High-frequency eddy current conductivity spectroscopy for residual stress profiling in surface-treated nickel-base superalloys

Recent research results indicated that eddy current conductivity measurements can be exploited for nondestructive evaluation of subsurface residual stresses in surface-treated nickel-base superalloy components. Most of the previous experimental studies were conducted on highly peened (Almen 10-16A) specimens that exhibited harmful cold work in excess of 30% plastic strain. Such high level of cold work causes thermo-mechanical relaxation at relatively modest operational temperatures; therefore the obtained results were not directly relevant to engine manufacturers and end users. The main reason for choosing peening intensities in excess of recommended normal levels was that in low-conductivity engine alloys the eddy current penetration depth could not be forced below 0.2 mm without expanding the measurements above 10 MHz which is beyond the operational range of most commercial eddy current instruments. In this paper we report the development of a new high-frequency eddy current conductivity measuring system that offers an extended inspection frequency range up to 50 MHz with a single spiral coil. In addition to its extended frequency range, the new system offers better reproducibility, accuracy, and measurement speed than the previously used conventional system.