In this study, the relation between fracture toughness and mechanical properties, grain structure, temperature and strain rate is analysed on the basis of a thermo-activation analysis of the plastic deformation of metals.As a result of the study, the relationships obtained are of sufficient accuracy in relation to standard cracked samples of many steels.These relations can be used as a tool when designing new alloys which have a strong resistance to crack growth.This study is limited to temperatures in the range 0 d T < 0.2Tm and BCC metals. NOMENCLATURE d = diameter of ferrite grain H, = constant with the units of activation energy KO, = value of KIC at T-0 K n = strain hardening index R = Boltmann constant T = absolute temperature Y = function taking into consideration the geometry of specimen and loading pattern C = speed of plastic deformation 4 = pre-exponential factor pc = distance from maincrack tip to microcrack a, = critical strength at microcrack formation [z(Oj -zO] = effective strain at 0 K corresponding to Ho H,, = work of overcoming barrier (energy of plastic deformation activation process)

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