Evolution of Damage in All-Oxide Ceramic Matrix Composite After Cyclic Loading

While structural ceramics usually display a brittle mechanical behavior, their composites may show non-linearities, mostly due to microcracking. In this work, we studied the evolution of the stiffness of a sandwich-like laminate consisting of an Al2O3-15%vol.ZrO2 matrix reinforced with Nextel™ 610 long fibers as a function of number of cycles N in uniaxial testing mode. We found that the stiffness of the composite degrades with increasing N, indicating increasing microcracking. However, synchrotron X-ray refraction radiography showed that the internal specific surface of inhomogeneities (mainly cracks and pores) decreases with N. A modeling strategy was developed, based on the combination of Voigt and Reus schemes for the calculation of equivalent stiffness of mixtures with the Bruno-Kachanov model for the calculation of equivalent stiffness in microcracked and porous materials. Through modeling we could estimate the initial microcrack density in the matrix (due to the simple thermal expansion mismatch between the two constituents) and the amount of microcracking increase upon cyclic load. We found that the stiffness in such a composite degrades dramatically already after as few as 20000 cycles, but then remains nearly constant. The combination of the mechanical testing, the quantitative imaging analysis, and the modeling provided insights into the damage mechanisms acting: microcrack propagation is far more active than microcrack initiation upon cyclic loading. This scenario corresponds to previous results obtained on porous and microcracked ceramics.