Following an intense period of test methodology development in the late 1970s and early 1980s, crack arrest has received very little attention in the research ommunity. Moreover, the available model for the crack arrest transition behavior of ferritic steel (i.e., the ASME KIa curve) is entirely empirical. In 1998 Wallin published an updated empirical model of crack arrest transition data having a form similar to his Master Curve treatment of crack initiation toughness. In this paper we provide a physical basis for the data trends noted by Wallin, thereby making the model more robust and enabling its confident application to situations not covered by available KIa data.