Fracture from artificial spherical pores, as well as natural defects,
in alumina in a grain-size range of 0.8-9.2 mu m has been studied expe
rimentally and compared with a fracture-mechanics model, Results from
fracture-strength measurements have been combined with detailed fracto
graphic analysis to elucidate the ensuing crack instability. Two exist
ing models of possible crack configurations have been extended and con
trasted. The semicircular crack as well as the circumferential crack b
oth are described as flaws in the stress-concentrating field of a sphe
rical pore. Surface correction terms afforded by the presence of the p
ore have been incorporated. A comparative computation shows that fract
ure occurs more likely from the semicircular crack configuration than
the circumferential crack configuration.