Bernd 



Certain types of nucleation sites were shown to be troublesome and so were 

 avoided in sonar transducer construction. These were: 



1. Fine cracks and joints. 



2. Pores created in metals by electric arc welding. 



3. Pores in ceramics. 



4. Nucleation sites in permeable materials such as rubberlike polymers. 

 Appreciable amounts of air may be contained within such materials. The air 

 diffuses through the material to reach the nucleation sites at the surface, keep- 

 ing them in action almost indefinitely. 



It was found feasible to determine 

 the suitability of a polymer for non- 

 cavitating duty by counting the number 

 of nucleation sites and measuring the 

 amount of gas contained in the mate- 

 rial. If only a small amount of gas was 

 in the material, and if it possessed 

 only a few nucleation sites, the result- 

 ant cavitation was not severe. The air 

 feeding the sites was soon exhausted, 

 stopping the cavitation without notice- 

 able damage within a minute or less. 

 Figure 2b shows this process taking 

 place in a porous Teflon surface not 

 considered usable. Here the duration 

 of cavitation was several hours. 



Fig. 2b - Cavitation caused by a 



porous Teflon surface An annoying source of trouble was 



the small bubbles that adhered tena- 

 ciously to the surface of materials that 

 were rough and possessed wetting 

 angles larger than 45°. (The wetting angle is the angle of the gas/liquid inter- 

 face of the bubble to the surface of the solid, measured through the liquid as 

 shown in Fig. 3.) Many bubbles would adhere to the surface when first sub- 

 merged in water. These materials, even when wet, also picked up bubbles from 

 the water. Such bubbles may take hours to dissolve. Smooth, low wetting angle 

 materials were therefore found desirable in order to achieve high tensile 

 strengths without difficulty. 



A rating of a few materials for noncavitating duty in the light of these ex- 

 periences is given in Table 1. Galvanic action is included as being undesirable; 

 low carbon steel was found unable to achieve high tensile strength because of it. 



Glass in the form of Pyrex beakers was used to face the acoustic trans- 

 ducers used in the laboratory. The transducers (18 kc/s) were small, and so 

 could readily be encapsulated in a class beaker. Wet, clean glass was outstand- 

 ing in being nonporous, without nucleation sites, while possessing a wetting 

 angle approaching zero. Polyurethane was used for large prototype sonar 



TIME - HOURS OF OPERATION 



82 



