Nonlinear ultrasonic techniques to quantify oxidation damage in carbon/carbon composite material

Abstract

This research applies linear and nonlinear ultrasonic (NLU) techniques to quantify oxidation damage in a carbon/carbon (C/C) composite material cut from an automotive friction disc ring. The oxidation as a result of gasification of carbon at high temperatures (T >∼ 500 °C) is a major degradation mechanism in C/C composites. This is the first research to successfully use a dry-contact mediator wedge setup for nonlinear ultrasonic applications, enabling repeatable in-situ Rayleigh wave measurements on slow wave velocity materials where submersion is impractical. The samples are subjected to thermal damage by exposure to a high temperature atmosphere over multiple holding times, and the oxidation damage is visualized using scanning electron microscopy (SEM). The sample mass is recorded and both linear (velocity and attenuation) and nonlinear (β′) ultrasonic measurements are performed after each holding time. Superior sensitivity to early stage oxidation affirms the suitability of nonlinear Rayleigh wave measurements for the quantification of thermally induced oxidation damage in C/C composites, while the double mediator wedge setup is well suited for high attenuation, slow wave velocity materials, demonstrating potential applicability for in-situ condition monitoring. The results show that a systematic oxidation starts after 3 min of exposure to the temperature in this C/C composite. The oxidation induced fiber–matrix debonding, matrix defects (appearing to be microcracking), and void growth are consistently detected by the Rayleigh wave velocity and acoustic nonlinearity. In addition to the higher sensitivity, the nonlinearity parameter has the capability of selectively measuring the strength related damage such as the observed fiber–matrix debonding, while the wave velocity can detect the oxidation voids. A combined use of these two parameters can provide useful information on the microstructural evolution in C/C composites subjected to high temperatures.