FIBER-MATRIX INTERPHASE CHARACTERIZATION IN COMPOSITES USING ULTRASONIC VELOCITY DATA

A nondestructive method is introduced to determine the effective elast ic moduli of fiber-matrix interphases from measured composite moduli. The composite moduli are obtained by measuring the angular dependences of ultrasonic longitudinal and transverse phase velocities in planes along and perpendicular to the fibers. Three independent interphase ef fective moduli are found using micromechanical multiphase models which were applied after averaging composite transverse moduli to account f or the composite orthotropy. Sensitivity analysis shows that errors in the calculated interphasial moduli are approximately ten times those in the composite moduli. Experiments are performed on Si3N4 ceramic an d Ti-24Al-11Nb intermetallic matrix composites reinforced with carbon coated silicon carbide fibers. The experimental interphasial moduli fo r the intermetallic composites agree with literature data after accoun ting for the interphase microstructure. The interphasial moduli for ce ramic composites are lower than those for the intermetallic composite due to imperfect mechanical contact between the interphase and the por ous matrix. The use of the method to assess the interphase degradation is demonstrated for interphasial oxidation damage. The analysis helps to determine the morphology of the damaged interphase.