Study On Pull-off Property Of Composite T-joint

As its light weight and high strength, composite has been widely used inaerospace field. Composite structure integrated design technology, which could cutthe additional mass of transition zone in the way of reducing number of parts andconnecting member, is an effective technical way in saving structural weight andassembly costs. Integrated design technology greatly reduces the aircraft structureand process separation surface, while the connections of separation surface aresuffering more stringent requirements. Moreover joint could be weakest point of theentire structure, so the design of joint is very important. Adhesive joints are theprimary means of connections; therefore the design of adhesive joints is significant.Aircraft stiffened skin structure is a kind of typical adhesive T-joints. It is animportant part of the whole structure but fewer domestic researcers are focus on it.Its pull-off property is one of its key properties. With this background, this papercarried out pull-off properties of adhesive T-joints.For the factors affecting properties of adhesive joints, this paper carried outthe characterization of T700/TDE-85mechanical property and adhesive processproperty. Tensile strength of1890.80MPa, tensile modulus139.78GPa, Poisson’sratio of0.3, and the interlaminar shear strength of78.67MPa of unidirectionalT700/TDE-85composite were obtained. According to the anisotropy of compositelaminates and low interlaminar shear strength, and compatibility with compositematrix, the adhesive J-133has been selected. In order to ensure a sufficient jointstrength, the surface treatment method is a combined method of surface mechanicalpolishing and degreasing, the fiber direction can be between0°and45°direction,thinkness of adhesieve layer should be between0.2to0.3mm, J-133adhesivecuring time at room temperature should be no less than7days. With refinedprocessing parameters, the adhesieve T-joints of typical sizing were processed andpull-off load of963.45N was obtained. Among process of the the pull-off test ofadhesive T-joints, first acoustic feedback, the initial load reduction, the visiblecracking, the pull-off limit load were observed. This paper selected two differentmodels of composite T-joint finite element for analysis. With the ananlysis of shellelement model, the stress concentration area of the T-joints is the center ofconnection between the skin and the web under the increasing of axial displacementload. The range of the stress concentration area becomes smaller, while stressbecomes higher. With using composites engineering equivalent engineering elasticconstants in solid element analysis and triangle fillment area definition, the stress concentration within triangle fillment area is observed obviously. It should be thecrack initiation. Analysis of adhesive joints half ribs degumming with the skin wascarried out, debond propagation direction is perpendicular to the direction ofcontinuous stress concentration zone and the concentration area was found on theelement boundary. Introduction of cohesive element to solid model can directlyreflect the pull-off properties of adhesive T-joints during damage propagation andthis analysis of progressive failure process in the joint design acted a good effect.