A creep analysis of the deflection test for evaluating oxide growth stresses

Abstract

During isothermal oxidation of one surface of a metal strip, growth-induced stresses will produce curvature of the metal/oxide composite sample. This observation has been used extensively elsewhere to evaluate the average stress developed within the oxide layer, but previous estimates have been based on an elastic stress analysis of the bending composite. At typical oxidation temperatures, creep deformation of both oxide and metal phases is much more likely. The present paper provides a stress analysis assuming such non-linear creep deformation. It considers the situation where the oxide experiences a net compressive stress and the metal a net tensile stress. The oxide neutral axis may be located either within the oxide layer or outside it, in the gas phase, depending on the thickness of the oxide and the relative creep strengths of the oxide layer and metal substrate. Evaluation of the stress distribution and average stress values in both the metal and oxide phases is provided for notional mechanical properties, within which the metal creep strength is varied widely. It is also shown how the assumption of elastic deformation can lead to large overestimates of stress values.