“TIREDNESS” OF METALS

Bay of Plenty Beacon, Volume 11, Issue 88, 7 October 1947, Page 7

“TIREDNESS” OF METALS

EXTENSIVE SURVEY AFOOT

METALLURGIST’S REPORT

One of the oldest most baffling problems in engineering science has been why certain metals get “tired” and fail from fatigue.

A new concept as to the reason for this “tiredness” has emerged from recent research conducted by one of the world’s leading makers of ball bearings, says the Dunlop Bulletin. No fundamental theory for the fatigue phenomenon—the strange behaviour of metals that accounts for more failures in moden engineering practices than any Other single factor—has previously been advanced, despite the fact that research on the problem has been going on for generations. For instance, what happens to the properties of a steel object, like an automobile axle or ball bearing, that causes it eventually to crack in a certain spot, when placed under repeated stress? The metallurgist who conducted the research believes he has found the answer—steel, like all metals, is crystalline. In the carefully heat-treated part of bearings these crystals are extremely minute, but they are still crystals. The pohesive forces are stronger within any one crystal than they are across the boundaries of adjacent crystals. The metallurgist states that it seems reasonable to conceive points where two, three or more crystals meet and where the cohesive forces are particularly weak.

In a running bearing the maximum stress is a little beneath the surface of the race or rolling member, in which' exertmely hard steel is normally used. If such an unfortunate crystal junction happens to be in this area of maximum stress, it may furnish the starting point for a fatigue crack. The metallurgist believes that is the reason why even the highest grade precision-made bearings occasionally fail.