Published: Jan 1970
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Increasingly severe requirements for materials in modern high performance applications have re-emphasized the need for fundamental understanding of erosion processes associated with cavitation and liquid impingement. In recent years, there has been disclosed a number of concepts which are proving helpful in the description of both types of erosion in a wide range of hostile environments (high temperatures, corrosive liquids, etc.). Such probings are providing a framework for development of useful methods of analysis and prediction of the response of materials under cavitation attack and liquid impact. In spite of the ingenuity of these ideas and investigations, however, the nature of the complex interactions among the many parameters involved inevitably has resulted in controversies which have yet to be resolved.
Perhaps the most important concepts which have made possible rational correlation attempts and show promise of a unified treatment of erosion are those of energy absorption. In both types of erosion, very useful results have been obtained based on static strain energy, but a more complete and critical evaluation of these ideas must await the accumulation of data on behavior of materials at high strain rates. Strain rates of interest are those associated with cavitation bubble collapse and with liquid droplet velocities typical of rain impact on high-speed aircraft and droplets in wet steam or vapor turbines.
Whether cavitation damage descriptions can be treated independently of the manner in which the pressures of collapsing cavities are applied—shock waves or internal jet formation—still requires investigation. Internal jet impact is analogous to droplet impingement. These problems are connected intimately with the hydrodynamics of cavitation bubbles, droplet deformation, and the physical properties of the liquid environment. In the latter connection, recent work in hot liquid alkali metals has added to the background information needed to achieve useful correlations.
The dependence of rate of erosion on exposure time and the existence of definite “zones” of erosion (incubation, accumulation, attenuation, steady state,) now seem clearly established for both cavitation and impingement attack. The mechanisms which account for this behavior, however, whether hydrodynamic, mechanical, metallurgical, or combinations of these, still require clarification. Very impressive correlations have been achieved for data in the so-called steady-state zone using strain energy concepts, but even here there is a question about the “steadiness” of this zone. Phenomenological fatigue theories have been adapted to describe the history of damage progression with good results. Here again further research is required to clarify whether the role assigned to fatigue is truly governing in the context of these theories or whether the correlations based on statistical fatigue theories are physically correct.
cavitation, erosion, impingement, evaluation, tests
President, Hydronautics, Inc., Laurel, Md.