دکتر فرزاد شهابیان مقدم

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.

In recent decades, shells made of composite materials have been used in modern structures under impact load. Multi-Layer Composites (MLC) and Functionally Graded Materials (FGMs) are the upgrades of composites that have been considered due to their suitable mechanical properties such as high resistance to weight ratio, flexibility and impact resistance. In this research, geometric nonlinear dynamic behavior on multilayer composite cylindrical shells with FGM core under impact load has been analyzed; Because it is necessary to know how structures made of these materials behave under such loads. For this purpose, the effect of FGM core volume fraction index and the effective parameters of multi-layer composites such as the angles of the layers and the number of layers have been investigated. The results of this study show that by increasing the volume fraction index, the maximum displacement of the shell decreases. The maximum displacement occurs in the CFC (CFRP/FGM/CFRP) shell with pure metal FGM and the minimum displacement occurs in the GFG shell with pure ceramic FGM. Evaluation of the different positions of the layers shows that selecting a 15-degree positioning angle causes less displacement and also decreases the displacement as the number of layers increases.

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 .

Damage detection is a necessary part for structural health monitoring (SHM), being beneficial to SHM and determining the severity of damage. Application of statistical pattern recognition methods for SHM has gained considerable attention to detect changes in a structure. One of the advantages of these methods is that only data from undamaged state is needed in training phase (unsupervised learning) as opposed to supervised learning where data from both undamaged and damaged conditions is required to train the model. There are different approaches used by researchers and the success of a certain one may depend on the type of structure or structural changes. Most of studies focused on the application of statistical pattern recognition methodologies for SHM utilize the time series analyses for extracting damage-sensitive features. These features are statistical properties of time series models that directly depend on damage. Extracting damage-sensitive features is a fundamental step in damage detection process because pattern recognition algorithms can identify the state of structure unless these features are just dependent on damage.  The change in an environmental and operational condition during the data acquisition process is one of the problems that causes damage features to be depended on factors besides existence of damage. This can lead to incorrect structural state identification. On the other hand, after extracting damage-sensitive features, the application of a statistical novelty detection methodology for decision making on structural state is a significant topic in SHM. This paper proposes a new application of random decrement (RD) technique in order to choose appropriate and accurate damage features which are independent from environmental and operational conditions of structure. The RD technique transforms time series data of the structural response to free decay vibration form that only consist of dynamic properties information by averaging them at specific time. Moreover, a novel statistical method named as Bhattacharyya measure is applied as a robust method for damage diagnosis. The Bhattacharyya measure determines the discrepancy between damage features from different structural states through partitioning data and utilizing numerical information of each partition. Herein, before extracting damage features, time series data are averaged through RD technique. Then the Autoregressive-Autoregressive with exogenous output model (AR-ARX) is used to fit a mathematical model to the averaged time series data and the residuals are considered as damage features. The Bhattacharyya measure is utilized for damage identification and localization. The data obtained from an experimental study on a three-story frame structure model are exploited to validate the accuracy and reliability of the proposed algorithm. Random excitation is applied by varying amplitude level of the input force, simulating various environmental and operational conditions. Damage is induced at two different locations. The proposed algorithm is conducted on data from various environmental and operational conditions at two different locations. A comparative study is also carried out to demonstrate the superiority of the proposed algorithm over some exiting techniques. Results show that the application of random decrement technique reduces the influence of operational and environmental condition due to averaging and normalizing data and correctly determines the state of structure. In addition, using Bhattacharyya measure improves the structural health monitoring results in damage identification and localization.

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.

Porous materials are widely applied in different engineering fields such as chemical engineering, materials engineering, environmental geomechanics, biomechanics, soil structure interaction and geomechanics. For this reason, dynamic and static analysis of the stress and strain of structures and continuum made of these materials have attracted the attention of researchers. To analyze these structures, numerical methods are required due to the fact that the governing equations are complicated. The meshless method was well implied for many structural problems in recent decades. In this method, the scattered nodes with regular or irregular distributions are used to discretize the field functions in local sub-domains. Because of using the nodes rather than the elements, the application of meshless method in some problems yields to more accurate results. To interpolate the fields' variables in terms of its nodal values, the radial point interpolation method (RPIM) with radial basis function (RBF) is used. The radial point interpolation method (RPIM) is employed to construct the shape functions. Because of the fact the interpolation function type affects the results of meshless approach, various interpolation functions are investigated. Then, the most proper one is selected for analyzing the porous material structure. To verify the presented method, the result of cylinders made of saturated porous materials using MLPG method under dynamic loading is compared to the analytical results. A good agreement between the presented method and analytical result is achieved. It is worthwhile to mention that damping should be included in dynamic analysis. Hence, the modeling of damping effect on the behavior of porous material is conducted. According to the obtained results, for the usual damping ratios of the porous materials, increasing the damping ratio leads to reduction in the displacement amplitude and stress. Also, it is observed that the period of the porous material are independent from its damping ratio.

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.

The increasing risks resulted by blast loads in current broad and thin plates necessitate the studies and evaluation of these challenges. Practical and cost-effective solutions such as plates enforcement with stiffener are suggested due to high displacement of structure as a result of applied loads. Moreover, application issues and the possibility of developing opening in plates and thus their change of behavior are considered important issues, for which solutions are suggested in this study. Functionally graded plates (FGM) with a power distribution in the direction of thickness are suggested as a solution to this challenge. Accordingly, the present study assesses parameters related to plates with opening including kind, percentage and the position of opening, the mass amount of explosive material, and the parameters related to plates with stiffener including type, count, arrangement of stiffeners, boundary condition and also the joint effect of stiffener and opening using Abaqus Software. The results indicate that opening decreases the plate hardness which consequently increases the amount of displacement. Plate displacement constantly reduces as a result of increase in opening percentage and hence decrease in the surface on which load is applied. On the other hand, the rectangle shape of stiffener has a better performance due to higher moment of inertia and easier application compared to other shapes. By studying the joint effects of these factors, it can be suggested that developing opening and application of an appropriate stiffener can significantly reduce the amount of plate displacement compared to other types.

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.

Practically there is a risk of explosion, and different structure's behavior should be examined against the effects of blast loadings. Sandwich panels are used for making 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 study, sandwich panel's behaviors, which are made of aluminum and steel, are examined against the blast loadings. In the process of analysis, three kinds of profiles, which are: rectangular, trapezoidal and triangular, are considered as panel cores. The results show that the panels with steel core, absorb less energy and damp by their core. Those panels which are completely made of aluminum or the panels with steel core 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 core are placed next to whole aluminum panels in order to damped energy level. Most of the deflections in panels that are whole aluminum made and steel core panels were belong to panel core with triangular profiles and the least deflection were for rectangular shaped profiles. In panels that only have steel sheets, unlike the other panels, the most deflection was for rectangular profiles and the least deflection was for triangular ones.

There are various factors in the analysis and design of the steel frames that uncertainties in any one of them can have a significant effect on the structural safety. The rotational stiffness of the bases and beam-to-column connections and also, the transitional stiffness of the bracing system can be considered as examples of these variables. In this paper, firstly, based on the exact solution of the governing differential equation for an Euler-Bernoulli beam, the stability matrix of portal and gabled steel frames with flexible connections and supports and non-prismatic members are derived. It should be added, the flexibility of connections and supports are molded by the linear rotational or transitional springs. The accuracy of the proposed approach in the deterministic stability analysis with numerical examples verified. Then, by using of the Monte-Carlo simulation method, the probability of failure of mentioned frames for the stability is estimated. The rotational stiffness of the beam-to-column connections and bases and also, the transitional stiffness of lateral support are considered as the random variables in the stochastic stability analysis. Accordingly, by using of the presented formulation the buckling loads of the simulated frames for the hundred thousand times obtained and the effects of various coefficients of variations and different probabilistic distribution functions for random variables in the buckling resistance of the foregoing frames estimated. Moreover, the sensitivity of the probability of failure for the stability of frames with respect to uncertainty of the stiffness of the connections and supports investigated.