Steel critical crack length

Therefore, the critical crack length is proportional to the square of KjJay, which is the basis for the ranking of the metal alloys in Table B.5. The ranking is provided in Table 8.3, where it may be seen that the medium carbon (1040) steel with the largest ratio has the longest critical crack length and, therefore, is the most desirable material on the basis of this criterion. 

Frost, N.E. A relation between the critical alternating propagation stress and crack length for mild steel. Proc. Inst. Mech. Eng. 173, 811 (1959)... 

The failure of many of the World War II Liberty ships is a well-known and dramatic example of the brittle fracture of steel that was thought to be ductile. Some of the early ships experienced structural damage when cracks developed in their decks and hulls. Three of them catastrophically split in half when cracks formed, grew to critical lengths, and then rapidly propagated completely around the ships girths. Figure 1.3 shows one of the ships that fractured the day after it was launched. 

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