C. Lo, Y. Shen, S. J. Lee, A. M. Frishman, N. Nakagawa, Iowa State University, Ames, IA
This paper reports on the recent progress in a swept high frequency eddy current (SHFEC) technique for nondestructive characterization of residual stresses in engine materials with specific application to shot-peened components. The methodology determines conductivity depth profile by model-based SHFEC data inversion, which can then be converted into residual stress profile using a material-based model that includes the piezoresistivity (PR) and other possible non-PR effects. Recent upgrades to the detection coil and measurement system has extended the frequency upper bound to 80 MHz. An improved algorithm based on perturbation with respect to small conductivity deviations and small skin depth (compared to coil outer radius) has been derived to replace Cheng-Dodd-Deeds forward model calculations, and is expected to reduce the computation time of SHFEC data inversion significantly. Microstructural characterizations on peened Inconel 718 samples show experimental evidences of shot-induced changes in texture. These have been incorporated into a theory of anisotropic piezoresistivity effect, which is being developed to reconcile the non-trivial relationship between conductivity and residual stress.
This work was performed at the Center for NDE at Iowa State University with funding from the Air Force Research Laboratory through S&K Technologies, Inc. on delivery order number 5007-IOWA-001 of the prime contract F09650-00-D-0018.
Summary: This paper reports on the recent advances in the development of a swept high frequency eddy current (SHFEC) method for residual stress characterization in shot-peened aerospace materials. These include (i) an upgraded SHFEC measurement system that can operate up to 80 MHz. (ii) An improved algorithm to replace Cheng-Dodd-Deeds forward model calculations for faster and more stable SHFEC data inversion. (iii) Experimental evidences of shot-induced texture changes in IN718 samples leading to the development of a theory of anisotropic piezoresistivity effect.
