Static bending tests
were performed at 0.5 mN/s until failure. The static fracture
load when the clogged layer was uppermost was generally
larger than when lowermost. This can be explained
from the geometry of the clogged layer and crack propagation
under tensile loading. During the bending test,
when the clogged layer was lowermost, the clogged layerunderwent tensile loading, and the notches on the clogged
layer functioned as a crack tip enabling the crack to propagate.
When the clogged layer was uppermost, the crack
on the notch of the clogged layer did not propagate,
because it underwent compressive loading. It appears that
these crack tips reduce the fracture strength under tensile
load. The fatigue test was carried out with the clogged
layer uppermost.