Cavitation, Tensile Strength, and the Surface Films of Gas Nuclei 



Gas on the surface of solid materials in contact with water was found to be 

 responsible for cavitation at very low stress levels. In fact, most solid mate- 

 rials were unable to achieve tensile strength with water unless air was first 

 removed from the surface of the material. Poor molecular bonding started to 

 be important in the 40 to 50 psi tensile stress range. 



Gas 



If only a few air bubbles were present on a surface in contact with the 

 water, cavitation was brief at high stress levels, ceasing as soon as the bubbles 

 were hurled away from the surface of the material by the violence of the cavita- 

 tion action. Operation at high stress levels thus provided a ready means of 

 removing gas from a surface to make it suitable for noncavitating duty. 



On the other hand, numerous nucleation sites may exist on a surface, seed- 

 ing cavitation for extended periods of time. Nucleation sites are usually small 

 pores or surface imperfections a few microns in size containing gas (17). De- 

 pending upon the characteristics of the material at hand, prolonged soaking may 

 often (but not always) dissolve gas to remove sites from action. 



Figure 2a shows the effect of a surface containing many nucleation sites on 

 the severity of cavitation. Both the water and water/solid interface were weak, 

 so that each could cause cavitation independently. The severity of the cavitation 

 was measured by the harmonics picked up in the water by a hydrophone. The 

 fundamental frequency of the signal fed the transducer was filtered out from the 

 hydrophone output. Increasingly violent cavitation increases the amplitude and 

 frequency range of the harmonics produced (25). As can be seen, the use of a 

 high strength surface that limited the source of cavitation to the water greatly 

 reduced the severity of the cavitation. 



0.1 1-0 



ACOUSTIC POWER AT FACE - W/cm^ 



Fig. 2a - Effect of a solid surface possessing 

 nucleation sites on the severity of cavitation 



81 



