Patent ID: 8164328

Claim:
A system for detecting damage below a surface of a cylindrical object, the system comprising: an eddy current probe comprising first and second eddy current coils, each coil being orthogonal to the other coil; a scanning mechanism to which the first and second eddy current coils are mounted, the scanning mechanism configured for moving the first and second eddy current coils in relation to the surface; and a dual frequency eddy current instrument configured for applying a current to the eddy current probe and measuring a resulting impedance as the eddy current probe is moved in relation to the surface; the dual frequency eddy current instrument further configured for either (1) concurrently applying a first current having a first frequency and a second current having a second frequency to the eddy current probe as the eddy current probe is moved in relation to the surface, or (2) alternatively applying a first current having a first frequency and a second current having a second frequency to the eddy current probe as the eddy current probe is moved in relation to the surface; wherein the first frequency is selected to penetrate the surface to a depth greater than or equal to a desired flaw detection depth, and the second frequency is selected to isolate a probe wobble and a surface effect, the second frequency being at least twice that of the first frequency; wherein the first eddy current coil comprises a first axis and the second eddy current coil comprises a second axis, and wherein the first and second eddy current coils are mounted to the scanning mechanism such that the first axis is aligned substantially with a circumferential direction of the surface as the eddy current probe is moved in relation to the surface, and such that the second axis is aligned substantially parallel to a longitudinal axis of the object as the eddy current probe is moved in relation to the surface, and wherein the first axis and the second axis do not intersect; wherein the scanning mechanism is further configured for varying a distance between the eddy current probe and the surface; and wherein the dual frequency eddy current instrument is further configured for rotating the measured impedance such that changes in the impedance caused by motion of the eddy current probe radially toward and away from the surface is along a predefined axis for both the first and second frequencies.