Rationale

Apparent negative crack growth (ANCG) is often encountered when performing elastic-plastic fracture toughness tests with the Elastic Compliance single-specimen technique. It consists of a decrease of specimen compliance (apparent reduction of crack size) during the early portion of the test, before the attainment of maximum force and the onset of ductile crack extension. This phenomenon is not observed on all materials and at all test temperatures, but only for certain materials and test temperatures (for example, austenitic stainless steels tested at cryogenic temperatures). Various investigations have shown that both experimental (friction, misalignment of the test setup, etc.) and material behavior (residual stresses, strain hardening, crack tip blunting effects, etc.) aspects can be invoked to explain the decrease of elastic compliance that translates into decreasing crack size in the early loading stage. Several authors have also shown that the minimum value of compliance corresponds to the initiation of ductile crack extension after crack tip blunting, and therefore the corresponding data point should lie on the construction line (what used to be called blunting line) having the form J = 2?Y?a. This is also the approach prescribed by the only international test standard that directly addresses the issue of ANCG, the Japanese standard JIS Z 2284:1998 for testing metallic materials in liquid helium. A comprehensive review and assessment of previous investigations and proposed approaches to tackle ANCG was published in Lucon, Materials Performance and Characterization 13, no. 2, 2024. (doi: 10.1520/MPC20230080) The current provisions of ASTM E1820-25a for addressing ANCG (stopping the test and checking the alignment of the loading train, as well as ignoring crack size values determined at forces lower than the maximum precracking force) are clearly insufficient, and therefore the following significant revision of E1820 is proposed here: (a) discarding all data points before minimum crack size; (b) placing the minimum compliance data point on the construction line.