Modelling the effect of temperature on crack onset strain of brittle coatings on polymer substrates

The effect of temperature on the crack onset strain (COS) of brittle coatings on polymer substrates was investigated through a series of temperature-controlled tensile tests carried out in situ under an optical microscope. It was observed that the failure of such materials under tensile strain was strongly affected by temperature, but the exact behaviour was heavily dependent upon the type of material used. Below the glass transition temperature, Tg, of the polymer substrate, an increase in temperature led to a decrease in crack onset strain. Above the Tg, the substrate softening effects and corresponding shrinkage behaviour had a presiding role, leading to an increase in COS at elevated temperatures. The experimental COS data were modelled as a linear superposition of an intrinsic COS and the internal strain taking into consideration the respective influences of temperature dependent energy release rate for crack propagation and thermal expansion behaviour. Using adjustable values of the coefficient of thermal expansion and toughness of the coating, the model was found to accurately reproduce the change of COS with temperature of two different coatings on aromatic polyester substrates. The proposed approach enables, for any thin film composite with known material properties, the COS at any given temperature to be predicted.