Research On Composite Thick Laminate Single-bolt Joint Strength And Analysis Of Failure Region

With the development of aircraft structure technology, composite materials are gradually taking place of metals applied in aircraft main force-bearing components due to large advantages in mass reducing. For composite thick laminate applied in main force-bearing components, mechanical joints are mainly used. Since the structure specialty for composites, influencing factors of joint strength are much more complex than those in metals. When research object expands to thick laminates, the original 2D modeling method and analysis results for thin laminates are not completely suitable, as load non-uniformity occurred in the direction of thickness. Strength check and the stress analysis of failure region for composite thick laminate mechanical joints are one of the main difficulties during aircraft structure designing.For this reason, numerical simulations and visible test methods for stress around bolt hole in the area of composite laminate mechanical joints researched by foreign and domestic scholars are analyzed in this paper, while regular designing rules are concluded. Towards two different definitions, a comprehensive definition for medium-thick and thick laminate is concluded. Meanwhile, physical meaning and mathematical properties of nine different failure criterions usually use in composites are analyzed, finding out the seriously conservative problem occurred in failure judging with classical strength criterions, which can be solved by characterized curve method. A 2D failure judging software for composites is developed to raise the efficiency of simulative strength checking.For thick laminates, 3D simplified models are built with MSC.Patran in this paper. Assumed central symmetry method and double assumed central straight line method are imposed to simplify external loads in single-bolt, single-lap and single-bolt, double-lap models. Nonlinear finite element simulation is carried out with MSC.Marc. Distribution law of failure region for different plys is studied from failure judging results by characterized curve method and Hoffman criterion, indicating that compressive damage is the main failure mode in single-lap joint while tensile and shearing damage in double-lap. Besides, distribution law of stress around hole for different plys is particularly analyzed, concluding priority failure angle range corresponding plys in each direction. Theory of ply failure priority for load bearing composites is detected.Through calculating load-displacement curve and compressive strength in two models, conclusions are obtained that, as to the structure unbalance loading, distribution of stress on compressive surface in single-lap model is seriously non-uniform, leading to a great decreasing in strength compared with in double-lap model. Meanwhile, in the range of tolerance, both models with three different clearances are calculated, in which compressive strengths are compared, concluding that strength is slightly decreasing and stiffness is increasing with the decreasing of clearance in same condition. Finally, compressive strength in double-lap model is compared with tested strength in reference, proving the reliability of 3D simplified model built in this paper, since the engineering accuracy is satisfied.