Ellis 



Fig. 12 - Cavitation on the end of a 15- 

 kilocycle magnetostriction transducer. 

 Note that shock waves radiate from 

 centers on the interface (300,000 frames 

 per second; 1 5-nanosecond exposure 

 time; taken with repetitively pulsed ruby 

 laser using back illumination; 3,6X). 



pressure field. In addition, it was de- 

 sired to use several atmospheres 

 pressure to collapse the cavity and 

 thus be able to actually damage mate- 

 rials. At Cambridge the objective was 

 to use low pressures to slow down the 

 collapse and permit easier observa- 

 tion. This, of course, also limited the 

 damage producing capabilities of the 

 collapsing bubble. At Caltech high- 

 speed photographic equipment has been 

 especially developed (31) to facilitate 

 observation of more rapid events. Fig- 

 ure 12 shows one of the first high- 

 speed movie sequences taken with 

 ruby laser illumination. The subject 

 is the specimen end of a magnetostric- 

 tion type cavitation apparatus operating 

 at about 15 kilocycles. The quality is 

 poor mainly due to excessive scatter- 

 ing of the beam by scratches, etc., on 

 the Lucite window. However, the rela- 

 tive sharpness of the cavitation- 

 generated shock waves are of interest. 

 This is due to the shortness of the ex- 

 posure time, which in this case is about 

 15 nanoseconds. The interval between 

 pictures is about 3 microseconds, and 

 resulting measurements of the shock 

 velocities indicate they are not much 

 greater than sonic. Since this was 

 merely a test sequence, quantitative 

 data was not taken, but it is of some 

 interest because all of the shock waves 

 appear to originate on the surface and 

 what can be seen of the surface cavita- 

 tion appears very unsymmetrical. 

 However, other pulsed laser photo- 

 graphs such as Fig. 13 have been taken of cavitation generated by a barium 

 titanate ring apparatus (22), and some of these cavities do appear to collapse to 

 small volume and radiate shock waves. It may be of significance that damage 

 by the barium titanate ring apparatus is less severe than by the magnetostriction 

 apparatus (32,33). 



148 



