Abstract in English:Abstract In this paper, dynamic analysis of two different weapon systems (35 mm Anti-Aircraft Barrel (AAB) and 120 mm Grooved Tank Barrel (GTB)) under the effect of statically unbalanced projectile has been performed with a new 12 DOF 3-D element technique using Finite Element Method (FEM). The muzzle deviations, which negatively affect the barrel shooting accuracy at firing, are calculated in a time dependent manner using Newmark β algorithm with high accuracy at both axes (y and z) considering the Coriolis centripedal and centrifugal forces. The effect of such fundamental physical parameters as shift from rotating center and angular velocity belonging to the unbalanced projectile on barrel dynamics are analyzed with this new and affective FEM. As a result, it was found out that 1% of a millimeter shift from projectile belonging to a weapon system leads to excessive vibration on both axes and compromises the shooting accuracy of the barrel.
Abstract in English:Abstract The externally bonded (EB) and the near-surface mounted (NSM) are two well-known methods for strengthening reinforced concrete (RC) beams. Both methods are unfortunately prone to fail prematurely through debonding when the amount of strengthening reinforcement provided is high. In response to this, a hybrid method that combines the EB and NSM method was introduced. The method allows the amount of reinforcement needed for EB and NSM methods to be reduced; this, in theory, should lower the interfacial stresses, thus reducing the possibility of debonding failures. While debonding failure can be prevented, certain amounts of debonding would still occur through the interfacial crack (IC) debonding mechanism which can affect the strength and stiffness of hybrid strengthened beams even if it does not directly cause failure. This paper presents a method to simulate IC debonding of hybrid strengthened beams using the moment-rotation approach. The proposed method allows a better prediction of maximum load and stiffness of the beams. The method is also less dependent on empirical formulations compared to the commonly used moment-curvature approach; this allows the method to be applicable to all material and shape of hybrid strengthening reinforcement, assuming correct material models are used. The proposed method was then used to perform parametric studies; among the important findings is the length of IC debonding tend to increase when FRP sheet with higher elastic modulus is used, thus negating most of the benefit from the higher modulus.
Abstract in English:Abstract Damage models are often used in finite element simulations of concrete structures. Evaluating the numerical quality of these computations consists in an important task which can be performed by using an error estimator. However, in the framework of nonlocal approaches these estimators seem to be much less abundant in the literature and their implementation requires considerable efforts. In this paper, a new error indicator with relatively less difficulties for its implementation is proposed. Based on local and least squares smoothing techniques, the convergence analyses of the developed indicator is carried out and discussed. The obtained results seem to be in good agreement with those obtained by the use of residual method. In addition the major difficulty to obtain an automatic step-by-step mesh adaptation is highlighted. Nevertheless for a chosen loading step, optimized calculations can be performed and example tests are presented.
Abstract in English:Abstract The use of viscoelastic materials (VEMs) has becoming more and more frequent both as vibration control in general or as parts of structural components. In all applications, the mechanical behavior of such materials can be predicted by the complex moduli (Young’s, shear or volumetric) and the complex Poisson’s ratio. Over recent decades, various methodologies have been presented aiming at characterizing complex moduli. On the other hand, the indirect identification of the Poisson’s ratio, in the frequency domain, proves to be underexplored. The present paper discusses two computational methodologies in order to obtain, indirectly, the complex Poisson’s ratio in linear and thermorheologically simple solid VEMs. The first of them uses a traditional methodology, which individually identifies the complex Young’s and the shear moduli and, from them, one obtains the complex Poisson’s ratio. The second methodology - proposed in the present paper and called ‘integrated’ - obtains the complex Poisson’s ratio through a simultaneous identification of those two complex moduli. Both methodologies start from a set of experimental points of the complex moduli in the frequency domain, carried out at different temperatures. From those points, a hybrid optimization technique is applied (Genetic Algorithms and Non-Linear Programming) in order to obtain the parameters of the constitutive models for the VEM under analysis. For the experiments described here, the integrated methodology proves to be very promising and with a great application potential.
Abstract in English:Abstract According to the conditions of today's world, design of resistant structures against blast loading is an important subject that requires special attention. Thus, given the benefits of optimization in engineering, development and assessment of optimization methods for optimum design of structures against blast is of great importance. In this research, the optimum design of steel frame structures against blast loading is investigated. For this purpose first an optimization methodology is proposed. In the proposed method the structural analysis is performed using nonlinear explicit finite element analysis. Based on the proposed method a framework is developed and three numerical examples are investigated using different numerical optimization techniques. Results of this study show that by using nonlinear explicit FE analysis as the structural analysis method and NLPQLP optimization technique as the optimization method, the current optimization problem can be performed effectively, because the procedure is relatively accurate and computationally inexpensive.
Abstract in English:Abstract Dynamic compressive tests of 3D braided composites with different braiding angle were carried out in the longitudinal, transverse and thickness directions respectively using the Split Hopkinson pressure bar (SHPB). The results show that the compressive properties present obvious strain rate strengthening effects in all directions. The 20° and 45° braided composite are most sensitive to strain rates in the longitudinal direction. The composites present the features of brittle failure at high strain rates, especially in the longitudinal direction. The composites with larger braiding angle have weaker mechanical properties in the longitudinal and transverse directions but stronger mechanical properties in the through-thickness direction. The braid angle has the greatest impact on the longitudinal mechanical properties. The compressive stress-strain curves in the thickness direction were similar to the hysteresis curve for both the 30° and 45° braided composites. The compressive failure modes vary with the loading directions and strain rate.
Abstract in English:Abstract This work presents a hybrid experimental technique for stress analysis in pipes due to soil-pipe interaction. It was used a variation of Digital Speckle Pattern Interferometry (DSPI) combined with a small indentation setup for acquiring radial displacement points. Data collected are fitted under the Airy stress functions method for the determination of displacements and strain components. The radial displacement data is fitted in to extract Airy stress function coefficients in a short combination selected specifically for the analysis of stress fields in pipes. This technique is of great utility as an auxiliary approach to the actual field data compilation.
Abstract in English:Abstract In the present paper, a new fifth-order shear and normal deformation theory (FOSNDT) is developed for the bi-directional bending analysis of laminated composite and sandwich plates subjected to transverse loads. This theory considered the effects of both transverse shear and normal deformations. In-plane displacements use a polynomial shape function expanded up to fifth-order in terms of the thickness coordinate to properly account the effect of transverse shear deformation. Transverse displacement is the function of x, y and z- coordinates to account the effect of transverse normal deformations i.e. thickness stretching. Hence, the present theory involves nine unknowns in the displacement field. The present theory does not require a problem dependent shear correction factor as it satisfies traction free boundary conditions at top and bottom surfaces of the plate. The governing differential equations and associated boundary conditions are obtained using the principle of virtual work. The plate is analysed for simply supported boundary conditions using Navier’s solution technique. To prove the efficiency of the present theory, the non-dimensional displacements and stresses obtained for laminated composite and sandwich plates are compared with existing exact elasticity solutions and other theories. It is observed from the comparision that the displacements and stresses obtained by the present theory are in excellent agreement with the results obtained by exact elasticity solutions compared to other higher-order plate theories available in the literature.
Abstract in English:Abstract This paper analyses the non-stationary free vibration and nonlinear dynamic behavior of the viscoelastic nano-plates. For this purpose, a size-dependent theory is developed in the framework of the consistent couple stress theory for viscoelastic materials. The previously presented modified couple stress theory was based on some consideration making it partially doubtful to apply. This paper uses the recent findings for the mentioned problem and develops it to analyze the nonlinear dynamic behavior of nano-plates with nonlinear viscoelasticity. The material is supposed to follow the Leaderman integral nonlinear constitutive relation. In order to capture the geometrical nonlinearity, the von-Karman strain displacement relation is used. The viscous parts of the size-independent and size-dependent stress tensors are calculated in the framework of the Leaderman integral and the resultant virtual work terms are obtained. The governing equations of motion are derived using the Hamilton principle in the form of the nonlinear second order integro-partial differential equation with coupled terms. These coupled size-dependent viscoelastic equations are solved using the forth-order Runge-kutta and Harmonic balance method after simplifying by the expansion theory. The short-time Fourier transform is performed to examine the system free vibration. In addition, frequency- and force-responses of the nanosystem subjected to distribute harmonic load are presented. The obtained results show that the viscoelastic model-based vibration is non-stationary unlike the elastic model. Moreover, the damping mechanism of the viscoelasticity is amplitude dependent and the contribution of the viscoelastic damping terms at higher forcing conditions becomes noticeable.
Abstract in English:Abstract The present study investigated the forming limit diagrams (FLDs) of aluminum alloy 6063 sheets using numerical and experimental methods at increased temperatures. In the numerical section, for the first time, the Ayada ductile fracture criterion and the second derivative of the large strain criterion were used. ABAQUS finite element (FE) analysis software was employed for the simulations. In order to determine necking time, after simulation, relevant data such as stress history, principal stresses, equivalent strain history, and large strain were extracted and the conditions for the necking criteria were investigated. To obtain the FLD in the experimental part, a Nakazima format was used. Experiments were conducted at temperatures of 25, 150, 200 and 250 degrees Celsius for the samples with equal lengths and different widths. Ayada criterion had better compatibility with the left side of the FLD (for small negative strains), while the second derivative of the large strain criterion had better compatibility with the right side of the diagrams (for small positive strains). The results also showed that with the increase in temperature, the FLD moved upward and sheet forming was improved. This improvement was almost similar for the temperatures of 150 and 200ºC, while the processing temperature of 250ºC led to significant improvement in forming, as compared to other temperatures.