FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1952
1952 - 0966.PDF
432 FLIGHT FATIGUE FAILURE —was Recognised Seventy Years Ago : A Poet's Understanding By RICHARD HOWARD THE news, reported in the previous page, of the obligatory replacement of structural components on certain airliners inevitably recalls Nevil Shute's book "No Highway" and the film based on it. It will be remembered that his plot centred on the possibility of fatigue failure of the tailplane front spar in the Reindeer, a hypothetical airliner. Not until one of these aircraft was over mid- Atlantic was it realized that the life of the component had almost elapsed. The central figure of the story, an R.A.E. scientist who had put forward a nuclear thesis to account for such failures, was on board. Would the tail stay on long enough for the aircraft to reach Gander ? This ingenious plot had a sound basis of fact, or at any rate of theory, for its author had had long experience in the aircraft industry. Yet it is a truism that in literature there is no such thing as a new plot . . . and perhaps the following unusual contribution—by an automobile engineer—proves it. AT first glance, the proverbial chalk and cheese may /% appear to have more in common than an eminent American poet and established scientific data. But, strangely enough, Oliver Wendell Holmes, who was a doctor by profession and an author by inclination, also combined these talents with a keen understanding of at least one fundamental engineering problem. Not only did he realize that it existed, but he also grasped its significance long before it received serious consideration by engineering research organizations, of which during his lifetime (1809-1894) there can have existed only a handful. This problem, the fundamentals of which are discussed below, is dealt with in his delightful and humorous poem The Deacon's Masterpiece; and, although it ascends in nights of fantasy, the poem has a basis that is nevertheless theoretically possible and, as the author points out, all strictly logical. But before his genius in dealing elegantly with the subject can be understood, the principal theme of the poem should be stated. It must also be related to present engineering concepts :— Have you heard of the wonderful one-hoss shay, That was built in such a logical way It ran a hundred years to a day, And then, of a sudden, it—ah, but stay I'll tell you what happened without delay, Scaring the parson into fits, Frightening people out of their wits— Have you ever heard of that, I say ? The failure of engineering components in service was, at the time the poem was written (about 1880), hardly recognized as a research problem and the causes were certainly not often the subject of investigations. As a result of later investigations, how ever, it is now recognized that metals which are subjected to loads that vary continuously from one value to another, or from one kind to a reverse kind, are liable to failure at much lower values than result from conditions of steady load. This kind of break down, as most readers of Flight are aware, is called "fatigue fail ure"; and a fatigue crack in the metal is usually the first signs that failure may ultimately take place. To Holmes, in considering his projected poem on the perform ance of the one-hoss shay, no kind of failure other than fatigue failure appeared to be significant. Indeed, the possible alterna tive kind—that commonly caused by abrasion and wearing out— hardly had a place in his thoughts. Attention was specifically directed to the fact that, in the construction of the vehicle, con sideration was to be given to the problem of avoiding failure caused by breaking down rather than to failure caused by wearing out:— Now in building of chaises, I tell you what, There is always somewhere a weakest spot, In hub, tire, felloe, in spring or thill, In panel, or crossbar, or floor, or sill, In screw, bolt, thoroughbrace—lurking still, Find it somewhere you must and will, Above or below, or within or without, And that's the reason, beyond a doubt, A chaise breaks down, but doesn't wear out. The proneness of metals to fatigue failure was first observed some time after Holmes had published his poem, and although the exact nature of this phenomenon is as yet imperfectly under stood, a fairly complete picture has been built up in the inter vening years of the conditions under which it is likely to take place. This kind of failure can occur, for example, in many engineering components and especially those with which vehicles are equipped. It could certainly have taken place in the one-hoss shay, in the same way as it now does in many mechanical com ponents, among which, for example, may be cited the axles, roller bearings, transmission and crankshaft. Such components are subject to loads which may vary in magnitude many times a minute and it is precisely this type of loading which may lead to fatigue failures at some time during the life of the machine in question. Although research into fatigue phenomena has to date been mainly restricted to metals, no such limitations were considered by Holmes. His writing implies that for the whole range of materials from which the Deacon's vehicle was to be built, there already existed sufficient data on fatigue characteristics to ensure a long service life :— "Fur," said the Deacon, "t's mighty plain Thut the weakes' place mus' stan' the strain." ... So the Deacon inquired of the village folk Where he could find the strongest oak, That couldn't be split nor bent nor broke ... Step and prop-iron, bolt and screw, Spring, tire, axle, and linchpin, too, Steel of the finest, bright and blue Thoroughbrace bison-skin, thick and wide; Boot, top, dasher, from tough old hide Found in the pit when the tanner died. That was the way he "put her through"— "There.'" said the Deacon, "naow she'll dew." Do ! I tell you, I rather guess She was a wonder, and nothing less ! • It is here that we come to the critical point both in the construc tion of the poem and also in the study of fatigue of metals. As might be inferred, the term "fatigue" conveys the impression of becoming tired, and one of the main factors in the study of fatigue is the observation of the tendency of metals to become tired over long periods of time. This period, known as the fatigue life, is the limiting factor which sets the bounds to the useful life of the component. For example, a broken axle in service is generally one which has reached the end of its fatigue life; its fatigue- resistance has become zero. It is, of course, evident that a high fatigue-resistance implies a long life and, correspondingly, that a low one implies a short life; but this statement also poses the question of exactly what is involved in the term "long life." Holmes had no hesitancy on this point; for although, according to his poem, the shay was built on Lisbon earthquake day, 1755, the Deacon plainly had in mind an abnormally long fatigue life :— But the Deacon swore (as Deacons do, With an "I dew vum," or an "/ tellyeouw") He would build one shay to beat the taown 'ri the keounty V all the kentry raoun'; It should be so built that it couldn' break daown. In practice, the experimental determination of fatigue life of components involves extensive testing before any idea can be obtained of the probable fatigue-life of metals or component parts. Once this experimental data is obtained, it forms the basis for the scientific prediction of the probable life of similar com ponents and similar materials. As the basis for predetermination of fatigue life, such data is, of course, of considerable practical value to design engineers, who are then able to prepare designs which can be predicted to have a certain specified life. (For example, with roller bearings, and also in the design of gears, there exist whole families of fatigue-life curves which enable intelligent prediction of probable life to be made for these com ponents. Thus, for example, transmissions for combat vehicles can be designed on a short-life basis, whereas transmissions for other vehicles can be designed for longer lives, but certainly not on the basis of 100 years.) As with much scientific data, there exist some unknown factors which tend to upset the exact fulfilment of the fatigue-life predic tion. Although fatigue-life curves exist, there is often also a diver gence in actual results from the predicted figures. This may
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
