All surfaces immersed in seawater are subjected to biofouling – the attachment and growth of organisms such as algae and barnacles. Biofouling results in increased fuel costs and is responsible for introducing species to non-native environments. Silicone-based coatings significantly reduce the force required for foul removal, but applying the coatings on ship hulls may result in a non-uniform thickness (i.e., a thickness gradient). The pull-off behavior of single-layer silicone coatings with a thickness gradient has been studied experimentally, and it was found that the release behavior (critical pull-off force and location of release initiation) depended on both the average thickness and thickness gradient. For thinner coatings, release typically initiated on the thinner side of the coating and then proceeded to the thick side. A finite element model (FEM) was previously created to study the effect of coating thickness and thickness gradient on the stress at the epoxy/silicone interface during tensile pull-off loading and results matched the experimental findings. However, the loading direction also plays an important role in the foul release process. This study used FEMS to assess the interface stresses in silicone coatings of non-uniform thickness when a transverse load was applied to an epoxy pseudobarnacle in both the thin-to-thick and thick-to-thin directions.

FEMs of single-layer silicone coatings with non-uniform thicknesses were created in ANSYS Mechanical. An epoxy stud, modeled as an isotropic linear elastic material with a Young’s modulus of 3 GPa and Poisson’s ratio of 0.37, was rigidly attached to the silicone coating. The silicone was modeled as an isotropic linear elastic material with a Young’s modulus of 4 MPa and Poisson’s ratio of 0.35 or 0.49. The average silicone thickness varied from 0.25 mm to 5 mm, and the thickness gradient ranged from 0 to 0.12. A fixed boundary condition was applied to the bottom surface of the silicone. After a transverse load was applied to the epoxy stud, the shear stress profile at the epoxy/silicone interface was output. Extensive mesh refinement was required to accurately produce a smooth stress distribution near the sharp reentrant corner of the epoxy. The model was validated against experimental results, and was used to study a wide range of coating thickness and thickness gradients.

At a Poisson’s ratio of 0.49, in all cases simulated the shear stress was highest at the edge of the coating where the transverse load was applied because of the stress concentration. This matches experimental results that showed that the release always initiated at the location where the transverse force was applied. The effect of the average thickness and thickness gradient was very different depending if the force was applied in the thin-to-thick or thick-to-thin direction. There was very little differences in the shear stress curves along the silicone/epoxy interface as a function of either average thickness or thickness gradient when the transverse force was applied in the thick-to-thin direction. This matches experimental results that showed that the maximum applied shear stress required to initiate release remained fairly constant over all geometries tested. However, when the transverse force was applied in the thin-to-thick direction, there were discernible changes to the peak shear stress as a function of both average thickness and thickness gradient. For instance, the peak shear stress increased with increasing thickness gradient. Qualitatively, results were similar at a Poisson’s ratio of 0.35. These results can be used to tune the geometric parameters of foul release coatings to reduce the critical transverse forces required to remove biofouling.


Assessing the Effect of a Thickness Gradient on the Stress Profile at the Epoxy/silicone Interface of Thin Coatings Subjected to Transverse Loading With Finite Element Analyses

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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