The failure of iron oxide scales at growth temperature under tensile stress

Acoustic Emission (AE) was used to study the fracture of oxide grown on mild steel. Samples were oxidised and mechanically deformed in air at 550 OC using strain rates of -and lo-' s-' in a bend test furnace having a ceramic 4-point bend jig. The results were compared with earlier observations made on oxides deformed at room temperature. In both cases equidistant cracks formed and the crack spacing decreased with increasing oxide thickness in agreement with a model based on interfacial slip. The adhesive strength of the oxide was lower and delamination was more evident at the growth temperature. The critical strains for failure were determined and compared with model predictions. In this context the voidage of the oxide scale appeared to be the single most important parameter and a KIc -value of 1.1 MN m-3/2 was obtained for iron oxide up to 550 OC.