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A tentative method is proposed for treating the scatter and size effects of fracture toughness of ferritic steels in the ductile-to-brittle transition regime. The method is aimed at the determination of the probability function for initiation of fast growing cleavage cracks under the plane strain condition. The practical relevance of this method has been checked with toughness data sets obtained from various CT-specimen sizes and CCT-specimens of a pressure vessel steel tested at two different temperatures in the transition regime. NOMENCLATURE a, a, = initial fatigue crack length B, = net thickness AB = necking of the specimen J, = J-integral at the instability point J,, =elastic part of J-integral J,, =plastic part of J-integral J, = scale parameter of Weibull distribution J, = initiation of stable crack extension k = ranking index N = total number of data of a data set Pf = cumulative failure probability r =distance from the fatigue crack tip rp = location of the uyr stress peak ahead of the blunted crack tip p = quantity for defining the border of in-plane constraint effects uyy = normal stress in the ligament u, = reference stress of the true stress/strain curve (estimated by uy) uy = yield stress J,,, = scale parameter of Weibull distribution, xx indicates specimen thickness J,,, = J-level up to which no in-plane constraint effects are expected

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