Modelling scale failure in tension (fracture and spallation)

Abstract

The paper addresses some important issues that relate to the prediction of through-thickness cracking and spallation that can occur in oxide layers subject to local tensile stresses that arise during cooling following periods in service where the oxide layers form and thicken. The issues are addressed in the context of steam corrosion of ferrous substrates that leads to the formation of three corrosion layers, namely, spinel, magnetite and haematite (on the outer exposed surface). For this system, the magnetite layer develops tensile stresses that lead to through-thickness cracking in this layer. The first issue concerns the failure criteria that should be used when predicting the formation of through-thickness cracking. A popular approach is to assume that an oxide layer develops through-thickness cracks when a critical tensile stress (the oxide strength) or strain (the oxide strain to failure) is encountered. Another approach applies fracture mechanics principles to defects that are assumed to exist in the oxide layer, although there is great uncertainty regarding the relevant defect size distributions that control behaviour. A third lower bound (and conservative) approach, that is discussed in some detail, is to consider the energetics of steady state through-thickness cracking that avoids the fracture energy issue of needing to know the defect size that initiates through-thickness cracking. The applicability of the three approaches is discussed with regard to predicting the progressive growth of through-thickness cracking in the magnetite layer, and the importance of residual stresses. Example predictions are made using a proven analytical stress transfer model that enables simulations to be made of progressive through-thickness cracking in the magnetite layer.The second issue that is discussed concerns the development of interface cracking from through-thickness cracks that can lead to the spallation of oxide layers. One key factor is the influence of the spacing of through-thickness cracks that can determine the size of oxide fragments that might be released during spallation. Another key factor is the determination of conditions for steady-state debond growth that is a critical factor when considering conditions for spallation of oxide layers.Issues that are considered to require further investigation are highlighted.