Bernd 



Alternately, the creation or utilization of tensile strength is a desirable 

 goal in order to improve the performance of devices normally limited in maxi- 

 mum output by cavitation, such as ship sonar, pumps, turbines, and propellers. 

 The gains, the prevention of cavitation damage, obtaining a greater output from 

 a given size or weight of equipment, i.e., an increase in return for a given eco- 

 nomic investment, are tempting. 



Hence, a specific objective was to learn how to operate sonar transducers 

 at power levels above the commonly accepted limit at which cavitation occurs, 

 but without cavitation. 



In ordinary water, the maximum tensile strength attainable should be lim- 

 ited by the strength of the weakest link in the system consisting of water, dis- 

 solved materials, undissolved gas, solids, and possibly other liquids that may 

 comprise the water and its immediate environs. The weakest links found to be 

 important in the to 100 psi stress range were small gas bubbles (gas nuclei) 

 in the water, gas bubbles on the surface of a solid, gas nucleation sites in cracks 

 or pores in the surface of the solid, and the strength of the molecular bond be- 

 tween water and solid. 



Various solid materials were therefore categorized and tested to determine 

 the minimum tensile strength at which cavitation would ensue as a consequence 

 of the conditions at the water/solid interface. "High strength" materials were 

 selected so that the water/solid interface would not be the weakest link in the 

 system. Sonar transducers covered with these high strength materials were 

 then used to stress water to the point of rupture, i.e., the inception of cavitation. 



The tensile strength of the water — once weak water/solid interfaces were 

 removed — was limited by the presence of gas nuclei in the water. Gas nuclei 

 are small bubbles containing air. When cavitating, gas nuclei are the end prod-' 

 ucts of cavitation voids produced in a low pressure area. Waves and splashing 

 at the surface, of water ingest small air bubbles to produce nuclei. 



In a body of water, the tensile strength should increase with depth. This is 

 due to several actions. Generally speaking, the tensile strength of the water is 

 inversely proportional to the maximum size of gas nuclei present. Large bubbles 

 that prevent tensile strength from occurring rise rapidly to the surface; small 

 bubbles rise slowly and so remain behind. However, small bubbles tend to dis- 

 solve. A high ratio of interface surface to gas volume, and internal pressure 

 created within the bubble by surface tension, favor the dissolving of small 

 bubbles. In addition, dissolving is promoted by increasing depth. Hence bubble 

 size decreases with increasing depth because of the relative rates of rise, and 

 because of dissolving. 



Tensile strength tests were made to see if this picture were so. Figure 1 

 gives the tensile strength obtained versus depth in two fresh water lakes, using 

 the specially constructed sonar transducers to stress the water. The tensile 

 strength did indeed increase with depth. A 6.7-db increase in power level above 

 the "normal" inception of cavitation was obtained at a 39.4-foot depth before 

 cavitation took place. Thus a 36.1-psi tensile strength was obtained at a 39.4- 

 foot depth, and the transducer cavitated as if it were at a 123.0-foot depth 



78 



