Active Lamb Wave Research Articles

Possible observation of acoustic emission precursors to catastrophic delaminations in composites

Measurements are reported of the elastic properties of carbon—carbon (C/C) composites during pyrolysis. This heat treatment is the first critical step during the processing of C/C components and constitutes the most frequent source of component failure, because of delaminations. Here, the use of active (Lamb wave) and passive (AE) acoustic sensors to study the various stages of microstructural evolution of C/C during pyrolysis is reported. The results of several experimental runs are presented in which AE precursors to delaminations were found. In some of the runs the precursors were used as indicators to slow down the time-temperature profile of the run and the delaminations were avoided. In other runs, the time-temperature profile was not changed and the components delaminated. The apparatus, sensors, and the technique of recognizing the precursors are described. A physical model is discussed for the source of the precursors based on the chemical and physical changes in the C/C component and their consequences on the microstructure as studied by micrographical and ultrasonic techniques. [The author acknowledges help from F. Montgomery of General Atomics in the measurements and the support of Rockwell International Science Center under a contract from the Office of Naval Research Contract N00014-87-C-0724.]