Experimental-based discussion for the separation of residual stresses and cold work in shot peened IN718 using high frequency Eddy current spectroscopy

Typical aero engine alloys, such as IN718, can be surface-treated by shot peening to induce near-surface compressive strains. To calc. the remaining operation time for those crit. aero engine components, a quant. nondestructive detn. of near-surface strain gradients has to be developed. We have demonstrated in the past, that it is possible to obtain a characteristic depth profile (surface and sub-surface) of the elec. cond. of shot peened specimen by using high-frequency eddy current techniques. The measured cond. profile is resulting from residual stresses, cold work, surface roughness, and the microstructure of the material. The objective is to measure residual stresses (sep. from other material properties) in such components after a defined life time. It can be assumed, that surface roughness and microstructure remain unchanged in IN718 materials over their lifetime, but cold work and residual stresses can change independently. Consequently, there is a need to clearly sep