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Increasing attention is being given to the observed variations in the time rate of weight loss during impingement and cavitation erosion testing. It is generally recognized that a fuller understanding of these variations is needed for more meaningful use of the test results and more insight into erosion mechanisms. A pattern often observed is that of an initial period of little or no weight loss, successively followed by a period of high rate of loss and one of diminished rate of loss. Some results, however, follow other patterns, such as a continuously declining rate or a continuing sequence of fluctuations. There has been no full agreement concerning the causes for, and the significance of, the various phases. Part I of this paper reviews the pertinent findings in the literature and discusses each of the following influences in some detail: (a) The effect of the geometrical changes in the eroded surface on the macroscopic flow, cavitation, or impingement conditions which determine impact severity. (b) The effect of the metallurgical and geometrical changes in the surface on the resistance of the surface to the impacts. (c) The relative significance of the various material-removal mechanisms, such as single-impact failure, fatigue failure, and corrosion. Part II develops a tentative statistical model of the erosion process for the case where fatigue is the predominant failure mechanism. The analysis predicts rate-time patterns similar to many of those observed, and it leads to the conclusion that the instantaneous erosion rates during nonsteady periods are strongly dependent on the scatter associated with finite fatigue life and with test parameters such as bubble or drop diameters. While the analysis predicts the eventual attainment of a steady-state rate independent of that scatter, this state is probably often unattainable in practice because the increasing roughness of the surface itself affects the erosion rate.
erosion, impingement, cavitation, work hardening, roughness, surfaces, mathematical models, statistical analysis, cracking, fatigue (materials)
Heymann, F. J.
Senior engineer, Westinghouse Electric Corp., Lester, Pa.