Variable Stiffness composites

Report
A study of failure criteria of
variable stiffness composite
panels
PhD Candidate: Yan Zhang
Department: AWEP
Section: FPP
Supervisor: M.J.L. van Tooren
Promoter: M.J.L. van Tooren
Start date: 01-06-2011
Funding: CSC
Cooperations: BIT
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%).
Figure 3. Fiber steered panel
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 closedform 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).
Aerospace Engineering
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.
(a)  contour
(b)  contour
Figure 4. Interlaminar shear stress distribution at the middle plane for
[<-30|0>/<60|90>/<-30|0>] layup
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:
Figure 2. Fiber orientation of the first and middle ply for [<-30|0>/<60|90>/<-30|0>] layup
 x y  y  xy
 2  2  2 1
2
X
X
Y
S
x
2
2
 xy
1 
 1
 1 1 
 t c
    t  c  x   t  c  y  2  1
t c
t c
t c x y
X X YY
X X YY
S
X X 
Y Y 
x
2
y
2
2
1
1
2
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.
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, 156-157: 15321536.
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: 11-18.

similar documents