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A procedure has been developed to unify cleavage toughness data in mode I and mixed-modeI/II loading into one consistent scheme. Mode I and mixed-mode I/II crack tip fields areshown to belong to the same family of fields, which are deviatorically similar and differmainly hydrostatically in the leading sectors around the plane of maximum principal stress.This observation allows the unconstrained mode I toughness data to be related to thetoughness under mixed-mode conditions for stress controlled cleavage. The procedure isverified with brittle fracture experiments on a plain carbon manganese steel at -90oC.