AbstractMechanical damage induced failure within protective coatings is one of the main reasons for loss of coating integrity. Thus, for applicability of any coating and sealing technology it is mandatory to know the stress/strain levels at which mechanical component failure will occur and it is important to understand which physical entities drive damage initiation and propagation. Within this work a model has been developed which allows correlating and study the effect of brittle porous coatings on the stress-strain curve evolution of plasma electrolytic oxidation (PEO) coated extruded Mg substrates. This is a great benefit as deriving material laws might be challenging since measured stress-strain relationships are a convolution of substrate and coating material contribution. The approach is based on a damage model which allows distinguishing between the substrate contribution model as a bulk body described by dedicated material laws, and the brittle coating contribution mathematically modelled as a boundary condition. The effect of coating thickness, the resulting steady state crack spacing and the contribution of coating porosity on the stress-strain curve is shown. The approach allows direct estimation of PEO coating barrier properties from slow-strain rate tensile testing.