Residual Stress Development in Marine Coatings Under Simulated Service Conditions

Abstract

The most common experimental method of measuring the residual stress in coatings uses a measurement of the curvature, from which the stress can be computed provided that the thicknesses and Young's moduli of the two components are known. In this chapter, this approach was used and the behavior of a thermoplastic coating and a thermoset coating were compared during solvent evaporation/curing. The curvature was measured using a strain gauge, permitting continuous monitoring. The method was further extended to examine the changes in residual stress when the coating was submerged in water and when it was removed again to dry out. The highest residual stresses observed in this study were caused by differential thermal contraction between coating and substrate. A temperature change similar to that between a dry dock in a warm climate and the open sea gave stresses of 4 MPa and more, a significant fraction of the failure strength. Other sources of residual stress are complex and are probably highly specific to the coating composition. When using bi-layered coatings the changes in stresses were moderated somewhat and it appears that a significant and beneficial reduction in the stress magnitude can be achieved by appropriate combination of thicknesses of the two layers.