Crack Paths 2012

Preferential crack path besides an inclusion

Figures 5 and 6 showed how the growth of small cracks from holes is greatly affected

by the local subgrain microstructure, namely the direction and size of the martensitic

laths along the path of the crack. To study the microstructure effects on crack growth

from a non-metallic inclusion an initiation site was cross sectioned and carefully

studied. A non-metallic inclusion had caused the failure of a smooth Q T steel test bar

‘N9A’ after more than 200Â106 cycles at the R ratio of -1. A S E Mimage and cross

section of this inclusion are shown in Figures 7 and 8. An “optically dark area” (ODA)

as defined in [1] is marked besides the inclusion. The cross-section images of the metal

matrix on different sides of the inclusion are compared in Figure 9.

The pictures in Figures 7-9 show that the O D Awas formed on the right hand side of

the inclusion where horizontal martensite laths can be seen below the O D Aregion

parallel to the direction of crack growth. The length of these laths equals to average

grain size. They are probably among the first ones to form and cross the whole prior

austenite grain in question. W eassume that the crack had first grown and O D Awas

formed on this side, where the microstructure provides a more preferential direction for

crack growth. Thus, our observations on the preferential crack paths from surface

notches (Figs 5-6) and subsurface inclusion (Figs 7-9) are in agreement. Furthermore, a

partial explanation to the formation and origin of the O D Ais proposed based on a

connection to microstructure.

Figure 7. The non-metallic inclusion that caused failure in a smooth test bar N9A.

The part of the fracture surface circled by a black line is the ODA.

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