Fracture of epoxy-based marine coatings as free films and substrated coatings under static tension

• Study of crack development in epoxy based coatings under static loads.
• Detection of coating cracking onset using digital image correlation.
• Comparison of strain to first crack in coatings as free film and on steel substrate.
• Fracture mechanics FE models for coating crack extension.
• Importance of coating toughness, defect size and residual stress to predict failure strain.

Factors controlling crack development in two formulations of heavily filled epoxy anti-corrosion coatings have been studied as free films and on steel substrates. Both coatings were nominally 300 μm thick. Tensile strength, Young’s modulus, strain to failure and fracture toughness of coating free films were measured at 23 °C. Tensile tests of both coatings on steel substrates were performed at 23 °C and strains to development of first coating crack and crack development with increasing strain were measured using extensometry and Digital Image Correlation (DIC). Finite element models of free film and substrated samples, incorporating non-linear stress–strain curves, were used to calculate residual stresses in substrated samples and JJ values of coating cracks. It was found that strain to first crack in coatings on substrates could be predicted using fracture mechanics models of coating cracks together with data on defect size, residual stress and toughness. Channelling crack development and factors influencing the relative ductility of free film and substrated coatings are discussed.