فصلنامه مکانیک هوافضا
سال هشتم شماره 1 (پیاپی 27، بهار 1391)

In this paper a general finite element formulation, using a modified augmented Lagrangian (AL) optimization method, has been developed to analyze the quasi-static frictional contact behavior of nano-indentation procedure.Based on the geometrical kinematic conditions of contacting parts, a novel improved AL approach has been used.This novel solution procedure for contact formulation overcomes the difficulty of dependence of the solution process on two user-specified parameters, i.e. the penalty parameter for contact constraints and the number of load steps. For overcoming the numerical difficulties of variational equality formulations, a new non-classical friction law has been used, in which the stick and slip conditions of classical law have been changed to a modified elastic friction law.Finally, the indentation procedure of rigid nano-indenters into an elastic layer has been evaluated to verify the results. The numerical algorithm in an applied example is accurately converged for stick and slip friction conditions and solution convergence is analyzed using automatic load stepping and contact load predicting.

In this paper the plastic deformation of single phase and two-phase polycrystal materials is studied both experimentally and by a FEM model computation, taking into account crystal plasticity. A fully coupled constitutive elastic-plastic crystal plasticity model is developed and implemented in the finite element model to define the single crystal material.A polycrystal lattice as an aggregate of single crystal grains with various orientations is both simulated and analyzed by means of unit cell method. The user-subroutine has been written for single crystals materials. Many experimental tests of polycrystal materials have been performed to trust the results of simulations. The stress versus strain dependence as obtained from this FEM-model appears to be in good accordance with experimental results.

Gas Turbines have wide applications in power plants and aerospace industries. Nowadays, based on welding of two different materials, some economic methods for repairing turbine blades have been developed. In this paper, a method, for determining creep life of a RTB (Repaired Turbine Blade) of two substances, based on transient FEM method had been developed. To find the effect of the difference between creep properties of materials of two substances RTBs, a simple 2D model under centrifugal forces was modeled. The base and filler metals were GH 4049 and Inconel 718, respectively. Creep is time dependent deformation of metals at elevated temperatures, therefore stresses are also time dependent. Hence, the life of RTB was estimated using damage fraction rule. These were performed using APDL ANSYS commercial software codes. To verification of the method, an experiment test has been established which defined the way critical elements behaved. In this experiment, pure lead was used as softer and Tin70%-lead30% was used as harder metal in creep. Experimental and numerical models were compared and good agreements between them were found. Rupture point was truly predicted and relative error was only 6% (traditional methods had 20% error). Results showed that the difference between creep properties of base and filler metals for supposed geometry and repaired location, leads to 90% decrement of designed life.

In this paper based on the plastic stress wave propagation theory, a new analytical model is presented for impact of the cylindrical projectiles on the rigid surface. In this analytical model, Johnson-Cook material behavior equation,Mie–Grueneisen equation of state, Johnson-Cook strain at fracture equation and kinematics equations of motion, are used. Also, the conservation equations of mass, momentum and energy in front of plastic stress wave are determined.For solving the governing equations, a computer program is developed. The values of stress, strain, strain rate,velocity and length and radius of rigid and deformed region of projectile were determined. Using new model proposed in this paper, the compressive stress at different plastic strain and strain rates were determined. The results of the new model have been compared with ABAQUS finite element simulations and available experimental results.It shows that the length and radius of deformed region of projectile predicted by the new model have good agreements with experimental results and ABAQUS FE simulations. Furthermore, the compressive dynamic stress is determined as a function of the final projectile dimensions and material behavior coefficients.

In this paper, linear elastic buckling analysis of rectangular composite sandwich panel with composite laminate faces and symmetric functionally graded material (FGM) core is presented. Unidirectional steady in-plane forces and simply supported boundary conditions are applied on the faces’ edges only. Hamilton principle and functional method are used to derive equilibrium equations, which are usually minimum functional of stored energy. In the mathematical formulation a new improved higher-order sandwich panel theory (IHSAPT) was used. First shear deformation theory (FSDT) is used for faces. Core displacement in various directions is modeled by polynomial function with indeterminate coefficients. It is assumed that the core is able to sustain shear and normal in-plane stresses. Temperature and humidity effects are neglected and the FGM core is modeled symmetrically. Finally, the effects of aspect ratio, thickness to side ratio, core materials’ modulus ratio, and various kinds of FGM distribution functions and powers on the critical buckling load are investigated. The numerical results show, that the effects of in plane stresses in the thick functionally graded cores on the critical buckling load cannot neglected. Also, the effects of type of FGM distribution functions and powers on the critical buckling load are considerable.

In this study effect of applying functionally graded core to decrease delamination of composite sandwich panel under transverse load is considered. The simply supported boundary conditions are applied on face's edges. Hamilton's principle and functional method are used to derive equilibrium equations. First, shear deformation theory (FSDT) is used for faces. Core displacement in various directions is modeled by polynomial with indeterminate coefficients.The material properties of FGM are functionally graded in thickness direction according to volume fraction power law distribution. It is assumed that core is able to sustain shear and normal in-plane stresses. Finally, the effect of FGM core on the displacement of sandwich panel under load in three directions are considered. the numerical results of present analysis has been compared with the ANSYS results slowing good agreements.

Rupture discs are used as high speed valves in gas guns and shock tunnels. In this article, grooved rupture discs are studied numerically as well as experimentally in order to design an aluminum diaphragm to be used in a gas gun. Instability and rupture of discs are the main concern of this paper. ANSYS and LSDYNA have been used in order to simulate deformation and rupture phenomena. Several diaphragms were constructed and tested in order to empirically determine rupture pressure. Several diaphragms with 0.3-1.7mm defect depth were made and their rupture pressures were obtained experimentally and then animpirical relation was developed. Finally, a semi-empirical relation was developed for grooved diaphragm, which was delivered from approximate burst pressure versus ultimate stress, diaphragm diameter, and groove thickness.

Crashworthiness and energy absorbers are including components used in some systems for energy absorbing and convert it into another energy. The purpose of this study is experimental and numerical investigation of collapse behavior of thin walled structure cylindrical tube with shallow spherical caps (combined thin walled structures) under quasi-static axial loading and studied effective factors on a collapse mechanism. In experimental approach,aluminum cylindrical with shallow spherical cap samples was made by the process of spinning. These samples are compressed between two rigid platens under quasi-static loading conditions and the collapse mechanism, the variations of crushing load and absorbed energy are determined. A numerical model is presented based on finite element analysis to simulate the collapse process considering the non-linear responses due to material behavior, contact and large deformation. The comparison of numerical and experimental results showed that the present model provides an appropriate procedure to determine the collapse mechanism, crushing load and the amount of energy absorption. The model is used to evaluate the effects of important parameters such as semi apical angle; height of the cylindrical region, and height of the spherical cap region on the mean crush load, energy absorption capacity, and collapse mechanism of cylindrical shells with shallow spherical caps. For example, results of this study can assist aerospace industry to reentry sounding rocket carrier payload based on combined structures.