Experimental results of damage development in and mechanical response of heat-treated
NicaloniCAS-II laminates subjected to monotonic flexure and axial loading and to cyclic tensile
loading are reported. The specimens were subjected to post-processing heat treatments at 900°,
1000°, and l100°C in air for 100 hours. Changes at the fiber/matrix interface/interphase due to
post-processing heat treatments were also characterized. The combined effect of fiber debonding
and transverse matrix cracking in both 90° and 0° plies plays an important role in damage
development in [0/90]45 Nicalon/CAS-II laminates, especially in developing the secondary damage
modes such as longitudinal matrix cracking and delamination. Frictional wear effects found in
cyclically loaded specimens may be responsible for the observed temperature profiles during the
intermediate stage of fatigue life. It is also believed that frictional wear is critical to the failure
of notch sensitive fibers. Different damage modes such as "brittle" matrix crack propagation and
"quasi-brittle" matrix crack propagation were observed in heat-treated specimens. Results obtained
from microanalysis using an analytical scanning transmission electron microscope equipped with
an energy dispersive spectrometer, and microindentation indicated that the changes of damage and
failure modes were directly related to the changes of characteristics at the fiber/matreix interface/interphase.
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