Date of Award
M.S. in Mechanical Engineering
Department of Mechanical and Aerospace Engineering
This investigation successfully uses an integrated NDE/mechanical test system to characterize damage initiation and accumulation in i6 Nicalon/MAS-5 and [0/90]3s SiC/BMAS ceramic matrix composite (CMC) systems at room and elevated (up to 900°C) temperatures. Extensive surface replication and photography are used to correlate changes in ultrasonic surface and longitudinal wave characteristics to damage mechanisms. The sudden decrease in the peak-to-peak amplitude of the surface wave signal coincides with the onset of matrix cracking at room and elevated temperatures. The amplitude of the surface wave signal decreases with increasing crack density. Matrix cracking first occurs in matrix rich regions where the fiber spacing is the widest. In both  i6 and [0/90]3s materials, testing at elevated temperatures delays the onset of matrix cracking. Testing at elevated temperatures results in an overall reduction in the number of matrix cracks for both  and [0/90] materials. The sensitivity of the surface wave to peak stress levels can be used to differentiate between the initiation and propagation of damage during interrupted testing. A portion of the surface wave response is attributed to the opening and closing of fiber bridged crack surfaces during loading and unloading. Measurements of the changes in surface wave amplitude as a function of crack density allow estimates of damage to be made through nondestructive means. The longitudinal wave demonstrates a sensitivity to changes in the overall stiffness, as opposed to the progression of matrix cracking. A shear lag model is used to deduce the changes in the interfacial sliding stress as a function of temperature for the  i6 and [0/90]3s material. The sliding stress decreases with increasing temperature for both materials. Predictions for the composite unloading modulus are not influenced by changes in the sliding stress. Predictions for the permanent strain indicate that the  i6 material possesses a very low debond energy.
Ceramic-matrix composites, Continuum damage mechanics, Nondestructive testing
Copyright 1998, author
Gural, Roger J., "In situ nondestructive characterization of damage progression in ceramic matrix composites at room and elevated temperatures" (1998). Graduate Theses and Dissertations. 3060.