ADVANCED FRACTURE MECHANICS TESTING OF DCI — A KEY TO VALUABLE TOUGHNESS DATA
Wolfram Baer BAM Federal Institute for Materials Research and Testing, Berlin Germany Copyright © 2014 American Foundry Society
A version of this paper was previously published in the 2013 Keith Millis Symposium Proceedings Abstract
The challenge of how to adequately characterize the tough- ness behavior of ductile cast iron (DCI) materials had been solved through the development and application of fracture mechanics concepts. Outstanding examples are found in the design and safety assessment procedures of sophisticated DCI components for wind turbines or transport and storage casks for radioactive materials. An essential requirement for these procedures is to have adequate fracture mechanics toughness data available.
Because DCI materials may substantially change their de- formation, damage and fracture behavior from ductile to brittle by increasing loading rate, decreasing temperature, increasing pearlite share and increasing stress triaxiality, the corresponding experimental measuring techniques and analysis concepts have to be chosen and adapted with delib- eration. Long-time experience in this field is imperative to provide valuable data.
Introduction
The conventional criteria for the selection of materials and the component safety evaluation are fundamentally supple- mented and amplified by applying fracture mechanical cri- teria. Thus, for the first time, it is possible to include the material toughness (which is defined as material resistance to crack initiation or crack propagation) into the evaluation of the fracture resistance. The engineer is now in a position to make a material selection that corresponds far better to the actual loading situation than a selection made on the basis of the impact energy or the notched bar impact energy.1
The notched bar impact energy is of unquestionable signifi- cance in the comparative estimation of a material’s tendency toward brittle failure and for the foundry’s in-house quality control. Nevertheless, it merely provides qualitative infor- mation regarding the material’s toughness behavior and is not suitable for component design and evaluation. In many cases, the characteristic deformability values (e.g. percent- age elongation after fracture) determined on smooth test pieces in uniaxial tensile tests are erroneously used synon- ymous to the term material toughness. Percentage elonga-
International Journal of Metalcasting/Volume 8, Issue 2, 2014
BAM has been dealing with mechanical and fracture me- chanical characterization of DCI materials for more than 30 years. This paper demonstrates state-of-the-art fracture mechanics test methods for the determination of quasi-static and dynamic fracture toughness. Among others, it is exem- plarily shown for a variety of DCI materials conforming to grade EN-GJS-400 how mechanical and fracture mechani- cal properties are influenced by temperature, loading rate and microstructure. A major lesson to be learned is that fracture toughness data of DCI should always be discussed, reported and used in correlation with microstructural pa- rameters.
Keywords: DCI, ductile cast iron, mechanical properties, fracture mechanics testing, fracture toughness data, micro- structure, loading rate, temperature
tion after fracture, however, can only give information on a material’s potential capability to use plastic deformation to reduce stresses without resulting in the formation of cracks.
Fracture mechanical concepts, on the other hand, cover quantitative correlations between the load acting on the com- ponent, the size of present or hypothetically assumed cracks or crack-like stress concentration spots (e.g. casting defects) and the material’s fracture toughness or crack resistance.2
As far as fracture mechanical analyses of DCI components are concerned, bodies of engineering rules and regulations such as the European SINTAP procedure3
generation of wind energy,6 are available. Fur-
thermore, subject-specific provisions for fields of applica- tion having high safety or availability requirements, such as nuclear technology, 4, 5 cal engineering7
mechani- or welding technology8 can be used.
One basic prerequisite of a fracture mechanical component evaluation is that loading as well as loading resistance, i.e. the material’s resistance, are quantified and available in terms of the same fracture mechanical concept and under corresponding loading conditions.
25
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97