Don't Let the War Fatigue Those Parts
Page 28
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THE contents of the letter reproduced above draw attention to a number of interesting and difficult problems, same connected 'with the issue directly raised, others arising from secondary but no less important considerations.
In any structure subjected to alternating stress, whether of simple or complicated form, certain components must inevitably undergo, in the first stage, some degree of work hardening. This will take place in big-end bolts and main-bearing bolts (stressed principally in tension) and in crankshafts And crankcases, which are subjected to complex forms of torsional stress.
Damage Curve Concept Nevertheless, if the stresses imposed lie below those figuring on the " damage curve " for the part in question, then that part will run indefinitely without failure. The damage curve may be likened to a fatigue curve, but, in general, lies a little below this. If, however, at any time and for any stress, peak loadings rise above the damage curve and above the elastic range of thematerial, then permanent deformation is caused and, sooner or later, failure is inevitable unless steps be taken to prevent it.
It will be recalled that a definite safety-first procedure is adopted in high-pressure apparatus, such as gas bottles, which are annealed regularly before recharging, so that any permanent deformation caused by the high pressures to which they are subjected is relieved. In theory, the parts of an internal-comleustion engine: could -also lie_ so treated, hut so great is the number of specialized steels used in a modern engine, that, unless manulac turers were to supply with every vehicle a full chart of the materials used and the heat treatments given to those materials, it would he extremely difficult in practice to administer regularly " tonic " heat treatments.
_ However, the thing is not impossible ; foolproof furnaces are available, numberless metallurgists are ready to give advice and, if necessary, assistance, and instruments are on the market which render heat-treatment processes practically automatic.
Could highly stressed engine parts be treated in this way their life could be immensely prolonged. It would seem, however, that a necessary step in the attainment of this ideal is a reduction in the number of materials specified, and, for vital portions of the engine structure, a national code of materials as few in number as possible. If this were done (as in fact in Germany has largely been done, both as to materials, designs and even dimensions), then the realization of the project would be brought considerably nearer.
At the moment, however, it would be almost essential for an operator desiring to put this scheme into practice to carry a heat-treatment shop and to run a laboratory and test shop.
We quite agree that gearboxes are not likely to show fatigue phenomena. We are, however, a little surprised to hear that trouble is not chef:Metered in' differentials, more particularly in planet pinion. It can only be concluded that these parts are designedWith sueh generous factors of safety that loads imposed upon them do not -exceed damage curve values.
A pOint of minor interest lies in the fact that case hardening, to which mcist of the differential components are subjected, does commonly raise fatigue values by preventing surface alteration, scoring and incipient fracture.
If a vehicle be run until softie part 'definitely breaks down, due to fatigue failure, it is quite conceivable that its runningqualities of life after reconditioning would indeed he affected. This is to be attributed to the fact that the fatigue process occurring in the part which ultimately fails is not an acute, but rather a clinenic, disease ; that is, it takes place gradually, usually due to a propagating track.
Progressive Loading
Thus, say in the case of a crankpin, a minute crack would commence at the surface, and the nontinal deflection of the crankshaft under load would, as aresult of this .crack, be increased, that is, the apparent deflection of the part would increase and as the crack grew larger and proceeded farther into the material, so would the deflection grow larger, throwing heavier loads on the main bearings and ' imposing secondary bending stresses on almost every component in' the structure.
Much of the damage caused by these consequential ills would be cured in reconditioning, but Obviously, in certain 'eases, this would, not be possible, mainly because the damage caused is so insidious that it hr extremely difficult to detect except by elaborate, lengthy and costly tests. Thus, cure for the ills -cited would'seem 'to be,' first Of: all, rigid .standardization of a ininimum number of materials, and the choice of such materials that elaborate treatment might safely be dispensed With,