Page 111 - Fiber Fracture

96 M.-H. Berger

Fig. 7. TEM image of Almax fibre. Numerous intragranular pores can be seen.

Fig. 8. Nextel 610 fibre is a dense a-alumina fibre with a grain size of 100 nm.

In the Nextel 610 fibre, the grains are smaller than the critical defect size and failure
is initiated from extrinsic defects such as pores or surface process flaws. The control
of the sizes of such defects leads to higher room-temperature strengths when compared
to Fiber FP and Almax fibre and distinct room-temperature fracture morphologies are
also obtained. As can be seen from Fig. 11 the failure surface shows two zones. Crack
propagation was at first stable and intragranular creating a first mirror zone which
fanned out symmetrically from the defect initiating the failure. In this fibre, failure
was induced by a pore with sharp edges, located at the near surface. The second zone
of the fracture surface corresponds to a mixed failure mode and was created during
catastrophic rapid final failure.
The high-temperature behaviours of these three fibres are controlled by their mi-