Two different methods, the unloading compliance and the AC-potential drop method have been applied simultaneously for crack length measurement during ductile crack extension in J-R curve determinations. The materials used in the comparison of the two test methods were a pressure vessel steel A533B C1.1 (Unified Numbering System [UNI] K12539), a comparable weldment, and a carbon-manganese steel OX522D weld. Specimen geometries used were 25-mm compact tension (ITCT) and 15-mm three-point bend (3PB), respectively.

The two methods applied give consistent results for the amount of crack extension. However, the location of the potential minimum is dependent upon material, specimen geometry, temperature, frequency and current. This leads to the conclusion that the ACPD method fails to indicate the initiation of ductile crack extension correctly. Reasons for this are discussed.