Fracto-emission and electrical transients due to interfacial failure

Interfacial failure is often accompanied by intense photon and electron emissions. When interfacial failure involves the separation of a conducting material from an insulator, transient electrical signals are generated in the conducting phase; these signals are ultimately due to contact charging across the interface, followed by charge separation during failure. We review the application of these signals as probes of the failure process in two adhesive systems: a steel rod embedded in an epoxy matrix, and a pressure sensitive adhesive on a copper substrate. The intensity of the photon and electron emissions (in vacuum) from the steel rod-epoxy system allows us to determine where debonding begins and the direction and approximate velocity of the resulting debond crack. Electron emission is particularly sensitive to the production of exposed fracture surface, and thus indicates when the debond crack reaches the epoxy surface. The peeling of pressure sensitive tape from copper yields a fluctuating current signal that reflects the rate of adhesive removal from the copper, and is thus particularly sensitive to micromechanical events along the peel front. Spectral analysis of these fluctuations show an interesting transition at peel speeds of about 6 mm/s. Numerical analysis of the current signal also shows fractal behavior, suggesting the presence of a chaotic process.