BS 7910:2019 Annex L — characteristic toughness $K_{mat}$ of welds / HAZ / inhomogeneous material via three-stage MML
For homogeneous ferritic steel, the Master Curve reference temperature T_0 follows ASTM E1921. Welds, HAZ and inhomogeneous parent material show extra scatter and low-tail outliers, so BS 7910 Annex L applies the Master Curve Maximum Likelihood (MML) three-stage method to a set of cleavage toughness data and derives a conservative reference index and the characteristic fracture toughness K_{mat} used by the Clause 7 assessment.
You enter a table of toughness test results (each with its test temperature). The three-stage MML procedure fits the Master Curve while accounting for the inhomogeneity: it screens for low-tail behaviour and gives a conservative index rather than the simple homogeneous-steel T_0. The result is the characteristic K_{mat} at the assessment temperature, shown with the Master Curve and its tolerance band.
The characteristic toughness also includes a crack front length correction (Eq. L.14): the longer the crack front, the more likely a weak spot lies on it, so K_{mat} is scaled down for longer fronts. The crack front length l is derived from the flaw type (§L.9.6 NOTE 3), or you can enter l directly:
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