Bernd 



transducers (3 to 4 kc/s) because it could be cast in large sizes, contained few 

 nucleations sites and little gas, and stood up well under cavitation. Stainless 

 steel, if smooth and degreased, was also satisfactory. All of these materials 

 have high bond strength to water. 



Bond Strength Between Water and Solid 



Solid materials were treated to remove gas from the surface. Physical 

 surface contaminants foreign to the material (oil, grease, dirt) were removed. 

 Thus the weak link under observation was the bond between water and solid. 

 Materials were selected over as wide a spectrum as possible. Correlations 

 were sought between known physical properties and bond strength so that entire 

 general categories of materials could be classified as being "cavitating" or 

 "noncavitating" on the basis of a reasonable number of tests. 



It was assumed that the tensile strength of the bond between water and solid 

 was obtained when increasingly severe degassing treatments did not raise the 

 tensile strength any further. An initial treatment was soaking in ethyl alcohol 

 or water and then if necessary subjecting the solid immersed in these liquids to 

 higher and higher pressures. This forced gas to dissolve. Assuming adverse 

 conditions of geometry and wetting angle, the pore size filled in overcoming the 

 surface tension of water in this way is approximately as given in Table 2. Little 

 change was noticed above 100 psi, implying that pore sizes much less than a 

 micron were not present as a rule. 



Table 2 

 Pore Size Filled in a Solid Immersed at Pressure 



In the supersaturated gas tests used for rapid screening, dissolved carbon 

 dioxide gas in the water was used to stress the bond until rupture occurred, as 

 evidenced by gas evolution. These tests placed the tensile strength of the bond 

 into groupings; i.e., 0-24, 24-48, 48-72, 72-96, 96-120, 120-135, or greater than 

 135 psi. The material under test was submerged in water, and the water was 

 pressurized with carbon dioxide until saturated. This application of pressure 

 also dissolved free air. The pressure was then decreased in uniform incre- 

 ments until gas evolved from the surface of the material. The duration of the 

 test was kept limited in time so that negligible amounts of gas would escape by 

 diffusion through the gas/liquid interface at the surface of the water, thereby 

 keeping the stress (degree of supersaturation) constant for each pressure 

 increment. 



84 



