J.C. McNulty, M.Y. He and F.W.
Zok, "Notch Sensitivity of Fatigue Life in a Sylramic™/SiC Composite at
Elevated Temperature," Composites Science and Technology, 61, 1331-1338,
2001.
|
|
| The effects of holes and notches on the fatigue life
of an advanced SylramicTM/SiC composite at 815°C is examined. At this
temperature, fracture occurs by an oxidative embrittlement mechanism,
common to most SiC-based composites. In unnotched specimens, embrittlement
is manifested at stresses above the matrix cracking limit, smc, leading
to fracture following relatively short exposure times (<~
100 h). As a consequence, a fatigue threshold is obtained at a stress,
sth = smc. This threshold is due to the absence of an easy path for
oxygen ingress when matrix cracks are not present. In center-hole
specimens, an analogous threshold is obtained, at a stress, sth =
smc/ke, where ke is the elastic stress concentration factor (= 2.5).
That is, once the cracking limit is exceeded at the hole edge, embrittlement
and fracture ensue. The threshold stress for center-notch specimens
with stress concentration ke = 7 is numerically similar to that of
the center-hole specimens with ke = 2.5, indicating some tolerance
to local stress levels above the global matrix cracking limit in sharply
notched geometries. Nonlinear finite element calculations of the stresses
in the center-hole and center-notch specimens are used to infer the
local conditions associated with the threshold. A key result is that
the damage tolerance and notch insensitivity normally associated with
inelastic straining cannot be exploited at temperatures at which the
embrittlement mechanism operates. The implication is that composite
structures with holes and notches must be designed extremely conservatively
to ensure long lifetimes (> 100 h). |
| |
|