Investigation and Modeling of Local Crack Arrest in Ferritic-Bainitic Steels Under Dynamic Loading

Abstract

Local crack arrest is usually irrelevant under quasi-static loading conditions in the ductile to brittle transition region. Elevated loading rates, however, allow cleavage fracture due to the dynamic embrittlement also at higher testing temperatures compared to static loading. This behavior is generally accompanied by local crack arrest events. In addition, adiabatic heating processes in the crack tip region increase local temperature as well, which further promotes crack arrest. This complex interaction between crack initiation and crack arrest at elevated loading rates substantially changes macroscopic fracture behavior, whereas its investigation is the core of this work. An experimental database of dynamic fracture mechanics experiments for the reactor pressure vessel steel 22NiMoCr3-7 is examined in this work that was previously tested at crack tip loading rates of about 103 to 105 MPa√m /s. Recent fractographic examinations and statistics regarding the occurrence and characteristics of local crack arrest incidences are shown for different loading rates and testing temperatures. Furthermore, an existing local probabilistic cleavage fracture model is used to describe macroscopic fracture behavior for the provided experimental database, and also compared to other assessment methods (i.e. Master Curve). The shortcomings in the numerical assessment methods can be linked to the amount of observed local crack arrest incidences, and a micromechanically motivated model modification is proposed to consider the mechanism of local crack arrest. The agreement between experimental results and numerical cleavage fracture assessment can be significantly increased by using the modified model.