Computations and Simulations in Mechanical Science
Volume:1 Issue: 2, Summer and Autumn 2018

In this paper, the prediction of fracture in 7075-T6 aluminum alloy is considered with focus on Lode Angle and Triaxiality effects. uniaxial stress-strain curve of 7075-T6 aluminum alloy are investigated experimentally for two different conditions; specimens with shock loaded pre-mechanical working and without pre-mechanical working. The experiments are done in order to compare variation of microstructures and material behaviors under these conditions. Also in this research, effects of shock loads on the generated defects are considered as well as material corrosion using metallography and Scanning Electron Microscopy (SEM). Results indicate that the pre-mechanical shock loading increases amounts of micro-mechanical strength and decreases the corrosion resistance of the material. In addition, using experimental data at different shock loads and the Hosford-Coulomb model, fracture locus of Al-7075-T6 are predicted in terms of the stress state parameters and plastic strain. The result shown good prediction of strain fracture for 7075-T6 aluminum alloy.

This paper considers biaxial buckling behavior of single-layered graphene sheets (SLGSs) using molecular dynamics (MD) and nonlocal elasticity theory.  Based on the MD simulations, Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS), an open source software, is used to obtain critical buckling loads. On the other hand, governing equations are derived using nonlocal elasticity and classical plate theory (CLPT) and solved using generalized differential quadrature method (GDQ). The small-scale effect is applied in governing equations of motion by nonlocal parameter. The effect of different side lengths, boundary conditions and nonlocal parameter are inspected for aforementioned methods. Results are obtained from MD simulations is compared with those of the nonlocal elasticity theory to calculate appropriate values for the nonlocal parameter. The nonlocal parameter value is suggested for graphene sheets with various boundary conditions.

In this paper, the gains of PID controller are obtained online using Model Predictive Control (MPC). In fact MPC tries to tune the PID-controller parameters by predicting system’s behavior in some time steps ahead. In this way, the nonlinear differential equation of system is approximated by a linear polynomial with unknown parameters. These unknown parameters are obtained using genetic algorithm to minimize the deviation between the real plant and approximated model. Moreover, multi-objective approach has been used to capture the parameters of MPC which are prediction horizon, control horizon and weight factor to minimize simultaneously two objective functions that are control effort and Integral time absolute error (ITAE) of the system response. Results mentioned at the end, obviously declare that the proposed method surpasses conventional MPC and PID-tuning method.

Buckling in cylindrical shells has been a major issue for researchers for more than a century. Cylindrical shells are often used in the production of aircrafts, racks, boilers, pipelines, cars, and some submarine structures. These structures may experience axial compression loads in their longevity and yield to buckling. Furthermore, these structures usually have disruptions, such as cutouts, which may have adverse effects on their stability.  In the present paper using finite element method, the buckling of cylinders with circular cutout of AA508 aluminum alloy under loading is investigated. The effect of some geometric parameters such as cutout position and cutout size on the critical load of buckling of these shells was studied. According to the results, with increase in the diameter of cutout the critical load of buckling sharply decreases. The results of the numerical analysis are verified by a series of experimental tests.

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.

With their special airfoil geometry, the blades adopted in hot section of gas turbines are highly susceptible to the damages induced by extreme thermal and mechanical stresses. The blades are usually made of super expensive super alloys. The present paper is an attempt to investigate the feasibility of using Functionally Graded Material (FGM) for the blades, and to study thermomechanical behavior of FGM blades and the influence of exponential function parameters on such behavior. On this basis, beginning with modeling the blades adopted in hot section of a gas turbine and applying centrifugal and high-temperature conditions developed as the gas flow hits the blade, the strain resulted under such conditions and the changes in FGM properties along the blade thickness upon varying different parameters were studied using finite element method. The results indicated that, FGM coatings, as compared to homogeneous material, can appropriately control the blade behavior under particular temperature conditions.