# A Review on the Erosion Mechanism in Cavitating Jets and Their Industrial Applications
## 3 Erosion Mechanism in Cavitating Jets  
Hydrodynamic cavitation enhances waterjets efficiency for cleaning and drilling applications. The performance of cavitating jets in those applications is defined by the cavitation erosion they can produce. When cavitation bubbles collapse, the vapor inside the bubble is compressed, generating shock waves. The other important phenomenon which affects material damage is the re-entrant jet: cavitation bubbles become unstable near the wall (located at about one jet diameter distance) and forms a microjet which penetrates into a material [39]. Shock waves and the re-entrant jet can co-exist during a short time interval when the cavitation bubble is at a maximum expansion near the wall [40; 41]. When the pressure which is generated by shock waves and microjets exceeds the strength of the material, it fails locally and incubation pits of different size (typically 2-3 hundred of microns in diameter) start to form across the affected surface. This mechanism leads to cavitation erosion and can result in significant material damage [42; 43; 44; 45].  
Special nozzles can be designed to maximize erosion rates. Nozzle geometry in addition to the nozzle material and its surface finish are crucial for a more effective cavitating jet. In most cavitating jets, cavitation starts inside the nozzle and can damage it during the operation. Therefore, the material selection depends on specific application and should be carefully conducted. For drilling applications or radioactive scale cleaning, when the same nozzle should be used for a long duration, it is usually manufactured from titanium alloys that can better resist the cavitation incubation process.  
[PERSON] et al. [39] discussed the relation between the cavitation impact and the material response when exposed to high intensity cavitation. It is known that in such case, the material is exposed to short duration pressure pulses capable of inducing plastic deformation in the subsurface layers and producing permanent micron-sized deformation pits [45]. [PERSON] et al. [39] illustrated the importance of determining the range of strain rates applicable for a given cavitation impulsive load configuration. The authors confirmed that the consideration of the strain rate effects allows not only a better understanding of the erosion behavior of different materials, but also helps to elaborate more accurate predictive models for long term erosion.  
The strain rate of cavitation impulsive loads can be expressed using the following equation:  
\[\dot{\epsilon}=\frac{\Delta\epsilon}{\Delta t}=\frac{\Delta l}{l}\times\frac {1}{\Delta t}=\frac{\Delta l}{\Delta t}\times\frac{1}{l}=\frac{V}{l}\]  
where \(\Delta\epsilon\) is the mean strain caused by an impulsive load, \(\Delta t\) is the peak duration, \(V\) is the displacement velocity of the impacted surface, and \(l\) is the length of the plastic zone resulting from an impulsive load. \(V\) and \(\Delta t\) can be evaluated experimentally [46; 47; 48]; \(\Delta\epsilon\) and \(l\) are usually estimated from a plastic deformation model of the material.  
The authors also showed by comparing the flow stress as a function of the strain rate from different studies that a strain rate of \(5\times 10^{3}\) s\({}^{-1}\) corresponds to the transition between low and high strain rate sensitive plastic flow. It was shown that for \(\dot{\epsilon}<\) 10\({}^{3}\) s\({}^{-1}\), where the plasticity is controlled by thermally activated processes and the dislocation dynamics, the effect of strain rate is weak. In contrast, for \(\dot{\epsilon}>\) 10\({}^{3}\) s\({}^{-1}\), where the plasticity is controlled by viscous drag on dislocation motion (velocity dependent), the effect of strain rate is more pronounced [49; 50].