فصلنامه مکانیک هوافضا
سال پنجم شماره 4 (پیاپی 18، زمستان 1388)

Application of functionally graded materials (FGM''s) to join materials with extremely different thermo-mechanical properties can reduce sharp interface stresses caused by weak chemical affinity and high residual stress concentration. So، if the functionality of gradient regions be designed properly، it can minimize the failure probability due to unstable crack propagation. It has been shown that if the material gradient is parallel to the crack، the asymmetric stress field results in mixed-mode fracture and crack deviation for its initial direction. Mixed-mode stress intensity factors (SIF''s) and phase angle are computed by finite element method for a four-point-bending problem assuming functionally and multi-layered gradient regions. In order to obtain an optimum design، the effect of slope and breadth of gradient region، crack geometry، and residual stress field on fracture parameters (including SIF''s and phase angle) are studied. Minimum value of complex stress intensity factor is obtained per 2. 25 for profile power and 9 mm for width of gradient region.

In this article، three-dimensional temperature distribution in polymer matrix composites، exposed to fire is studied. Considering the variation of the thermal properties of composites during the heating process، equations are solved using a finite difference method. Also، strong variation of temperature through the thickness of composites is observed. More heat transfer in the longitudinal direction is observed than that of the transverse direction. A semi-empirical model for simulation of the distribution of temperature in composites is presented. Experiments are performed on glass/epoxy composites and the results are compared with the theoretical results. The morphology of delamination are elliptical and circular for unidirectional and woven laminates، respectively.

In this paper، the procedure of a novel computational solution for the formulation and analysis of nano-indentation contact problems between a viscoelastic half-space and a rigid indenter of any arbitrary geometry is presented. The main objective of this paper is to develop the relations which link the contact pressure distribution، the load on the indenter، and the indentation depth with assumption that the surfaces are frictionless. The viscoelastic indentation problem has been solved numerically employing a proper formulation in constitutive equations and applying a new matrix inversion approach satisfying the boundary conditions. The computational approach has been validated by comparing the numerical results to the analytic ones for loading and unloading process of a spherical nano-indenter.

In this research، crack propagation in mode II is considered. Relation between stress in front of crack tip and velocity is extracted by means of Wiener-Hopf technique. As a result، by comparing the solutions of modes I and II، it is verified that in mode II the effect of dilatational wave in increasing stress is smaller than that in mode I.

In this paper، previous analytical models for analyzing dynamical progressive buckling of square tubes under axial loading have been modified. In the improved model، the dissipated energy due to shortening of tube walls and the effects of inertia of previous folding layers are considered. The mean crushing force، which is predicted by the analytical model، has acceptable consistency with the results of experimental tests and in most cases their accuracy is higher. 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 in absorb energy of a square tube.

In this research، a free-free Timoshenko beam was used as a mathematical model to investigate static and dynamic characteristics of a typical type of aerospace structures. For this purpose، stepped Timoshenko beam was modeled، considering the shear deformation and rotary inertia parameters. The external force was combination of a follower force and the control system force in the direction of the force. The conservative and non-conservative parts of the acting force were taken into account in this analysis. Hamilton’s principle was used to develop the governing equations. The whole mathematical model was solved using finite element method. The results were validated by comparing a simple uniform beam with other references. It was concluded that، for models without control system، small variation in mass distribution or section properties can cause significant changes in critical follower force، and also could change the characteristics of the instability from static to dynamic or vice versa. For beams with control system، the conjunction of the first non-zero frequency with the horizontal axes was remained almost fixed for different IMU locations and control gains. However، the system behavior and the characteristics of the instability depended on the IMU location and the control gains. Finally، the effects of the variation in diameter ratios on the critical controlled follower forces were studied and presented in this paper.

In this study، the extended finite element method (XFEM) was used for modeling 2-D cracks in order to investigate the significance of the crack tip functions in the enrichment procedure. Two codes were prepared to simulate fracture of plane problems using constant strain triangle (CST) element and 4-node quadrilateral (Q4) element. Considering these elements، new and simple schemes were proposed for enrichment procedure. The stress intensity factors (SIF) for several benchmark problems were calculated، using the J-integral and the interaction integral methods. The results show that the elimination of crack tip functions has no noticeable effect on the accuracy of the computed stress intensity factor (SIF) for plane problems. It was also shown that the interaction integral method is more efficient than the J-integral method for computation of SIF in such problems.