Abstract
Cavitation erosion during the incubation period was investigated via pitting tests conducted on three different materials: an Aluminum alloy, a Nickel Aluminum Bronze alloy and a Duplex Stainless Steel. Pitting tests were conducted in a cavitation tunnel in the velocity range 45-90 m/s at a constant cavitation number. The test section was made of a straight nozzle 16 mm in diameter discharged into the radial 2.5 mm space between two flat walls. Cavitation appears in the form of a toroidal cavity attached to the nozzle exit and damage on the samples facing the nozzle is concentrated in a circular ring centered in the cavity closure region. The exposure time was adjusted to avoid pit overlapping. The material surface was examined using a conventional contact profilometer which allowed us to identify the pits, count them, and measure their main characteristics such as depth, surface area, and volume. From these the pitting rate, the coverage rate, and the depth of deformation rate were defined. Pits were classified according to their diameter. For all materials and operating conditions, pitting rate appears to follow an exponential law in relation to the pit diameter. This law depends upon two parameters only, which were identified as the coverage time (i.e. the time required for the surface to be covered by erosion pits) and a characteristic pit diameter i, which corresponds to the pits whose contribution to the coverage process is the highest. Scaling laws for pitting were derived accounting for both material properties and flow velocity, and a procedure to make pitting test results non-dimensional is proposed. The influence of the material on pitting test results was analyzed. It is shown that the damage is not correlated in simple terms with the elastic limit determined from conventional tensile tests and it is conjectured that other parameters such as the strain rate might play a significant role and should be included in the analysis. The effect of flow velocity on both parameters and i was analyzed and a classical power law was found for the influence of the flow velocity on pitting rate for all three materials. Finally, some analysis and discussion is given concerning distributions of pit volume and pit depth.
Highlights
This paper is devoted to the incubation period of the cavitation erosion process which precedes material removal and mass loss
The present work shows that the distribution of pits with diameter can be approximated by an exponential law
Scaling laws for pitting tests can be analyzed on the basis of both parameters
Summary
This paper is devoted to the incubation period of the cavitation erosion process which precedes material removal and mass loss. Other devices have been developed for special purposes such as vortex cavitation generators [13,14], the Hopkinson bar type impact device [15], the magnetic impact testing machine [16], the rotating disk cavitation apparatus [17,18] or submerged cavitating jet devices [19,20,21,22] Some of these techniques were standardized and resulted in American Society for Testing and Materials (ASTM) Standards such as G-32 “Test Method for Cavitation Erosion Using Vibratory Apparatus” and G-134 “Test Method for Erosion of Solid Materials by a Cavitating Liquid Jet”. The analysis of pitting tests on different materials carried out in this paper is an important step in the development of a technique to assess the cavitation intensity from pitting tests
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