The toughness-curve (T-curve) behavior of a composite that is composed of 3
0 vol% spherical, polycrystalline, fine-grained alumina agglomerates disper
sed throughout a constant-toughness 50-vol%-alumina-50-vol%-mullite matrix
was compared to that for a coarse-grained agglomerate version of the compos
ite, with the intent of identifying the operative toughening mechanisms in
the latter. The T-curve behavior was evaluated using the indentation-streng
th method, and the underlying T-curves were deconvoluted from experimental
data using an indentation-fracture-mechanics model. Compared to the T-curve
of the coarse-grained composite, the toughness of the "fine-grained" compo
site increased from a similar initial toughness and over a similar crack-le
ngth range but to a lower saturation value. The T-curve of the fine-grained
composite can be explained as being derived predominantly from unbroken br
idging elements in the crack wake, set up by agglomerate-induced crack-path
deflections in the matrix. This mechanism is proposed to function to the s
ame degree and account for the majority of the observed toughening in the c
oarse-grained composite, with the remaining toughening increment being deri
ved from intra-agglomerate grain bridging. Calculations suggest that unbrok
en bridging elements do not behave as simple elastic cantilevered beams tha
t bend uniaxially with increasing crack-opening displacement.