Fifth International Symposium on Marine Propulsion smp’17, Espoo, Finland, June 2017 Experimental and Numerical Study of Polyurea Failure under Cavitation Jin-Keun Choi 1,2 , Pauline Marlin 1 , Georges L. Chahine 1 1 DYNAFLOW, INC., Jessup, Maryland, USA ABSTRACT Observation of polyurea failure in a cavitation field suggests the strong occurrence of temperature rise in the material in addition to mechanical load. In the present work, the thermal response of polyurea under cavitation loads is studied both experimentally and numerically. In the experimental study, the temperature in a polyurea coating is monitored during exposure to a cavitating jet. Significant temperature rise in the material is clearly measured and is seen to directly increase with the cavitating jet velocity and to depend on the thickness of the coating and on the substrate material. Measurements of the material deformation and failure are seen to correlate very well with the temperature rise in the material. Numerically, the response of a viscoelastic material to the pressure generated by a bubble collapse is examined using a finite element method. Heat generation in the material is predicted from the energy dissipated by the plastic work in the material, and heat conduction is then estimated and found to have a time scale much longer than heat generation. The numerical modeling explains well the observed temperature evolution including the effects of coating thickness and substrate material. Keywords Cavitation, Erosion, Polyurea, Thermal Failure, Heat Conduction. 1 INTRODUCTION Polymeric materials are becoming more widely used as coatings on ship hulls and propellers for purposes such as anti-fouling and drag reduction (Korkut & Atlar 2009; U.S. Navy 2012). With increased applications of these coating materials on propellers and rudders, it is necessary to investigate their resistance to cavitation. Among various polymeric materials, polyurea has been presented by its proponents as being of particular interest due to its easy application and good performance against shock from explosions (Amirkhizi et al 2006). In recent years, we have conducted extensive cavitation erosion tests on various polymeric and other coatings and observed different modes of failure with polyurea showing a combined mechanical and ‘thermal’ failure (Choi & Chahine 2015). This is explained by the fact that repeated cavitation bubble collapses on the polyurea impart repeated stresses resulting in cycles of deformation and viscous and plastic work, which generate heat in the material. Since polyurea is a poor heat conductor, the generated heat accumulates to different extents depending on the thickness of the coating and on the heat conduction properties of the substrate. The shear modulus of polyurea is very sensitive to temperature, and the material becomes softer as the temperature increases. In the end, the material cannot withstand the stress and starts to flow like a molten material as shown in Figure 1. In this paper, the thermal response of polyurea to cavitation loads is studied both experimentally, using the cavitation generated by cavitating jets, and numerically, using a structural finite element method code. The temperature in the polyurea is monitored over time for different cavitating jet pressures and different polyurea coating thicknesses and different substrate materials. 2 EXPERIMENTAL STUDY 2.1 Polyurea Sample Preparation A circular polyurea sample molded in a 6 mm thick Plexiglas holder and equipped with a PVDF transducer, as shown in Figure 2, was used for the initial tests. The PVDF pressure measurements are not reported in this paper. The diameter of the polyurea coating area was 1 inch, and the thickness of the polyurea was 2 mm. The polyurea was prepared at the University of Massachusetts at Lowell, by mixing Isonate 2143L and Versalink (Amirkhizi et al 2006). To monitor the temperature distribution in the material, four K-type thermocouples (Omega ® Model No. 5SC- KK-K-30-36) were implanted at 2.5 mm intervals, with the first thermocouple positioned at the jet axis. The 0.25 mm diameter thin thermocouples were installed from the bottom of the sample through holes made in the substrate. After inserting the thermocouples, the hole was sealed with silicon adhesive E6800 to prevent the 2 Currently at Naval Surface Warfare Center Carderock Division