A high-frequency eddy current inspection system and its application to the residual stress characterization

This paper reports on an on-going project toward developing electromagnetic residual-stress characterization method. Specifically, we present recent work on an eddy current technique and model-based inversion method, specific to nickel-based superalloys. In our approach, we treat residual stress measurements essentially as a "layer-substrate" problem (a shot-peened layer on an alloy substrate), to which the swept-frequency EC technique is applicable. To this end, a high-sensitivity eddy current system with operating frequency up to 50MHz has been recently developed and validated. We present a description of our instrumentation that includes proprietary probes fabricated by the PCB technology, and laboratory-grade instruments under software control. The paper also describes the experimental procedure for performance studies, supported by preliminary data that distinguish clearly between surfaces before and after shot-peening for nickel-based alloys. In addition, we present forward and inverse model validations which will determine their reliability in the subsequent use in model-based residual stress profile inversion.