WHY METAL BREAKS

Evening Star, Issue 22722, 9 August 1937, Page 15

WHY METAL BREAKS

It is an erroneous idea that fatigue crystallises a metal, so making it weak. There is no rational basis for the idea that a shaft breaks owing to fatigue. As a rule a superficial scratch, a flaw, or some high local stress initiates a crack, and then, under the influence of repeated stresses, the fissure gradually extends through' the metal until, finally, the sound material left is insufficient to carry the load. The part breaks, apparently suddenly, but in reality the process may have been, going on for months. A close examination ot the broken part will probably reveal traces of a long-standing weakness, and the freshly-broken metal, often just, a small portion of the whole, can easily be distinguished by its brightness. A defective axle shaft may drive under normal conditions for a very long time, but go immediately the car runs on to a patch of sand or tries to cross a rubble splash. The sudden stress finds the weakness, or the weakness may become so pronounced that the breakdown actually occurs as the car is running apparently normally on.. In most cases, however, it will be noticed that some sudden strain or jerk causes the shaft to go.