47 



be noted that the value of k computed from the experimental results includes 

 the effect of nonuniform jet, if any, and the (small) effect of friction.) 

 The important result is evidently that once the mixing process can be accur- 

 ately characterized for diverging conduits, an analysis of this type should 

 lead to quite accurate predictions of the points at which damage might be ex- 

 pected for such systems. 



THE MECHANISM OP COLLAPSE AND THE ASSOCIATED DAMAGE 



INTRODUCTORY REMARKS 



Among all the many aspects of the cavitation phenomenon, perhaps the 

 greatest amount of controversy has been in connection with the origin of the 

 damage associated with cavitating liquids. Although there is general agree- 

 ment among investigators that, both from theoretical and experimental observa- 

 tions, cavitation damage may be attributed to purely mechanical action leading 

 to eventual fracture of brittle materials and fatigue failure of ductile ma- 

 terials, there is still a small group who are convinced that the damage is 

 chemical or electrochemical in nature associated with or hastened by the cavi- 

 tation. It will not be possible in this survey to examine critically all the 

 arguments pro and con the latter theories. The discussion will be concerned 

 primarily with the results which indicate that the pressures developed in the 

 collapse of a cavitating region are sufficient to cause damage, while only a 

 few remarks will be made concerning the ideas of associated chemical action. 

 A reference will also be made to more recent ideas on cavitation damage which 

 associate damage with the corrosive action of electrical currents within the 

 solid, itself, generated by crystal deformation under repeated impact. It 

 should be noted that the earliest observers attributed the observed corrosion 

 to chemical action. However, it should be emphasized that a great deal of 

 confusion arose on this point since damage on propellers after service at sea 

 might be associated not only with mechanical damage due to cavitation but also 

 with chemical corrosion associated with sea water. However, controlled experi- 

 ments have demonstrated that mechanical erosion is the only tenable explana- 

 tion for the majority of cases. This point will be elaborated further. The 

 hydrodynamic and thermodynamic processes involved in the collapse of cavitated 

 regions and the origin of damage from this point of view will be discussed 

 first. Secondly, a description of the possible mechanism which may account 

 more clearly for some anomalous results will be proposed. Finally, some re- 

 marks will be made on the role of electrochemical phenomena in the cavitation 

 process. Any discussion of metallurgical aspects of the problem are beyond 

 the intended scope of the discussion. 



