Evaluation of Specimen Geometry and Testing Parameters for Hydrogen-Assisted Fracture Measurements

Abstract Elastic-plastic fracture mechanics testing is a mature methodology standardized in various standards, arguably the most complete being ASTM E1820. Testing in gaseous hydrogen does not change the basic mechanics of the test method (e.g., specimen geometry), rather method-independent test results for environment-assisted fracture require careful control of the environmental boundary conditions (e.g., pressure and testing rate). ASTM E1820, Standard Test Method for Measurement of Fracture Toughness, provides clear guidance on appropriate specimen geometry and crack conditions to achieve size-independent fracture results (e.g., specimen thickness and initial crack length respectively); however, the standard does not provide guidance for testing under environmental conditions. The ANSI/CSA CHMC1 standard provides state-of-the-art guidance to establish appropriate boundary conditions for testing in gaseous hydrogen environments, namely gas purity, soak time prior to testing and testing rate (while referring to ASTM E1820 for the mechanics of the test). In this study, the influence of specimen thickness (nominal W/B of 2 and 4) and initial crack length (nominal a/W of 0.5, 0.6 and 0.7) on hydrogen-assisted fracture were evaluated, using the standard compact tension (CT) geometry and monotonic loading with electric potential difference to monitor crack extension as described in ASTM E1820. Testing rate is also evaluated in the context of the guidance from ANSI/CSA CHMC1. Collectively, these results establish clear geometric bounds and procedures for standardized elastic-plastic fracture mechanics measurements in gaseous hydrogen environments using the compliance method and CT geometry