CRACK EXTENSION IN THE DUCTILE-BRITTLE TRANSITION REGION W. Ehl 1 ), D. Munz 1 • 2 ), A. Brückner 2 ) !)University of Karlsruhe, Z)Nuclear Research Center Karlsruhe The extension of cracks in the ductile-brittle transition region of the reactor steel 20 MnMoNi SS was investigated with compact specimens of different size and specimens with surface cracks. The specimens failed by cleavage started up to 1 mm ahead of the crack front. The stable crack extension could be described by a size- independent J 1 R-na-curve. Jlcl at the onset of cleavage cracking showed a large amount of scatter and was dependent on the size of the specimens. A weakest link model, which included the effect of stable crack extension, could not explain the size effect. Therefore it was concluded that the constraint at the crack tip changes during stable crack extension and is dependent on the specimen size. INTRODUCTION Fracture toughness of ferritic steels in the ductile-brittle transition region is characterized by a large amount of scatter and a large effect of specimen size. Both effects are related to the transition from cleavage cracking at low tempera- tures to ductile tearing at higher temperatures. At a given temperature in the transition range different specimens may fail by different fracture modes: cleavage fracture without prior stable crack extension, cleavage fracture following some amount of ductile tearing or ductile tearing beyond the maximum load. The tendency to cleavage cracking not preceded by ductile tearing increases with decreasing tem- perature and increasing size of the component /1/. The scatter of cleavage fracture and the size effect has been explained by a weakest link model /2/. In addition, a change of stress state (plane strain to plane stress for decreasing thickness) may contribute to the size effect. A complete des- cription of the fracture has to separate the constraint effect and the statistical effect and it has to include the stable crack extension preceding cleavage cracking. Also should it be possible to predict the behavior of components with two-dimensio- nal cracks, e.g. semielliptical surface cracks from tests performed with simple specimens, such as compact specimens. ln this paper results will be presented which should contribute to solving these problems. Sorne of the presented results were also presented at the Sixth European Conference of Fracture /3/. 233
CRACK EXTENSION IN THE DUCTILE-BRITTLE TRANSITION REGION
May 19, 2023
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