Modeling of Detection of the Stress
Distribution in Shot-Peened Turbine Metals
Inclusive Dates: 11/15/04 03/15/05
Background - Advanced jet engine components are often subjected to shot-peening to induce a residual surface compressive stress. If the surface of the component part remains in compressive stress throughout its life, then there is little likelihood that cracks will initiate and lead to catastrophic failure. However, the compressive surface stress tends to relax as a function of service life, and thus at some time during the component's life, it can become susceptible to crack initiation. Because of the heavy use of advanced engines in the Gulf wars, the United States Air Force (USAF) is trying to extend the life of engine components. This can be done in two ways. First, when inspecting the components, develop a more sensitive and reliable defect detection technology so that no defect can grow to a critical dimension prior to the next inspection. A second and more advantageous approach would be to have the ability to verify that the component's surface is still in compressive stress. Therefore, the USAF needs a reliable way to measure the compressive residual stress distribution near the surface of the part.
Approach - One way to measure the distribution of a physical property at different depths is to use measurement of eddy currents at different frequencies. Because of the skin effect, the layer carrying the current becomes increasingly shallow from the surface down as frequency is increased. Thus, by varying the frequency, one effectively measures the electrical conductivity at different depths. If one knows how the stress affects the conductivity, one can use the conductivity measurement at different depths to measure the stress distribution at different depths. In this project, the objective was to get to the forefront of eddy current measurement of conductivity distributions with depth, and thus be in a position to use it as a method of measuring the shot-peened residual stress distribution in turbine engine components.
Accomplishments - The objective was achieved by review of the literature and by formulating a method of inverting the eddy current impedance data to obtain the residual stress distribution at various depths. This formulation is an improvement on the very approximate determination of the residual stress distribution presently available in the literature.