Fracture Behaviour of Nodular Cast Iron
The fracture process of nodular cast iron is either fully duc- tile (Figure 1a) or partly brittle (Figure 1b), where the load- displacement record consists of a series of crack initiation and crack arrest events. The fracture process is controlled by factors like temperature, strength, and pearlite content. The fully ductile behaviour is similar to the upper shelf fracture behaviour of steel, but the partly brittle behaviour differs from the behaviour seen in steel in the transition region. In this region, the fracture energy for steel is controlled by the energy required for cleavage crack initiation. In the case of cast iron, the fracture energy is also controlled by the en- ergy required for crack propagation as shown in Figure 1b. In terms of tearing resistance, the fully ductile case will lead to a classical full tearing resistance curve, whereas the partly brittle case will lead to a limited tearing resistance curve as schematically shown in Figure 2.2
The limited tearing resis-
tance curve differs from cleavage fracture toughness in steel in that the limited tearing resistance curve requires a “con- stant” crack driving force to propagate the crack, whereas a cleavage crack propagates as long as the crack driving force is above the crack arrest toughness. This difference reflects on the Charpy-V fracture toughness correlation in the “tran- sition region”. Unlike steel, nodular cast iron should have basically the same correlation between Charpy-V and frac- ture toughness in both the transition region and the upper
shelf region. For this reason, transition temperature concepts are not descriptive for nodular cast iron.
The proper fracture toughness values describing nodular cast iron are the full tearing resistance curve, some defined point on the tearing resistance curve like JIc value of the limited resistance curve (Kinst
(KJIc
non-linearity in the tests, it is advisable to assess the fracture toughness in terms of the J-integral that can be presented in the form of KJIc
. Nodular cast iron preferably should not in any way descriptive of the critical fracture toughness. = PQ in a test, is the LEFM KIC
be tested according to ASTM E399 or equivalent standards, nor should the fracture toughness be taken as the value cor- responding to maximum load. Both of these values can lead to fully incorrect descriptions of the materials true fracture toughness. Only if Pmax
value
Cast iron is traditionally known as notch insensitive mate- rial, because its fracture energy is less affected by different notches than steel. The reason for this apparent insensitiv- ity is, however, the opposite. Cast iron is very sensitive to notches. Even a moderate notch is sensed as if it were close to a sharp crack. This appears as an apparent insensitivity, since all notches are experienced as being close to cracks. The reason for this lies in the graphite nodules, which fail at small strains. This results in a microcrack region in front of the notch (Figure 3). This microcrack region overwhelms
) or the instability ). To account for
Figure 1. Fracture behaviour of nodular cast iron.
Figure 2. Schematic presentation of definitions for “limited tearing resistance” (full line) and “full tearing resistance”2
(dashed line). 82
Figure 3. Schematic description of the reason why nodular cast iron experience a notch as being sharper than nominally is the case.
International Journal of Metalcasting/Volume 8, Issue 2, 2014
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