Study On Load-carrying Capacity Of Large-Scale Complicated Composite Structures

Fiber reinforced resin matrix composites are widely used in the field of Aeronautics and Astronautics, with the advantages of high specific strength, high specific stiffness, anisotropy of materials, and so on. For the bearing capacity evaluation of the component level composite structures, because of the characteristics of large-scale, complexity of connection and molding technology, lead to the structure failure mode with diversity, while some local failure can even bring about an early failure, that cause the main control factors resulting in final failure behavior of the whole structure to become uncertain, thus the error of the numerical prediction results and the experimental results is hard to achieve in less than 15%. Moreover, it is a trend to gradually reduce the dependence on experimental test in large scaled composite structure design process, and it is very necessary to further improve an effective numerical prediction method of the bearing capacity for large, complex composite structures.Focusing on finite element numerical analysis, a high precision effective numerical calculation method for the load bearing ability prediction of large complex composite structure is developed. The main idea is proposed based on the concept of "models group", namely hierarchical analysis strategy. When using this analysis strategy, the failure behavior of composite structures obtained by a large number of experiments, a global equivalent simplification model and a certain number of corresponding comprehensive local analysis models will be integrated into consideration."Models group" analysis process can be described as follows. Firstly, load transfer and distribution behavior, basic strength criterion, stiffness degradation information, and the boundary of sub models are acquired by preliminary calculation of the global equivalent model. Secondly, the local models such as laminate failure model and the interface failure model are established respectively. Thirdly, the stress or displacements acquired by the whole analysis is applied to the boundary of each sub model. Then, the main failure modes and the critical factors which would cause the final failure of the whole structure can be obtained by comparing the number of failure units, the failure rate, and the failure time of each sub model. Finally, a comprehensive model can be established which only focuses on the critical factors and ignores the secondary factors.The effective and precision of secondary fine analysis were controlled by model modification according to the results of composite laminate test and composite thin wall stiffened plate test. Using "models group" analysis strategy, the global-local analysis of the aircraft tail box structure was carried out, damage characteristics of each crucial local details of box structure which subjected to bending load were researched, and the critical factors which would cause structure final failure were investigated, thus the load-carrying capacity of structure was evaluated. Based on the result of "models group" analysis and orthogonal experiments, the sensitivity to delamination imperfection with various forms is studied for tail box key bearing area. Research result shows that beam web of tail box is more sensitive to the depth of delamination imperfection than other factors, the damage influence factor increases with the increase of the radius of imperfection, and the delamination growth is more sensitive to the location of imperfection in the region near the symmetry plane of the beam web.