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Strength of flfcatertals.
The strength of materials may be divided into Tensile,
Crushing, Transverse, Torsional, or Shearing, and besides
this, the elasticity of the material or its resistance against
deflection must also be taken into consideration in figuring for
strength.
Tensile Strength.
From experiments it is known that it it will take from
40,000 to 70,000 pounds to tear off a bar of wrought iron one
inch square. Therefore we usually say that the tensile strength
of wrought iron is from 40,000 to 70,000 pounds, according to
quality. The average is 50,000 to 55,000 pounds. The tensile
strength of any body is in proportion to its cross sectional area
;
thus, if a bar of iron of one square inch area will pull asunder
under a load of 40,000 pounds, it will take 80,000 pounds to pull
asunder another bar of the same kind of iron but of two square
inches area. The tensile strength is independent of the length
of the bar, if it is not so long that its own weight must be
taken into consideration. Table No. 25 gives the load which
will pull asunder one square inch of the most common materials.
No part of any machine should be strained to that limit. A
high factor of safety must be used, sometimes from 4 to 30 or
even more, which will depend upon the kind of stress the mem-
ber is exposed to, as dead load, variable load, shocks, etc. Dif-
ferent factors of safety are also used for different kinds of
material. ( See page 274).
nodulus of Elasticity.
The modulus of elasticity for any kind of material is usually
defined as the amount of force which would be required to
stretch a straight bar of one square inch area to double its
length or compress it to nothing, if this were possible. But a
more comprehensive definition is to say that the modulus of
elasticity is the reciprocal of the fractional part of the length
which one unit of force will, within elastic limit, stretch or com-
press one unit of area. For instance, if the modulus of elasticity
for a certain kind of wrought iron is 25,000,000, it means that it
would take 25,000,000 pounds of pulling force to stretch a bar of
(213)
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