We will now imagine it bent to form a circle, thus:
The centre line is still the same length as before being
bent; but the top line, being farther from the centre of the
circle, is now longer than the centre line. That can be due
only to the strain of elongation produced by the stress of
tension. The wood between the centre line and the top
line is then in tension; and the farther from the centre,
the greater the strain, and consequently the greater the
tension.
The bottom line, being nearest to the centre of the circle,
is now shorter than the centre line. That can be due only
to the strain of crushing produced by the stress of compression.
The wood between the centre and bottom lines is
then in compression; and the nearer the centre of the circle,
the greater the strain, and consequently the greater the
compression.
It then follows that there is neither tension nor compression,
i.e., no stress, at the centre line, and that the wood
immediately surrounding it is under considerably less stress
than the wood farther away. This being so, the wood in
the centre may be hollowed out without unduly weakening
struts and spars. In this way 25 to 33 per cent. is saved in
the weight of wood in an aeroplane.
The strength of wood is in its fibres, which should, as far
as possible, run without break from one end of a strut or
spar to the other end. A point to remember is that the
outside fibres, being farthest removed from the centre line,
are doing by far the greatest work.
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