Revised Incremental J-Integral Equations for ASTM E1820 Using the Crack Mouth Opening Displacement.

The incremental J-integral equations used in ASTM E 1820 were developed utilizing the work input to the test specimen, and utilize the load versus load-point/line displacement (LLD) data obtained from the fracture test. The analysis of Ernst et al. [Fracture Machanics: Thirteenth Conference, ASTM STP 743] was then used to obtain crack growth corrected estimates of the J-integral as needed for an accurate J-R curve evaluation. For the compact type specimens, the crack mouth opening displacement (CMOD) gage was moved to the load line so that an adequate estimate of LLD could be obtained from the clip gage and utilized for compliance-based crack extension and incremental J-integral estimates. For the single edge-notched bend [SE(B)] specimens, however, it has been necessary to measure both CMOD for compliance-based crack length estimates and LLD for the crack growth corrected J-integral calculations. Additional experimental complexity and expense are thus added. ASTM E 1820 includes a "basic" procedure which allows evaluating the J-integral using CMOD without crack growth corrections if the result is used only for evaluation of initiation toughness. Recently, Annex A 16 based on work of Wallin and Laukkanen [Engineering Fracture Mechanics, Vol. 7 I, 2004, pp. 160 I-1614] has been added to ASTM E 1820 that presents a multiple-step procedure to obtain a crack growth corrected J-R curve from the basic method data. A more direct approach was proposed recently by Zhu et al. [Journal of ASTM International, Vol. 5, No. 5, 2008, paper ID: JAI 101532] using an incremental procedure to obtain the desired, crack growth corrected J-R curve from the CMOD data of SE(B) specimens with two geometry factors η CMOD and γ CMOD similar to the parameters presently used in the existing LLD-based J-integral analysis. Using fracture toughness test data for SE(B) specimens, detailed comparisons and discussions are made between the proposed method and the ASTM E1820 CMOD methods in determination of J-R curves for HY80 steels. As the basis of comparison, the "accurate" solutions obtained from the standard LLD-based incremental J-integral method are provided for all specimens considered. The difference between the CMOD and LLD methods are then quantified. The results show that the proposed CMOD incremental J-integral equations are simple and effective, and thus are recommended here for use in ASTM E1820 for testing J-R curves and for reducing test costs and complexity. [ABSTRACT FROM AUTHOR]

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