A study of failure criteria of variable stiffness composite panels Fiber reinforced composites has been widely used in the field of Aeronautics, Astronautics and automotives because of its own outstanding features, such as high specific strength, high specific modulus, designable performance and integral forming easily. This application can significantly reduce the weight and improve flight performances. Advanced fiber placement technology is applied on the production of many commercial aircrafts, such as Boeing 787 Dreamliner (50%), Airbus A380 (20%), and A350 (50%). Interlaminar stresses It is known that, as a function of spatial location, the stiffness distribution of variable stiffness laminates is nonuniform, and this might result in large gradients in in-plane stress fields, which contributes to the amplification of the interlaminar stresses, and could lead delamination to the dominant failure mode of these laminates. A method to approximate these interlaminar stresses involving the use of closed- form expressions of in-plane stresses and equilibrium equations will be developed and applied to variable stiffness composite panels. Meanwhile, numerical simulation for the stress and strain analysis of variable stiffness composite panel is applied (with Abaqus 6.11). Publications Y. Zhang, F.F. Xiong, S.X. Yang, X.N. Mei. (2011) “Optimization design of composite wing structure of a minitype Unmanned Aerial Vehicle”, Advanced Materials Research, : Y. Zhang, F.F. Xiong, S.X. Yang, (2011). “Numerical simulation for composite wing structure design optimization of a minitype Unmanned Aerial Vehicle”, The Open Mechanical Engineering Journal, 5: Figure 1. Material used in Boeing 787 Dreamliner Variable Stiffness composites Benefits of the directional properties of advanced composites could be fully utilized by varying the fiber angles of layup continuously from point to point, which resulting in stiffness properties that change as a function of location, and so this laminates are termed variable stiffness composite panels. In this manner, it is possible to redistribute the loads, in order to respond more adequately to planar stress variations and also divert the loads from the most sensitive regions, such as holes and notches, leading to high efficiency of composite structure. Figure 2. Fiber orientation of the first and middle ply for [ / / ] layup PhD Candidate: Yan Zhang Department: AWEP Section: FPP Supervisor: M.J.L. van Tooren Promoter: M.J.L. van Tooren Start date: Funding: CSC Cooperations: BIT Aerospace Engineering Figure 3. Fiber steered panel Failure criteria There are already a number of previous articles working on the failure criteria and structural response of composites under different boundary conditions during the last four decades. However, most of these failure theories only focus on in-plane stresses, without taking account of out-of-plane stresses, such as interlaminar shear stresses. Tsai-Hill and Tsai-Wu failure theory, which are widely used, are as following: Tsai-Hill failure criterion: Tsai-Wu failure criterion: A failure theory for prediction of failure initiation taking account of the interlaminar shear stresses, will be extended and applied to variable stiffness composite laminates. Pagano’s three layer case will be investigated for both constant stiffness and variable stiffness cases as the reference cases. A set of analyses with different layup will be carried out to verify the prediction accuracy of the modified failure criterion. Figure 4. Interlaminar shear stress distribution at the middle plane for [ / / ] layup