METALS THAT TIRE.

Auckland Star, Volume LXVI, Issue 98, 27 April 1935, Page 8 (Supplement)

METALS THAT TIRE.

THEORIES ABOUT "FATIGUE."

MORE LIKELY IN PURE METALS PROFESSOR ANDRADE'S DISCOVERY. Tho discovery that .even the purest metal can get "tired" has been made by Professor E. N. da C. Andrade, of University College, London. It is a conclusion which is of considerable importance to engineers as well as to all those whose lives depend 011 the smooth working of machinery.

"Fatigue" in a metal means its liability to fracture under continually repeated stress, and there aro few machine or structural components, the working conditions of which can be so adjusted as to exclude this possibility.

Oil the contrary, tho whole tendency to-day is towards higher engine speeds and working stresses, and 011 a conservative estimate SO per cent of all the failures in modern engineering practice can lx> attributed to fatigue, The essence of fatigue is that, under continued strain, a metal may yield to forces considerably smaller than would normally break it.

Breaking as a result of fatigue is accompanied by a "slipping" of successive layers of crystals in the metal. This slipping can be seen under the microscope, although there is in some cases a certain amount of difficulty in interpreting the photographs obtained. Hitherto it lias been generally believed that "slipping," and therefore fatigue, was caused by the presence of impurities in the metal. Now Professor Andrado has taken tho purest metal which 110 could find, mercury frozen solid at a temperature of some 00 degrees below zero, and 110 finds that "fatigue" is actually greater than if tho metal is less pure.

Professor Andrade estimates that his mercury was pure to within one part in one hundred million, and a well-known commercial firm to which it was submitted could find 110 trace of impurity whatever, in itself a remarkable tribute to his skill.

Evidence of this slipping 111 pure solid mercury has been obtained in a scries of beautiful photographs, taken beneath the microscope, which the most expert criticism has found 110 cause to interpret differently.

The places along a rod of solid mercury at which slipping takes place are not uniformly spaced, and it has not yet been possible to decide whether they represent pre-existing weaknesses in the surface of the mercury. It is possible however, that the effect may be connected with another discovery recently mado by Professor Andrade —that of "invisible cracks" in most forms of solid material. It is these cracks, it is believed, which make the practical strength of most materials so much less than the theoretical strength, as calculated from tho "binding forces" between the different atoms which make it up.

111 anv case it now appears certain that metallurgists have been 011 the wrong track in thinking that they could reduce fatigue by eliminating impurities. If a solution is to be found of one of tho pressing problems of modern engineering, Professor Andrade has shown that it must be along other lines.