THE INFLUENCE OF IMPOSED STRAIN-RATE ON FRACTURE OF SURFACE OXIDES

The mechanical properties of chromium rich scales formed on 304 stainl ess steel have been investigated as a function of deformation rate and operating temperature. At 900-degrees-C at slow strain rates <10(-6) s-1 no cracking was observed at strains up to 10%. At rapid strain rat es in excess of 10(-4) S-1 oxide cracking was found to be independent of strain rate and controlled by the fracture toughness of the oxide. In the intermediate region, with strain rates between 10(-4) and 10(-6 ) s-1 in the temperature range 700-950-degrees-C, the behaviour is det ermined by the creep deformation and fracture mode of the oxide. The m echanism of surface oxide failure is examined and an equation to predi ct cracking density over the full range of both monotonic tensile and creep fracture modes is suggested.