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In this paper, the new theory is proposed to investigate the fracture behavior of cracked composite materials. Based on this theory, crack is created and distributes in the isotropic matrix. Therefore, contrary to the previous fracture theories of composite materials presuming that crack growth occurs in anisotropic homogenous material, the new theory assumes that crack growth occurs in the isotropic matrix. It states that the isotropic matrix is affected by fibers in the composite structure of the material. In this approach, fibers are considered as isotropic matrix reinforcements and the reinforcement effects are defined as coefficients in stress state of the isotropic matrix. The coefficients are called reinforcement factors and derived via three different approaches to study the arbitrary crack in 2D materials. The reinforcing effects of fibers are obtained when tension across and along fibers and shear loadings exerted on the body. The three methods demonstrate that the reinforcement factors depend on elastic properties, crack growth location and the crack and fiber orientations. However, the method, derived from the micro-mechanic approach, displays their dependence on the fiber volume fraction. Comparing the results of these cofficients with the existing fracture theories illustrates the efficiency and ability of the reinforcement factors in investigation and explanation of the fracture behavior of orthotropic materials. [ABSTRACT FROM AUTHOR]

In this study, flutter of functionally graded carbon nanotube (FG-CNT)-reinforced composite wing carrying a distributed patch mass is analyzed and presented. Wing is modeled by a rectangular plate with cantilever boundary conditions in supersonic flow. To evaluate the displacement fields of the moderately thick plate, First-order shear deformation theory (FSDT) and chebyshev polynomials series are applied. In supersonic airflow simulation effect, the firstorder piston theory was used and differential equation governing the system was adapted, using the Hamilton principle. In this study, 4 different types of CNT are considered through the thickness. CNT distribution patterns are as uniform, decreasing, decreasing-increasing, and increasing-decreasing. Finally, the effects of size, mass, and location of the distributed patch mass as well as various CNT distributions and fiber orientation angle in a two-layer antisymmetric composite on flutter boundaries were studies. In comparisons with the results of previous studies, a good agreement is observed. The results showed that the flutter boundary reduced with increasing mass ratio and increased in longer length of added mass. By increasing orientation’s angle of CNT fiber of anti-symmetric composite, the flutter boundary is raised and has different behavior for different distribution patterns. [ABSTRACT FROM AUTHOR]

The big deformation of composite structures under dynamic loads is one of the most disadvantages of these structures that cause to loss of stiffness of them. The using of fiber metal laminated shell that named FML in abbreviation is one of the ways to decrease the adverse effect of dynamic load. In this study the optimum fiber orientation of composite layers of the FML circular cylindrical shells are determined to more decrease the adverse effect of dynamic loads. For this purpose the fiber orientation of composite layers of the FML circular cylindrical shells are changed frequently and each cases being subjected to axial compressive load and with use of ABAQUS program the tension of all composite layers are calculated for all cases. Then with use of MATLAB program the fiber orientation that cause to maximum stiffness based on maximum tension fracture criterion is selected. The free vibration analysis is used for determination the accuracy and performance of design process. The results of free vibration analyses show that determination of the optimum fiber orientation cause to improvement of the FML shell natural frequency. Energy method and high order shear deformation theory is used to define the equation of motion. Full Calculus method is used for optimization in order to apply the exact result. [ABSTRACT FROM AUTHOR]

Composites materials are widely used because of their high specific stiffness and strength. Although the methods and techniques of designing these materials are well developed, however, the research on the behavior of these materials under impact and dynamic loadings are still under progress. In this article the crushing and energy absorption of composite tubes under axial dynamic loadings are studied. In order to calculate the absorbed energy and the crushing load, composite tubes with circular and tubular cross-sections are modeled using LS-DYNA software. In order to simulate the crushing behavior of the tubes the laminated-composite material model is utilized. Also the effects of fiber orientation on the energy absorption are studied. The results show that the energy absorption of the tubes with tubularcross-section is less than those of the circular cross-section. The results show that for [θ/-θ] configuration, the minimum energy is absorbed for θ = 15o. The results of simulation for the tubular cross-sections are compared with theexperimental results and a very good correlation is achieved.