فهرست مطالب mojtaba sadighi

In this paper, the effect of including SiC nanoparticles in the weld zone on the maximum shear strength of friction stir lap welded 7075 aluminum alloy is investigated, both experimentally and numerically. This objective is carried out by studying the effects of rotational and transverse speeds, tilt angle, the shape of the tool and the penetration depth. The numerical investigation is based on developing an FE model by means of Deform and ABAQUS to simulate the welding procedure, which results are verified by the experimental findings. Experimental procedure is designed based on Taguchi method. The verification of the developed FE model shows that the simulation results are in proper agreement with the experimental findings. In general, including SiC nanoparticles in the weld zone can increase the maximum shear strength up to 24%, compared to the case of where the specimens are welded without SiC. Furthermore, applying the threaded tapered tool leads to higher shear strength in comparison with the squared shape tool, i.e., the strength of the specimens welded by threaded tapered tool are 4 to 5% higher without SiC inclusion and 4 to 7.5% higher with SiC, compared to the same case welded by squared tool. In addition, while the rotational speed has the highest influence on the findings, the tilt angle does not affect the results that much.

Electromechanical impedance spectroscopy can be used for damage localization by estimating the electromechanical impedance spectrum with numerical or analytical models. The existence of several sources of uncertainty, however, leads to a significant mismatch between the numerical and experimental results. Therefore, uncertainty quantification for high-frequency coupled electromechanical vibration response of the piezoelectric patch is necessary. Polynomial chaos expansion is an efficient method for assessing uncertainty when dealing with time-consuming models. For the probabilistic analysis of modal features of the impedance spectrum, surrogate models derived by polynomial chaos expansion were used. The statistical moments and probability distributions of the quantity of interest were computed analytically using surrogate models. By post-processing the coefficients of polynomial chaos expansion models with relatively minimal computing cost, global sensitivity analysis was performed to rank the relevance of input variable variation on response variance. According to the results, due to the common uncertainties in the material properties and geometry of the piezoelectric patch, the coefficient of variation in the peak amplitudes is substantially higher than the peak frequencies. In addition, modal frequencies are most sensitive to mechanical properties (compliance and density), whereas modal amplitudes are most sensitive to mechanical damping, electrical permittivity, and the piezoelectric constant.

Corrosive environment can significantly effect on the mechanical behavior of composite structures which can improve by using hydrophobic nanoparticles. The aim of this study is to investigate the improvement of the mechanical properties of incorporated glass fiber reinforced polymer (GFRP) composite specimens with 3 wt. % of nanoclay and nanosilica in a corrosive environment. Filled GFRP composites were fabricated using the mechanical stirrer following by sonication method, and the hand layup method. After preparing the neat and incorporated GFRP according to ASTM standards, all the samples were immersed in 5% sulfuric acid solution for 0, 1, and 3 months. As the immersion time increased, the samples containing nanosilica absorbed more water than the other samples. The tensile and compressive tests were performed at different immersion times to obtain the ultimate tensile and compressive strength and tensile modulus. The results showed that by adding nanoparticles, the mechanical properties were increased, which GFRP containing nanoclay showed a better behavior in the corrosive environment. By adding 3 wt. % of nanoclay, the ultimate tensile and compressive strength and the tensile modulus decreased after one month of immersion by only 0.34%, 1.81% and 2.95%, respectively, and after three months of immersion only decreased by 0.43%, 10.88% and 6.95%, respectively. Finally, SEM images of all specimens were examined to investigate the fracture mechanisms and the corrosion behavior of fabricated nanocomposites.

The aim of this study is microstructure modeling and deformation and damage analysis of aluminum-based metal matrix reinforced by Tic particles by using the Peridynamic theory and experiment. The particulate composite was fabricated by mixing aluminum and TiC powder and then the mixture was hot-extruded. Tensile tests were carried out to validate the Peridynamic model. Four representative volume elements were extracted from surface images of specimens. The location of Particles in the matrix was obtained by image processing. Due to restrictions on bond-based Peridynamic, state-based Peridynamic was utilized for modeling. Overall stress-strain curve, the destitution of equivalent stress and plastic strain, distribution of damage parameter, the total plastic stretch of all interactions, and the number of damaged interactions were used to analyze the results. At matrix surrounded by particles, matrix/particle interface, and narrow particles stress concentration were detected. The damage was initiated at these regions, but the damage was mostly propagated in matrix and matrix/particle interfaces. In the loading process, several damage mechanisms were initiated and propagated, and finally, a principal crack was created that led to the final fracture. By comparing scanning electron microscope images of the fractured surface, modeling, and experimental result, it is shown that the developed Peridynamic model can precisely predict the progressive damage behavior of particulate composites.

Exploiting the nonlinear nature of structural damage through piezoelectric patches is one of the latest concepts of early detection of damage. Support loosening as one of the prevalent defects in engineering systems is a source of contact acoustic nonlinearity. Vibro-acoustic modulation has proven to be a promising method for revealing structural nonlinearity characteristics. Requiring a large number of cycles to be solved in the time domain to reach the steady-state before evaluating the results in the frequency domain, makes using existing computational methods such as finite element method to model this phenomenon very expensive. This paper is concerned with a novel numerical method called Fourier spectral element to investigate the vibro-acoustic modulation in the Euler-Bernoulli beam caused by contact nonlinearity. Three piezoelectrics were attached to the beam as the probe, pump and sensor. The numerical outcomes are compared against the experimental results to check the validity of the developed method. The results show that the Fourier spectral element not only provides a high convergence rate but is also capable of simulating the phenomenon of modulation with sufficient accuracy.

The low velocity impact (LVI) response of pure and glass fiber reinforced polymer composites (GFRP) with 0.1, 0.3 and 0.5 wt% of functionalized single-walled carbon nanotubes (SWCNTs) was experimentally investigated. LS-DYNA simulation was used to model the impact test of pure and incorporated GFRP with 0.3 wt% of SWCNT in order to compare experimental and numerical results of LVI tests. All tests were performed in two different levels of energy. In 30J energy, the specimen containing 0.5 wt% SWCNT was completely destructed. The results showed that the incorporated GFRP with 0.3 wt% SWCNT has the highest energy absorption and the back-face damage area of this sample was smaller than other specimens. TEM images from specimens were also analyzed and showed the incorporation of well-dispersed 0.1 and 0.3 wt% of SWCNT, while in specimens containing 0.5 wt% of CNT, tubes tended to be agglomerated which caused a drop in LVI response of the specimen. The contact time of impactor in numerical and experimental results was approximately equal; however, the maximum contact forces in LS DYNA simulation results were higher than the experimental results which could be due to the fact that in the numerical modeling, properties are considered ideal, unlike in experimental conditions.

Since osteomyelitis is a serious and dangerous disease, it requires immediatetreatment with antibiotics or bone substitute replacement in orthopedic surgeries.Therefore, a porous polymeric-ceramic was fabricated using hydroxyapatite(HA) and polymethylmethacrylate (PMMA) composed with elastin as an idealscaffold for bone tissue engineering applications. The current study is aimed atinvestigating the effects of various amounts of elastin biopolymer on porous bionanocompositescaffold using the freeze-drying (FD) technique. The morphologyand phase analysis of the prepared scaffold are analyzed using scanning electronmicroscope (SEM) and X-ray diffraction (XRD) techniques. The biologicalperformance of the porous tissue is evaluated in simulated body fluid (SBF) andsodium chloride (SC) solution. The tensile test is used to measure the elasticmodulus and tensile strength of the porous tissue before soaking in the SBF. Theobtained result is simulated using micromechanical model from the experimentalvalues. The elastic modulus of samples decreases from 1.18 MPa to 0.69 MPa,and porosity evaluation is in the range of 70-85% with addition of 10 wt% and 15wt% elastin to PMMA-HA bio-nanocomposite. The biological behavior indicatesthat a thick apatite layer precipitate on the surface of the sample with 10 wt%elastin beside increases alkaline group with constant pH concentration. Accordingto the obtained porosity and elastic modulus results, suitable micromechanicalmodel is assessed. The comparison of micromechanical model is assessed, anderror rate was less than 10%; therefore, optimum model is introduced as the bestmicromechanical model for porous bone substitute.

Porous materials especially metallic foams are novel materials with high energy absorption and strength to weight ratio capability. In the present paper we investigate quasi-static uniaxial compression and crushing behavior of closed-cell graded aluminum foams and foam-filled tubes, both numerically and experimentally. To model the mentioned specimens, we place cubes with several densities and strengths to generate functionally graded specimens. Specimens are considered as to be graded with two and three layers and non-graded single layer, with and without tubes. Various standard uniaxial compression experiences are conducted for numerical model calibration and validation and also for non-linear mechanical properties and hardening characterization. To enhance strength and energy absorption capability and also tailoring purpose, we layout the cubic foams in tubes with square profile. The 3D Voronoi diagrams approach is manipulated to model stochastic foam microstructure. Also Novel unit cell is proposed based upon Kelvin cell. We implement the hybrid finite element analysis and Voronoi diagram using Python script and Abaqus 2017 commercial FEM-based code for more convenient modeling and efficient analysis. Finally, and after numerical model calibrations, numerical and experimental results showed good agreement.

Fiber-reinforced thermoplastic composites have drawn much attention due to higher toughness in comparison with thermosets. Polycarbonate is of particular concern because of low weight, high thermal and impact resistance. In this paper, effect of manufacturing parameters such as temperature, pressure and time on mechanical properties of Polycarbonate/Glass laminate is investigated using Taguchi method. In order to evaluate effects of various parameters, an experimental design planned using L9 orthogonal array with three level of time, pressure and temperature with the help of Minitab software. For this, composite beams having arrangement [0]8 were produced in film staking procedure in a hot-pressing apparatus. The 3-points bending tests were carried out to assess manufacturing quality. Comparison of flexural modulus and flexural strength of various specimens were made and the results are commented upon and discussed. Results show that the temperature of 215 Celsius degree results higher mechanical properties than other temperatures because of preventing resin degradation and adequate viscosity. Furthermore, results indicated that the time factor was more significant, than time and pressure, on curing quality. Optimum values of factors such as time, pressure and temperature to achieve higher flexural modulus and flexural strength are determined using analysis of Signal to Noise Ratio (SNR).

Sandwich structures are used in applications that required a combination of high rigidity and low weight same as aerospace, marine and automotive. Large and/or complicated sandwich structures are often manufactured by connecting pre-fabricated sandwich panels by means of connections, adhesive or bolts. In present study, two types of metallic connections were used to join two sandwich panels with glass-epoxy face-sheets and aluminum honeycomb core. Connections have the same material and different geometries and were bonded to the sandwich structures using the same epoxy as used to manufacture the face-sheets. Two groups of specimens were made and tested under bending loading. Also, a finite element simulation using LS-DYNA were performed to predict the behavior of sandwich structures. A good compliance between numerical and experimental results was observed. The effects of increasing the length and the thickness of the connections on the maximum force and energy absorption were investigated to examine the influences of involved parameters on bending response of a sandwich plates.

Large and/or complicated sandwich structures are often manufactured by connecting pre-fabricated sandwich panels by means of connections, adhesive or bolts. In nearly all sandwich constructions certain types of joints have to be used for assembly but little is known about their mechanical behavior. This paper deals with the investigation of the behavior of two aluminum joints with different geometries under low velocity impact tests. These two joints are used to connecting sandwich panels with glass-epoxy skins and aluminum honeycomb core. The joints and sandwich panels are connected by means of epoxy resin. After construction of the specimens, low velocity impact tests were performed on the specimens. Finite element analysis were used to simulate the behavior of sandwich panels with connection. Verification of the numerical results was performed by comparing the numerical and experimental results. There was a good compliance between numerical and experimental results. Also, the effect of increasing the length and the thickness of the connections on the behavior of the sandwich panel was done through a parametric study using the FEM model.

Using fabrics as reinforcement of composites considerably leads to improve some of mechanical properties. One of the important mechanical properties is resistance to impact forces. Therefore, in the present study, the impact behavior of composites reinforced with weft-knitted spacer fabrics has been studied. Due to the out of plane nature of impact force , the through-the-thickness reinforcement of composite play a key rule in undergoing the impact forces. Weft-knitted spacer fabrics are adequate structures to reinforce through-the thickness of composite due to the existence of spacer yarns. In this study, at first, principle of low velocity impact behavior of composites was studied. Then, a semi-empirical model was generated to predict the impact behavior of composites considering the structural parameters of weft-knitted spacer fabrics as reinforcement of composites. In order to validate the proposed model, weft-knitted spacer fabrics with different types of spacer yarn's orientation were produced and used as reinforcement of composites. The low-velocity impact test was carried out on the prepared samples. A good correlation was found between theoretical and experimental results.

Sandwich structures are widely used in aerospace, automobile, high speed train and civil applications. Sandwich structures consist of two thin and stiff skins and a thick and light weight core. In this study, the obligatory mandate of a sandwich plate contact constitutes a flexible foam core and composite skins with a hemispherical rigid punch has been studied by an analytical/empirical method. In sandwich structures, calculation of force distribution under the punch nose is complicated, because the core is flexible and the difference between the modulus of elasticity of skin and core is large. In the present study, an exponential correlation between the contact force and indentation is proposed. The coefficient and numerical exponent were calculated using the experimental indentation results. A model based on a high-order sandwich panel theory was used to study the bending behavior of sandwich plate under hemispherical punch load. In the first method, the force distribution under the punch nose was calculated by the proposed method and multiplied to deformation of related point in the loading area to calculate the potential energy of the external loads. In the second method, the punch load was modeled as a point force and multiplied to deformation of maximum indented point. The results obtained from the two methods were compared with the experimental results. Indentation and bending tests were carried out on sandwich plates with glass/epoxy skins and a styrene/acrylonitrile foam core. In the bending test, a simply support condition was set and in the indentation test the sandwich specimens were put on a rigid support. Indeed, in this position the punch movement was equal the indentation. The comparison between the analytical and experimental results showed that the proposed method significantly improved the accuracy of analysis.

A numerical and experimental low-velocity impact behavior of sandwich plates have been presently studied with regard to the compressibility and viscoelasticity features of their cores. Face sheets were assumed to be anisotropic composites or isotropic aluminum materials and a viscoelastic behavior has been considered for core. The boundary conditions are assumed to be simply supported or rigid. Abaqus, as FEM software, and its python script programming feature, have been used to model the specimens. To model hyper-viscoelastic nonlinear behavior of the core, Ogden hyper-foam elasticity and Prony series approach are manipulated. To solve the numerical problem, dynamic explicit solver option with sufficient solving amplitude has been used. Prony series have been used to model the core time-dependent behavior. In conjunction with a simple indentation experiment, FEM used to formulate a novel method for finding the Prony series coefficients. By performing some low-velocity impact experiments, the impact force and displacement of the composite sandwich plates have been investigated. The results indicate that increasing the structural damping increases the contact time and missing energy and decreases the stored energy of the system. The structures with composite face sheets have a minimum ratio of upper face sheet displacement to lower face sheet displacement in comparison to those with the isotropic face sheets. Impact behavior of isotropic face sheet specimens are more flattened than that of the composite face sheets. In addition, the specific energy stored in the sandwich plates with composite face sheets, on different supports, is greater than that stored in the aluminum face sheets.

In this article two dimensional sliding contact of a rigid cylindrical punch on a functionally graded (FG) semi-infinite medium is studied. The modulus of elasticity in the graded layer is calculated based on TTO model approximation. This model defines a parameter q which considers the microstructural interactions between the constituting phases. The governing equations are solved by finite difference (FD) method by means of MATLAB software. The effects of different parameters such as non-homogeneity­, q, the dimensions of the punch, the thickness of the graded layer and the coefficient of friction are investigated on the force-displacement diagram of the punch, stress distribution and modes I and II stress intensify factors (KI and KII). The results show that the contact force and the stress distribution under the punch are affected by the thickness of the graded layer, radius of the punch and the non-homogeneity­ coefficient. But the variation of q has no effects on the contact force and the stress distribution under the punch. In the absence of the friction, KI is always non-positive and the crack has no tendency to grow under this mode. Increasing the friction coefficient, increases KI but decreases KII.

A novel geometrically nonlinear high order sandwich panel theory for a sandwich beam under low velocity impact is presented in this paper. The equations are derived based on high order sandwich panel theory in which the Von-Karman strains are used. The model uses Timoshenko beam theory assumptions for behavior of the face sheets. The core is modeled as a two dimensional linear elastic continuum that possessing shear and vertical normal and also in-plane rigidities. Nonlinear equations for a simply supported sandwich beam are derived using Ritz method in conjunction with minimum potential energy principle. After obtaining nonlinear results based on this enhanced model, simplification was applied to derive the linear model in which kinematic relations for face sheets and core reduced based on small displacement theory assumptions. A parametric study is done to illustrate the effect of geometrical parameters on difference between results of linear and nonlinear models. Also, to verify the analytical predictions some low velocity impact tests were carried out on sandwich beams with Aluminum face sheets and Nomex cores. In all cases good agreement is achieved between the nonlinear analytical predictions and experimental results.

An experimental study of the high velocity impact on 2/1 GLARE panels has been carried out. Four GLARE 3 panels with (Al/0/90/Al)، (Al/02/902/Al)، (Al/04/904/Al) and (Al/08/908/Al) configurations were fabricated. All panels had same aluminum thickness. Ballistic tests were conducted using flat ended steel projectiles. The failure modes and energy absorption mechanisms were characterized by observation the front and rear sides and also sectioning the panels. Some global and local failure modes due to high velocity impact were identified. The ballistic limit velocity and energy absorption capacity of each panel set was determined experimentally. Specific perforation energy was applied for comparison the ballistic efficiencies of each target. It was found that the (Al/02/902/Al) sequence has the superior specific perforation energy.

sandwich structures are widely used in aerospace, high speed trains and marine applications because of lightweight and high in-plane and flexural stiffness. Sandwich structures consist of two thin face sheets and a core. Face sheets usually are made from highly stiff and highly strong materials; In general, the face sheets may be of different metal or composite layers. Both metal and composite face sheets have advantages and disadvantages, and searching for new materials with better properties is in progress. In this paper flexural behavior of a new generation sandwich beams with fiber metal laminate (FML) face sheets were investigated experimentally. Three groups of specimens with different layer arrangements of face sheets consist of (Al/GE (0-90)/GE(90-0)/Al), (Al/GE(0-90)/Al/GE(90-0)) and (GE(0-90-0-90-90-0- 90-0)) and 40 kg/m3 polyurethane foam core were made and tested. The results show that sandwich beams with FML face sheets have better resistance against local loads, while composite faces are weak against intense loads. Also, FML faces are lighter than metal face sheets and have better connection to foam core. Also, a simple classical theory was used to predict the force-deflection behaviour of sandwich beams in elastic region. Good agreement between the experimental results and analytical prediction were obtained. Sandwich beams with FML face sheets have larger elastic region than beams with composite face sheets therefore agreement between the analytical andexperimental results in these specimens are in larger area