Page 130 - Fiber Fracture

FRACTURE CHARACTERISTICS OF SINGLE CRYSTAL AND EUTECTIC FIBERS 115
a" 2000
E A120,1Y,AIs0 ,I Eutectic
5
on 1500
E
L
iZ
aJ ~OOO1> AI 0
c1
'Z 1000
c

500

0

Strain Rate, s-'
Fig. 3. Tensile strength of single-crystal (0001) A1203 (squares) and directionally solidified A1203/Y3A15
OI2 (circles) fibers as a function of strain rate at 1000°C. Each data point represents the mean of 40 tests
and error bars represent the standard deviation.

A possible contributor to the loss in strength at high temperatures is static fatigue or
delayed failure due to stress-enhanced slow crack growth. This would require that the
fiber strength be markedly dependent on loading rate and load application time. Hence,
successful structural exploitation of these fibers requires a detailed understanding
of fiber strength behavior as a function of stress and time so that accurate failure
predictions can be made. The time to failure behavior for uniaxially loaded commercial
(0001) A1203 was tested (Sayir, 1993). This experimental study showed that the failure
of (0001) A1203 fibers was due to slow crack growth at 1000 < 1400°C under static
stress and variable stress rate conditions. In the present work fibers grown by the
laser-heated float zone technique that had superior room temperature strength were
tested. The strong dependence of strength on strain rate for the high strength (0001)
A1203 at 1100°C is shown in Fig. 3 (squares). The dynamic response of high-strength
fibers is identical to commercially produced fibers even though their room temperature
strength was almost a factor of two higher. The data strongly indicate that single-crystal
(0001) A1203 failure is dependent on the slow crack growth at elevated temperatures
and therefore any meaningful improvements to fiber strength at high temperature must
consider strength as a function of dynamic loading conditions.
The results of the stress rupture test at 1400°C are shown in Fig. 4. The threshold
stress intensity factor below which no crack propagation occurs seems to be around
400 MPa as shown in Fig. 4. To understand slow crack growth in A1203 fibers and
make accurate lifetime predictions, one needs to know the crack velocity as a function
of stress intensity factor at high temperatures. It is well established that for a given