Dynamic Piezoresistivity Calibration for Eddy Current Nondestructive Residual Stress Measurements

It has been recently demonstrated [M. P. Blodgett and P. B. Nagy, J. Nondestruct. Eval. 23, 107 (2004)] that eddy current conductivity measurements can be exploited for near-surface residual stress assessment in surface-treated nickel-base superalloy components. To quantitatively assess the prevailing residual stress from eddy current conductivity measurements, the piezoresistivity coefficients of the material must be first determined using known external applied stresses. These calibration measurements are usually conducted on a reference specimen of the same material using cyclic uniaxial loads between 0.1 and 10 Hz, which is fast enough to produce adiabatic conditions. Therefore, the question arises whether dynamic calibration measurements can be used or not for accurately assessing the sensitivity of the eddy current method for static residual stress. It is demonstrated in this paper that such dynamic calibration measurements should be corrected for the thermoelastic effect, which is always positive, i.e., it increases the conductivity in tension, when the material cools down, and reduces it in compression, when the material heats up. For low-conductivity titanium and nickel-base engine alloys the thermoelastic corrections are relatively modest at ≈5–10%, but for high-conductivity aluminum alloys the difference between the adiabatic and isothermal properties could be as high as 50%.