Shape of the Ductile‐to‐Brittle Transition in the Whole Regime: An Experimental Review

Ferritic and ferritic/martensitic steels exhibit a ductile‐to‐brittle transition (DBT) whose large scatter in cleavage toughness makes structural integrity assessment statistical. This review focuses on the experimental, data‐driven and event‐based evidence bearing on the probabilistic description of transition toughness, and on the shape of the toughness–temperature curve; the extensive numerical constraint‐modelling literature is deferred to a companion review. Treating the transition micromechanism critically, we show that the weakest‐link Master Curve survives recent fractographic findings, and that the Weibull exponent is derived from small‐scale‐yielding crack‐tip scaling and confirmed experimentally, not assumed. Using model‐free machine‐learning analyses of raw toughness data, we argue that the exponential Master Curve and the saturating Unified Curve differ decisively only beyond the transition window, that saturation is real rather than imposed, and that prior ductile crack growth is the deciding variable. Constraint, active volume, loading rate, and warm pre‐stressing are reviewed accordingly.