Calibration functions to determine crack size using the direct current electric potential difference method for standard fracture toughness specimen geometries

Guidelines for the use of direct current electric potential difference methods for the determination of crack size have been included in ASTM E1820, Standard Test Methods for Measurement of Fracture Toughness, since 2020. These guidelines, however, do not include calibration relationships for a disk-shaped compact tension specimen or a specific procedure to identify the onset of crack growth. In addition, the proposed location for the voltage measurement points often interferes with the knife edges used for a clip-on gage, especially for small specimen sizes. In this article, calibration relationships that can be used to infer crack size from potential difference (PD) measurements were established for common fracture toughness specimen geometries. A general mathematical framework was developed. Calibration relationships were established with the support of finite element analysis. For compatibility with the use of a clip-on gauge, a more convenient location of the voltage measurement points was
chosen. A procedure to identify the point of crack initiation on the PD versus displacement plot is proposed. The onset of crack growth is defined by the point for which the difference between a third-order polynomial fitted to the PD versus displacement data and the tangent line through the origin exceeds 0.5 %. The calibration relationships were validated by using the reference method of ASTM E1820-24. The predicted crack extension not only met the validity requirements for the PD method but also met the stricter requirement for the unloading compliance (UC) method. The critical fracture toughness (JQ) obtained with the PD method was in good agreement with the UC and energy normalization methods. The normalization data reduction method significantly overestimated the JQ value measured by the PD method.