Ultrasonic Characterization of Composites

The results of recent ultrasonic studies are presented on the microstructure of high temperature composites. The acoustic sensors consist of an acoustic emission (AE) detector, an ultrasonic generator/receiver and a scanning acoustic microscope (SAM). With the aid of high temperature waveguides some of these sensors are shown to detect the in-situ changes in some of the key properties of plates of Carbon/Carbon during pyrolysis in the range of temperatures from 25 'C to 900 °C. In particular, changes in stiffness as a result of microcracking and decomposition are readily detected and are shown as useful to guide/revise the time-temperature profile of the carbonization heat treatment. The acoustic data show a variety of features giving insight into the microstructural changes as the heat treatment proceeds. Among these are: the observation of a minimum in the temperature dependent modulus and a maximum in the AE counts at about 650 °C. Microcracking is shown to be significantly influenced by the relative roles of the reinforcing materials vis-a-vis the matrix. The results point the way towards the use of sensors toward process monitoring, understanding and control in a manufacturing environment.