Page 323 - Engineered Interfaces in Fiber Reinforced Composites

304 Engineered interfaces m fiber reinforced composites

1.21 I

-0.4
0 0.1 0.2 0.3 0.4
(a) Coating thickness, t/a

1.2-

v)
u)
E 0.8-
+ --.
v)
7 0.6-
5 ---___
2 0.4-
- ---.
$ 0.2-
-a -.
2 0- -.
-
-.
-0.2 Increase in ai -*. --.
-0.4 -
0 0.1 0.2 0.3 0.4
(b) Coating thickness, t/a
Fig. 7.14. Normalized radial residual stresses as a function of coating thickness, ?/a, for varying
coefficients of thermal expansion (CTE) of the coating, a, = 10,70,130 x 10-6/oC: (a) Young’s modulus
ratio Ei/Em = 0.333; (h) Ei/Em = 1.0. After Kim and Mai (1996a, b).

summary, if the residual stresses at the interfaces are to be reduced substantially, the
interlayer material should have a high (~i value, unless its Young’s modulus is much
lower than that of the matrix (i.e. Ei/E,,, less than approximately 0.1 for the CFRP
studied). This conclusion is considered to be consistent with the criteria proposed for
Sic fiber-Ti3Al + Nb matrix composites (Arnold and Wilt, 1992).
In the light of the work presented in the foregoing from both the theoretical and
experimental viewpoints, three concepts of engineered interfaces have been put
forward to explain the roles of the thermoplastic coating, and their physical/
chemical requirements for three different functional coatings are proposed by Kim
and Mai (1996b):