Experimental and Numerical Study on Cavitation Erosion of Polyurea Coatings

Jin-Keun Choi (NSWC Carderock)

Polymers are becoming more widely accepted as coating materials on ship hulls and propellers for the purpose of anti-fouling, drag reduction, and increase of maintenance intervals [1,2]. With the increased applications of these coatings on propellers and rudders, the resistance of polymeric coatings to cavitation erosion becomes of more interest. Among various polymeric materials, polyurea has been presented by its proponents as being of particular interest due to its convenience of application [3].
Observation of cavitation erosion of polyurea coatings has shown that failure is due to synergetic effects between the locally imparted impulsive loads and temperature rise of the material [4]. Repeated cavitation bubble collapses impart cyclic strain on the polyurea, resulting in viscoelastic and plastic work that generates heat in the material. Since polyurea is a poor heat conductor, the generated heat accumulates quickly in the coating. The shear modulus of polyurea is sensitive to temperature, and the material becomes weaker as the temperature increases. In the end, the material cannot withstand the stress and starts to flow like a molten material.

This presentation is a summary of a few papers on the cavitation erosion of polyurea [4,5,6], which address the material response in both experimental and numerical approaches. The temperature in the polyurea was monitored during exposure to a cavitating jet, and the temperature rise in the material was clearly seen to depend directly on the cavitating jet pressure, the thickness of the coating, the type of coating, and on the substrate material. Failure of the material was seen to correlate well with the temperature rise in the material. Numerically, the response of a viscoelastic material to a bubble collapse was modeled using a multiphase flow solver coupled with a finite element method (FEM) structural solver. Heat generation in the material was estimated from the energy dissipated by the plastic work in the material, and the predictions were compared with the experimental observations. 

Acknowledgments: This work was conducted at Dynaflow, Inc. and supported by the Office of Naval Research under contract N00014-11-C-0378 N00014-15-C-0052.

References

1. Korkut, E., Atlar, M., “An Experimental Study into the Effect of Foul Release Coating on the Efficiency, Noise and Cavitation Characteristics of a Propeller”, 1st Int. Symposium on Marine Propulsors, SMP09, Trondheim, Norway, June 2009.
2. “Propeller Coatings”, <http://greenfleet.dodlive.mil/files/2012/06/Propeller-Coatings.pdf>, U.S. Navy, Energy, Environment & Climate Change, 2014.
3. Amirkhizi, A. V., Isaacs, J., McGee, J., Nemat-Nasser, S., “An Experimentally-Based Viscoelastic Constitutive Model for Polyurea, Including Pressure and Temperature Effects”, Philosophical Magazine, Vol. 86, No. 36, pp.5847–5866, December 2006. 
4. Choi, J.-K., Chahine, G.L., “Experimental and Numerical Study of Cavitation Erosion Resistance of a Polyurea Coating Layer”, 4th International Symposium on Marine Propulsors, SMP’15, Austin, Texas, USA, June 2015.
5. Choi, J.-K., Marlin, P., and Chahine, G.L., “Experimental and Numerical Study of Polyurea Failure under Cavitation”, 5th International Symposium on Marine Propulsion, SMP’17, Espoo, Finland, June 2017.
6. Marline, P., Chahine, G.L., “Erosion and Heating of Polyurea under Cavitating Jets”, Wear, No. 414-415, pp. 262-274, 2018.