Materials and Methods of Construction in Light Structures Page: 30 of 38
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N.A.C.A. Technical Memorandum Jo. 515
small permanent deformations even with very small loads
and more or loss for every material. In testing materi-
als it has therefore been found necessary to establish,
for such permanent elongations, arbitrary minimum limits,
such as the measure of the elasticity and proportionality
limits, i.e., in other words, any determination of the
proportionality limit, independently of arbitrary assurmp-
tions is practically impossible. Even from these rela-
tions, it is obvious that the problem of fatigue cannot
be regarded as definitely solved. Experience shows that,
in airplane parts which are exposed to vibrations, unex-
pected breaks, some of them dangerous, repeatedly occur,
as they never occur in the same way with wood.
The physical properties of metals and wood differ fun-
damentally. All metals have a crystalline structure, i.e.,
the material is not homogeneous and its strength character-
istics cannot possibly be uniform as regards the component
particles. On the contrary, cellulose, the chief con-
stituent of wood, is an amorphous substance. It does not
have, like metals, separating surfaces between adjacent
crystals whose cohesion, always weaker than that of the
rest 'of the material, can be still further weakened by
mechanical action. In other words, the metal, being of a
crystalline structure, is gradually weakened by the en-
gine vibrations, while the wood, being amorphous, is not
5. As regards the corrosion of light metals in com-
parison with the weathering of wood, very few exact and
comprehensive data are yet available. An unprejudiced
judge, however, receives the impression that the corro-
sion of the light metals is as important as the weathering
of wood, at least in our climate, and requires correspond-
ing precautionary measures. Like wood, no metal structure
can last long without a good protective covering of paint
The defect most commonly imputed to wood, namely,
its.liability to warp, generally plays a very subordinate
role in accurate investigation. Previously it had always
been said, for example, that ordinary wood-and-wire fuse-
lages very easily become distorted. More acoarate inves-
tigation nearly always shows, however, that no changes
have occurred in the length of the wood, but that the
troubl.e is almost always caused by the slackening of the
brace wires due to poor terminal fastenings. Aside from
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Rohrbach, Adolf. Materials and Methods of Construction in Light Structures, report, May 1929; (https://digital.library.unt.edu/ark:/67531/metadc65364/m1/30/: accessed May 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.