فهرست مطالب mehran kadkhodayan

In this work, mechanical vibration analysis of rotating bi-directional functionally graded Euler-Bernoulli nanobeams is investigated, which has not already been studied deeply based on the latest authors’ knowledge. Material properties vary along the thickness and axis directions based on power-law distribution. The nonlocal elasticity theory of Eringen (NET) is utilized for modeling small-scale effects. Different boundary conditions are considered as clamped-clamped (C-C), clamped-simply (C-S), and clamped-free (C-F). Governing equations and associated boundary conditions are derived based on minimum total potential energy, and the generalized differential quadrature (GDQ) method is employed for the solution process. Convergence and verification studies are accomplished to affirm this work, and in the continuation, the effects of various parameters, namely hub ratio, rotation speed, and power indexes along x and z directions on the dimensionless natural frequencies, are investigated. It is revealed that the decrement made by the different values of  in the natural frequency, parameter is more effective than the reduction caused by the , especially for the higher rotation speed.

The equal channel angular pressing process is one of the most effective severe plastic deformation processes to produce ultrafine grained steels metals. Also, the upper bound theory is one of the reliable theoretical tools to forecast deformation strain and forming load. In this research, analysis of equal channel angular pressing technique in an arbitrary channel and corner angles using upper-bound theory was performed and a general and user-friendly equation for predicting the forming force is proposed according to the geometry of process and elastic-plastic properties of work-piece material. By comparing the amount of obtained theoretical load with experimental forming force resulted from applying process on 7075 Al alloy, has been observed very good agreement between the results. This guarantees the reliability of the achieved general equation for equal channel angular pressing force. According to the results of the present research, experimental and theoretical forming load of equal channel angular pressing process on this material under conditions of channel angle 135°, corner angle 20°, billet diameter 10 mm, and billet length 90 mm were obtained equal to 48 kN and 55.04 kN, respectively. Furthermore, by increasing the channel angle from 60 to 150 under a constant corner angle of 20°, process load is decreased equal to 41.3% from 86.4 kN to 50.7 kN. In addition, by increasing the corner angle from 0° to 40° under a constant channel angle of 135°, the negligible reduction of a load equal to 2.5% was observed from 56 kN to 54.6 kN.

In this research, two new methods that improve the drawing depth of deep-drawing processes have been introduced. In the first technique, by creating ridges on the punch surface, the stress concentration is decreased on the blank near the punch edge, in turn increasing the drawing depth. The second method is based on the principle of reducing resistant force in the flange area between the die, the blank-holder and the blank that can decrease the required forming energy. By using the ridges on the flange surfaces of die and blank-holder, the contact surface is reduced, which in turn can decrease the force required for blank forming. The simulation results of finite elements are compared to the experimental data. It is found that the ridged punch may delay the blank rupture and significantly raise the drawability.

Equal channel angular pressing (ECAP) is one of the most effective processes to produce ultra-fine grain (UFG) and nano-crystalline (NC) materials. Commercially pure titanium (CP-Ti) has a significant potential to be used as a biomedical and implant material because it shows excellent biocompatibility properties. This material has the low static and dynamic strengths. By applying the ECAP process, the strength of CP-Ti could be developed. The elastic recovery during unloading or spring-back phenomenon is one of the most sensitive parameters in sheet and bulk metal forming processes. This phenomenon leads to some unfavorable geometrical and dimensional changes in the final products and it must be decreased. In this study CP-Ti of Grade 2 is ECAPed at the room temperature via a channel angle of 135° for 3 passes. The microstructural analysis and mechanical tests such as the tensile and three-point bending tests are all performed on the ECAPed CP-Ti. The microstructural evolution reveals that by applying the ECAP, coarse grain (CG) structure develops to UFG structure. Moreover, the results of the mechanical tests show that applying the ECAP significantly increases tensile and bending strengths of the CP-Ti. Investigation of springback in three-point bending of unECAPed/ECAPed CP-Ti is conducted by experimental and finite element simulation methods using the Abaqus software. The results of this study reveal that by applying the ECAP, spring-back values increase. Thus, to eliminate the disadvantages of springback phenomenon, this should be considered in design and manufacturing of products include bent made of ECAPed material.

Nowadays, fiber-metal laminate composites are highly used by designers due to their good mechanical properties and low weight particularly in aerospace industry. The Fiber Metal Laminate shows improvement over the properties of both aluminum alloys and composite materials. In this study, bending properties of these composites are investigated both numerically and experimentally. Since these multi-layers have a very limited formability and deformation of fibers is pure elastic, the fibers cause some kind of springback that is known as a defect. In the current study, six types of specimens are prepared with variable fiber angles and thickness of layers for bending test. In addition, the bending test is optimized through the Taguchi design experimental method, until the results become independent of die and press parameters. The effects of design and fabrication parameters of fiber- metal composite on the springback are investigated in detail. The results show that the springback increases linearly with increasing punch radius and with decreasing pressure forming due to the reduction of the plastic deformation. On the other hand, by increasing the punch speed, the springback increases slightly because forming by the high-rate punch speed increases the amount of elastic recovery in aluminum sheet and induces larger springback. Moreover, when the fibers are parallel to the bending axis and the thickness of outer layers increase, the bend radius and springback decrease. The obtained results are compared partially with the FE numerical results.

Equal channel angular pressing is one of the most effective severe plastic deformation processes for fabrication of ultrafine grained or even nanostructured materials. Among the metallic biomaterials, commercially pure titanium exhibits the best mechanical properties, compared with other alloys. In this study, the effect of work-piece cross section on the mechanical properties of commercially pure titanium produced by this process has been investigated. The work-pieces in two types of cross section(square and circular) are pressed one pass in the square channel with angle 120° at room temperature and effects of cross section on the forming load, grain size, hardness, strength and toughness was studied. Finite element simulation by using the ABAQUS software has been performed for forecasting the forming load, equivalent plastic strain and investigation of effects of geometry parameters of die channel on these. The simulation results have shown good agreement with experimental results. Through analysis of results, it is found that by using the work-piece with circular cross section at equal channel angular pressing process, not only decreased the required pressing load, but also significantly improved the mechanical properties of the materials such as hardness and strength as compared to using the work-piece with square cross section.

Given the significance of energy absorption in various industries, light shock absorbers such as honeycomb structure under in-plane and out of plane loads are in the core of attention. In this research an analytical equation for plateau stress is represented, taking power hardening model into consideration. The equation of specific absorbed of graded honeycomb structure with the locking strain and strain energy equation is represented. The structure made from five aluminum grades is simulated in ABAQUS/CAE for elastic-perfectly plastic and power hardening model, according to the results; numerical value of absorbed energy is compared to that of analytical method. A drop weight test on a graded honeycomb structure was performed. Based on the numerical simulation results, the experimental and numerical results showed good agreement. Based on the conducted comparisons, the numerical and analytical results are more congruent for power hardening model rather than elastic-perfectly plastic one. In the first step of optimization, by applying SQP method and genetic algorithm, the ratio of structure mass to the absorbed energy is minimized. In the second step, regarding the optimum value of parameters obtained in the first step, the material property of each row is changed. According to the optimization results, while keeping the mass of structure as constant, the structure capacity of absorbing energy is increased by 18% in the first step and 264% in the second model, compared to the primary model.

In this paper the three dimensional dynamic analysis and stress wave propagation in thick functionally graded plate subjected to impact loading is studied. Material properties (elasticity modulus and density) are assumed to vary continuously through the thickness direction of the plate according to a simple power law distributions and the Poisson’s ratio is assumed to be constant. The equations of motion are based on three dimensional theory of elasticity. The three dimensional Graded Finite Element Method (GFEM) based on Rayleigh-Ritz energy formulation and Newmark direct integration method has been applied to solve the equations in time and space domains. It is assumed that in dynamic loading the upper surface of the plate is subjected to a pressure load that varies linearly with time, and suddenly is unloaded at a specified time. This unloading acts as an impact loading. Afterward, the time histories of displacement through the thickness, stresses in three dimensions and velocity of stress wave propagation for different values of power law exponents, various boundary conditions and thickness to length ratios have been investigated. The obtained results are in agreement with available data in literature.

In this paper, a novel technique on friction aided deep drawing using tapered blank holder divided into eight segments is proposed to overcome defect of friction aided deep drawing using four segments tapered blank holder technique. A taper blank holder is designed to be of two parts: stationary part with 5 degree taper angle and moving parts divided into eight tapered segments. The main function of this tapered blank holder device is adopting the frictional force between the blank and the blank holder segments to work in the useful drawing direction. At first, the drawing mechanism of eight segments tapered blank holder technique and inflow of material in the flange portion of blank are investigated and compared with four segments tapered blank holder technique by ABAQUS software to show the merits of the proposed process. Then, the finite element analysis of springback is investigated by the ABAQUS software. Effect of different process parameters such as initial blank thickness, punch profile radius, blank holder force, friction coefficient and hardening models on springback prediction are studied. A successful deep cup with drawing ratio up to 3.67 can be produced without any defect by using this new technique only in one die set. The cost and time of die fabrication in this technique are less than the conventional deep drawing.

This paper investigates the forming of sheet metal forming under cycling loading by considering the Bauschinger effect. Different proposed plasticity models which can handle this kind of deformation process have been reviewed in details. For instance, isotropic, kinematic and combined forms in the linear and non-linear cases including the well-known Yoshida and Chaboche's models have been studied. The ability of the best proposed model has been examined in the forming of reverse cup drawing and the obtained results have been compared with some other experimental results. The normalized axial stress, von-mises stress and the punch forces for both first and second stages have been calculated for different materials and thicknesses.