5.8.2 Erosion progression
When a drop impinges on to the blade surface, a very high pressure shock is generated for a short period. The pressure wave causes plastic deformation of the material and repeated plastic deformation leads to an increase in the internal stress.
After a certain time, the stress concentrations in some areas exceed the tensile strength of the material, and cracks form. During this incubation, or primary period of erosion, there is little or no weight loss. When the cracks spread and join, however, the result is material breakdown. Tiny pieces of surface material are released and, during this secondary period, erosion rate increases to a maximum.
After the surface has become uniformly pitted, there follows a tertiary period during which the material lost by erosion falls logarithmically with time and after about a year may almost cease. The probable explanation for this is that the pores in the surface of the blade where erosion has taken place are partly-filled with water. This cushions the impact of the drops striking the blade and the impact forces are only transferred to the material in a damped form.