Evaluation of cavitation-induced pressure loads applied to material surfaces by finite-element-assisted pit analysis and numerical investigation of the elasto-plastic deformation of metallic materials

Abstract

Implosion of cavitation bubbles close to a material applies a high pressure load on the surface that leads to cyclic, elasto-plastic deformation followed by damage and loss of material. The load strongly depends on the flow conditions and its experimental determination is extremely difficult. This study presents a method for quantitative calculation of the pressure loads induced by collapsing bubbles. This method is based on the analysis of pits on the material surface formed within the incubation period. The pits are footprints of collapsing bubbles and are measured by atomic force microscopy (AFM). An inverse algorithm based on finite element method (FEM) simulations is then used to determine the pressure load that is necessary to form the measured pits. The pressure fields, which are assumed to be axially symmetric (bell-shape profile), were calculated for cavitation pits formed in pure copper. The pits were induced by short-term exposure to cavitation in an ultrasonic cavitation testing device. Additionally, the elasto-plastic deformation of copper was numerically (FEM) investigated for a given cavitation load. It was found that the deformation is mostly elastic and that the maximum stress is located in a subsurface region. The maximum pressure of the cavitation load, the resulting maximum plastic strain in the material, and the ratio of the elastic to total deformation work correlate well with the ratio of pit width to pit depth. In order to evaluate the simplified assumption of a static pressure profile (bell-shape), the calculated pressure loads were critically compared to those determined by a detailed single bubble simulation with a compressible CFD flow algorithm. The maximum pressure profiles of the highly-transient CFD results show partly significant deviations dependent on the non-dimensional bubble stand-off distance to the wall. An improved pressure load profile and transient effects will be considered next in the FEM algorithm.