Page 64 - Fiber Fracture

MODELS OF FIBRE FRACTURE 49

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REACTION ZONE THICKNESS, c (nm)
Fig. 11. SiC/W monofilament strength as a function of the reaction zone thickness, c.

(~3.4 GPa), and r (~7 km) and c are given in Fig. lla. The monofilament strength
predicted by both equations is plotted in Fig. 11 as a function of the reaction zone
thickness. If c is below 260 nm (as is often the case) failure occurs after the tungsten
core has yielded plastically (Eq. 8). The monofilament strength is controlled in this
situation by the tungsten yield strength, and the dependence on the reaction zone thickness
is weak. On the contrary, the critical condition for brittle fracture provided by Eq. 7
becomes dominant when c = 260 nm and the monofilament strength decreases with
c-1/2.
Further attempts to improve the strength of the SiC/W monofilaments were directed
to creating a weak interface between the interface reaction zone and the Sic mantle
through the deposition of a thin (250 nm) Tic layer (Faucon et al., 2001). Under such
conditions, the annular crack at the interface was deflected along the weak TiC/SiC
interface before penetrating into the Sic mantle (Fig. lob). The interface strength was
characterised by the debonding length, d, and the critical condition for crack propagation
was computed using an axisymmetric finite element model. The results in terms of the
fibre strength are plotted as a function of the d/r ratio in Fig. 12, and they show that the
blunting effect of a weak interface was significant when the decohesion length was of