniformity of optical intensity along the cavity length is evaluated with flatness parameter. The dependence of this parameter on coupling coefficient, reflectivity and grating phase at the facets is also studied. This investigation shows that in some structures reflected waves of the facets cause optical intensity along the cavity length to have more uniformed distribution than a DFB laser with anti-reflective facets. It is also shown that flatness parameter is very sensitive to grating phase .Thus it is necessary for designing a DFB laser to consider the effects of reflected wave and grating phase at both ends of cavity in order to increase the stability of the laser against SHB (Spatial Hole Burning) effect. The effects of reflectivity and grating phase on longitudinal distribution of photon and carrier density above threshold are investigated, too.

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Multiple flaws are frequently occurred in actual components, such as pressure vessels and power plants. These flaws will in some circumstances lead to more severe effects than single flaw alone. Assessment of the interaction behaviour is based on an evaluation of the alignment and combination of these multiple flaws. In the current standards, multiple cracks are treated as an equivalent single crack if the distance between two cracks satisfies a prescribed criterion. First, this study introduces the current alignment and combination rules for through cracks. Following, to investigate the effects of the interaction of cracks, brittle fracture of a plate containig two adjacent cracks is simulated. The effect of cracks distances and crack lengths on stress intensity factors is evaluated. Also, crack growth analysis is simulated based on linear elastic fracture mechanics approach. The extended finite element method has been utilized to model the problem. This method enables the domain to be modeled by finite elements without explicitly meshing the crack surfaces, and hence crack propagation simulations can be carried out without remeshing. Based on the results, a new alignment and combination rule is proposed.

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Bolt joints play an important role in the industries, so the estimation of fatigue life of bolts is an essential task. The aim of present study is estimation of fatigue life of connection bolts of two flanges in reinforced cylindrical shell with cutout. Two groups of data are needed for mentioned bolt: fatigue properties of bolt and value of stress of bolt due to applying load to structure. So, two paths have been gone. First, the fatigue properties of bolt have been measured in laboratory according to ISO 3800 standard. For this purpose a specific fixture was designed and manufactured which provided testing different bolts. By doing fatigue experiments, the fatigue properties of mentioned bolt such as fatigue limit and Basquin’s equation constants (fatigue strength coefficient and fatigue strength exponent) have been measured. Fracture mechanism and fracture surface have been investigated, too. Afterward, in the next step the value of stress in bolt that is subjected to mix loading has been calculated by using of FE modeling. Because of problem complexities, cost of three dimensional analysis of this problem increases, so analysis of the problem has been performed by shell-to-solid sub-modeling technique. At the end, by calculating the nominal stress of bolt from FE modeling and using fatigue properties witch obtained from experiments, life of the mentioned bolt has been estimated.

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Curved thin-walled structures are extensively used in many industrial applications including aviation industries. These parts have a specific role in manufacturing of helicopter main rotor blade. Fracture mechanics parameters are needed for life estimation and life extension of curved thin-walled structures. Standard methods of fracture mechanics testing are not applicable to these parts because of curvature and thinness of the specimens. So new test methods for these specimens is necessary to be investigated and developed. Vast trying has been done by scientists to overcome these problems and some theories and experimental methods such as the theory of fracture of thin-walled curved plates, the conventional burst test method and some new non-standard test methods has been initiated and developed. In the present paper, first of all, the theory of thin-walled curved plates has been briefly presented to link the behavior of curved specimens to the flat ones and the conventional burst test (BT) method has been accordingly introduced. Then, the newly developed non-standard test methods such as compact curved tension (CCT), pin loading tension (PLT), three points bending (TPB), double edge notched tension (DENT), internal conical mandrel (ICM) and X-specimen have been reviewed. Finally, a comparison between mentioned methods had been done to determine the most appropriate one.

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Rolling bearings is one of the important elements in rotating machinery that they are under rolling contact fatigue, so for better estimation of remaining life of these elements a model which is considered multiple moving loads based on the linear fracture mechanics has been proposed. Considering the effect of the multiple moving loads is a key element of this model that is not considered in literature. The contact surface is modeled by a half-plane with an edge crack. Rolling contact is simulated by moving a load distribution on the boundary of a half-plane. The calculation of the crack path and stress intensity factors is carried out by the step-by-step process on the basis of the singular integral equations method. The effect of the friction coefficient and considering two concentrated moving load on the crack growth path for different angles between the initial crack and boundary of the half-plane and remaining life of a surface under the fatigue rolling contact for 75XГCT steel is analyzed. The results of this analysis are compared with results from previous model. According to the results of the remaining life obtained by increasing the numbers of moving loads on the contact surface the remaining life decrease and get closer to the real state. Furthermore, the remaining life decreases with increasing friction coefficient.

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The instability behavior of stiffened cylindrical shells and determination of the corresponding buckling loads under axial compression, according to the extended range of structural applications of them in various fields of engineering, has been paid a lot of attention from researchers and extensive amount of studies have been performed on it so far. Because of a lack of the general closed form responses due to complexity of the governing equations and analyses process, using the FE software codes as the main technique of the stiffened shell's buckling load determination is inevitable. Accordingly the present paper has been studied the reinforcement effects of ring and stringer and also compared the buckling loads which are evaluated by analysis of the FE numerical modeling in ABAQUS software with instability results that obtained from a general analytical equation derived by other references via applying the simplifying assumptions to the governing equations. Furthermore an attempt has been performed for extraction of the finite element instability load vs. structure reinforcement correspondence that enables the designers to accurately determine the instability load of structure for other values of structure's stiffening volume without performing additional FE analyses which are much more expensive in term of computer time.

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The unidirectional composite DCB specimen is considered as two finite length Timoshenko beams, attached together along a common edge except at the initial delamination length. Because of symmetry, only one half of the specimen is considered, which is partly free and partly resting on an elastic foundation. The problem is analytically solved by considering Timoshenko beam resting on Winkler and Pasternak elastic foundations and fracture toughness is generally derived. In the prior researches on this specimen using Timoshenko beam theory, the effect of the ligament length on the energy release rate was ignored. This research presents the solution for finite ligament length. Besides, the effect of ligament length on energy release rate and its minimum value that makes the energy release rate independent of the ligament length, is presented. For the special case when the ligament is large compared with the beam thickness, a closed form solution is derived for Timoshenko beam resting on Winkler elastic foundation. The analytical results are compared to prior researches on this subject and a good agreement is observed. The fracture toughness and compliance obtained by Timoshenko beam resting on Winkler elastic foundation predicts more accurate results with respect to experimental results.

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Unlike metals for which the fracture characterization methods have been standardized in the context of linear elastic and elastic plastic fracture mechanics theories, for polymeric materials the linear and especially nonlinear theories of viscoelastic fracture mechanics has not been completely developed due to complexities regarding the viscoelastic nature of these materials. For rubbers, even the rate independent theories based on nonlinear finite elasticity have not been widely used. In practice, most researchers make use of the same methods as applied for metals to rubbers. In this paper, the common methods of fracture characterization of rubbers based on J integral and the different challenges regarding them are reviewed. Specificly, the energy dissipation effects in regions far from the crack tip and the correction methods proposed to compensate for these effects are discussed. Performing fracture toughness tests on SENT specimens of a rubbery material based on polybutadiene, it is shown that the well-known multiple specimen method for determination of Jc has a strong sensitivity to experimental errors that exhibits itself as initial crack length dependence of Jc values and is just usefull when testing numerous specimens and removing the experimental errors. On the other hand, the locus method of dissipation correction, gives a single reliable Jc value using a fewer number of specimens and with a considerably lower sensitivity to the experimental errors. Also, using this method the specimen length dependence of Jc values reported in the literature is removed, and hence, it is possible to obtain a dimension independent Jc value.

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Compressor and their blades are one of the most important parts of gas turbines. Based on recent reports, failure of compressor’s blades was one of the major cause in malfunctioned t56 gas turbines. In this study, propagation rate of a crack within the compressor blade of a T56 jet engine has been investigated. To this end, centrifugal and aerodynamic forces acted upon the blade has been calculated and their corresponding stress field has been simulated in ANSYS software. Spots at the maximum risk of foreign object damage and corrosion had been located, and their bending and tension stresses had been calculated via employed simulation. Subsequently, an initial half elliptical crack has been created on all of previously located spots, and their stress intensity factor using Raju-Newman method has been determined. Finally, by using Paris law fatigue life and crack growth rate of each cracks has been extracted, individually. Results indicate a drastic decrease in fatigue life of blades when crack located close to the blade’s root. Furthermore, cracks located on the suction surface has remarkably shorter fatigue life than those which are located on pressure surface, in comparison.

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Ceramic materials are desirable in structural applications because of their strength at high temperatures, low thermal expansion, and excellent wear resistance. However, ceramic structures are generally brittle and will fail due to inherent flaws that can be introduced into the material through processing, handling, or while in service. In the strength test samples of ceramic materials, the observed strength depends on the sample volume and the test method, and the results have significant scattering. On the other hand, it is important to recognize the variability of the strength of ceramic materials for structural design, and in the design process, this variability usually requires a probability-based failure criterion, which, due to the brittle fracture of ceramics, usually uses the Weibull distribution to represent their strength. With the help of Weibull distribution, the concept of effective volume is defined and by using it, the strength of a test sample with a specific geometry and loading is predicted from another sample that has a different geometry and loading. In this research, the effective volume for a truncated cone under internal pressure has been derived analytically. With the effective volume for this configuration and a sufficient number of flexural strength tests with the help of small and low-cost flexural specimens, it is possible to predict the tensile strength of the ceramic truncated cone without conducting expensive tests. for a problem with a specific material property and dimensions, the effective volume was calculated numerically and it had a 3% error compared to the analytical value.