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214 STRENGTH OF MATERIALS.
one square inch area to double its length, if this could possibly
be done; but it means also—which is exactly equivalent
—
that one pound of pulling force will stretch a bar of one square
inch area one 25-millionth part of its length, or one pound
compressive force will shorten the same bar one 25-millionth
part of its length, and that two pounds of force will stretch or
compress twice as much, three pounds thrice as much, etc.
Strength of Wrought Iron.
From experiments it is known that wrought iron can not
very well be stretched or compressed more than one-thousandth
part of its length without destroying its elasticity ; therefore if
a bar of wrought iron has 25,000,000 as its modulus of elas-
ticity, one pound will stretch it 25 o oV o o o of its length and it
would take 25,000 pounds to stretch it toVo of its length. Thus,
25,000 pounds would then be said to be its strength at the limit
of elasticity for that kind of iron ; 80 to 100 per cent, more will
usually be the ultimate breaking load.
The pull or load which such a bar can sustain with safety
will depend a great deal on circumstances, but it must never
exceed 25,000 pounds per square inch of area. It must not
even approach this limit if the structure is of any importance or
if the load is to be sustained for any length of time, or if it is,
besides the load, also exposed to shocks or jar.
Strength of Cast Iron.
Cast iron of good quality has a modulus of elasticity of
15,000,000 pounds, but if strained so it will stretch ysW of its
length its elasticity is usually destroyed. For instance, a bar of
cast iron of one square inch area is exposed to tensile strain,
its modulus of elasticity being 15,000,000 pounds and its elas-
ticity being destroyed if it stretches jsV of its length, what
then would be its strength at limit of elasticity? One pound
will stretch it
T
__
7J
i__^_ of its length, therefore it must take
10,000 pounds to stretch it y^Vo of its length; thus we would
say that 10,000 pounds is its strength at limit of elasticity. It
is not always that cast iron is of as good quality as that; very
frequently its elasticity is destroyed if it is exposed to a tensile
stress of 6,000 pounds per square inch of area ; thus the strength
of cast iron at its limit of elasticity is often found to be only
6,000 pounds instead of 10,000 pounds. Besides, it is very
often found that a pulling force of 10,000 pounds will stretch
a bar of one square inch area one twelve-hundredth part of its
length, and this, of course, gives the modulus of elasticity
12,000,000 pounds. Frequently cast iron is of such quality that
it cannot be stretched over 2 sV ^ * ts l engtn before its elas-
ticity is destroyed. Cast iron is very variable in quality, and
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