What term is used to describe a permanent and cumulative deformation of the turbine blades of a turbojet engine?

Creep

Explanation:

Creep is a type of deformation that occurs when continuous and long term mechanical stresses are being subjected to a material. Material deformation is observed even if the stresses are below the yield strength of the material,. The effect of creep is more pronounced if there is an application of heat.

In a study of turbine blades it has been shown that at the location where creep is present there was micro-structural degradation. In other places there was no micro-structural degradation.