May 24, 1997

Article at New Scientist

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NEWS | The plane truth about holes

Wilson da Silva, Sydney

DESPITE their solid appearance, modern aircraft are riddled with holes—each one a potential weak spot. Now a novel method for making holes, invented by Australian defence scientists, could make planes stronger and extend their working lives.

A Boeing 747 has more than half a million holes—tiny ones for rivets and large ones for doors. Landing and take-off stresses generate micro-cracks which radiate from the holes, eventually causing metal fatigue. If not repaired this can lead to disaster. But checking each one is expensive and time consuming, requiring planes to be grounded for long periods.

Now Albert Wong and Nik Rajic of the Defence Science and Technology Organisation’s Aeronautical and Maritime Research Laboratory in Melbourne have found a way to strengthen rivet holes and extend the life of a plane. 

The new method replaces the drills and stamps used in conventional aerospace manufacturing. Instead, clamps are placed on the top and bottom of a metal sheet being pierced. Compression punches, or mandrels, press into the sheet fromopposite sides—but stop before they meet. The bottom mandrel is withdrawn, leaving the top one to finish the job.

The compression created by the bottom mandrel runs in the opposite direction to that created by the perforating mandrel from above, forcing compressed metal to “ooze” sideways into the hole walls. “The radial outward flow of the metal isthe trick,” says Wong. “The metal on the edge is highly compressed giving improved fatigue resistance.”

In tests on aluminium sheets the new holes lasted 2.5 times as long as standard perforations. They also survived 50 per cent longer than holes strengthened using a process known as cold expansion.