TIRED METAL.

Star (Christchurch), Issue 19060, 3 May 1930, Page 23 (Supplement)

TIRED METAL.

A bridge collapsed in New Jersey because it was tired. Its steel beams and girders appeared to be in excellent condition, but the professor who examined the remains with a microscope declared fatigue of the metal to be the cause of the’breakdown. Metals, like animals, become tired, and accumulate fatigue poisons, though they are of an extremely different kind. The fatigue of metals is not unlike decay. Metals are made up of crystals, but they are different from the geometrical and brittle crystals of sugar or salt, which give way as soon as they are stretched. Metal crystals will yield, and they are not all of the same size or shape. When the metal is strained or bent each crystal undergoes a change of shape merely by slipping. Whole layers of crystals' si in oyer one another, and each straining, with the slipping of the crystals over one another, alters the surface of the crystals. Some of the crystals grow larger, some of the smaller ones are broken. If this gliding of crystals is forced on the metal too often the crystals become brittle. The gliding refuses to occur. A crack begins. This is more likely to happen if the strain Distress on the metal is continually changing, as with a revolving axle. The strain is always being altered in direction. There is so much slipping of crystals that presently the crystals break down. The same thing happened in the steel parts of the Jersev Bridge If a bridge were built and left always supporting the same weight it would last for years. But when traffic goes over it unceasingly the load is always changing, the steel crystals are always being shifted to and fro. and at last they grow tired and break.