Crack trapping effect of persistent grain boundary islands

In the polycrystalline Fe-Si alloy, when a cleavage front transmits from one grain to another, it first penetrates stably across the grain boundary at a number of breakthrough points (BTPs) that distribute along the front quasi-periodically. As the critical energy release rate is reached, unstable crack jump occurs and the persistent grain boundary islands (PGBI) between the BTPs are left behind the verge of propagating, bridging across the crack flanks, which leads to a 10-30% increase in fracture resistance. In this article, this process is investigated through an energy analysis. The influence of the size/spacing ratio of PGBI on the grain boundary toughness is discussed in detail.