Application of acoustic emission to detect damage mechanisms of particulate filled thermoset polymeric coatings in four point bend tests

In this study, particulate filled thermoset polymeric coatings with two different coating thicknesses were subjected to four point bend loading while being monitored by acoustic emission in order to establish the failure modes of the filled polymeric coatings. The polymeric coatings were approximately 260 μm and 350 μm in thickness on a steel substrate, and filled with 9% by volume of calcium silicate. After testing the thinner coating was found to exhibit more microcracks on the surface of the coating while the thicker coating showed more extensive delamination between the coating layer and the steel substrate. A noticeable load relaxation was observed during the plastic deformation of the coating samples which was coincident with an increase in acoustic emission. This was linked to the development of macrocracks and their propagation through the coating thickness. The number and amplitude of the acoustic emission events were investigated and correlated with the failure mechanisms present during the four point bend test, such as debonding of the embedded fillers, delamination of the coating layer from the substrate and the initiation and propagation of cracks. This then allows the failure modes of particulate filled thermoset coatings and their location to be identified from the acoustic emission detected during the four point bend test.