METALS SUBJECT TO FATIGUE

Dominion, Volume 14, Issue 306, 20 September 1921, Page 5

METALS SUBJECT TO FATIGUE

. i REST NEEDED AFTER CONSTANT STRAIN. It is not generally known that metals —iron and steel in particular—afo subject to fatigue after constant strain, and need an occasional rest, just as an overworked human bring requires a holiday every now and then to recuperate from his toil (writes an engineer in tho “Daily Mail”). ' u A case in point occurred recently at Mapping Station, where two lives were lost through a runaway goods wagon colliding with a stationary motor train. Tho disaster was caused by tho drawbar hook coupling the wagon to the remainder of tho goods train breaking on a gradient, leaving tho wagon free .to run back on its course with increasing impetus. Experts who examined the broken pieces of the hook could find no flaw of any kind in the metal, and were unanimous in declaring that the metal, brittle through fatigue, had collapsed beneath excessive shock, possibly received during shunting operations. It was impossible to know beforehand that the hook needed a rest, although had fate ordained that the wagon should be held up for a few days, the rested metal woiud in all probability hive recovered and tho accident have been averted. A good vest will work wonders with a piece of petal suffering from fatigue, and give it a new lense of usefulness. A few years ago a Bessemer steel rail collapsed under the weight of « goods Irniil and broke into several pieces. It wan found that'the rail had done gallant iKTvieo for 22 years, and badly needed a real. Had it been replaced by another rail, and lain aside for. a few months, it would have returned to its normal sialo and done duly for many more Lord Kelvin was tho first to make this dlueovery, and, by experiments, prove Ila Iriif'h. By keeping iron wres in a idnlo of constant oscillation for six days ho iliHi'overcd that they, had lost a contddorn’blo amount of their elasticity. A dnv’s rest was giyen, when the wires wore found to have returned to their origii'inl rendition. Repeated experiments proved this result to be an absolute fact wHh 'Tcgai’d (o wrought '.ron and stfrel. Singularly enough, although a wrought iron bar will recover 10 per cet in elnstic.ily after a tlir/o weeks’ respite from hard work, cast iron improves m sti’ongfli if {subjected to constant sheiks —re much so that experiments have demonstraled that, cast-iron bars ' .'d gain as much as 100 per cent, 'n strength after being subjected Io a. succession of shocks.

There have- been instances in the past of building! giving way through the midden collapse of a supporting girder owing Io tho unceasing strain the metal lias had to endure. Happily, in this age of concrete ami steel, such cares ore rare, as the metal used n-iwadavs is invariably to-d.i-l to withstand n fur greater strain timn it will bo called upon to bear when in use.

The knowledge, has undoubtedly benefited tho travelling public, as the railway companies now make ?. point of Inking up tho rails on all .nueh-used lines every few years to give the old metals a needed rest and restore thoir elasticity for another period of years.