Brittle behaviour in laboratory-scale mechanical testing

Abstract

Instrumented tests on various geometrically similar sizes of test pieces in notch tension, bending and impact notch bending are described for two grades of mild steel. Data on variation of the ductile-brittle transition with size, stress state and strain rate are presented to add to the extensive results already published on these steels (A.A.C.S.S. 1960,1962). The self-consistency of the effects of these variables with yield stress and temperature is studied in the light of present concepts of fracture. The results show, for example, that decrease in size in slow notch bending can decrease the 50 % crystalline transition temperature by 100 degC. Agreement between, say, V notch Charpy impact and 3 in. notch tension data appears to be largely fortuitous, strain rate and size having nearly equal but opposite effects. At low temperatures maximum loads pass from well above to gross yield loads or less. The nature of this transition is different for the two steels, but in each case the temperature for the transition is dependent on size, stress field and strain rate. There is no direct relation between the normally observed appearance or energy transition temperature and this much lower temperature load transition, thus adding considerably to the difficulty of interpretation of brittle fracture data from laboratory-scale testing and its application to large structures.