Cyclic and sustained loading behaviors of oxide/oxide Nextel™720/alumina composite with double edge sharp notch

This study investigated an oxide/oxide CMC consisting of Nextel™720 (meta-stable mullite) fibers in alumina matrix, N720/A, with 0°/90° fiber orientation having double edge sharp notch under sustained and cyclic loading conditions at 1200 °C in laboratory air environment. Monotonic tensile tests at 1200 °C were also conducted. Fracture surfaces were examined to analyze failure and damage mechanisms. Comparisons with counterparts from unnotched geometry showed N720/A is mildly sensitive to the sharp notch under monotonic tensile, creep and fatigue loading conditions. The ultimate tensile strength of the composite was reduced by about 15% in the presence of the sharp notch. The rupture strength of the sharp notched geometry was reduced by about 15% of unnotched geometry for a given rupture time. The fatigue strength was reduced by about 20% of unnotched geometry for a given number of cycles to failure. Deformation under cyclic loading condition had contributions both from fatigue and creep. Damage mechanisms were identical under cyclic and sustained loading conditions.