Modeling of Cavitation Erosion Resistance in Polymeric Materials Based on Strain Accumulation

Cavitation erosion damage in elastomeric coatings is inherently different than in metallic surfaces. Low cycle fatigue appears as a reasonable underlying mechanism for initiation and progression of cavitation erosion in metallic surfaces. Therefore, the actual time rate of bubble collapses is somewhat irrelevant, while their total number determines the state of the surface. This not only allows for observation of the progressive damage, it also enables accelerated testing. In contrast, polymeric coatings may essentially show no signs of damage in one regime, while deteriorating rapidly and uncontrollably in a slightly higher intensity of testing. This threshold behavior may not be explained by low cycle fatigue. Instead an approach inspired by strain accumulation analysis in viscoelastic polymers was adapted to include post-yield behavior of polyureas. In this approach, the strain recovery in the polymer after an impact load representative of a bubble collapse may not be fully completed before the next cycle of loading. Therefore at high loading frequency, the un-recovered strain tends to accumulate faster. On the other hand, at very high frequencies, the viscoelastic material may not allow as much initial deformation due to higher stiffness affected by the rate of loading. The competition of these two effects leads to critical loading scenarios that may induce catastrophic damage. When the material may be loaded beyond its yielding limit, this effect is a lot more important and sensitive, as the larger strains induced post-yield will need longer times to recover. A computational model is presented to show the distinct thresholds in loading frequency and loading level in contrast with coating material characteristics, including both stiffness and relaxation spectrum. The results are compared with the observations of cavitation erosion damage in polyurea coatings.