Abstract: Low stress repeated impact experiments and tests are done on medium carbon steel specimens and stainless steel spe- cimens who are coated with high strength Co or Ni alloy. The results show that plastic deformation happen in the coat mater- ial and a part of substrate material near the coat. The mean plas-tic deformation of every impact, along with the increment of impact frequency, is getting large at first and than turn small. And under accumulating, there is macroscopic plastic deform-ation along with the material hardening phenomenon. The value of total deformation and the size of deformation area are related to stress value of impact and materials strength. The degree of deformation hardening is attenuating from surface to inner, and this phenomenon only occurs at the surface of impact contact and the next following length, forming an “easy deformation area”. The analysis considers that repeated impact deformation is a kind of cumulative fatigue damage. When the peak stress is same, the absorbed energy of material under the repeated imp-act contact load is more than under the static fatigue load. And absorbability is in inverse proportion with the distance from the impact point. Repeated impact can intensify the mobility of atom, reduce critical shear stress, and make the enablement and multiplication of dislocation easy.

Key words: Coated part, Low stress, Plastic behavior, Repeated impact load