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Problem statement: Many modern structures are made from thin shells. Design of these elements depends to a large extent on their buckling behavior which is hugely affected by the initial geometric imperfections. Approach: For axially compressed isotropic circular cylindrical shells, axisymmetric localized geometric imperfections were found to reduce severely the buckling strength. Among various axisymmetric shapes of localized defects that were investigated, the entering triangular form was recognized to yield the most adverse case. Since multiple localized defects may be present in the same shell structure and interact, studying their mutual effect on the buckling load is of great importance for shell design. Results: In this study, the effect of two interacting entering triangular localized axisymmetric initial geometric imperfections on shell buckling strength under uniform axial compression was modeled by means of the finite element method. A special software package which was dedicated to buckling analysis of quasi axisymmetric shells was used in order to compute the buckling load either via the linear Euler buckling analysis or through the full non linear iterative procedure. A set of five factors including shell aspect ratios, defect characteristics and the distance separating the localized initial geometric imperfections had been found to govern the buckling problem. A statistical approach based on the Taguchi method was used then to study their relative influence on the buckling load reduction. It was shown by comparison with the single imperfection case that further diminution of the critical load was obtained. Conclusion/Recommendations: In the range of investigated parameters, the distance separating the localized geometric imperfections and imperfection wavelength were found to yield major influences on the critical load. Further studies must be performed in order to assess shell buckling strength in the presence of more than two defects and to state the relative influence of the intervening factors

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The role of wind energy is so promising as a source of future energy all over the world. However, whether the unpredictable nature of wind speed fluctuations and the stability of the power systems be affected by a high penetration of wind power remains an unanswered question. Therefore, an accurate analysis study of the effectiveness and robustness performance of the doubly fed induction generator (DFIG) is one of the challenges in wind turbine applications. The present works tackle the issue of grid connection of DFIG modeling and the dynamic operation to evaluate the capabilities and their impact on the pitch angle, current and voltage stability. The model is simulated using MATLAB/Simulink software, and the curves’ results are indicated based on the system that was connected to eight wind turbines in the wind farm. The results show the dynamic features of the DFIG response to the necessary output variables. On the other hand, its show us a view of the waveforms obtained, which means that an adequate study for choosing the type of control suitable for the DFIG is necessary.

Unlike conventional materials, composites have become an optimal option for a range of modern, industrial, clinical, and sports applications. This is combined with their noteworthy physical, thermal, electrical, and mechanical properties, as well as low weight and cost investment funds in certain cases. This review article attempts to give an overall outline of composite materials, regularly polymer-matrix composites (PMCs) and metal-matrix composites (MMCs). Polypropylene (PP) polymer and aluminum alloy were selected as matrices for this concentrate in light of their appealing properties and their use in different applications. Various studies address the different build-up materials, material handling, and the various properties. Mechanical characterization is an important cycle process for the development and design of composite materials and their components. It includes the determination of mechanical properties, for example, stiffness and strength according to standard test techniques (i.e., tensile, compression, and shear test strategies) distributed by the ASTM and EN ISO associations. Comparable to the determination of fatigue strength and fatigue life for composite materials. With respect to mechanical properties of composite materials, this paper reports several variables and limitations that affect mechanical property estimates, including material constituents, manufacturing process, test parameters, and environmental conditions.