The notched coating adhesion specimen: a fracture test for coatings and accelerated screening test for adhesion
A simple adhesion test method is proposed to provide estimates of the debond toughness of adhesive bonds. Notched coating adhesion (NCA) specimens consist of single substrates coated with thin layers of adhesive. The coating is notched to induce initial debonds, and the specimen is then loaded in a manner to produce tensile stresses in the coating. The substrate strain at which the coating debond propagates is then used to determine the critical strain energy release rate. Yielding of the substrate is permitted, and does not complicate the calculations. The specimen geometry results in a mode mix which drives the debond to the interface, thus obtaining a measure of interfacial behavior. Because of the geometry and testing method, the technique is simple, inexpensive and may be conducted quickly. The properties of the coating and the residual stresses of the bond must be known to predict the bond strength. Since accurate data on these properties are not always readily available, the test may be limited as a method to screen adhesive systems. Besides being a screening test, the NCA can be used as an accelerated test to study the durability of adhesive bonds. NCA specimens reach moisture equilibrium quickly because of the short diffusion path. By significantly reducing the amount of time needed for the adhesive/substrate interface to reach equilibrium moisture conditions, the time required to obtain estimates of performance in humid environments is greatly reduced. If one assumes that moisture at the interface is the cause of bond degradation, these simple tests offer the potential to rapidly estimate the durability of a given adhesive/substrate system. Accelerated durability studies were conducted on model steel/epoxy systems, and the results were compared to the results for double cantilever beam tests.