The objective of the investigation presented in this Technical Report was to quantify the required material toughness for the operational safety of round steel chains. The determination of this toughness must take place under unfavourable boundary conditions, i.e. at the lowest permissible operating temperature on damaged (precracked) chains. On the basis of characteristic properties determined on chains and three-point bend (TPB) specimens and the notch impact energy, a load-bearing and brittle fracture transition temperature concept was elaborated. From the correlation of the fracture stress of chains, fracture mechanics characteristic properties and the notch impact energy, minimum values of the notch impact energy can be derived which yield adequate safety against fracture for the damaged chain. Furthermore, in order to exclude brittle failure of chains, the position of the lowest permissible operating temperature relative to the NDT (nil-ductility transition) temperature was determined. Finally, the linkage of the determined fracture mechanics properties to notch impact energy values and the definition of the requirements for these values furnishes a simple test and acceptance procedure for chain materials and chain types.