Ultrasonic cavitation damage characteristics of materials and a prediction model of cavitation impact load based on size effect

High-speed micro-jet produced by cavitation collapse near the wall is the main mechanism of material damage, and cavitation pit is the most typical damage feature. The reason why high-pressure and high-speed micro-jet can only cause nano- and microscale cavitation pit is that the micro-jet is a short-term impact load of nano- and microscale, and the material shows size effect during the formation of pits. To further explore the cavitation damage characteristics and deformation mechanism of materials, the theoretical framework of indentation test and J-C constitutive model were adopted, and the size effect of materials during the process of cavitation pit formation was mainly considered, and the prediction models of cavitation impact load, impact pressure and velocity of micro-jet were established. The results showed that the equivalent stress and strain of cavitation pit and the impact pressure and velocity of micro-jet are only related to the diameter-to-depth ratio of pit without size effect, and also to the diameter of pit with size effect. Larger diameter and deeper depth of the pit infers greater cavitation impact load, and the influence of the pit diameter is more obvious. When considering the size effect, there is an additional size effect coefficient: 1+54hpα2μ2bdp2σJC2. In the selected size range of pit, the cavitation impact load, impact pressure and velocity of micro-jet predicted with size effect increase by 0.9408%–322.5% compared with those without size effect. The maximum increase ratio appears at the minimum of diameter-to-depth ratio of pit (dp = 2 μm and dh = 2 μm), that is, the smaller the pit diameter is and the greater the depth is, the greater the increase ratio is. Ten typical cavitation pits were selected for inversion analysis. The impact pressure and velocity of micro-jet with and without size effect are 473–1131 MPa and 355–848 m/s, and 427–604 MPa and 320–453 m/s, respectively. The predicted values increase by about 11%-88% when considering the size effect, and the micro-jet velocity predicted is closer to that observed by high-speed cameras, which confirms the necessity and rationality of size effect in the inversion analysis of cavitation pits.