فصلنامه مکانیک هوافضا
سال نهم شماره 4 (پیاپی 34، زمستان 1392)

The element-free Galerkin method which is enriched intrinsically is applied for fracture analysis of functionally graded materials under mode I steady-state and transient thermal loading. The stress intensity factors are evaluated by means of both equivalent domain integral and displacement correlation technique. Continuum functions and the micromechanical model are used to describe the distribution of material properties. For thermal shock analysis, the modal decomposition method which is a semi-discretization approach is implemented to obtain the transient temperature field. The accuracy of numerical results is verified using the available reference solutions. Also, few parametric analyses are performed to study the effect of material gradation on the stress intensity factors. The results imply that the magnitude of the stress intensity factor reaches to its peak at a short while after the thermal shock which indicates its significant role in the fracture failure.

In this paper, Abramowitc and Jones model for analyzing dynamic progressive buckling of circular tubes under axial loading have been modified. In the improved model, the dissipated energy due to shortening of tube wall and the effects of inertia of previous folding layers on new one is considered. The mean crushing force and energy absorption, which is predicted by the theoretical model, has higher accuracy than the previous theoretical model. Based on the results of this paper, it is observed that the impact velocity and the mass of striker (neglected in pervious analytical models) are important in mean crush force and absorb energy of circular tube. Also in this research, it has been observed in the experiential tests that in most of the aluminum specimens, the high impact speeds, dynamic progressive buckling and in some samples in which the speed of impact was lower, dynamic plastic buckling happened. In this paper, final reduction in axial length, energy absorption and dynamic average crushing force in specimens in which dynamic progressive buckling has been occurred, is compared and discussed. Based on the results extracted in this thesis, it has been specified that theoretical results have acceptable consistency with experimental results.

In the present paper, nonlinear analysis of lateral deflection of bidirectional functionally graded circular plates whose material properties vary in both transverse and radial directions is presented. The governing equations of the plate are derived based on the classical theory and von Karman’s nonlinear strain-displacement relations. Due to bidirectional variations of the material properties, influence of the radial derivatives of the material properties and the transverse variations of the properties on the rigidities have appeared in the governing equations. The governing equations are solved by the finite difference method. Incorporation of the finite difference form of the boundary conditions on various points of the boundary is also discussed. Finally, behaviors of plates fabricated from isotropic, transversely-graded, and two-directional-functionally graded materials are investigated and present results are validated against results of the special cases reported in the available well-known references and results of Abaqus software.

In this paper, mechanical buckling analysis of open circular cylindrical shell reinforced by single-walled carbon nanotubes subject to axial loading is studied. Based on the first order shear deformation theory, the equilibrium and stability equations have been derived using the total potential energy equations and Euler equations. To estimate the material properties, the rule of mixture has been used. The governing equations are solved by considering the boundary conditions of problem and the effects of geometrical parameters and material properties on the critical buckling load have been studied. In order to validate obtained results, comparison study with other available literature has been carried out.

Ductile failure is one of the subjects that many researches on which have been done, up to now. X100 pipeline steel is one of the recently developed materials for gas transfer pipe production. Regarding the metallurgical structure and different treatments, this material has a correct balance between strength (yield stress of 690MPa) and toughness (150% true strain at failure in simple tension). Moreover this material is severely anisotropic in plastic deformation. In this article Continuum Damage Mechanics (CDM), which is a new method with high capabilities, is used to predict the failure of X100 steel. At first the CDM model is modified to take into account the effect of anisotropic plasticity (Hill quadratic plasticity), then according to damage formulations, an FE subroutine is developed to model the failure behavior of this material. Material parameters which are needed in damage and plasticity models are extracted from a series of experiments. Experiments and simulations on smooth and notched specimens show that this model has a good capability to predict the failure in different loading conditions.

V notches are one of the most possible cases of initiation of crack due to the stress concentration. On the other hand study of stress field around notches and cracks is very complex. In this study Digital Image Correlation method was used to obtain displacements fields around V notches with a crack within PMMA specimens. First stress intensity factor was calculated using the data extracted from DIC and the relation between displacement fields and SIF from William’s equation and use of linear least square technique. Since in the defined problem crack length and V notch depth have the most considerable effects on SIF, effect of these parameters on the results was investigated. Finally experimental results were compared with those obtained from FE and good agreement was observed.

In this paper, the ballistic performance of Kevlar plain-woven fabric impregnated with a colloidal shear thickening fluid (STF) is investigated. STF is composed of silicon dioxide nanoparticles and polyethylene glycol and is a non-newtonian fluid behavior defines as the increase of viscosity with the increase in the applied shear rate. The ballistic limit and specific ballistic energy of double and quadruple ply fabric systems impregnated with 15, 25 and 35 wt% STF particle concentration are compared to neat fabric. Results show those targets with 35 wt% STF particle concentration yield the highest ballistic limit for double and quadruple ply fabric. The double ply system with 35 wt% STF particle concentration showed the greatest improvement in specific ballistic energy over neat double ply systems. High speed photography showed that the neat Kevlar experience more localized deformation on impact. A finite element model was created using Ls-Dyna software and it was shown that the increased ballistic performance for shear thickening fluid impregnated Kevlar fabric is at least due to the increased friction between projectile and target.

Composite materials are used frequently in industrial structures such as airplane and space vehicles. Strength and probability of failure is an important challenge for designer engineers. In this paper a practical method to determine ultimate strength of a composite laminate is explained, this method based on reliability prediction. Strength of each layers and total strength is calculated using First Order Reliability Method and failures sequence is determined by Branch & bound method. In this study two modes for failure are assumed: fiber fracture and matrix fracture. At first step, stress distribution is calculated by FE software (ANSYS Package) then according to failure mode and first order reliability method, reliability index is determined. At the second step, probability of failures mode in all layers are determined. At the end, the important sequences for the failures are determined using branch and bound method. Comparison of this method result with Monte Carlo simulation shows accuracy of this method to predict failure mode and probability of composite laminate failure.