farzad shahabian

The stochastic meshless local Petrov–Galerkin method is employed for dynamic analysis of multilayer cylinders made of fully saturated porous materials considering uncertainties in the constitutive mechanical properties. The multilayer porous cylinder is assumed to be under shock loading. To approximate the trial functions in the radial point interpolation method  (RPIM), the radial basis functions (RBFs) are utilized. The Monte Carlo simulation is used to generate the random fields for mechanical properties. The results are obtained for various random variables, which are simulated by uniform, normal and lognormal probability density functions with various coefficients of variation (COV), changing from 0 to 20%. The obtained results from the presented stochastic analysis are compared to those obtained from the analysis considering deterministic mechanical properties. The results show that the uncertainty in mechanical properties has a significant effect on the structural responses, especially for big values of COVs.

Experimental observations reveal that the classical continuum theory cannot accurately describe the mechanical behavior of micro/nanoscale structures. In fact, the size-effect will arise when the order of structure dimensions is the same as the material characteristic length. The current work presents free vibration and stability of axially functionally graded (AFG) tapered micro-beams with random properties. The size-dependent behavior of the micro-structure is modeled by the modified couple stress theory. The mathematical formulations are developed based on the Euler-Bernoulli beam model and von Kármán geometric nonlinearity. The minimum total potential energy principle is employed to obtain governing differential equations and the corresponding boundary conditions. The governing equations are solved by the Galerkin method. Due to the complexity of the fabrication process of FGMs, their mechanical and structural properties may vary from sample to sample significantly. Hence, achieving the desired FGMs specification is almost impossible and they are not deterministic, inherently. To incorporate uncertainties in the mathematical model of this study, a First-Order Second-Moment (FOSM) technique is applied to estimate the reliability index of the micro-structure, stochastically. Finally, numerical examples are presented for both deterministic and reliability analysis to show the effects of geometry, length scale parameter, material distribution, and axial load on the natural frequency of vibration and the reliability index of the AFG tapered micro-beam. It can be concluded that by increasing the coefficient of variation (COV) of random variables, the reliability index will decrease. Indeed, by enhancing the length scale parameter, a higher natural frequency of vibration is expected.

The idea of using a meshless method for geometrically nonlinear problems due to its advantage of eliminating mesh distortion has been attracted many researchers. In this paper, the nonlinear wave propagation analysis in two-dimensional functionally graded (2D FG) thick hollow cylinders is studied using the meshless method. For this purpose, the meshless local Petrov-Galerkin (MLPG) method is developed for geometrically nonlinear dynamic analysis based on the total Lagrangian approach. The radial point interpolation, which possesses the delta function property, is used to construct the shape functions. Since the cylinder is assumed to have large deformations, the neo-Hookean hyperelastic model is employed for constitutive modeling of material. The incremental-iterative Newmark/Newton-Raphson technique is applied to iteratively solve the nonlinear equations of motion. The 2D FG cylinder is analyzed under uniform and non-uniform mechanical shock loading. The mechanical properties of the cylinder are assumed to vary nonlinearly through the radial and longitudinal directions which are simulated using two-dimensional volume fractions. Rayleigh damping is utilized to model energy dissipation in analyses. Numerical examples demonstrate the applicability and accuracy of the present approach in tracking the nonlinear wave propagation in two-dimensional FG thick hollow cylinders. The effects of grading patterns on the time history and wave propagation are discussed in detail.

Reactive powder concrete (RPC) is one of the ultra high performance concrete ( UHPC ) with superior physical and mechanical properties . Reactive powder concrete is produced using cement and very fine powdered materials including crushed quartz , silica fume and low water to cement ratio using super plasticizer . In this study , an optimum mixing ratio of RPC has been used , in which water / binder ratio ( W / B ) was equal to 0.15 and the ratio of super plasticizer to cement ( SP / C ), was equal to 3 % . For a long time , the idea of adding fibers to various types of concrete has been considered to improve its physical and mechanical properties . In this research , by performing several experiments, the influence of adding steel fibers on the compressive and tensile strength of RPC has been investigated . The results show that the average compressive strength of samples with 1 , 2 and 3 percent volumetric fibers increases about 6 , 20 and 5 percent , respectively , compared to non - fibrous specimens. This increase in tensile strength is 46 , 73 and 66 percent , respectively for the same amounts of steel fibers .

Functionally graded materials (FGM) are some kind of composite materials that due to the continuity of mixture of constituent materials, have more effective mechanical properties than composites which leads to eliminating interlayer stress concentration. The most application of these materials is in thin structures such as plates and shells. This research presents a Tamura-Tomota-Ozawa based model to obtain the elastoplastic behavior of Functionality graded materials under impact loads. Also, based on this model, the ceramic phase of FGM was considered as an isotropic elastic material and the metal phase was considered as an elastoplastic material.Several parametric studies have been conducted to assess different aspects of such material behavior. The results show thatthe maximum displacement of the shell has increased by increasing the volume fraction index and the thickness ratio, and it has decreased  by increasing the aspect ratio. It was also observed that the thickness ratio(32%), volume fraction index(30%), aspect ratios(23%) and shell curvature (16%) parameters affect the maximum displacement of the shell. The elasto-plastic response of FGM shells is similar to homogeneous shells and the TTO model can describe the mechanical behavior of FGM shells beyond the elastic range where the FGM response is mainly governed by the plastic region of the metal phase.

Functionally Graded Materials (FGMs) are kinds of composite materials that due to the continuity of mixture of constituent materials as Functionally Graded, have more effective mechanical properties. Also, due to some executive needs, make opening and stiffener in shells will be necessary. Therefore, In this paper, the effective parameters on the free vibrations of Functionally Graded plates with opening and stiffener have been studied. In order to do this, ABAQUS finite element software has been used, in the first, the effect of important parameters such as volume fraction index, geometric dimensions and plate support conditions have been analyzed. The results show that the thickness and volume fraction index have the highest and lowest effects, Respectively, on the natural frequencies of each of the FGM plate modes and with the increase of simple support conditions in the edges of the plate (the decrease of fixed conditions), the natural frequency of the plate decreases. In the following, the effect of opening and stiffener on the natural frequencies of the plate has been observed, that Circular opening has a greater effect on the natural frequency of the plate than the square opening, and also the stiffener thickness has a greater effect on The natural frequency of the FGM plate than its height.

At present, certain types of concrete such as ultra high performance concrete (UHPC) provide some concrete properties such as high resistance. Reactive powder concrete (RPC) is one of the UHPC concrete with superior physical and mechanical properties. Reactive powder concrete is produced using cement and very fine powdered materials including crushed quartz, silica fume and low water-to-cement ratio using super plasticizer. In this study, a mixing scheme of RPC, in which water/binder ratio (W/B) is equal to 0.15 and the ratio of super plasticizer to cement (SP/C), is equal to 3%, with interest of the selection of native materials was determined. Subsequently, by performing several experiments, the compressive strength, rupture modulus and tensile strength of RPC have been investigated at different ages. In order to determine the resistance of RPC at different ages, samples with W/B and SP/C were made up to the mixing scheme and tested after 3, 7, 14, 28 and 56 days of normal operation. Each group consisted of six samples, and the mean values, standard deviation and coefficients of variations of the samples were determined. The relationships have been proposed to predict the compressive and tensile strength of reactive powder concrete in different ages, which are accurate and can be used in practice.

Functionally Graded Materials (FGMs) are kinds of composite materials that due to the continuity of mixture of constituent materials, have more effective mechanical properties which leads to eliminate the interlayer stress concentration. The most common usage of such materials is in thin-wall structures, such as plates and shells. One of the most effective factors in behavior of such structures especially in single-curved shells, thermal loads or Impact loads is caused by explosion. Also, due to some executive needs, make opening in shells and their behavioral changes are important and suggesting solution will be necessary. Therefore, in order to prevent large displacement and resistance improvement, using shells made of FGM and suitable stiffeners, will be suggested. In this study, ABAQUS finite element software has been used to survey the Effect of opening and stiffener on geometric nonlinear dynamical behavior of single-curved FGM shells under the blast loads. In order to do this, the effect of volume fraction index, the effect of different openings and stiffeners has been studied. Results show that by increasing the volume fraction index, the maximum amount of displacement of the shell decreased. Making opening in the center of the shells, has better function in contrast with making opening distribution in the level of shells. By increasing moment of inertia of longitudinal and circular stiffeners, the maximum displacement has been decreased. Also, by utilizing opening distribution and longitudinal stiffeners, the maximum amount of displacement can be reduced.

Functionally Graded Materials (FGMs) are kinds of composite materials which exhibit continuous variation of material properties from one surface to another. With regard to the continuity of mixture of constituent materials, FGMs have more effective mechanical properties compared to laminated composite materials, which leads to eliminate the interlayer stress concentration. The most common usage of such materials is in thin-wall structures, such as plates and shells. Plates due to their extensive surface and low thickness, not only have low resistance against the blast loads, but also experience large displacements. Hence, utilizing FGM plates because of their constituents materials which have a power distribution in the direction of thickness, will lead to resistance improvement and displacement reduction. In this research, ABAQUS finite element software has been used to study geometric nonlinear dynamic response of Stainless steel-Silicon nitride FGM plates against the blast loads. The effect of volume fraction index, plate aspect ratio, plate thickness, the amount of explosive material and its distance and also the effect of Replica scale with Hopkinson scale are investigated in this study. The results show that with rising in the volume fraction index (from zero to infinity), increasing in the distance of explosion center to the plate (from 500mm to 900mm) and plate thickness (from 6mm to 18mm), the maximum amount of displacement of the plate have decreased by %29/1, %43/64 and %87/74 respectively. On the other hand, by expansion of the plate dimensions ratio (from 1 to 2) and increasement in the amount of explosive material (from 5 to 25gr), the maximum displacement of the plate have increased by 1/06 and 3/09 respectively. Also, it has been observed that changes in materials properties along with plate thickness don’t have any effect on scaling.

Structural health monitoring is an economical and reliable strategy for infrastructure condition assessment. In recent years, researchers have tried to propose algorithms based on statistical pattern recognition techniques. Studies show these algorithms can be successfully used to detect structural damage. Variability of operational and ambient conditions during data acquisition should be considered as an important factor in applying statistical pattern recognition methods in practical applications. This paper studies the efficiency of statistical pattern recognition methods on the damage detection of structures under various operational and ambient conditions. The data is obtained from an experimental study on an eight degrees of freedom mass spring system. Ambient vibration is applied to the mass spring system using random excitation. In order to simulate various ambient conditions, the amplitude level of the input force has been varied. By applying the statistical pattern recognition methods, the ability of these methods to damage detection under various ambient conditions is discussed. Two common approaches of statistical pattern recognition are considered. These approaches are autoregressive model accompanied with using control chart and Mahalanobis distance for outlier analysis. Results show the importance of considering the statistical pattern recognition methods for structural damage detection under various operational and ambient conditions.

Structural damages and subsequently considerable economic losses that ever made in catastrophic seismic events caused reaserchers and codes gradually move from prescriptive design to performance-based design. Seismic fragility analyses are one of the most important tools in performance-based seismic design of structures that lead to production of fragility curves and functions. Fragility function is a capable tool for probabilistic assessment of the seismic vulnerability of the structures. In this study, in order to assess seismic vulnerability of corrugated steel shear walls and compare their seismic vulnerability with simple steel shear walls, fragility functions have been developed and fragility curves have been constructed. In this regard, the effect of the corrugation angle has been studied in corrugated models. Construction of these curves has became feasible through conducting incremental dynamic analyses. For this purpose, inter-story drift and peak ground acceleration have been used as damage measure and ground motion intensity measure, respectively. Assessment of resulted curves show that in low seismic intensity, probability of damage in corrugated models is reduced by increasing the corrugation angle; while, in high seismic intensity, probability of damage in corrugated models has been increased by increasing the corrugation angle. On the other hand, results indicate that corrugated models have more appropriate seismic performance than simple model in low seismic intensity.

The behavior of different structures subjected to blast loads is always an important factor to study. Sandwich panels have a wide range of applications in different fields of engineering and the construction of some industrial and military structures. These panels are made of two sheets and an intermediate core. The core has a significant role in reducing the deflection and enhancing energy absorb of structure. In this article, the effect of the shape of the corrugated panel and the type of panel materials is investigated. Hence, the reaction of aluminum and steel sandwich panels subjected to blast loads has been investigated. In the process of analysis, four kinds of profiles, which are: rectangular, trapezoidal, triangular and elliptical, are considered as panel cores. the most deflection was for rectangular profiles and the least deflection was for triangular ones. Those panels which are completely made of aluminum or the panels with steel cores can sustain more strain energy by their back sheets. all the panels that are completely made of aluminum have more damped energy than others. Those panels which have only aluminum sheets or aluminum cores are placed next to whole aluminum panels in order to damped energy level. In the study of the joint effect of the type of materials and the shape of the core panel, the panel with the upper and back of the steel and aluminum core with the triangular shape has the most desirable.

Today, with the increasing damages of explosion, it is of great significance to assess the performance of structures against such damages that established by explosive load. Accidental explosions exert great and intense dynamic forces to surrounding structures. Recently, composite shells have been used in structures to protect them against explosion which is due to its high resistance to volume ratio, flexibility and resistance to shock forces. Thus, it is essential to assess how structures protected by such materials behave against these forces. In this paper, we have used Abaqus software to analyze data pertaining the behavior of composite shells against explosive loads. We assessed the various parameters affecting the behavior of CRFP and E-Glass Epoxy and how they were affected by explosive load. Some of the parameters assessed include loading, curving rate, number of layers and size of interior angle. In practice, it is necessary to include openings in the composite shell, thus it is important to evaluate the effect of these openings on the behavior of the composite shell. The survey showed that use of the opening has fallen down shift. The reason for this phenomenon is reduction of area that effected by explosive load in composite shell

The support vector machine (SVM) is a relatively new machine learning method which is increasingly being applied to engineering problems and have yielded encouraging results. Because of complex behavior of elastoplastic of web panels of plate girders under patch loading, almost none of the proposed methods provides consistent and accurate predictions of patch load capacity. Consequently, alternative solutions are required to overcome these limitations. In this paper SVM models are developed for predicting the ultimate resistance of plate girders subjected to patch loading. The training and testing patterns of the proposed SVM models are based on well established experimental results taken from literature. Finally a comparison is made between predictions obtained from the SVM models and a traditional method for determining patch loading resistance. The comparison confirms that the SVM models developed in this paper, outperform the traditional method.

This paper deals with the dynamic analysis of stress field in cylindrically layered structures reinforced by carbon nanotube (CLSRCN) subjected to mechanical shock loading. Application of meshless local integral equations based on meshless local Petrov-Galerkin (MLPG) method is developed for dynamic stress analysis in this article. Analysis is carried out in frequency domain by applying the Laplace transformation on governing equations and then the stresses are transferred to time domain, using Talbot inversion Laplace techniques. The mechanical properties of the nanocomposite are mathematically simulated using four types of carbon nanotube distributions in radial volume fraction forms. The propagation of stresses is indicated through radial direction for various grading patterns at different time instants. The effects of various grading patterns on stresses are specifically investigated. Numerical examples, presented in the accompanying section 4 of this paper, show that variation of * CN V has no significant effect on the amplitude of radial stresses. Examples illustrate that stress distributions in cylindrical layer structures made of a CNT type  are more sensitive rather than other grading pattern types of CNTs. Results derived in this analysis are compared with FEM and previous published work and a good agreement is observed between them.

Localized edge loading or patch loading of bridge girders, is frequently encountered in practice. The behavior of girders under patch loading represents complex stability and elastoplastic problems. In situation where the location of the patch load is fixed, transverse web stiffeners can be used to provide increased resistance, but for economic reasons should be avoided wherever possible. For moving load in some structures such as bridges, it is not possible to provide transverse stiffeners at all critical locations. One option is to use longitudinal stiffeners in these situations. In this article, ANSYS finite element software was used for structural analysis to investigate the ultimate resistance of plate girders subjected to patch loading. The finite element model was validated with experimental results taken from literature and found to be more accurate. Extensive parametric study was also performed to investigate the ultimate resistance of longitudinally stiffened plate girders. Furthermore, the ultimate resistance and the optimum position of the longitudinal stiffeners were presented in closed-form which showed satisfactory correlation with the theoretical results.