Advances in Coating Adhesion Testing Using the Blister Test, Digital Image Correlation and Finite Element Methods

Adhesively bonded aluminum joints are widely used in multiple engineering sectors, especially the automotive industry. In long-term service, environmental conditions and stresses caused by factors such as internal deformation, residual stresses, or corrosion, cause adhesion strength degradation. Quantifying the adhesion strength is a complex task. While many different adhesion tests are available, the Blister Test (BT) is the only one that provides a measure of the normalized energy required to initiate and propagate a crack at the polymer/substrate interface. The adhesion strength of coatings on aluminum substrate for automotive applications was investigated using the Blister Test (BT) along with Digital Image Correlation (DIC), and Finite Element Methods (FEM). The BT was performed by pressurizing a fluid through a hole in the substrate, which causes blistering on the coating, ultimately leading to delamination. This test is performed after exposure of samples to an accelerated corrosion exposure. The DIC system obtains a full field displacement history of the de-adhesion mechanism using high speed cameras by tracking small subsets of the material in its field of view. This full field displacement history obtained from the BT is compared against theoretical displacements obtained through FEM and an inverse optimization algorithm, allowing the quantification of adhesion strength. Along with the BT, the effects of the surface treatments will be appraised through filiform corrosion (FC) testing. While there have been multiple investigations in the field of adhesion strength using the BT, the incorporation of a DIC system allows recording of the full field shape of blister evolution and debonding propagation. The use of FEM and inverse optimization algorithm to determine the adhesion strength allows for these calculations without making typical assumptions such as constant curvature and small deformations. The appraisal of FC which is prevenient in the automotive industry, along with the BT allows a better understanding of the effects of various surface treatments on adhesion strength and corrosion.