Validity requirements of circumferentially notched tensile specimens for the determination of the interfacial fracture toughness of coatings

A simple method was developed for evaluating the interfacial fracture toughness of coatings on substrates using circumferentially notched tensile (CNT) specimens. Mild steel cylindrical substrates of 0°, 15°, 30°, 45° and 60° notch angles with electroplated nickel were tensile tested. A well defined pre-crack was introduced at the interface for the quantitative evaluation of adhesion. In situ acoustic signals and scanning electron microscope were used to analyze the crack initiation and propagation. Finite element analyses were used to evaluate the critical interface energy release rate. The size of the plastic zone was determined for different notch angles to validate application of the linear elastic approach in determining the interfacial fracture toughness. The validity requirements have been proposed for this specimen, considering the yield strength of the coating and substrate, pre-crack position, notch angle and plastic zone size. The obtained interfacial fracture toughness values using CNT specimens was found to be very close to the values obtained by others using standard specimens.