INVESTIGATION OF FAILURE OF BRITTLE LAYERS UNDER COMPRESSIVE STRESSES USING ACOUSTIC-EMISSION

Oxides formed on mild steel at 550-degrees-C and brittle lacquer layers on mild steel substrates were tested in compression in four point bend tests at room temperature and their failure studied using acoustic emission to monitor crack formation. Scale spallation and failure mechanisms were interpreted using standard electron microscopy techniques. Initial failure occurred by loss of interfacial adhesion for oxides and by loss of cohesion within the layer for brittle lacquer. At higher strains loss of adhesion of the oxide and cohesive failure in the brittle lacquer resulted in spallation. The critical strains for both failure initiation and spallation decreased with increasing oxide thickness according to a predicted power law and the energy release for interfacial failure was found to be 20 J m-2. Acoustic emission was found to be a useful tool in explaining the different.failure mechanisms observed.