Eddy current offers a powerful tool for investigating residual stress and other metallurgical properties

Residual stress alters the natural conductivity of a metal only slightly; t ypical values of Delta sigma/sigma range from 5 to 100 parts per million (p pm). To detect such small effects, an eddy current bridge must be extremely stable and free of thermal or noise effects. By analyzing the conductivity as a function of frequency the residual stress distribution in the surface layers can be compared to standard samples whose stress distribution is kn own from X-ray analysis. It is not yet determined what the relationship bet ween conductivity and stress is, only that is it repeatable. Many different shot peened metals have been measured successfully. Changes in conductivit y which produce changes in the eddy current signatures have also been obser ved in samples of welded aluminum, heat treated steel, and others.