System and method for mapping residual surface stresses in a value ring

A system for mapping the magnitude of residual compressive surface stresses over a surface of an annular ferromagnetic component, comprising:

• an eddy current probe having a detection coil for emanating a fluctuating electromagnetic field that penetrates no more than about 0.10 millimeters beyond the surface of the component, wherein the maximum width of the detection coil is no more than about 20% of the width of the ferromagnetic component scanned to avoid interaction between said fluctuating magnetic field and edges of said component, and a probe circuit connected to said coil for both conducting a fluctuating electric current through said coil and detecting changes in the impedance of the coil;
• means for scanning said detection coil over the surface of said annular metallic component in a 360.degree. path around a central annular portion of said component while monitoring changes in coil impedance detected by said probe circuit, including a turntable means for rotating said annular metallic component, and a support arm for positioning said detection coil against the surface of the annular metallic component while said component rotates, and
• processor means electrically connected to said scanning means and said probe circuit for converting said changes in impedance into signals generating a map of the magnitude of radially oriented residual surface stresses over said central annular portion of the surface of the component.

A method for mapping the magnitude of radially oriented residual surface stresses over a surface of an annular metallic component by means of an eddy current probe having a detection coil for emanating a fluctuating electromagnetic field that penetrates little beyond the surface of the metallic component, the maximum width of said probe coil being substantially smaller than the radial width of the annular component, a probe circuit connected to said coil for both conducting a fluctuating electric current through said coil and detecting changes in the impedance of said coil, and a turntable means for rotating said annular metallic component comprising the steps of:

• rotating said annular metallic component with said turntable means while annularly scanning said detection coil in a 360.degree. path around a central annular portion of said annular metallic component;
• detecting changes in the impedance of the coil while said coil scans said component;
• converting said detected changes in impedance to changes in the surface stresses of the component, and
• correlating said changes in said surface stresses to locations on the component to map said surface stresses on at least a portion of said component.