The brittle-to-ductile transition for cleavage fracture of steels has been examined with integrated considerations of micromechanical and macromechanical features. The evolution of transition temperature approaches to fracture-safe design has been based on concepts that metal ductility factors should override mechanical constraint factors, in the higher temperature range of the transition. The transition temperature range, determined by dynamic fracture tests, has provided the necessary guidance for the development of improved steels. Fracture mechanics concepts emphasize that macroscopic fracture toughness is controlled by mechanical constraint and flaw severity factors. While true, within limits, there has been an unwarranted extension of these principles to signify that the transition temperature does not have a basic significance to fracture processes. Contrary to popular beliefs, these concepts are not in opposition: metallurgical factors determine the intrinsic metal ductility, and mechanical parameters serve to describe the response of the metal to specific stress states. Recent investigations of the effects of large section size have demonstrated that increased mechanical constraint results in shifts of the transition temperature, as predicted by fracture mechanics theory. However, these shifts are of relatively small magnitude and, more importantly, do not eliminate transition temperature characteristics.
The rapid changes in fracture toughness which develop over a narrow temperature range indicate that practical engineering use of fracture mechanics must be based on transition temperature tests of simple types. Conventional fracture mechanics test are not practical for transition temperature definitions because of large size, prohibitive expense, and the very steep-slope temperature dependence of KIc and KId defined fracture toughness tests. For such use, the tests must be indexable to KIc and KId parameters. The Dynamic Tear (DT) test represents the most advanced engineering test which provides accurate indexing of the true transition temperature range and the specific interval in this range for which fracture mechanics applies. The temperature interval of high fracture toughness which is outside the range of fracture mechanics capabilities is also defined. Size effect can be interpreted and related to expected transition temperature shifts.
All of these factors have been integrated into simple reference diagrams which index flaw size-stress relationships for fracture initiation in the transition range. The Fracture Analysis Diagram (FAD) procedure is now extended to cover the full range of thickness. Its validity is confirmed by these additional studies and is no longer based entirely on experience factors. Conversely, the experience factors serve to validate the predictions of fracture mechanics theory and continue to provide engineering assurance that the analytical methods can be relied on.
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| Number of Pages : | 46 |
| Published : | 1969 |
