abolfazl darvizeh

In this paper, the effect of multi-walled carbon nanotubes (MWCNTs) on the ballistic limit of fibers metal laminates (FML) Exprimentaly investigated. For this purpose, the MWCNTs were added with weight percentages of 0.2, 0.4, and 0.6 to pure epoxy resin and homogenized by mechanical and ultrasonic homogenizers. Then the FMLs were fabricated by fiber glass, 2024-T3 aluminum alloy sheets, pure epoxy resin, and modified resin with MWCNTs using a hand lay-up process. In the end, Ballistic tests on the samples were conducted by using a conical nose projectile. The experimental results show that the ballistic limit of FMLs is increased by adding MWCNTs. Also highest in this increase was observed in samples containing 0.4 weight percentages of MWCNTs, but in the 0.6 weight percentage, agglomeration of nanoparticles caused a reduction in the mechanical properties. The microstructural investigations using Electron microscopy show that the addition of MWCNTs improves the interfacial adhesion between the epoxy matrix and the reinforcing fibers.

Connecting point of the longitudinal veins and cross-veins in wing is called Joint.  In some insect wing joints, there is a type of rubber-like protein called Resilin. Due to the low Young's modulus of this protein, its presence in the wing can help to change the shape of the wing during flight. Today, using composite structures in flying vehicles in order to achieve the desired shape of wing is considered. The purpose of this study is the multi-objective optimization of artificial wing by arranging Resilin joints in the artificial wing of Micro air vehicles (MAVs). The amount of torsion and bending of the flapping robot wings is considered as the objective function to improve the flight performance of robots. Two types of artificial wings have been investigated, and considering pareto points, the optimal arrangement of Resilin joints has been achieved.  The result of this study shows that in both wings, with the presence of Resilin in the joints, the amount of torsion has increased to 38.65 degrees.

Most natural materials are composites that can exhibit a wide range of material properties, such as the elastic modulus. Insect cuticle is one of these natural materials. The material gradient of the cuticle drastically varies in different insect body parts. Considering the small size of cuticular samples, conducting experimental tests is very challenging, expensive, and time-consuming. Understanding how cuticular structures work and their graded properties can help to design and develop engineering materials with enhanced properties. In this paper, the finite element (FE) method is used to investigate the function of the cuticle in the membrane of dragonfly wings. In this regard, first, the distribution of materials on the wing membrane is investigated using images obtained by confocal laser scanning microscope (CLSM). Then, in order to estimate the stress and strain values subjected to displacement, multiple stiffness functions are applied for a geometric model of a cantilever beam, which represents the membrane. The results show that the presence of a graded stiffness has a significant effect on the mechanical behavior of the cantilever beam. A comparison of the results obtained from the analysis of different stiffness functions showed that The quadratic function is introduced as a more suitable distribution for stiffness, in terms of having the least stress and strain in this structure, compared to other stiffness functions. This study can provide a proper and applied platform for further interdisciplinary research in this area.

Damage is a progressive physical process, which at last leads to failure. During the damage process, material experiences some changes such as nucleation, growth, and coalescence of microscopic voids and microcracks. If the metal material is under the plastic strain, some defects like voids and microcracks are created. Increasing the plastic strains, these defects develop and eventually crack is created in the material which causes fracture in it. Therefore, measuring the damage is very important and various methods have been proposed to determine the damage in materials yet. In this paper first, different methods of estimating the ductile damage are presented and then, ductile damage for stainless steel 316L is determined, using the measurement of elasticity modulus and also measurement of velocity of ultrasonic wave propagation methods. Finally, value of critical ductile damage parameter for the material is calculated by each of these methods and compared.

One of the main challenges in nanoindentation tests is to work-out a method in order to obtain the material properties through the test results. Using a hybrid method which combines the experimental results of nanoindentation tests with FEM analysis is considered as one of the main solutions for this problem. In order to calculate the mechanical properties, an explicit method was developed on the basis of modified dimensional analysis method. The main advantage of this method is to provide unique answers without any need for iteration so that it would minimize the calculations significantly. In this paper, the mechanical properties of Titanium in plastic phase (yield stress and strain hardening Module) are calculated by utilizing this method. For the first time, the dimensional analysis was used for the bilinear constitutive equation (with two dimensionless parameters), and in the finite element analysis, the general form of the Johnson-Cook equation was utilized. The most important property of plastic state, namely, the yield stress, was extracted in proper agreement with reference values. In the process of solving, a new method for error analysis based on calculating the resultant errors and determining the extremum error in the entire range of parameters was successfully developed and applied. Finally, in order to calibrate the solution, it was also proposed to set up the critical conditions of the problem, such as the indenter tip radius. It was concluded that the use of a radius more than 200 nm leads to a more consistent response to the experiment.

At present paper, an equivalent model with different dimensions and also with different dimensions and material in comparison with main body for strain rate sensitive structures subjected to high rate loading is presented by using the novel finite similitude method. The finite similitude method provides performing a test on the model instead of the original sample. This method is used to obtain the properties of model and to reverse the obtained results for model to main body by using the principles of nature (the law of conservation of mass, the law of conservation of momentum, the law of conservation of energy and the law of conservation of entropy) which is always true for any system. The relationships for both pure dimensional and simultaneously dimensional/material scaling of strain rate sensitive structures are presented. To evaluate the efficiency of the proposed relationships, the numerical results are obtained for impacted circular plates. It should be mentioned that the numerical results are obtained by using the finite element software LS-Dyna in which the strain rate effects are considered into account by using the Cowper-Symonds and Johnson-Cook constitutive equations. The results indicate that the scaled plate to one tenth of its original dimensions and also made of different material in comparison with original plate predicts the response characteristics of the original plate with a very good accuracy.

In this paper, the static behavior of a cracked Timoshenko beam resting on an elastic foundation is investigated through the approximate method of Galerkin. The crack is simulated using a rotational spring whose stiffness is obtained in terms of geometric and material characteristics of the cracked structures. The governing differential equations are derived by considering a discontinuity in the axial direction of the beam. In the solution of the problem by use of the Galerkin’s method, the beam is divided into two parts, as the first part is defined from the support of the left-hand side to the crack position and the second is assumed from the crack position to the right-hand end of the beam. For each part, a deflection function is considered with five unknown parameters. Ten unknown parameters for two parts are determined using four boundary conditions, four continuity conditions in the crack point and two equations of the Galerkin integral. The deflection equation throughout the beam is obtained from the known parameters. The results are reported in two boundary conditions including simply supported-simply supported (SS-SS) and clamped-free (C-F) and the validity of the presented solution is accepted with some references. The results show that the elastic foundation decreases the side effect of the crack in the structure.

In this research, an analytical method is presented for predicting the viscoelastic and dynamic behavior of polymer nanocomposite. The analytical model is achieved by coupling the SUC micromechanical model with standard linear solid model. Boltzmann superposition principle is used to develop the constitutive equations. First, the strain associated with a relaxation experiment is considered, and then by using the idea of linearity as embodied in the Boltzmann superposition principle, the resulting stress history is predicted. Eventually, the creep function corresponding to the relaxation modulus is obtained and the hysteresis loop for nanocomposite material is represented. Creep response is sinusoidal in time and a function of stress history. Loss and storage modulus and material behavior in Laplace domain are obtained using standard linear solid model and SUC micromechanical model, respectively. Standard linear solid model is achieved by paralleling the Kelvin model with Maxwell model. The model is validated with experimental results. Effects of different interphase thickness, CNT volume fraction and phase angle on hysteresis loop is studied. Obtained results reveal that increasing the CNT volume fraction and phase angle leads to decreasing and increasing the nanocomposite hysteresis loop area, respectively. Also, Interphase thickness contains considerable effects on the nanocomposite dynamic behavior.

In the present work, a fuzzy logic approach was used in order to determine the effect of thermal aging on the surface hardness of phenolic resin reinforced by woven carbon fibers (CFP). For this purpose, the specimens made from CFP composites with different fiber volume fraction (20%, 30%, 40%, and 50%) were exposed to different thermal aging fashions (refrigerated at -30 ºC, room temperature at 25 ºC, and thermally aged at 50 ºC).The surface hardness values of both the pristine and aged specimens were then experimentally determined using Barcol hardness testing machine. The hardness values for CFP composites were also predicted by two types of fuzzy inference systems (FIS), i.e. Mamdani and Sugeno methods. Results of the developed fuzzy logic models were compared with the experimental results and it was found that the Sugeno method performed better than Mamdani method.

In this paper, the effect of different environmental conditions on the impact properties of fiber metal laminates hybridized with nanoclay is studied. For this purpose, the fiber metal laminates were first laminated by aluminum alloy sheets, glass fiber, pure epoxy resin and modified resin with nanoclay using hand lay-up process. The influence of different types of environmental conditions including cryogenic aging (at temperature of –196 °C in LN2), high-temperature aging (at temperature of 130 °C in dry air), and hygrothermal aging (at temperature of 90 °C in distilled water) on the impact properties of the specimens made with pure epoxy resin and modified resin was investigated using response surface method in various levels. A suitable model was developed to predict the effect of aging type and nanoparticle content on the impact strength of specimens. The results obtained suggest that the cryogenic aging has a most effective role reduction of the impact properties of the specimens. While htgrothermal aging has a less effective role in decreasing the impact properties of fiber metal laminates. Additionally, the result of main effects analysis showed that the detrimental role of different types of aging in reducing the impact properties is more effective than the positive role of nanoparticles in improving the impact properties of fiber metal laminates.

In the present work, experimental and numerical results of the effects of different pressure curve on the thickness variation of sheet metal and distribution of radius and hoop strain for effective stress-strain curve have been presented. A series of experiments are carried out using a hydroforming apparatus by exerting different pressure curves including pendulous, steeped, saw and continuous. In each series, the effect of changes in pressure curve on thickness distribution was quantitatively measured. Different pressure curves such as continuous, stepped, and pendulous was produced in experiments. The ABACUS software was implemented to simulate the effect of changes in pressure curve. A good agreement between the experimental and numerical results was observed. The results show that stepped pressure produces more uniform distribution in sheet metal thickness. Mechanical behavior of sheet metal during plastic deformation phase under stepped pressure, produced satisfactory results, and using this type of pressure could control the effects of friction between the die surface and sheet metal specimen much better. Also, constant time duration of pressure pulses in stepped and pendulous curves leads to decreasing of maximum pressure needed for deformation of sheets.

In this article, the effect of hygrothermal aging on mechanical properties of fiber metal laminates (FMLs) and E-glass/epoxy (GE) composites is investigated. First, FML and GE specimens were built using wet lay-up technique under vacuum pressure. Hygrothermal aging simulation was then carried out on both specimen types in distilled water at a constant temperature of 90 °C for 5 weeks. The resulting behavior of degradation for both types of specimens caused by hygrothermal aging was evaluated by bending and Charpy impact testing. As expected, because of the protective role of aluminum layers, FML specimens showed remarkably lower water absorption after hygrothermal aging compared to the glass/epoxy composites. Experimental results also revealed that the flexural properties of both the FML and GE laminates were affected by the hygrothermal aging, whereas a lower level of deterioration in impact strength was found.

The aim of this study is to investigate effects of isothermal aging on mechanical properties of fiber metal laminates (FMLs) and glass/epoxy composites. For this purpose, both materials were fabricated using the wet lay-up manufacturing technique under vacuum pressure. Both the glass/epoxy composites and the FML specimens were then subjected to isothermal aging (130°C, dried air) for up to 5 weeks. After the isothermal aging, the specimens’ weight loss, caused by thermo-oxidative conditioning, was evaluated. Bending and Charpy impact tests were conducted on both the unaged and aged specimens to examine the isothermal conditioning’s effect on the mechanical properties of studied materials. Experimental results revealed that isothermal aging severely affected impact strength in the form of embrittlement and reduced ductility. However, no significant reduction was found in the flexural stiffness of isothermally aged FML and glass/epoxy specimens.

In the new sheet metal forming process as incremental sheet forming and spinning forming, this is not perfectly true in Marciniak-Kuczyinski model to assume that sheet deformation occurs in the plane-stress state indispose there are normal compressive stress and through-thickness stress. In this type of forming processes, the obtained limit strains refer to improving the sheet forming. However, in researches the effects of through-thickness shear stresses, also known as out-of-plane shear, has been studied less. The generalized forming limit diagram is a great curve that includes all six components of the stress tensor. In this paper, the effect of normal comprehensive and through-thickness shear stresses on the limit strain AA6011 aluminum sheet using a modified M-K and the anisotropic Yield function, Hill 48 and by using numerical solutions of nonlinear equations, Newton-Raphson method. The first the forming limit diagram was drawn with the assumption that the through-thickness shear stresses and then the effects of normal comprehensive stress and through-thickness shear stress on the limit strains were proved and the generalized forming limit curves were obtained. The results show that forming limits can be increased significantly by both normal compressive stress and through-thickness shear stresses. Also, the effects of normal stress on increasing the formability of sheet compared with the effects of through-thickness shear stress is greater.

Composite materials such as fiber reinforced polymeric laminates are used extensively in various engineering applications. Of the greatest impediments to the use of these materials in advanced applications is the reduction of their mechanical properties and structural integrity when they are exposed to high temperatures. In this paper, the effect of nanoclay addition on impact properties of composite and fiber metal laminates before and after exposure to high temperature shock have been investigated. For this purpose, the nanoclay particles were added to pure epoxy resin using mechanical mixer, high-speed mechanical homogenizer, and ultrasonic homogenizer. Then, both the composite laminates and fiber metal laminates 2/1 were laminated by aluminum sheets, pure epoxy resin and modified resin with nanoclay and glass fiber using hand lay-up process. The effects of using nanoclay on the impact strength of composite laminates and fiber metal laminates before and after exposure to temperature of 230 ° C were studied. According to the results obtained, it was found that nanoclay has effective role in maintaining impact properties of the specimens. Additionally, as a result of protective role of metallic layers, the thermal shock induced degradation in impact properties of fiber metal laminates was decreased.

Experimental investigation and numerical modelling of dynamic loading effect on the density, strength and microstructure of the pure and composite parts fabricated from iron powder has been studied in the present paper. Experimental tests have been performed using drop hammer apparatus. Diametral compression test has been used to evaluate the strength of fabricated parts. Also, numerous micrographs have been provided using scanning electron microscopy to investigate the influence of impact loading on the microstructure of compacts. The obtained results show that with increasing compaction energy, density and strength of pure parts increase in three ranges of low, medium and high pressures with different rates. Also, the obtained results by compaction of iron powder with different ceramic powder contents under equal pressure reveal that adding more than 5 percent of ceramic content, sharply decreases the density of composite parts. Examination of graphs taken from the microstructure of compacts reveals that propagation of compressive stress waves through the powder column, causes plastic deformation of particles and forms mechanical inter-locking on a completely uniform structure. Furthermore, a mathematical expression has been presented using dimensional analysis along with genetic algorithm method to predict the values of density of produced pure parts. In this method, after constructing dimensionless numbers based on dimensional analysis approach, these numbers have been used as the inputs of genetic algorithm modelling method. Comparison of the values predicted by this equation with those obtained by experimental values, shows that the results obtained by this model, agree with experimental results surprisingly.

In the current study, low velocity off-center impacts of glass/epoxy laminates considering different impact locations are investigated experimentally and numerically. Low velocity impact tests are performed using an instrumented drop-weight machine and the composite specimens are formed through the use of the vacuum infusion process. To simulate low velocity impact properties of the composite, the finite element software ABAQUS/Explicit is employed. The damage model is implemented in the FE code by a user-defined material subroutine (VUMAT). In order to effectively describe the progressively intralaminar damage for composite laminates, two three-dimensional progressive damage models are presented exponentially and linearly and for predicting damage initiation of composite plates, 3D Hashin’s failure criterion is chosen. Both damage models are established as functions of energy dissipated by damage in addition to introducing the characteristic length for each three dimensional solid element. The contact force-time histories and peak loads are obtained to compare the numerical and the experimental results at several impact energy levels and three different impact locations of the composite plates. In addition to these achievements, the comparison of the numerically predicted damage pattern and damage size and those observed experimentally can verify the efficiency of the present models

In this article, a new method for finite element modeling of planar multi-material structures, using only an input image, has been presented. This method which is implemented in a Matlab program is based on digital image processing technique. After importing a digital image by the user, the program automatically converts it into a binary format. The inner part and the boundary of the desired structure in the input image are recognized based on the difference between the colors of their pixels. These two regions are meshed separately by connecting their pixels to each other. The output of the program is a file in INP format which includes the coordinates of the nodes of the finite element model, their connections and the material of each part which has been defined by the user during the modeling process. The obtained results indicate the high accuracy of the method in the modeling of planar composite structures with complicated geometries in a very short time. Further, the results from the numerical analysis of the output model are in a good qualitative and quantitative agreement with the previous experimental data. The presented method in this article provides a strong background for future realistic modeling of planar composite structures.

The effect of strain hardening modulus on axisymmetric buckling of circular cylindrical shells made of aluminum and steel materials is investigated, so the nonlinear dynamic equations of cylindrical shells with different strain hardening modulus for cylindrical shells made of aluminum material and different linear and multi-linear strain hardening modulus for cylindrical shells made of steel material are solved for three types of boundary conditions and two types of loading. It is found that under investigated conditions, changing of strain hardening modulus for cylindrical shells made of aluminum and steel materials transmit the buckling shape from plastic buckling to progressive buckling.

In this paper, a discontinuity in beams whose intensity is adjusted by the spring stiffness factor is modeled using a torsional spring. Adapting two analyses in strong and weak forms for discontinuous beams, the improved governing differential equations and the modified stiffness matrix are derived respectively. In the strong form, two different solution methods have been presented to make an analogy between the formulation of the Euler-Bernoulli and Timoshenko theories that indicates the influence of the shear deformation in discontinuous beams. The flexural stiffness of discontinuous beams is corrected by using the Dirac’s delta function. In the weak form, the reduced stiffness matrix is derived from the strain energy equation established by the continuity, kinematics and constitutive equations. The linearity assumption of the geometry and material is considered to construct the kinematics and constitutive equations respectively. The continuity conditions mathematically connect two divided parts of the Euler-Bernoulli beam for which an improved Hermitian shape function is employed to interpolate displacement field. An application shows the comparison and validation of the results of the strong and weak forms, and also the quasi-static behavior of discontinuous beams.

In this paper, the internal inversion process of metallic cylindrical shells under dynamic axial loading is investigated experimentally and numerically. Experimental tests are performed on the steel tubes in a gas gun and the required force for internal inversion is obtained using the measurement system of impact loadings. Also, numerical analysis is carried out by the finite element software ABAQUS and the accuracy of simulated models are validated with the experimental results. In this paper, all geometrical properties of the tubes and die are assumed to be constant and the effect of the projectile mass and velocity is investigated on the shortening and energy absorption of the tubes which are affected by axial impact in the internal inversion process. Therefore the projectile is shot directly to the specimen with different masses and velocities. It is observed that if the projectile mass remains constant, increasing in the impact velocity has almost no effect on the constant inversion load and just increase the tube displacement but if the impact velocity remains constant, increasing the amount of projectile mass causes increasing in the constant inversion load besides of increasing in tube displacement. Comparing the results of numerical simulations with the experimental results shows a good agreement between them.

Nowadays, availability, durability, reliability, weight and strength, as the most important factors in optimum engineering design, are responsible for the widespread application of plates in the industry. Buckling in the elastic or elastoplastic regim is one of the phenomena that can be occurred in the axial compressive loading. Using Galerkin method on the basis of trigonometric shape functions, the elastoplastic dynamic buckling of a thin rectangular plate with different boundary conditions subjected to compression exponetiail pulse functions is investigated in this paper. Based on two theories of plasticity: deformation theory of plasticity (DT) with Hencky constitutive relations and incremental theory of plasticity (IT) with Prandtl-Reuss constitutive relations the equilibrium, stability and dynamic elastoplastic buckling equations are derived. Ramberg-Osgood stress-strain model is used to describe the elastoplastic material property of plate. The effects of symmetrical and asymmetrical boundary conditions, geometrical parameters of plate, force pulse amplitude, and type of plasticity theory on the velocity and deflection histories of plate are investigated. According to the dynamic response of plate the results obtained from DT are lower than those predicted through IT. The resistance against deformation for plate with clamped boundary condition is more than plate with simply supported boundary condition. Consequently, the adjacent points to clamped boundary condition have a lower velocity field than adjacent points to simply supported boundary condition.

This paper investigates the effects of constructional elements on the biomechanical behavior of desert locust hind wing. First, the microstructure of the insect wing is investigated using scanning electron microscope. The results of the scanning electron microscopy are used to develop finite element models of the wing with different constructional elements. The presented models are studied under the inertial and aerodynamic loads applied during flight and the obtained stresses and displacements are assessed. The results show that longitudinal veins, longitudinal and cross veins, corrugations, corrugations and longitudinal veins and finally a combination of corrugations and longitudinal and cross veins cause averagely 4, 25.75, 4.34, 184.54, 768.5 times decrease of the achieved principal stresses in comparison with a wing without the mentioned constructional elements. Constructional elements of the locust wing play an important role to uniform the pattern of stress distribution in the wing during flight. Further, the existence of the mentioned constructional elements causes a decrease in the variation of the stress within a stroke-cycle. In addition, it is shown that the inertia and aerodynamic forces have less effect on the wing deformation than the elastic ones. The results of this research may be helpful in the development of lightweight structures with high strength.

In this paper, a scanning electron microscope (SEM) is used for microstructural investigations of a woodlouse shell. Finite element (FE) method is employed to study the dynamic behavior of the shell subjected to the impact of a cone-shaped projectile. Despite of small thickness, the shell, as a composite material, enables the insect to bear large external forces. The woodlouse is also able to roll up into a complete sphere to protect itself from danger. In order to study this defense mechanism, the external loads are applied to the shell in different configurations: when the shell is in (1) normal and (2) rolled-up forms. The simulations are performed at different velocities and at different impact angles. Comparisons of the results obtained from different simulations indicate that the defense mechanism of the woodlouse has an important role in decreasing the stress concentrations. Indeed, it is a defense mechanism which effectively increases the load-bearing capacity of the insect shell. The results of the present research may be useful in the design and manufacture of modern engineering structures with a high strength to weight ratio.

In this paper, with the assumption of constant material properties, the nonlinear behavior of beams is studied using the finite element method. To this end, two approaches are represented: in the first approach, the beam is modeled by one dimensional elements of second order that is formulated according to continuum mechanics relationships based on the Lagrangian strategy, while in the second approach based on the Eulerian strategy the nonlinear behavior of the beam is investigated by making use of two dimensional elements. In both approaches, the second configuration, strains and stresses in the beam are obtained via the calculation of deformation gradient.