Liberty ships, thousands of which were constructed during World War II, were welded rather than riveted. Many were mistakenly made from a steel that became brittle at the temperatures of the Atlantic in winter. They had square hatches with sharp corners from which a crack could start and propagate across the deck which, being welded, allowed a continuous path not interrupted by riveted plates. It is said that alert seamen, observing such a crack to start, would seize a power drill and drill a hole at the crack tips, effectively blunting it and reducing the stress concentration. If the crack when the seaman first saw it was 1 m long, and the largest drill he had was in diameter, what is the stress concentration at the end of the crack once the hole was drilled? You will find the equation for the stress at a distance r from the tip of a crack with a length 2a in Appendix B, Table B10.

Data From Appendix B10

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2r=25 mm