Dedicated CT Tests and Numerical Simulations to Assess Crack Arrest by Quasi-Laminar Flaws

Abstract

Abstract Following the detection of a large number of quasi-laminar flaw indications in the core shells of two Belgian Reactor Pressure Vessels (RPV), an extensive Safety Case demonstrating the Fitness-for-Service for continued operation of both reactors was elaborated. These indications were attributed to hydrogen flaking induced during the component manufacturing process. The conservative Flaw Acceptability Assessment demonstrated that the identified indications do not jeopardize the integrity of the reactor vessel in all operating modes, transients and accident conditions. However, beyond this specific case, the need to better understand the behavior of quasi-laminar flaws has been brought to light. In that framework and as a defense-in-depth investigation, the question of quasi-laminar flaws subsequent behavior in case of fracture initiation is raised. In the hypothetical case of brittle failure initiation, it is expected that the presence of quasi-laminar flaws in the propagation path can stop the flaw propagation. To illustrate this statement, exploratory compact tension (CT) tests are designed such as a mode I pre-crack, from which initiation occurs, subsequently interacts with quasi-laminar flaws in the propagation path. In each of the CT tests, the measured load-displacement curve shows evidence of crack arrest. Besides these experimental results, 3D extended finite element analyses (XFEM) are conducted to confirm and provide additional insights about this phenomenon, in particular the link with the mixed-mode fracture toughness. Moreover, the simulations allow defining an additional test matrix aiming at evaluating the relation between the mixed-mode fracture toughness and the ratio between mode II and mode I stress intensity factors. The present paper presents the experimental results and describes the numerical simulations that are performed to illustrate the capacity of quasi-laminar flaws to induce a crack arrest phenomenon.