فهرست مطالب amin farrokhabadi

This study introduces a novel lattice structure, whose unit cell design draws inspiration from the fusion of honeycomb patterns and the DNA found at the core of cells, constructed from PLA material. This structure underwent tensile testing along the X and Y axes. Additionally, the paper presents a new analytical-numerical approach that combines Timoshenko beam theory, mechanics of materials principles, and finite element analysis to determine the mechanical properties and forecast failure in cellular structures. This method was corroborated using the ABAQUS commercial software. Research indicated that a closer ratio of thickness to unit cell length, specifically 1/10, leads to more precise predictions for the mechanical behavior of the cellular structure under tension along the X axis. The findings showed that, in comparison to the Y axis, the X direction exhibited a 7% increase in load-bearing capacity and an 8% increase in maximum yield stress, yet the equivalent stiffness was 75% lower

Today, composite materials and sandwich structures have been studied due to their unique properties such as strength to high weight, corrosion resistance, ability to absorb energy and sound. One of the most important applications of layered composites is their use in the construction of sandwich structures. Recently, sandwich panels have been widely used in engineering structures due to the mechanical properties they show in various fields such as bending behavior, shear, impact response and dynamic loads. Fatigue and quasi-static have been used. In this regard, this research has experimentally investigated the mechanical behavior of composite sandwich panel with multi-layer corrugated core subjected to quasi-static three-point bending. Parameters such as bending stiffness and maximum bending load tolerance for corrugated core geometries (square and trapezoidal) by changing the core from square to propeller and also the effect of adding polyurethane foam were studied. The purpose of adding corrugated composite inside the core is to strengthen it and, as a result, strengthen the sandwich structure against bending loads without significantly increasing the weight of the structure. Composite plates and corrugated cores are made by vacuum resin transfer technique and using E-glass woven fabric and ML506 epoxy resin with 15% hardener. The experimental results show that the multilayer composite sandwich panels strengthen the structure in the quasi-static three-point bending process. The main damage and failure mechanisms of sandwich panel samples during the load bearing process are: matrix cracking, fiber breakage, delamination, local indentation, global bending, crushing and buckling of cell walls and skins.

In this research, the process of surface modification, fabrication, and examination of the mechanical and electrical properties of a semi-biodegradable composite prepared from polypropylene and hemp fiber fabric has been discussed. Production of composite samples was done by dissolving polypropylene polymer in a xylene solution and then combining it with fiber fabric under pressure and temperature curing of a hot press. To improve the interface between the fibers and the matrix and to improve the mechanical properties of the produced samples, the surface of the hemp fiber fabric was modified using a 6% sodium hydroxide solution for 12 hours. To determine the mechanical properties of the produced samples, two tensile and three-point bending tests were performed. The elastic modulus, yield stress, ultimate stress, elongation, and toughness were measured from the tensile test and were analyzed and investigated. The bending of the composite was obtained from the bending test. To determine the electrical properties of the composite, the dielectric constant test was performed using a network analyzer in the x band. The maximum tensile strength and Young's modulus obtained in this research are 8 MPa and 1.6 GPa respectively. The maximum dielectric constant and loss tangent of the produced samples in the x-band is equal to 3.48 and 0.24, respectively. Finally, to validate the results obtained in this research, the results of the tests have been compared with the results of other authorities in this field.

In this study, a 4×4 square network structure made of titanium has been optimized under tensile force using relevant parameters. The network structure is a quadrilateral structure with a side length of L and ϴ, and the fracture order of the walls has been compared using MATLAB software and simulation with Abaqus software, and the results of the fracture order of the structure match each other. In the present study, the objective function in optimization is to increase energy absorption and minimize the maximum stress, and the effect of parameters such as side lengths and various angles in this structure has been investigated. 100 different cases have been obtained for values of L and ϴ with output of area under the curve (energy absorbed) and maximum stress and strain using MATLAB software. With input data (L and ϴ) and output data (energy absorbed and maximum stress), a neural network has been trained and a regression model has been used in the neural network to achieve a prediction accuracy of over 99%, which is a high level of accuracy. The relationship function between input and output of the neural network has been obtained using MATLAB software, and the optimization of this 4×4 network structure has been carried out using the genetic algorithm. The objective function in this study is to increase energy absorption and minimize the maximum stress so that the network structure has the highest strength considering the examined parameters.

This research examines the vibrations of multi-layer aluminum-metal fiber cylindrical shells with embedded piezoelectric layers that have fluid-structure interaction based on the three-dimensional elasticity theory. Using the state space approach, the equations of motion for simple support boundary conditions were obtained. The natural frequencies of the multi-layer metal fiber cylindrical shell containing the driving fluid were obtained by solving the special frequency equation. The effect of different parameters such as length to radius, fluid velocity, ambient wave number and radius to thickness was investigated for aluminum reinforced with carbon fibers. The volume ratio of composite/metal was considered constant. So far, no research has been done on multi-layered aluminum fiber-metal cylindrical shells with embedded piezoelectric layers that have fluid-structure interaction. It is a very new topic. The consistency and convergence of the results of this research was confirmed by comparing the results of natural frequencies reported in the literature, and the high accuracy of the results of this research is expressed.

This article examines the effect of layer arrangement and fiber angles on the vibration behavior of cylindrical shell (FML) in GLARE and ARALL samples and on Pasternak elastic bed with different boundary conditions using 3D elasticity theory (composite/metal volume ratio is fixed in taken). The governing equations are obtained using the potential energy minimization principle. In order to ensure the accuracy and convergence of the results of this study, the numerical solution was done using the Finite Element Method (FEM). The results show that the layering has a great influence on the vibration behavior of the FML cylindrical shell. The results are in good agreement with other literature. The use of three-dimensional elasticity theory along with the analysis of different samples for the vibrations of reinforced FML shells, which is presented for the first time in this research, is one of the most important innovations of the current research, and these results can be used as a suitable criterion for analyzing and optimizing FML shells. It can be used with any number of metal and composite layers, any fiber placement angle and under any boundary conditions.

This paper investigates the mechanical performance and energy absorption capacity of bi-material three-dimensional lattice structures via experimental and numerical approaches. At first, fused deposition modeling was used to manufacture the outer part of the proposed three-dimensional lattice structure with TPU material. Using a syringe, epoxy resin is injected into the inner part of the manufactured lattice structure. Then, quasi-static compression tests were conducted to analyze the mechanical properties and energy absorption capacity of the bi-material three-dimensional lattice structure. As the nonlinear numerical study, the elasto-plasto-damage behavior was implemented in finite element analyses which track the nonlinear response of considered structures. This model is capable to investigate the differences in tensile and compressive properties of the materials as well. The comparison of the load-displacement curve of structures under compressive loading has been compared. The numerical models exhibit an acceptable prediction about the linear and nonlinear responses of the proposed three-dimensional lattice structure. The results reveal that not only does the use of hybrid structures provide more energy absorption and improve mechanical properties, but also the rational combination of two materials makes the bi-material three-dimensional lattice structure with the optimum energy absorption and stiffness, in comparison to those usual lattice structures with a single material.

In this paper, the strength analysis of composite sandwich structures under buckling load as well as composite polyurethane foam core in composite sandwich structures has been studied experimentally. For better performance of the shell and core in the sandwich structure as well as to increase their strength against buckling compressive load, a reinforcer with new geometry of hourglass and square with foam and without foam has been used. Having a negative Poisson's ratio and greater flexural stiffness as well as maximum load bearing capacity is the reason for using hourglass geometry. The new VARTM method has been used to create better and more uniform quality samples. The results of this study show that corrugated sandwich structures with hourglass filled with polyurethane foam withstand more load against buckling load and also have good efficiency in energy absorption. In a foamless composite sandwich structure, the core is distorted (separation of the shell from the core), and local buckling occurs due to the empty core. While in the sandwich structure with foam, due to the presence of foam in the core, the distortion of the structure is prevented. Also, despite its light weight, foam is able to withstand the compressive load due to local buckling. As a result, the structure has only suffered general buckling.

In this study, a biodegradable composite by ML506 as matrix and kenaf fibers as reinforcement was designed, manufactured, and finally, its mechanical properties were investigated. The test samples were manufactured by the Vacuum-assisted resin transfer molding (VARTM) method. To determine the mechanical properties, a tensile test and three-point bending test were performed. Young modulus, yield stress, ultimate stress, maximum elongation, and toughness were obtained from the tensile test. Flexural strength and flexural chord modulus of elasticity were obtained from the three-point bending test. To determine the electrical properties, a dielectric constant test in X-band was performed. The loss tangent, as well as composite products, were obtained. The maximum tensile strength and tensile modulus were achieved in this research respectively equal to 62.42 Mpa and 4.43 Gpa. The flexural strength and flexural modulus were achieved in this research respectively equal to 99.19 Mpa and 5.619 Gpa. The dielectric constant and loss tangent of the samples produced in the X -band were 4.74 and 0.053, respectively. Finally, to validate the results in this research, the result of the tests were compared with the result of the other similar studies.

In the present study, the effect of natural microfibers (cork particles) on the mode I fracture toughness of plain-woven laminated composites has been investigated. For this purpose, double cantilever beam (DCB) specimens manufactured using hand lay-up method with stacking sequence of [0]28. To investigate the effect of cork particles on fracture toughness, samples with two different weight percentages (1% by weight and 3% by weight) were manufactured and the experimental results were compared with one obtained from sample with pure epoxy resin. Experimental results show that as the amount of cork particles increases, the onset of crack growth requires more energy. The amount of improvement in initiation fracture toughness for the DCB sample with 1% and 3% cork weigh has been increased by 67.15% and 71.96%, respectively which is due to the role of the cork in the resin rich area near the crack tip that arrested the delamination growth. Unlike the initiation fracture toughness, the propagation value is reduced by adding cork particles to the resin.   During delamination growth, due to the agglomeration of micro fiber at delamination interface and role of stress concentration of these particles, hence, micro-cork fibers have not been able to increase the propagation fracture toughness and in some cases have slightly reduced the propagation fracture toughness of the delamination. Also, in order to investigate the mechanisms of damage, the fracture surfaces of the samples were scanned using scanning electron microscopy.

Due to the impossibility of performing destructive tests, the need for new and Non-Destructive Tests (NDT) in this area is strongly felt. In the present study, using the non-destructive method of Automated Ball Indentation (ABI) test method, some mechanical properties of 4340 annealed steel and 7075 raw aluminum and prepared from aged aircraft are determined. To perform calculations related to ABI method, the explicit relations routine was coded in FORTRAN and verified. Then samples of uniaxial tensile test of 4340 annealed steel and 7075 raw aluminum and prepared from aged aircraft are prepared and their stress-strain curves are obtained. Also, ABI tests are performed on 4340 annealed steel samples and 7075 raw aluminum and prepared from aged aircraft and the results of the two methods were compared. The comparison of results showed that the calculated error of yield strength and ultimate strength was less than 9 and 14%, respectively. With increasing force, this value in some areas reached less than 0.7%.

Unlike other materials, composite laminates have more uncertainty sources, such as uncertainty in the material properties; hence it is very essential to predict the probability of the occurrence of various types of damage modes in composite laminates due to random behaviors of composite structures. Matrix cracking induced delamination (MCID) is one of the catastrophic modes that can occur in composite laminates. In this paper, a new algorithm for probabilistic prediction of MCID based on the concept of energy release rate and its critical value was developed and by applying this proposed framework, the reliability analysis of MCID damage was performed. The limit state function was formulated using a general failure criterion which was developed by the authors of this article, previously. To represent the performance of the proposed algorithm, the probability of the occurrence and growth of MCID in a quasi-isotropic laminate including 45°, 90°, -45° and 0° plies under different loading conditions and various stacking sequences was extracted by using first and second order reliability methods (FORM and SORM),. The verification of the obtained probabilities was performed using Monte Carlo simulation (MCS). In addition, some significant results were validated using several experimental data, qualitatively. The effect of variables such as the ply thickness, the level of longitudinal uniaxial stress and the presence of general in-plane stresses on the probability of occurrence and growth of MCID was investigated.

Composite lattice structures are widely used in the aerospace, automobile and marine industries due to their benefits such as high stiffness and strength with light weight. In this study buckling strength after impact of grid stiffened composite panels was investigated experimentally. For this purpose, grid stiffened composite panels with iso-grid reinforced lattice and three types of skin thickness of 6, 12 and 18 layers were designed, fabricated and tested. E-glass fibers and epoxy resin was used for both ribs and skin. At first, the specimens were subjected to a low velocity impact and then the buckling test was carried out. From results it is concluded that with increasing the thickness of skin, failure mechanisms such as fiber breakage, delamination, matrix cracking consumes more energy. But, by considering the weight, the buckling strength per weight of the specimen was maximum for the specimen with 12 layers for skin. More explanation about failre mechanisms are included in the manuscript.

In this study, by using an energy-based method, growth of induced delamination due to matrix cracking has been studied in symmetric composite laminates subjected to constant in-plane stresses as well as thermal stresses. Two unconstrained and generalized plane strain states have been analyzed here. Matrix cracking as a primary assumption has been supposed in both states and the impact of matrix cracking and delamination on the stiffness degradation is calculated. Then, some thermoelastic constants are proposed. By relating stiffness matrix elements using these constants, a simple equation due for energy release rate due to delamination formation is derived. The accuracy of developed relations is examined using ANSYS finite element software. The obtained results reveal that there is good agreement between the extended and FE approaches

Due to the mismatch of mechanical properties in composite laminates, propagation of delami-nation is considered as a severe damage mechanism in beams with various lay-up configurations. Delamination can be generated due to matrix cracking propagation or it can also be initiated due to the manufacturing process before using composite beams. Using a micromechanics model, this study is aimed to investigate the bending moment required for matrix cracking and induced delamination in cross-ply composite beams. To that end, a unit cell is selected from the lamina surface in a composite beam containing matrix cracking and delamination. Later, the governing equation of stress and displacement fields are extracted in the unit cell to calculate the strain energy release rate due to the propagation of matrix cracking and induced delamination.  In order to validate this method, the stiffness variations in Carbon-Epoxy cross-ply laminate [90/02]s is examined and the obtained results are compared with the numerical results. It concluded that there is a favorable agreement between the results of the proposed micromechanics model and available numerical results.

Sandwich structures are made of two thin skin with high mechanical properties and a thick core that have weak strength. Due to high strength and stiffness to weight down, sandwich plate are used too much in engineering structures such as ship hulls, turbines, etc. In this paper, the effect of skin/core debonding on the dynamic response of sandwich structures with composite skins and a PVC foam core is investigated experimentally and also numerically. The separation size affects strongly on the natural frequency so that with a reduction in structural stiffness due to the separation zone between the upper skin and the core occurs, the natural frequencies decreases. Increase the length and depth of separation reduces the local stiffness and thereby reduces the natural frequency of the structure. The composite skins are bonded to the foam using VIP (Vacume Infusion Process) method because of VIP method can make more qualify specimens. In order to validate the obtained results, the results of modal test and also the effect of separation length on the natural frequencies and mode shapes in ABAQUS software are used.The results show good agreement between the numerical and experimental tests respectively

High ratio of strength to weight in carbon/epoxy composites causes to their applications in several structures, especially aerospace structures. In addition, to enhance the reliability in such structures, investigating damages in composites is essential. One way to detect defects in composites is to utilize the acoustic emission approach. Thus, the objective of the present research is to find failure mechanisms in open-hole laminate composite specimens with 〖[0_3/〖90〗_2/0_2]〗_s layup under cyclic loadings at different displacement amplitudes, using the acoustic emission. First, the standard specimen was examined and elastic waves due to failures in the specimen were detected by acoustic emission wide-band sensors. Two methods have been utilized to detect the failure percent, including Pocket wavelet transform and Fuzzy clustering approaches. Results from these methods were compared to micro-structure images by the scanning electron microscopy. Obtained results in this research indicated the appropriate efficiency of the acoustic emission approach to detect the type of failures and their percent in laminate composites.

Studying the behavior of composite materials reveals that various types of failure modes occur when material experiences different loading conditions, which may have a significant impact on performance and properties of a structure. In this research, we study the mechanical response of orthogonal multi-layers by considering different failure modes at micro-scale and their development in macro-scale. For this purpose, the effect of the emergence and growth of fiber separation and subsequent formation of matrix cracks are investigated in the micro-scale. Furthermore, interlayer separation caused by leaving the matrix are studied in macro-scale. To model the separation of fiber matrix which is the first dominant failure mode, the sticky area method is used. The model verification and obtained results are compared with the previous research. Then, XFEM method is used to take into account the failure mode of matrix. Finally, using of the sticky area method, we are able to simulate the separation of matrix layers. The FE-program Abaqus via its user scripting interface (Python) are employed in this research for modeling of fibers embedded into matrix.

Matrix Cracking and induced delamination in symmetric cross-ply, [0m/90n]s, and angle-ply, [θ/-θ]s, laminated composites under axial and shear loading are studied by multi-scale micro-meso approach. The stress transfer model is implemented to predict the stress and displacement distribution incorporated with the ply-refinement technique in laminates. By considering crack closure condition, a series of useful inter-relationships between thermo-elastic constants for damaged and corresponding undamaged laminates are derived. By using both of stress and displacement fields and these inter-relationships the properties of damaged laminated structure are achieved. These obtained properties are used to calculate the energy release rate in load control condition for the initiation and growth of matrix cracking and induced delamination in cross-ply symmetric laminates with [0m/90n]s layup. The obtained results of mechanical properties degradation show a good agreement with the results of experiments, variational approach and finite element methods (FEM) existed in literature. By comparing the results of the energy release rate of single-layer method with variational approach and finite element methods, it was observed that the results of proposed single-layer have a considerable error due to differences in slop of stiffness versus crack density.Finally, the accuracy of single-layer micromechanical approach in predicting the mechanical properties and inaccuracies in predicting of energy release rate was confirmed

Sandwich structures are consisted of two thin skins with high mechanical properties and a thick core with lower mechanical properties and weight. Due to high strength/ stiffness to weight ratio, these structures are used extensively in engineering structures such as aerospace structures, ship hulls, turbines blades, etc. Skin/core debonding is one of the major failure modes in these structures. In this paper, debonding resistance of sandwich panels with composite skins and a core consisted of PVC foam and a corrugated composite laminate is investigated both experimentally and numerically. Square geometry is considered for corrugated composite laminate and obtained results are compared with reference specimen with simple core made of PVC foam. The three point bend test with attached ENS fixture is used to perform the standard experimental test. The results have shown that in square specimen with 3 and 6 layer skin before the separation between skin/core, the specimens are failed from the middle of the upper skin, but for 8 layer skin, the skin/core debonding are accured before other modes of failure. The maximum skin/core debonding resistance for square specimen are increased 269.26 percent. Specimens are modeled in Abaqus and results show a reasonable agreement between experimental and numerical result.

In this study, the longitudinal and hoop tensile strengths of an industrial ±55° Glass Reinforced Epoxy (GRE) pipe with eighteen layers as well as the associated failure mechanisms are determined. To obtain the longitudinal and hoop tensile strengths values, three specimens are cut from the studied GRE pipe in each direction. A comparison is done between both the strength values, and the fracture pattern of the specimens is studied. Determining the different failure mechanisms which are created during both of the tests, the acoustic emission technique is used. The acoustic emission energy as an important damage parameter in determining the different failure mechanisms of the specimens is depicted for both of the tests and is related to the obtained results from the stress-time curve. A high magnification camera is used to verify the failure mechanisms characterized by the acoustic emission method.

One of the most important issues about the composites behavior in different loading conditions is the initiation and propagation of various damage modes that have significant effects on the application of these materials. Fiber/matrix debonding is one of the first damage modes that appears in different composites and causes the formation of other damage modes like matrix cracking. In the present study, by using the cohesive zone model (CZM) as well as an extended finite element method (XFEM) and by applying a transverse loading on different representative volume elements (RVE’s) in micromechanical scale, the effects of initiation and propagation of different damage modes like fiber/matrix debonding and matrix cracking will be studied. To this aim, the authors start by studying the behavior of cohesive zone model and validating the applied method by simulating the previous researchs. Then, the effects of cohesive zone on different volume elements will be studied and the results will compare with each other. Finally by entering the effects of matrix cracking initiation and propagation using the extended finite element method, effects of cohesive zone damage and matrix cracking will be studied simultaneously based on finite element method and using Abaqus software

In the present study, using a precise shear lag parameter in an extended shear lag model, by considering the effects of out of plane shear stresses, the stress fields distribution as well as strain fields and displacement distributions will be obtained for a typical [0m/90n]s cross ply composite laminate containing a specified matrix cracking density. Then, the stiffness degradation due to existence of matrix cracking in these cross-ply composite laminates will be evaluated and specific damage parameters, which affect the stiffness matrix of composite ply, will be defined. Furthermore, using the concept of fracture mechanics by applying two different criteria including the maximum stress and strain energy release rate, the matrix cracking initiation and evolution as well as induced delamination propagation will be studied. Finally, a closed form relation will be presented which predicts the evolution of matrix cracking under uniaxial loading conditions in cross-ply composite laminates. At last, the obtained results by present study will be compared with available semi-analytical and experimental results. The obtained results reveal that the proposed closed form relations by the authors have a less difference with experimental results in comparison with the previous semi analytic results.