Rationale

The ASTM E1820-21 standard covers "alternative methods" of measuring crack extension, besides the elastic compliance technique, in section 8.7 and in Annexes A15 (Normalization Data Reduction Technique) and A18 (electric potential difference methods). For these alternative methods, a confirmatiory test is required, whereby an additional specimen is loaded to a value of load-line displacement corresponding to a ductile crack extension of 0.5 ± 0.25 mm. If the measured crack extension does correspond to 0.5 ± 0.25 mm, then subsequent tests can be considered qualified. In three of the four sections where this confirmatory test is mentioned (8.7.1, A14.7.6, A15.3.2), however, it is currently claimed that, if the measured crack extension is 0.5 ± 0.25 mm, the test and its J_Ic value is "qualified according to this method". In section A18.16.3, however, the language used is different ("The measured crack extension shall be 0.5 ± 0.25 mm in order for the subsequent tests to be qualified according to this method."). Since it's unlikely that a test interrupted at a crack extension of about 0.5 mm satisfy the spacing requirements of section A9.6.4 (at least one point between the 0.5-mm and 1.5-mm offset lines), the language in 8.7.1, A14.7.6, and A15.3.2 should be modified to match the language in A18.16.3.