Fracture toughness is an important material property used to characterize material resistance against crack tearing. In ASTM E1820 standard fracture toughness test method developed for metallic materials, the elastic unloading compliance method is recommended for measuring incremental crack growth and evaluating fracture resistance curve from a single specimen test in the conventional test conditions. For dynamic loading or other harsh conditions, on the other hand, ASTM E1820 suggests the normalization method as an alternative approach for estimating incremental crack length during stable crack tearing and for evaluating the J-integral resistance (J-R) curve from a single specimen test.
The normalization method is a semi-analytical methodology that was developed in the 1990s based on the load separation approach with use of load-line displacement (LLD) for a single edge notched bend (SENB) or compact tension (CT) specimen. This method has been extensively utilized to develop J-R curves using SENB and CT specimens for different steels in various loading and environmental conditions. Recently, the LLD-based normalization method was simply extended to use crack mouth opening displacement (CMOD) in determination of J-R curves for SENB and single edge notched tension (SENT) specimens. Based on the historical work for the LLD-based normalization method, this paper develops a fundamental basis for use of the CMOD-based normalization method. Experimental data measured from a single SENB specimen test are then used to evaluate the normalization method in determination of J-R curves using both LLD and CMOD. Finally, how to use the LLD-based or CMOD-based normalization method is suggested for developing a J-R curve.