فصلنامه مکانیک هوافضا
سال دهم شماره 4 (پیاپی 38، زمستان 1393)

In this paper, dynamic analysis of doubly curved composite shells under low velocity impact is studied analytically. The governing equations based on the first-order deformation theory (FSDT) are derived for simply supported boundary conditions. The contact force history is predicted using two models of complete and improved spring-mass. Considering the displacement components as the doubly Fourier series, equations of motion shell and impactor are solved analytically. By writing code in Matlab softwar, using Galerkin method, the dynamic response of shell is obtained.0 In this investigation, the effect of geometrical parameters, such as curvature changes, aspect ratio (curvature length ratio), fiber orientation, mass and velocity of impactor, with constant impact energy, on the impact response of shell is determined by two models.

A convenient, experimental technique for evaluation of V-notch mode I stress intensity factor using the method of phase shifting shadow moiré is described and demonstrated. In this study, the governing equations that relate the elastic solution and phase shifting shadow moiré for mode I V-notch tip are derived in terms of material constants and the stress intensity factors. Then, a set of phase shifting shadow moiré experiments were conducted on a polymeric tensile specimen and fringe patterns were captured. Through utilization of image processing techniques, phase map of fringe patterns was extracted and by analyzing the phase map, required data for determination of stress intensity factors were obtained. The obtained stress intensity factors by using phase shifting shadow moiré method, demonstrates a good agreement with the FEM analysis of specimen.

In this article, it is attempted to present a study of the design, analysis and optimization of a projectile carrying a payload from the moment of loading until final release. Thus the study consists of the following: First of all, the ballistics of the projectile must be solved and then diagrams of pressure-time and speed of the barrel and the maximum accelaration exerted on the projectile must be considered. The barrel speed is the required parameter in the trajectory section and to analyze the structural section the maximum accelaration is considered. Next, the structure of the projectile in the shell is considered in terms of stress based on the maximum speed gained from the internal ballistics. Also, the minimum thickness of the shell is assessed considering that the structure of the projectile should not fail. In the next step it is attempted to strike a relative balance between the first two sections and finish the design cycle. Finally, the optimization of the design cycle in terms of the weight and thickness of the projectile is considered.

The purpose of this article is the conceptual design of a two-stage crew launch vehicle with side boosters. At first, in order to achieve a suitable design point, the statistical design technique is used, and then statistical design process is validated by using two degree of freedom simulation and doing energetic-mass calculations. Then, multidisciplinary design optimization approach is applied for initial conceptual design optimization. The preferred structure for multidisciplinary design optimization is all-at-once and Genetic Algorithm (GA) is used as the optimizer algorithm. In order to achieve more accuracy, Simplex method is employed using GA’s results as a combined algorithm. Having performed the optimization process, a mass decrease of about 4 tons from missile gross weight was attained with respect to normal simulation results, and as already known, the decrease in gross mass undeniably leads to a consequent decrease in the cost of producing and launching missiles.

In the present paper the penetration mechanism of the projectile in the fiber metal laminates are investigated experimentally. Glass/epoxy composite laminates and fiber metal laminates made from thin aluminum sheets and glass/epoxy composite in different thicknesses were fabricated with the use of hand layup method. Ballistic test were conducted on plates with the projectile velocity of 720 m/s. Steel plates were placed behind the composite and glare plates to measure the energy absorption capacity of them. With the use of an analytical- experimental method and measuring the deformation of the steel back plate after impact, the absorbed energy in composite and glare plates were determined. The energy absorption capacity of the composite and glare plates were compared. The results showed that in the same plate thicknesses, the energy absorption capacity of the glare plates are higher than composite plates. Finally with the help of experimental results an analytical method to estimate the required energy for penetration of the projectile into glare plates were developed.

One of the simple criteria for estimating the formability of sheet metals is use of forming limit diagram (FLD). Determining the amount of accuracy and the range of proper prediction of this criterion is one of the most important challenges of researchers and engineers. In the present research first, employing the forming limit diagram of St14 steel, number of practical experiments such as tensile test of standard, notched, and opened specimens, tube hydroforming and deep drawing processes are simulated, and possibility of crack initiation and fracture in them are predicted. Then, to evaluate the forming limit diagram damage criterion, the above experiments are also practically performed and the practical results are achieved. Finally, comparing the results of numerical simulations and empirical observations, the amount of accuracy and proper range for fracture prediction of sheet metals by the FLD damage criterion is revealed.

In this paper, development af a new analytical model for expressing the penetration of conical projectiles into fiber metal laminates is presented. For this purpose, the Paul&Zaid theory i.e. an analytical model for calculating the residual velocity of projectile after perforation of metal and Wen theory i.e. an analytical model for calculating the residual velocity and ballistic limit of laminate of composite in impact, are combined. Then the result of this new analytical model is compared with numerical results and a good correlation has been observed, comparing the analytical model presented in this paper to the numerical results.

In this paper, a new analytical model has been presented for energy absorption of aluminum-honeycomb sandwich panels under high velocity impact. In high velocity impact, striker with initial velocity penetrates into target and perforates it completely and exits by residual velocity. The panels consist of hexagonal honeycomb core sandwiched between two aluminum skins. In analytical model cylindrical projectile with flat ended have been considered. It is supposed that aluminum skins failure by mean resistive pressure. Energy absorption of honeycomb has been determined by wierzbicki model. Energy balancing equation has been employed for determination the ballistic limit and residual velocity of striker. The results of ballistic limit and residual velocity of striker computed by new model have good agreement with experimental results. Also the effects of projectile mass and diameter and cell diameter of honeycomb in energy absorption of sandwich panel has been investigated.