AeroMat Home      Exposition      To Register      ASM Homepage
Back to "Session 1: NDI Methods for Residual Stress Characterization" Search
  Back to "Nondestructive Evaluation/Health Monitoring/Prognostics" Search  Back to Main Search

Tuesday, May 16, 2006 - 10:00 AM
NEHMP061.3

Development of a Residual Stress Depth Profile Measurement Instrument

D. Stubbs, J. D. Hoeffel, W. C. Hoppe, R. T. Ko, J. R. Sebastian, University of Dayton Research Institute, Dayton, OH

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. In the past few years experimental 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 components. At the present, there is interest in enhancing engine life by taking credit for these residual stresses that extend fatigue life, however, the stresses must be accurately measured in order to be of benefit.

Recent 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 frequency can be used to calculate residual stress as a function of depth in the material.

The Air Force has recently awarded a competitively-bid contract to the University of Dayton to demonstrate the feasibility of incorporating the EC NDT residual stress measurement technique into a working instrument. The program goals are to create an EC instrument 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:

1) Electrical conductivity measurements accurate to within 0.1 percent relative to the nominal alloy conductivity,

2) Measurement times of less than 5 minutes,

3) EC frequencies from 100 kHz to 50 MHz.

This presentation will describe the program goals, approaches, and expected results. Information about the current state of research in using EC NDT for residual stress measurement will be presented including limitations of the technology. The potential for quickly moving the technology to the depot floor will also be discussed.


Summary: 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 maintenance processes have been implemented at Air Force depots during the past twenty years to help ensure safe usage and extended life. In the past few years numerous experimental programs have investigated the implicit relationship between near surface residual stresses, introduced during manufacturing by processes such as shot peening, and the fatigue life of engine components. At the present, there is interest in enhancing engine life by taking credit for these residual stresses that extend fatigue life, however, the stresses must be accurately measured in order to be of benefit. Recent Air Force sponsored 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. Further, this work has shown that, under carefully controlled laboratory conditions, conductivity measurements as a function of frequency can be used to calculate residual stress as a function of depth in the material. Based on the success of the recent research, the Air Force has recently awarded (late October 2005) a competitively-bid contract to the University of Dayton Research Institute to demonstrate the feasibility of incorporating the EC NDT residual stress measurement technique into a working instrument. The program goals are to create an EC instrument design and working prototype 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: 1)electrical conductivity measurements accurate to within 0.1 percent relative to the nominal alloy conductivity (~ 0.0015 percent IACS), 2)individual measurement times of less than 5 minutes, 3)EC frequencies from 100 kHz to 50 MHz. This presentation will describe the program goals, approaches, and expected results. Information about the current state of research in using EC NDT for residual stress measurement will be presented including limitations of the technology. The potential for quickly moving the technology to the depot floor will also be discussed.