J.C. McNulty and F.W. Zok, "Low
Cycle Fatigue of Nicalon Fiber-Reinforced Ceramic Composites," Comp. Sci.
Tech., 59, 1597-1607, 1999..
|
|
| An experimental investigation of damage and fracture
in fiber-reinforced ceramic composites under low-cycle fatigue has
been performed. Several different composites have been studied, each
reinforced with ceramic grade Nicalonô fibers but with varying fiber
architectures and matrix materials. Correlations have been established
between the presence of matrix cracks within the longitudinal tows
or laminae, the hysteresis in the stress-strain response, and the
propensity for fatigue fracture. Fatigue was found to be most prevalent
when both matrix cracks are present and the stress-strain response
exhibits hysteresis. In such cases, a fatigue threshold was obtained,
typically at a stress of = 65-80% of the ultimate strength of the
pristine composite. Furthermore, the interface sliding stress diminished
with cycling, reaching a saturation level of = 1/3 to 1/2 of the initial
value. This reduction is predicted to cause a fatigue strength reduction
to = 80% of the ultimate strength, broadly consistent with the measured
thresholds. The inference is that the degradation in fatigue strength
is attributable in large part to the degradation in the sliding stress,
with the balance being associated with a degradation of the fibers
themselves. The latter conclusion has been corroborated through measurements
of the fracture mirror radii on the fibers in both pristine and fatigued
specimens. |
| |
|