Further Investigation of Normalization Method in Determination of J-R Curves Using Different Fracture Specimens

Abstract

Fracture toughness is an important material property used to characterize material resistance against crack tearing and to assess structural integrity for a variety of engineering structures, including nuclear pressure vessels and oil and gas pipelines. ASTM E1820-18 provides the standard fracture toughness test methods for metallic materials and recommends the elastic unloading compliance method for measuring crack length and determining the fracture resistance curve from a single specimen test in conventional conditions. For dynamic loading or harsh environmental conditions, however, the compliance method is difficult to use. Accordingly, ASTM E1820-18 provides the normalization method as an alternative approach for estimating crack length during stable crack growth and for developing the J-integral resistance curve in a single specimen test. The normalization method developed in the 1990s is a semi-analytical methodology based on the load separation approach and uses load-line displacement (LLD) for single edge notched bend (SENB) specimens or compact tension (CT) specimens. This method has been extensively utilized to develop J-R curves using SENB and CT specimens for various steels in different loading or environmental conditions. Recently, this LLD-based normalization method was extended to use crack mouth opening displacement (CMOD) to determine J-R curves for SENB specimens and single edge notched tension (SENT) specimens, but it was not investigated further. To better understand the CMOD-based normalization method, this paper revisits the historical efforts to develop the LLD-based normalization method and then develops a fundamental basis for using the CMOD-based normalization method. Different experimental data are used to evaluate the viability and validity of both LLD- and CMOD-based normalization methods in developing J-R curves for different steels using SENB, CT, and SENT specimens. Based on this evaluation, an adequate normalization method is recommended for each fracture specimen type to use in fracture resistance testing.