The nominal cleavage strength of steel and its importance for welded structures 1

A certain property of steel called the “nominal cleavage of strength” can be determined by means of tensile test pieces on the edges of which a brittle alloy has been welded. On exposing these test pieces to an increasing tensile stress, cracks are continuously formed in the brittle layers of welding. At a certain critical stress a fracture will be initiated at one of the sharp crack fronts. A characteristic property of a steel, the nominal cleavage strength is defined as the highest nominal tensile stress a steel can withstand in the presence of a crack without the initiation of fracture at the crack front. The nominal cleavage strength decreases continuously as the temperature decreases just as the conventional yield point increases continuously with decreasing temperature. If curves of these strengths are drawn the point of intersection would appear to be a transition temperature for the steel, defined as the temperature above which plastic deformation can occur in a large volume of material despite the presence of a crack, but below which only minor local plastic deformation is possible immediately in front of the crack. By means of the testing method called “NC-testing” (NC = nominal cleavage) results have been obtained that show a certain correlation to the results of impact tests as well as to those of other brittle-fracture tests, e.g. Tipper, Van der Veen and others. Moreover, the critical stresses for initiation and propagation of brittle fractures are shown by means of NC-curves as well as the influence of residual welding stresses. It is also shown that the quality of a steel with regard to its brittle-fracture tendency cannot be simply expressed by an impact transition temperature as the critical stresses quoted above are not directly connected with the position of this temperature.