A Calibration Technique for the D.C. Potential Drop Method of Monitoring Three Dimensional Crack Growth

Abstract

One of the major problems in the experimental study of fracture is the accurate determination of defect size, shape and growth in metallic specimens or components. Various NDT techniques exist for identifying defects but only ultrasonic techniques have been applied to accurately sizing defects and monitoring their growth. Recently Potential Drop techniques have also become available for measuring the one dimensional growth of defects in two dimensional specimens. If one wishes to measure the two dimensional growth of a defect in a three dimensional body such as a surface crack, then the techniques of measurement are not well established. The authors have developed Potential Drop techniques for the measurement of growth of long shallow defects in line pipe girth welds and for elliptical surface cracks in tensile specimens. For long shallow defects, multiple probes along the defect have proven to be effective in determining the defect profile as it grows in fatigue or ductile tearing. Such detail is often necessary since defect growth may not be uniform.