The U.S. Air Force has a strong strategic and economic interest in extending the safe life of the gas turbine engines in its inventory. Many research programs have investigated the relationship between near surface residual stresses, introduced during manufacturing by processes such as shot peening, and the fatigue life of engine component alloys. At the present, there is interest in extending engine life by taking credit for residual stresses that extend fatigue life; however, this requires accurate, nondestructive measurement of the residual stresses.

In recent years, research programs have established that residual stresses in some nickel-based engine alloys can be accurately and repeatably calculated from electrical conductivity measurements acquired using eddy current (EC) nondestructive testing (NDT) methods. This work has also shown that, under laboratory conditions, conductivity measurements as a function of eddy current frequency can be used to calculate residual stress as a function of depth in the material.

In November 2005 the U.S.A.F. awarded a competitively-bid contract to the University of Dayton to demonstrate the feasibility of incorporating the EC NDT residual stress measurement technique into an instrument designed for eventual depot deployment. The program goals focused on creating an EC instrument/system that can reliably and repeatably measure electrical conductivities which allow calculation of residual stress as a function of depth in nickel alloys. Key performance requirements include:

- electrical conductivity measurements accurate to within 0.1 percent relative to the nominal alloy conductivity

- measurement times of less than 5 minutes

- EC frequencies from 100 kHz to 50 MHz

- instrument design anticipates eventual usage in a depot environment as part of an automated inspection system

This presentation will describe the program goals, instrument design, and experimental results. The potential for moving the technology to the depot floor will also be discussed.