فهرست مطالب مصطفی لیوانی

In this article, to investigate the severity of damage to the helicopter pilot under the crash impact, the numerical simulation of the actual helicopter cockpit seat has been done. To limit the complexity of the problem, a fully vertical impact was considered as a reference scenario for the assembly consisting of a collapsible helicopter seat, an energy absorption section, and a humanoid dummy. For this purpose, the three-dimensional model of the chair in the laboratory was studied in two states with and without the addition of the energy absorption mechanism. A honeycomb structure was used in the seat cushion section. The results of the research showed that the use of UPVC damper to test the crash impact with a certain mass based on biomechanical criteria, an average 80 % decrease in serious injuries to the skull and spine has been achieved. Also, based on the results obtained in this research, although the aluminum honeycomb structure meets the criterion of damage to the head and is within the permissible range, it has rejected the criteria of damage to the spine and permissible acceleration range. However, compared to the case without honeycomb, it has performed much better and has been able to absorb the energy of the crash impact.

In this study, the effect of material imperfection on the free vibration response of perfect and imperfect FG plate is studied. A new hyperbolic shear deformation function is presented in this paper. The new hyperbolic function is chosen in such a way that the degree of the function is reduced as much as possible, despite the sufficient accuracy, so that the calculation speed is greatly reduced. The properties of the FG plate varied along the thickness according to power law. The material composition in production process cannot be completely in accordance with the expected pattern, which leads to the production of imperfect FG material. The governing differential equations are derived using the Hamilton’s principle. The obtained equations were solved using the Navier method with simple boundary conditions. The effects of important geometric and mechanical parameters of perfect model and two types of imperfect model, including length to thickness ratio, length to width ratio, wave number and power-law exponent on natural frequency response of imperfect FG plate are investigated. To verification, the analytical results obtained in this study are compared with the results presented in the literature, and in this comparison, a good agreement was obtained, which shows the correctness theory, deriving and solving equations.

The experimental and numerical study on flexural behavior of new type sandwich structures with glass-epoxy skins and a combinatorial core consist of foam and corrugated composite with rectangular geometry was investigated in this paper. After sample production by vacuum assisted resin transform molding technique; samples were tested according to standard and then the related load-deflection curves were obtained. Finite element analysis was applied for determining maximum deflection of samples by ABAQUS software. In experimental three series of samples with combinatorial core and three different thicknesses for rectangular shape corrugated composite in a foam core and one series of sample with simple foam core; have been product. In order to experimental accuracy of problem; three same samples have been product in each case and the average data have been used for results. It has been shown that the flexural stiffness was increased with increasing in thickness; and the flexural stiffness to mass ratio was increased so on; but the rate of this increasing was decreased from 3 to 5 layers of corrugated composite laminate in PVC foam. Finally the experimental and numerical results were compared and a reasonable agreement between them has been observed.

In this study, the effect of FG porous on the free vibration behavior of sandwich plate with piezoelectric face sheets and FG core is studied. A new hyperbolic shear deformation function is presented in this paper. The properties of the FG core varied along the thickness according to power law. The properties of the FG material change with the power distribution as a function of the changes in properties along the thickness. The structure of porous materials has cavities and pores. Therefore, in this research, a linear porous model and two types of nonlinear porous models are considered for the property change function. The governing differential equations are derived using the Hamilton principle. The obtained equations were solved using the Navier method for simple boundary conditions. The effects of important geometric and mechanical parameters including thickness to length ratio, length to width ratio, ratio of FG core thickness to piezoelectric face sheets and the effect of electrical potential on the natural frequency response of porous FG sandwich and piezoelectric face sheets have been investigated. To verification, the analytical results obtained in this study are compared with the results presented in the literature, and in this comparison, a good agreement was obtained, which shows the correctness theory, deriving and solving equations.

In this study, the effect of material imperfection on the free vibration response of FG sandwich panel with piezoelectric face sheets is studied. A new hyperbolic shear deformation function is presented in this paper. The properties of the FG core varied along the thickness according to power law. The material composition in production process cannot be completely in accordance with the expected pattern, which leads to the production of imperfect FG material. Hence in this research, one perfect model and two types of imperfect models have been considered. The governing differential equations are derived using the Hamilton’s principle and solved using the Navier method. The effects of important geometric and mechanical parameters of perfect model and two types of imperfect model including thickness to length ratio, length to width ratio, FGM core to piezoelectric layer thickness ratio and electrical potential on natural frequency response of imperfect FG sandwich panel with piezoelectric face sheets are investigated. To verification, the analytical results obtained in this study are compared with the results presented in the literature, and in this comparison, a good agreement was obtained, which shows the correctness theory, deriving and solving equations.

 In this paper, buckling analysis of composite sandwich cylinders with transverse flexible core is investigated using an improved higher order sandwich panel theory (IHSAPT). The face sheets of sandwich cylinder are made of laminate composite that lay up symmetric and anti-symmetric. The case study is under axial compression load and external pressure. The first order shear deformation theory and second Frostig’s model is proposed for deriving displacement field of face sheets and core, respectively. Lagrange’s principle considering initial stresses is used for extracting governing equations. The Buckling problem of the shell has been numerically solved by using the Galerkin’s method. In this research, the effects of some important parameters including core radius to cylinder thickness ratio, length to core thickness ratio, face sheet to cylinder thickness ratio and length to core radius ratio on the buckling response of composite sandwich cylinder are studied. The obtained results indicate that by decreasing face sheet to cylinder thickness ratio and length to core radius ratio, and increasing core radius to cylinder thickness ratio buckling load of composite sandwich cylinder are increased.

In this paper, static strength analysis of superalloy Rene-80 is studied experimentally in both of the coated and uncoated cases. Superalloy Rene-80 is widely used in manufacturing aircraft turbine blades. The service temperature of this alloy is in the range of 760–982 °C. Although this superalloy has good mechanical properties and acceptable protection against oxidation and hot corrosion, it is coated to improve its resistance to surface degradation factors such as oxidation, hot corrosion and erosion at high temperatures. In this paper, penetrating aluminide coatings were applied to this superalloy with two separate methods of penetration-powder and penetration-slurry, respectively, under the brand names Codep-B and IP1041slurry, respectively and the effect of these coatings on the tensile properties of Rene-80 under ehe temperature range of 25-982 ° C was investigated. For this purpose, the samples according to ASTM-E8 standard are produced and after coating (the two methods mentioned above) along with uncoated samples are carried out a tensile testing in accordance with the ASTM-E21 standard. The results of the present study showed that at low operating temperatures, uncoated samples have better tensile strength than coated samples, but this behavior is reversed at high operating temperatures, which indicates the importance of using nickel-based superalloy coatings for extreme heat environments. Also, the model with IP1041slurry coating at high operating temperature has better yield and ultimate strengths than powder penetration coating.

Ring rolling process is used in the production of seamless rings.The most important advantages of these rings are favorable grain flow and crack growth resistance. Aircraft engine spools are usually produced by means of this process. Failure of the work rolls during forming of spools leads to unacceptable geometrical structural and irreparable damages. Evaluation of the work rolls life of the ring rolling machine has a direct effect on the properties of the output ring. Initially, different types of life estimation and cumulative damage assessment methods were studied. By making use of Abaqus simulation, the loading history was extracted and applied to the work rolls. According to the loading history, stress-life method and cumulative damage relations were performed. The results of the research shows that the main roll is significantly affected by mechanical stresses. On the other hand, the mandrel is affected by thermal stresses by applying the thermo-mechanical forces. In general, in order to extend the work roll’s life, of the work rolls and reduce the effects of the thermo-mechanical stresses, the uniformity of temperature during work rolls operation should be maintained. So that proper cooling will have a significant effect on the service life. In addition, the effect of bending stress on the mandrel life is not significant.

In this paper, the creep strength of Rene 80 superalloy in both uncoated and coated states is studied experimentally. The operation temperature range of this superalloy is 760–982 °C, in order to increase its resistance to surface degradation factors such as oxidation, hot corrosion and erosion at high temperatures, a coating is applied on it and its use without coating is not recommended. In this paper, diffiosion aluminide coatings were applied to this superalloy by two separate methods of pack cemantation and slurry under the brand names of Codep B and IP1041, respectively, and the effect of these coatings on the creep life of this superalloy at 982 °C and metallography is examined by SEM was investigated. For this purpose, samples according to ASTM-E8 standard were produced and after coating along with uncoated samples were subjected to creep test according to ASTM-E139 standard. Control parameters including temperature, stress and service life of uncoated samples are evaluated according to C50TF28 standard. Uncoated creep samples lost their strength after an average of 33 hours and rupture occured. Codep-B coated samples lost their strength after on average of 45 hours and slurry-coated samples lost their strength after on average of 51 hours. The results showed that the coatings increased the creep life of the coated samples compared to the uncoated samples by at least 12 hours.

In this research, for the first time, the free vibration of the functionally graded beam containing transverse crack under clamped-clamped boundary condition studied based on Reddy high order theory. It was assumed that mechanical properties of beam vary exponentially in the thickness direction. To drive the closed form solution for natural frequencies of functionally graded beams containing a transverse crack, Ritz method was used. The effects of slender ratio, the location of crack, depth of crack and material gradients on natural frequency were also examined. Finally, the results of the analytical method were analyzed and compared with results of modeling by ABAQUS software and other studies. This research shows that due to more complex equations of Reddy beam theory, using Reddy beam theory for a thick beam and Timoshenko beam theory for a thin beam are more suitable. In addition, a relation between the depth of crack and the necessity of using functionally graded beam is confirmed.

In this research, the free vibration analysis of doubly curved composite sandwich panels with variable thickness is studied using higher order sandwich panel theory. For the first time, considering different radii of curvatures of the face sheets in this paper, the thickness of the core is a function of plane coordinates (x,y). In addition, in the current model, the continuity conditions of the transverse shear stress, transverse normal stress and transverse normal stress gradient at the layer interfaces, as well as the conditions of zero transverse shear stresses on the upper and lower surfaces of the sandwich panel are satisfied, which is unique. The formulation is based on an enhanced higher order sandwich panel theory and the vertical displacement component of the face sheets is assumed as a quadratic one, while a cubic pattern is used for the in-plane displacement components of the face sheets and the all displacement components of the core. The equations of motion and boundary conditions are derived using the Hamilton principle. The effects of some important parameters including composite layup sequences, length to width ratio, varying properties of the face sheets materials, Face sheet thicknesses ratio and varying materials of the face sheets were investigated. The results are validated by the latest results published in the literature.

In this paper, the three dimensional bending analysis of a multi-directional functionally graded annular sector plate is considered. The material properties are assumed to be graded in all three spatial directions following introduced power law functions. Governing Equations and boundary equations are derived based on three-dimensional theory of elasticity. The differential quadrature method is employed to discretize and solve the governing equations. Numerical results for different gradient indices are calculated. The results obtained from this research show that the direction of material variation has a noticeable effect on bending of an annular sector plate. In fact it indicates that the direction of material variation in a multi-directional annular sector plate can use as an effective variable for design optimization. Also the results of this paper can be used as benchmark for following studies. Moreover three dimensional bending analysis for an annular sector plate even for a common functionally graded plate (one directional FGM) has novelty and doesn’t considerate in literature. Comparisons are made with the solutions resulted by simulation in ABAQUS and show good agreement. Comparisons are made with the solutions resulted by simulation in ABAQUS and a good agreement was observed between the results.

In this study, weight optimization of the prismatic core sandwich panel under the transverse and longitudinal loadings has been independently investigated. To solve the optimization problems corresponding to the mentioned loadings, the Improved Constrained Differential Evolution (ICDE) algorithm based on the multi-objective constraint handling method has been used. The constrains of the problems are buckling load and yield stress. By comparing the results of the ICDE with those obtained by the other evolutionary algorithms based on the penalty function method in the previous studies, it can be seen that the results of the transverse loading obtained in this study are equal to those of the previous works, but the results of the ICDE in the longitudinal loading are better. In addition to its better results and ease of Implementation, another advantage of ICDE is that there is no need to the process of trial and error to fine tuning of the penalty function parameters. It can be observed that for the both types of loading, values of the panel height, the thicknesses of the face and core in addition to the weight index are increased by increasing the load index to avoid yielding and buckling of the panel. But in a constant value of the load index, by increasing the core corrugate number, the weight index has a lowering behavior in the transverse loading and a rising behavior in the longitudinal loading. However, the thicknesses of the face and core have a lowering behavior and the panel height has a rising behavior.

This study dealt with the flutter and biaxial buckling of composite sandwich panels based on a higher order theory. The formulation was based on an enhanced higher order sandwich panel theory that the vertical displacement component of the face sheets were assumed as quadratic one while a cubic pattern was used for the in-plane displacement components of the face sheets and the all displacement components of the core. The transverse normal stress in the face sheets and the in-plane stresses in the core were considered. For the first time, the continuity conditions of the displacements, transverse shear and normal stress at the layer interfaces, as well as the conditions of zero transverse shear stresses on the upper and lower surfaces of the sandwich panel are simultaneously satisfied. The aerodynamic loading was obtained by the first-order piston theory. The equations of motion and boundary conditions were derived via the Hamilton principle. Moreover, effects of some important parameters like lay-up of the face sheets, length to width ratio, length to panel thickness ratio, thickness ratio of the face sheets to panel, fiber angle, elastic modulus ratio and thickness ratio of the face sheets on the stability boundaries were investigated. The results were validated by those published in the literature. The results revealed that by increasing length to width ratio, length to panel thickness ratio and elastic modulus ratio of the face sheets, the stability boundaries were decreased and the largest nondimensional buckling load was occurred at the angle ply sandwich panel.

In this study, single-objective and multi-objective optimization of curved sandwich panel with composite face sheets and magneto-rheological core have been done to maximize the first modal loss factor and minimize the mass by using genetic algorithm. The studied sandwich panel was curved with simply support boundary condition. In order to derive the governing equations of motion, an improved high order sandwich panel theory and Hamilton's principle were used for the first time. The face sheet thickness, core thickness, fiber angles and intensity of the magnetic field have been considered as optimization variables. In single-objective optimization, the optimized values of variables were calculated. The results showed that the structures tend to have thick core and thin face sheets which seems physically true. As the magneto-rheological fluid placed in the core, it has a significant effect on the increasing of the modal loss factor. For the multi-objective optimization the Pareto front of optimal technique was presented. Then for the first time at this field, the set of optimal points are selected based on TOPSIS method and it was showed that in the case of similar size and mass, modal loss factor of double-curved panel is more than sigle-curved.