The fracture mechanical loading can be determined by means of analytical models or finite element methods. This will not be discussed in this paper.
In order to experimentally determine fracture mechanical material properties of DCI under quasi-static loading, test standards such as ISO 12135,9 39911
ASTM E 1820,10 or the procedure ESIS P212 ASTM E (equal to ISO 12135 to a
large extent) can be used. This is basically justified by the scope of these standards, which is “metallic materials,” but compliance to the numerous validity requirements must be proven in any case.
In case of rapid and impact loading (dynamic loading), the above mentioned standards9, 10, 11, 12 ASTM standards10, 11
contain special annexes on rapid load-
ing conditions. Basically, the loading situation always shall be analyzed to verify the quasi-static formulae and proce- dures given by the standards remain applicable. Otherwise, special, not yet standardized procedures must be applied to ensure inertia effects and potentially present larger kinetic energy amounts are considered adequately. There is not yet a special, general standard on dynamic fracture mechanics testing of metallic materials. When the most recent edition of ASTM E 1820 was published in 2011, it was the first time ever that a standard procedure on the determination of dy- namic fracture toughness values using precracked Charpy specimens (Annex 17) had been published. The correspond- ing ISO draft standard 2684313
is considered up to date. It
should be noted that it is, unfortunately, not possible to sim- ply call on the instrumented notched bar impact test as it is standardized in ASTM E 229814
or EN ISO 1455615 determine the dynamic fracture toughness parameter KId
to .
For instance, real fracture mechanics tests require specimens containing fatigue precracks, not simply the commonly used Charpy test pieces. Furthermore, plenty of additional re- quirements regarding the analysis procedure and the validity requirements must be met.
With steel alloys, the design engineer faces a large variety of definitions of fracture mechanics values that often are claimed to be material characteristics. Although the data- base for DCI materials is much weaker, plenty of different definitions of “characteristics” can be found in the literature. As has been shown for DCI,16
the value of elastic-plastic
fracture mechanics crack initiation toughness may vary up to 60% even when the same set of data is analyzed simply using different definitions from different editions of several ASTM and ISO test standards. Therefore, the design engi- neer must be particularly careful and the material character- istics always must be verified.
Usually, fracture mechanical examinations are performed using small test pieces taken from the casting or the semi- finished product. In order to avoid accidental errors or non- conservative safety evaluations, the transferability of the determined characteristic values to the component shall be
26
Apart from loading factors such as temperature and loading rate, the fracture mechanical material properties of DCI un- der quasi-static, dynamic and cyclic mechanical loading are decisively determined by the microstructure. With regard to the microstructure, a differentiation shall be made between the metallic matrix on one hand and the distribution and morphology of the embedded graphite particles on the other. As will be demonstrated in this paper, fracture mechanical toughness properties may be more sensitive to changes in the graphite morphology while strength and ductility proper- ties are not.
This paper is focused on the determination of mechanical properties and fracture mechanics toughness data under qua- si-static and dynamic loading and how these parameters are influenced by temperature, loading rate and microstructure.
Characterization of Microstructure
Appropriate metallographic analyses are considered a major prerequisite for the discussion of the influence of microstructure on mechanical and fracture mechanical properties. Graphite classification by comparative visual analysis as standardized in ISO 945-121
is valuable for a
foundry’s quality assurance system. Nevertheless, to dis- cuss microstructure property relations, a more detailed, quantitative graphite classification by image analysis is considered mandatory.
International Journal of Metalcasting/Volume 8, Issue 2, 2014 also are adapted. The
ensured on the basis of the validity requirements formulated in the test standards.
In the case of linear-elastic material behavior and with re- gard to the initiation of instable crack propagation, fracture toughness values KIc
conforming to the standards9, 10, 11, 12
elastic-plastic material behavior applies and the initiation of stable crack propagation is regarded, the only material char- acteristic to be considered independent of specimen size and transferable to the component is the physical crack initiation toughness. Physical crack initiation toughness values do not comprise any crack growth (e.g. Ji
are considered independent of specimen size and transfer- able to the component (under dynamic loading KId
). When
determined by the DC potential drop technique comes close as per ISO 12135 do not ful-
to the physical crack initiation. The characteristic values JIc as per ASTM E 1820 or J0,2BL
fill this requirement unless some additional validity require- ments are met. 9, 10
For the experimental determination of fracture mechanical characteristic values of DCI under cyclic loading, the stan- dards ASTM E 64717
and ISO 1210818 lection of recent references19, 20 can be used. A se- report on further aspects of
fracture mechanics testing as well as material data for DCI under cyclic loading.
termined on the basis of fractographic measurement of the so-called stretched zone). The parameter Ji/EP
as per ISO 12135, de- which can be
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