What size hole should be drilled at the ends of a progressive crack?

• A. 1/4 inch
• B. 1/8 inch
• C. 3/8 inch
• D. 1/2 inch

Answer: (A)

Stopping a progressive crack relies on stop-drilling: drill a hole at the crack tip to blunt the end and remove the sharp notch that drives crack growth. The hole size has to be large enough to blunt the tip and reduce the stress concentration, but not so large that it unnecessarily weakens the structure. A quarter-inch diameter hole achieves that balance for common thicknesses, giving a solid blunt tip to prevent reinitiation under service loads while preserving as much intact material as possible. If the hole were smaller, the crack tip could still propagate; if it were larger, the remaining section would be weakened more than needed.